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MONOGRAPHS OF THE BOREAL ENVIRONMENT RESEARCH

16

Liisa LepistoPhytoplankton assemblages reflectingthe ecological status of lakes in Finland

Yhteenveto: Kasviplanktonyhteisot Suomen jarvienekologisen than kuvaajina

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ContentsSummarised publications and authors contributionAbstract.1 Introduction

1.1 Phytoplankton1.2 Terminology

1.2.1 Plankton1.2.2 Classification of phytoplankton1.2.3 Water blooms and eutrophication1.2.4. Ecological status

1.3 Lakes1.4 Aims of the study

2 Materials and methods3 Phytoplankton in different types of lakes

3.1 Long-term eutrophication of a shallow lake3.2 Effects of forest fertilization on a brown-water lake3.3 Development of the trophic degree in two man-made lakes

3.3.1 The Lokka reservoir3.3.2 The Porttipahta reservoir3.4 Problems caused by increased phytoplankton3.5 Phytoplankton as an indicator of the status of lakes in Finland

3.5.1 General3.5.2 Oligotrophic lakes3.5.3 Acidic lakes3.5.4 Dystrophic lakes3.5.5 Mesotrophic lakes3.5.6 Eutrophic lakes3.5.7 Hyper-eutrophic lakes

4 Problems in the analysis of phytoplankton

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Phytoplankton assemblages reflecting the ecological status of lakes in Finla

Summarised publications and the authors contributionThis thesis is based on the following papers, which are referred to in the teI-Vu:I Lepisto, L., Rike, A. & PietilSinen, 0.-P. 1999. Long-term changes of peutrophicated boreal lake during the past one hundred years (18931998).223244.

II Lepist, L. & Saura, M. 1998. Effects of forest fertilization on phytoplawater lake. Boreal Environment Research 3: 3343.III Lepisto, L. 1995. Phytoplankton succession from 1968 to 1990 in the sPublications of the Water and Environment Research Institute 19: 142.IV Lepisto, L. & Pietilinen, 0.-P. 1996. Development of water quality anties in two subarctic reservoirs and one regulated lake. In: K. V. Rao (ed.).conference on aspects of conflicts in reservoir development & managemenCity University, London, UK. 553566.V Lepisto, L., Antikainen, S. & Kivinen, J. 1994. The occurrence of GonyDiesing in Finnish lakes. Hydrobiologia 273: 18.VI Lepisto, L., Lahti, K., Niemi, J. & Fardig, M. 1994. Removal of Cyanophytoplankton in four Finnish waterworks. Algological Studies 75: 1671VII Lepisto, L. & Rosenstrm, U . 1998. The most typical phytoplankton tlakes. Hydrobiologia 369/370: 8997

Liisa Lepisto has since 1965 participated in the microscopy work of phytlakes in Finland and is responsible for the phytoplankton data in papers I In paper I L. Lepist was responsible for planning of the examination, flished data, and for processing the phytoplankton data. All authors tookdata processing, interpretation of results and writing in their own area of e

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Phytoplankton assemblages reflectinthe ecological status of lakes in FinlaLiisa Lepisto

Finnish Environment Institute, P.O.Box 140, FIN-0025Lepist, L. 1999. Phytoplankton assemblages reflectinlakes in Finland. Monographs of the Boreal Environme

ABSTRACTA review is presented of studies on phytoplankton reflecting theman- made lakes in Finland. Attention is paid to phytoplankton qlake types, and to changes in phytoplankton assemblages caused bwaste water loads and construction of man-made lakes. The respTuusulanjrvi to eutrophication during the twentieth century ancaused by water protection efforts since the late 1970s was investion in the nutrient load in 1979 changed the phytoplankton assecyanophytes were replaced byN- fixing cyanophytes. The totalspecies indicating more oligotrophic conditions reappeared. Thplankton to the fertilization of the catchment area of a humic foreconsidered. Only in the subsequent spring was an increase in phNo other biomass maxima were observed in the lake until five ysemen became abundant. After the construction of man-made lakvia several stages; the original river plankton, such as pennate didiatoms, especially during periods of strong water level regulatiflected the water quality of the reservoirs. In the meso-eutrophicbecame abundant, especially during warm weather conditions. T

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8 Lepisto Monographs of the Borea

1 Introduction1.1 PhytoplanktonStudies on phytoplankton assemblages of lakes inFinland were initiated in the late I 890s by KaarloLevander (1900). In the early 1910s, HeikkiJrnefelt (e.g. 1932, 1934, 1956a) started his fundamental studies on phytoplankton, coveringlakes from southern Finland as far north as Lap-land. He classified the trophic status of lakes according to their phytoplankton assemblages(Jarnefelt 1952, 1956a). On the basis ofJarnefelt s studies, the monitoring of water bodiesin Finland was started by the water authorities inthe early 1960s. The first monitoring networkcovered 150 lakes (Heinonen 1980). In additionto the monitoring data of water authorities, several authors have published phytoplankton datafrom Finnish lakes. For example, Ilmavirta (1980,1983), Arvola (1983, 1984) and Salonen et at.(1984) studied humic lakes and focused on the influence of water colour on phytoplankton assemblages. Eloranta (1976, 1986, 1995) examined thephytoplankton of lakes in central and southernFinland, including lakes situated in the nationalparks which could be regarded as reference lakesfor monitoring. Ilmavirta et at. (1984) studied thephytoplankton of 151 lakes in eastern Finlandduring the summer stratification. Spatial and seasonal variations of phytoplankton of the oligotrophic Lake Paajarvi, Lammi, southern Finland,was studied by Ilmavirta and Kotimaa (1974).The quantity and quality of phytoplankton ofLake Pyhajarvi, Sakyla, SW - Finland was brieflydiscussed by Sarvala and Jumppanen (1988) andof Lake Ala-Kitka, NE-Finland by Kankaala et at.(1990). The horizontal distribution of phytoplankton and relationships between phytoplank

water body in low dtention. However, witself as an increasphytoplankton becoof the process of euimpacts, such as anplankton species, tprolonged water bland Waisby 1975, P

The dynamics ofplankton in oligotrfrom those in eutropton is often charactmer and autumn a1967, Ilmavirta 198Trifonova 1986,Eloranta 1995). Inability there are usuvariations in the aThe species assembexact timing, and hvidual species mayinant species at awill not always be tpopulation densitiesof magnitude (Reyn

The compositionblage (Fig. 1) doesbut also on physicaation, turbulence achemical factors (eand on biological faand loss rates amonation and competit1967, Harris 19861989). These factorswhen considering tphytoplankton (Jacand due to their co

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10mg L

LuCuCuE0

1

8

6

4

2

0

A cidic Oligotr.Lake types

Cyan. Crypt. Dinop. [lChrys. LiDiat.

Fig. 1 The composition and quantity (wet weight mg L) of phytoplankton (June (not hyper-eutrophic) with some typical taxa in 19811997 (VII, database of FEI).cryptomonads, Dinop. = dinoflagellates, Chrys. = chrysomonads, Diat. = diatoms, Chlomum semen (Raphidophyceae) is included in the group others. Figures according to T

Many of them are only facultatively planktic(Hutchinson 1967, Round 1981). Plankton couldbe regarded as the community of prokaryotic andeukaryotic organisms, such as bacteria, algae,protozoa and larger zooplankton adapted to sus

The plankton of lsegregated from platon) and rivers (pota1986). The term euused to refer to the

0

--,i, , k-c.LI

Dystr. Me

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10 Lepist Monographs of the Boreal

1.2.2 Classification of phytoplankton

Representatives of several groups of planktonicalgae and bacteria, as well as the infective stages of certain actinomycetes and fungi are suspended in freshwater. Algae, although a muchused term, does not have any exact systematicmeaning, and should be regarded as a loose blanket term for primitive cryptogamic photoautotrophs, as proposed by Reynolds (1986). Heterotrophic species, such as Katablepharis ovalisSkuja, have often also been included in the phytoplankton. Furthermore, several phytoplanktonspecies are considered to be capable of mixotrophy (Ramberg 1979, Salonen and Jokinen 1988,Tranvik et al. 1989, Jansson et al. 1996, Robertsand Layborn-Perry 1999).

A discrete group among algae are the cyanophytes, also referred to as Cyanophyceae, Cyanobacteria, Cyanoprokaryota or simply blue-greenalgae. Cyanophytes share affinities with theprokaryotic organization of bacterial cells; theylack both the structural organization of chromosomes within a separate nucleus and the discretepigment-containing organelles (plastids or chromatophores) characteristic of many plants. Theidentification of naturally occurring taxa as a botanical approach is proposed to depend on morphological, physiological, genetic, and ecologicaldata (Anagnostidis and Komrek 1985). As wasnoted by Reynolds (1986), there are difficulties inmatching wild cyanophytes to cultured type-strains, because their morphologymay alter underlaboratory conditions from that of the initial isolate.

In addition to cyanophytes, fresh waters alsocarry a mixture of cryptomonads (Cryptophyceae), dinoflagellates (Dinophyceae), chrysomonads (Chrysophyceae), diatoms (Diatomo

1.2.3 Water bloomNaumann (1912) incolouring for an awhole water columnunderstood as the sutic cyanophytes. Inbloom is used to refecyanophytes or occBotryococcus.

The term eutrophdescribes a trophic cment due to the high1986). During the pment of many of therect consequence omade by growingcreased waste wateroff from agriculture1980, Kauppi 1984kolainen 1989, Ekroles of nitrogen andtrophication were rethe study of eutrop1955, VollenweiderThe term natura

describes very gradutus of a lake over lonshort in a geologicasilts derived fromwashed and accumlake volume diminithe amount of nutri

picoplanktonnanoplanktonmicro- or netplanktomeso- and macropla

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Phytoplankton assemblages reflecting the ecological status of lakes in Finlan

with surface waters. Good ecological status is defined in terms of the biological communities, thehydrological characteristics and the chemicalcharacteristics. Biological communities with ecological variability but with minimal anthropogenic impact are the target of protection (Commission of the European Communities 1999).

1.3 LakesLakes in Finland have mainly been formedthrough three events: cracking of the bedrock,transport of boulders and soil by water and ice,especially during the glacial period, and deglaciation, resulting finally in the development of a lake(Jbrnefelt 195 8a,b). The lake type depends in general on the nature of the weathering bedrock in thedrainage area, on the loose soil particles: clay andcarbonates in the bedrock cause eutrophy; moraine and sand cause oligotrophy. In Finland, naturally eutrophic lakes are mainly situated incoastal areas with wide clay plateaus, but are alsofound elsewhere in areas where clay is abundantin the soil or where phosphates occur in the bedrock.

Oligotrophic lakes are situated in the vicinity ofthe Salpausselka formation, in areas where the soilmainly consists of sand or c ru st, and in Lapland(Maristo 1941, Jarnefelt 1958a). Oligotrophiclakes are more or less rich in humic compoundsand are characterized by low phytoplankton biomass and no occurrences of water blooms (Teiling 1916). Dinoflagellates, chrysomonads, somegreen algae, especially the genus Botryococcusand desmids are abundant (Jrnefelt 1952, 1958a,Hutchinson 1967, Rosen 1981, Brettum 1989).

A moderate degree of eutrophication is described as mesotrophy (Hutchinson 1967). Meso

dense algal surfaceblooms (Naumann 1toms mainly domina1916). Eutrophic dinPeridinium) and greenalso often abundant (H(1952) also includedcategory. When the dcreases the number of(Heinonen 1980, Elobiomass exceeds 10 mand the diversity mayHeinonen 1980, klanThienemann (1925

the lake category. Jbthe oligotrophic and eas dys-oligotrophic atrophic lakes. The lattally accepted (Roundterized by brown or yecaused by allochthononating from the drainapounds are of varyi1958a, Wetzel 1983, Pis typical in lakes situgenerally associated wage basin. Many of thweakly exposed tomeromixis during sprithe productive layersdevelopment of thermastricts the availabilityalgae. Consequently thcryptomonads and flavirta 1983, Smolandersupport their growthnutrient-rich hypolimbut nutrient-poor ep1984). Large dystroph

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Later on, the biota and the nutrient levels of theselakes change towards oligotrophy (Kinnunen1982, Krzyzanek et al. 1986, Carnier 1992). Water level regulation with strong unidirectionalflow affects phytoplankton by displacing the water with suspended phytoplankton assemblages,perennating cells and resting spores downstream.Under these circumstances spring growth may bedelayed because the inocula ofphytoplankton issmall, although nutrients and light are adequate(Reynolds 1986).Acidification was recognized as a severe environmental problem in the 1980s both in Europeand in North America. In southern Finland, smallclear water lakes with catchment areas mainly ofinfertile forest soil or granite bedrock are particularly sensitive to acid deposition (Kauppi et al .1990). However, many lakes in Finland are naturally acidic and dystrophic or have been so for atleast the past 100 years due to dissolved humicsubstances derived from the surrounding soils(Rask e t a l. 1986).

1.4 Aims of the studyThe aims of the present study were:1) to evaluate the typical phytoplankton assemblages in different lake types

2) to evaluate the response of phytoplankton assemblages to anthropogenic stresses in terms

of long-term owell as to the deman-made lakeswater level regu

3) to evaluate thea monitored ensignificance ofchanges, and fin

4) to evaluate the kphytoplankton.

2 Materials andThe phytoplanktonbased on the data sitored lakes of the(FEI), with a totaland two reservoirs60 and 69 (1Vua brown-water foritored man-madefrom four lakes exfor water works (Vlakes. Lakes werephic, dystrophic, mper-eutrophic usingical and chemical1). The acidic lakethe HAPRO proje(Kauppi et al. 1990

Table 1. Some water quality variables (mean, maximum and minimum) in the studiedOctober) from the depth of one metre, n = number of the lakes / and the number ofForsberg and Ryding (1980), in which acidic lakes were not included.Lake types n Total P Total N W

jig L jig L mg

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Table 2. The sampling time, depth, method and preservatives used in the investigMethods are those used in the monitoring programme of FEI , with the exception of (II).PubI. Date Sampling depth Sampling meNo. Year Month >02 m 02 m net RuttI 18931995 VIX 09m x x xII 19881994 VTX x xlIT 19681990 VTTX 0lOm x xIV 19651994 VTTx 0lOm x xV 1982, 1986 VTI x xVI 1991 VXT 112m x xVTT 19821994 VII x x1) Since 1971 in general2) Preserved with acid Lugols solution; formaihyde (35 %), as 16 % final solution3) Sampling from the pipes of raw and treated water

Phytoplankton has been sampled by the Regional Environmental Centres since 1963 (Table2) , and the biomass as fresh weight has been estimated by microscopy using the Utermhl (1958)technique. During the analyses all the observedtaxa of a permanent area of the chamber bottomare counted and the counting results, includingthose taxa observed in low numbers, are converted to biomass using coefficients. Phytoplanktonquantity as chlorophyll a concentration (TV) hasbeen determined since 1973. The data from thestudy lakes (TVTI) and additional phytoplanktondata from the database of FET were examined tofind the average biomass of the most importanttaxa during spring, summer and autumn, and theseasonal succession of phytoplankton in the different lake groups. Partly unpublished data fromthe database of FET were used when consideringthe long-term changes of cyanophytes, diatomsand total phytoplankton, and the proportion ofdifferent algal groups in lakes in Finland. Acidic

3 Phytoplanktonlakes3.1 Long-term eutrlakeLake Tuusulanjbrvi isFinland. The naturallywater source for the1950s and 1960s. Wicentrations the lakephytoplankton biomasing water blooms of cyobserved. Several whave improved the w1980s (I).In 1921-1932 the t

was on average 1.4mgincreased to 22.4 mgcharacterized by stronfluctuations. The max

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14 Lepista Monographs of the Borea

Table 3. Mean and standard deviation of phytoplankton biomass (wet weight, mg L1(tg L1 ) in 19811997 (JuneAugust) in the studied lake groups and in man-made ltaxa. SD denotes standard deviation.Lake types Samples Taxa Biomass mg L Sam

n n Mean SD nAcidic 11 77 0.3 0.1 -Oligotrophic 165 436 0.3 0.2 181Dystrophic 197 519 0.9 0.7 603Mesotrophic 144 556 2.3 2.0 456Eutrophic 35 313 7.6 7.7 212Hyper-eutrophic 146 486 13.2 12.4 276Lokka 37 277 3.0 2.4 39Porttipahta 31 249 1.0 0.9 751) in 19872) man-made lake

tions in summer 1997 Aphanizomenon spp.caused dense water blooms. This emphasizes thatweather is an essential regulating factor in the formation of algal blooms (I).

Soon after 1910, the cyanophytes Anabaena,Aphanizomenon and Microcystis were ratherabundant, although no algal colouring of thewater was visible (Jarnefelt 1937, 1956b). Atthat time eight species indicating oligotrophywere included in the phytoplankton assemblage.Dinobryon bavaricum Imhof and D. divergensImhof were rather abundant, and the cyanophytesCoelosphaerium kuetzingianum Nageli andWoronichinia naegetiana (Ung.) Elenkin, bothoccurring in oligo-mesotrophic waters (Brettum1989), were also common, as were the oligotrophic diatoms Cyclotella kuetzingiana Thwaitesand Tabellaria flocculosa (I). Water bloomscaused by Anabaenaflos-aquae (Lyngb.) Brbisson, A. inacrospora Klebahn and A. spiro ides

1959 the water wa(Kangas 1961). A(1993) and Lamprobably causes acaying cyanophytieach summer in thEspecially in theload into the lakealternatively domas Planktothrix agAutacoseira itaticaTabe ltaria floccuconditions (Hutch1976 during the hisilicate depletionwas absent until 19

Ptanktothrix agphytoplankton untload was stopped.in 1981 , but since

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significant biomass of cyanophytes was observed.Obviously, this was partly due to the exceptionally windy and rainy spring and early summer. During strong turbulence, diatoms are able to outcompete cyanophytes (Reynolds 1980). In the1980s diatoms were clearly reduced compared tothe previous decade of vigorous diatom growth,which obviously depleted the silicate. The large-sized Aulacoseira granulata (Ehr.) Simonsen wassucceeded by the narrow A. granulata v. angustissima (0. MUller) Simonsen (I).

In the 1990s, more than 10 years after theend of waste water input to the lake, cyanophytesdecreased markedly although they still dominatethe phytoplankton. Aphanizomenon flos-aquae(L.) Ralfs became abundant (Tables 46), butcryptomonads, dinoflagellates, especially chrysomonads and also green algae increased. As a newphenomenon the small-sized Rhodomonas lacustris Pascher & Ruttner produced a biomass peak inMay 1994. There was a shift from Microcystisdominated, usually hepatotoxic (Watanabe et al.1986, Sivonen et al. 1990), blooms to Aphanizomenon dominance in the 1990s. Lower phosphorus and nitrogen concentrations not only decrease the intensity of mass occurrences but favour less harmful populations of Anabaena andAphanizomenon (e.g. Ekman-Ekebom et al. 1992,Willn and Mattson 1997, Rapala 1998).

In the 1990s, 22 new taxa, mainly of cyanophytes, were identified. Such taxa were Aphanothece, Cyanodictyon, Radiocystis, and Snoweliawhich form small bacteria-like colonies and aretypical for naturally eutrophic shallow lakes(Komrkovd-Legnerovd and Cronberg 1994).Furthermore, Anabaena mucosa Kom.-Legnerova& Eloranta and the diatom Skeletonema potamus(Weber) Hassle, which earlier was assigned to thegreen alga Gloeotila according to Turkia and

which is considered tent input (Watson anLarge diatoms werediatoms, a phenomensilicate concentrationtle et al. 1987). Thein the material alsoin taxonomy which mtion of small bacte1990s (I).

The high total pLake Tuusulanjhrvi athe 1920s. In fact tharea and is naturallyCompared to the mephorus and nitrogenand 400 jig L-1 res1998), nutrient concejrvi are still in the 1tal median phosphoruan nitrogen 1200 jigeven hyper-eutrophy.ity in the hypolimnioaeration, the internaloperative and the phmains at a high levelwater load to the laketion of inorganic nisuggests that nitrogenfact, the low median Dbelow seven (the optRedfield et ci. 1963)conclusion.

3.2 Effects of forebrown-water lakeThe forests in the cat

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the hypolimnion nutrient concentrations, organicmatter, water colour and conductivity increasedgradually. Five years after the fertilization, inSeptember 1994, Gonyostomum semen caused asecond maximum, with total biomass 2.9 mg L-and chlorophyll a concentration 10 .Lg L-1 Phytoplankton biomass and chlorophyll a concentrationwere in agreement with the relatively high phosphorus concentration reflecting meso-eutrophicconditions, according to criteria by Heinonen(1980) and Wetzel (1983). Except for these twomaxima, phytoplankton biomass and chlorophylla concentration remained rather low (II).

The fertilization obviously caused an immediate increase of cryptomonads, accounting for Ca.60 % of the total biomass. Cryptomonas spp. arefavoured by N+P addition (Jansson et ai. 1996)and are potentially mixotrophic (e.g. Tranvik etal. 1989, Roberts and Layborn-Perry 1999). Afterthe first spring, cryptomonads contributed lessthan 20 % to the total biomass (II). During thestudy period small flagellates, such as the prymnesiophycean Monochrysis parva Skuja and thegreen alga Monomastix sp., were typical. Monochrysis parva is obviously capable of bacterialconsumption (Holen and Boraas 1996). The chrysomonads Maiiomonas, Pseudopedineila andSynura were also abundant. The large-sized flagellate Gonyostomum semen is able to maintainits growth in brown-water lakes by migrating between the epilimnion and the hypolimnion, wherenutrients are concentrated (Cronberg et ai. 1988,Arvola et ai. 1990, Eloranta and Raike 1995).Gonyostomum appears to be able to consume bacteria (Holen and Boraas 1996).

One year after the fertilization Merismopediawariningiana Lagerheim, typical for nutrient-poor, dark and acidic waters (Brettum 1989), wasabundant. Other cyanophytes, such as Anabaena

nella formosa Hassatrophic lake groupcase ira italica, rathlake group, is rare i

3.3 Developmenttwo man-made la3.3.1 The LokkaThe reservoirs Lokstructed by damminof the Kemijoki wawatercourse in Finltion, the reservoirsof October to the eJune. The filling ofing the spring andvolume is at its minlation of the reserv19771981.

The Lokka reserterms of surface a1978). It remains lamum water level, alow (mean depth 5The reservoir is conswamps, and the amake the water colNenonen 1972, Virtpletions release phoment. However, onlare in a bioavailablemic compounds (BoIn May 1968 th

rather similar in thvoir to that in Septebefore any reservoi

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creased when the extent of water level regulationwas diminished in 1981. In the 1990s the averagephytoplankton biomass decreased to 1.9 mgreflecting mesotrophy (Table 3). The interannualbiomass variations also decreased (III, IV).Small colony-forming cyanophytes, e.g.

Eucapsis alpina Clements et Shantz probably derived from metaphyton in the flooded peaty bogs(Komrek and Anagnostidis 1999), chrysomonads and cryptomonads were the main constituents of the phytoplankton assemblage in therecently filled reservoir. The heterotroph protistDesmarella moniiiformis Kent was also a typicalspecies (III, IV). This species prefers waters withhigh concentrations of organic compounds andabundant bacteria (Jbrnefelt 1961, Tikkanen andWilln 1992). Furthermore, the commonly observed Dinobryon bavaricum is closely linked tomixotrophy (Bird and Kalff 1987, Straikrabovand Simek 1993). Many cryptomonads, such asRhodomonas lacustris, are also mixotrophs(Haffner et ai. 1980), and are effectively grazedby zooplankton (Porter 1973). The green algaMonoraphidium contortum (Thur.) Kom.-Legnerov was very abundant, obviously as a relictfrom the River Luiro (III, IV). Reynolds (1988)classified M. contortum as a true river phytoplankton (potamoplankton) which tolerates high-frequency hydraulic disturbances (III, IV). Arvola (1980) described it as a pioneer species able tosurvive in a new labile ecosystem. With the passage of time, diatoms increased and strong waterlevel regulations favoured species, such as Aulacoseira ambigua (Grun.) Simonsen and A. italicav. tenuissilna (Grun.) Simonsen (III, IV). Silicatedepletion in summer 1988 obviously caused thinand bent valves of the cells of Aulacoseira (Fig.2). Stoermer et al. (1985) and Kling (1993) reported similar thinly silicified and distorted

its shape is oblong andlevel is at its minimuand Nenonen 1972,there is little organicthough the Lokka respahta reservoir, the watoplankton compositiodiffer to some extent fvoir. The N:P ra tio ilakes in Finland, and plimiting nutrient fo r p

In the early historywhich was constructeLokka reservoir, phytoligotrophy but incrthe strong water leve

Fig. 2 Aulacoseira ita(5.7.1988).

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18 Lepistd Monographs of the Boreal

seems to be abundant in deep waters with adequate nitrogen and low phosphorus (database ofFEI). Cyanophytes are less abundant partly due tothe consistently high mineral nitrogen concentration. No mass occurrence of cyanophytes was observed in 1988 (III, IV).

The different morphology and different waterquality of the two reservoirs affects the phytoplankton. In the Lokka reservoir, phytoplanktonproduction seems to be regulated by both nutrients (P and N) alternatively. However, the concentrations of phosphorus, which are higher thanin mesotrophic lakes (Table 1), maintain a phytoplankton assemblage typical for mesotrophic conditions. In the Porttipahta reservoir, nitrogen concentrations are relatively high and consequentlyphosphorus regulates the growth of phytoplankton. Phosphorus concentration is equal to that ofmesotrophic lakes on average (Table 1) but phytoplankton biomass reflects oligo-mesotrophy.The strong water level regulation promotes theoccurrence of diatoms. Especially in the Lokkareservoir warm weather periods may favour massoccurrences of cyanophytes.

3.4 Problems caused by increasedphytoplanktonOligotrophic lakes are suitable as raw watersources for waterworks using a minimum of onlysand filtration. The chemical quality is notchanged during treating of the raw water (VI). Microscopical phytoplankton analyses indicate thereduction of total phytoplankton biomass in thetreated water to be on average 76 % and that ofcyanophytes 14 %. However, occasionally cells ofcyanophytes, such as Snowella spp., are flushedfrom the sand filter with the treated water. Prob

ment with almost tton, cells of Microctothrix agardhii areduring the peak occthe euglenid Tracheciently removed byever, the abundant cily toxic (VI).

The patchiness onificant during a waness the species coespecially of cyanfrom that of the bloothe water intake. Thand simultaneouslyworks non-toxic opossible that no watPlanktothrix is abunthe raw water intakwater blooms but is(Lindholm 1992).

A simple waterof high quality. Saenough if the lake uually changes to mochanges of phytoplwater body are alreation becomes a prbe considered as waDavis (1964), who sfrom the incomingLake Erie in 1919

3.5 Phytoplanktostatus of lakes in3.5.1 General

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In fact the 1980s was on average characterised byrather heavy precipitation (Finnish Meteorological Institute 1991). Only in mesotrophic lakes diddiatoms increase the total biomass, obviously as aresult of slight eutrophication. In eutrophic lakesphytoplankton biomass has clearly decreased dueto water protection procedures (I). According tothe database of the Finnish Meteorological Institute, June was on average colder and more windyin 19901996 compared to the previous decade, which might have influenced phytoplanktondevelopment. Due to the cold June the summerstratification development might be delayed aswell as the development of phytoplankton springmaxima, or the maxima could be even totally pre

Oyanophyta0 1963-79w 1980-895

C)011g. 011g.- Mes. Eutr. Lo Po

mes.

Diatoms0 1963-79w 1980-89[1990-96

n__LII EL011g. 011g.- Mes. Eutr. Lo

mes.

10mg

aS0

mg

T 15

vented especially inHowever, weather covariables affecting ph

In the studied lakplankton biomass gento 4 mg L- but in eumately 10 mg L-1 wSwedish investigationphytoplankton quanttrophic status (Rosention of 40 Tyrolean(Rott 1984).3.5.2 OligotrophicIn the studied oligotro(VI, VII , database ofis low, on average 0.varies mainly in Maykeeping with this, chllow , on average 2.7 iplankton is composedof chrysomonads (32and diatoms (21 %). Cportance in oligotrophof the classes PrasinoTribophyceae (groupThe total number436 taxa in 162 samonads, such as theales, Bitrichia chodalykos planctonicus Mmermann, D. divergecum, are typical for oltaxon of chrysomonathey were abundant asof the chrysomonads wcate oligotrophy, suc(Jrnefelt 1952, 1956

10mg

aS0

P0

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20 Lepisto Monographs of the Boreal

Table 4. The dominant spring taxa in different lake types in 19811997 given as avera+ = observed, - = not observed, n = number of samples. The two dominant taxa showlake group with only one sample.SpringTaxon Oligotrophic Dystrophic Mesotrophic E

9lakes,n= 3 4lakes,n=21 9lakes,n= 10 lAphanothece chlathrata - + - -Cryptomonas - 0.10 - -Cryptomonas spp. 1,2) 0.02 - - -Cryptomonas spp.) - - - 0Cryptomonas - - 0.29Rhodomonas lacustris 0.02 0.02 0.06 0Gymnodinium spp. 0.01 0.02 0.04 -Chrysochromulina spp. + - 0.02 0Mallomonas akrokomos + 0.02 0.02 -Mallomonas allorgei - 0.03 - -Mallomonas caudata + 0.03 + 0.Ochromonadales 0.06 0.02 0.01 -Mallomonas crassisquama + 0.11 + -Pseudopedinella spp. - 0.05 0.03 -Synura spp. + 0.03 + 0Aulacoseira ambigua - + 0.17 -Aulacoseira granulata - - - -Aulacoseira alpigena - - - (7Aulacoseira islandica 0.01 0.03 0.12 0Aulacoseira italica - 0.01 0.09 0A. italica v. tenuissima + + 0.04 (7Cyclotella spp. - - 0.07 -Rhizosolenia longiseta 0.02 0.02 0.02 -Stephanodiscus spp. + - + 0Asterionella formosa + 0.05 0.01 0.Diatoma tenuis 0.01 + + -Fragilaria crotonensis + + + 0Fragilaria (Synedra) Spp. 0.01 + + 0Tabellaria flocculosa 0.01 0.08 0.02 0

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Table 5. The dominant summer taxa in different lake types in 1981-1997 given as avera+ = observed, - = no t observed, n= number of samples. The two dominant taxa shownSummerTaxon Oligotrophic Dystrophic Mesotrophic Eu

9 lakes,n= 16 2 10 lakes,n =197 6 lakes,n= 152 5Aphanocapsa reinboldii 0.01 0.02 0.03 0.5Microcystis aeruginosa + + + 0.2Microcystis flos-aquae - - - 0.1Microcystis viridis + + + 0.0Microcystis wesenbergii - - + 0.0Microcystis sp . - - + -Snowella lacustris + + + 0.2Anabaenaflos-aquae + + 0.03 0.6Anabaena lemmermannii + + 0.01 0.0Anabaena solitaria + + + 0.0Aphanizomenon spp. + 0.01 0.14 0.3Planktothrix agardhii + + 0.04 0.0Cryptomonas spp. 2) 0.03 0.13 0.24 0.7Rhodomonas lacustris 0.03 0.05 0.07 0.1Ceratium hirundinella 0.01 0.01 0.01 0.0Gymnodiniumspp. 0.01 0.01 0.03 0.1Chrysosphaerella spp. 0.01 + + -Mallomonas akrokomos + 0.01 0.02 +Mallomonas caudata + 0.02 0.01 0.0Ochromonadales 0.02 0.01 0.02 0.1Pseudopedinella spp. 0.02 0.02 0.03 0.0Uroglena spp. 0.02 0.01 0.02 0.0Synura spp. + 0.01 0.05 0.0Acanthoceras zachariasii + 0.03 0.08 0.1Aulacoseira ambigua + 0.03 0.07 0.7Aulacoseira distans 0.01 0.02 0.06 0.0Aulacoseira granulata + - + 0.2Aulacoseira italica 0.01 0.06 0.15 0.3Rhizosolenia eriensis + 0.01 + +Rhizosolenia longiseta 0.01 0.10 0.04 0.0Stephanodiscus spp. + + + 0.0

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Table 6. The dominant autumn taxa in different lake types in 1981-1997 given as aver+ = observed, - = not observed, n= number of samples. The two dominant taxa shownAutumnTaxon Oligotrophic Dystrophic Mesotrophic E

9 lakes, n= 8 4 lakes, n= 21 4 lakes, n=11 1Aphanocapsa reinboldii + + 0.01 0Microcystis aeruginosa + - + 0Microcystis viridis - - - -Microcystis wesenbergii - - 0.02 -Snowella lacustris 0.01 - + -Anabaena flos-aquae + + 1.06 0Aphanizomenon spp. + + 0.01 0Cryptomonas spp. 1,2) - 0.06 - -Cryptomonas spp. 2) 0.03 - 0.11 0Rhodomonas lacustris 0.02 0.01 0.03 0Gymnodinium Spp. 0.03 + 0.01 0Peridinium umbonatum + + 0.02 -Dinophyceae + 0.01 + -Chrysococcus spp. - + + 0Mallomonas akrokomos + + 0.01 0Mallomonas crassisquama - 0.01 - -Mallomonas punctifera 0.01 0.01 - -Ochromonadales 0.03 0.01 0.01 -Pseudopedinella spp. + 0.03 0.01 +Uroglena spp. + 0.01 0.02 0Synura spp. + 0.01 0.01 -Acanthoceras zachariasii + + + 0Aulacoseira ambigua - + 0.01 0Aulacoseira distans 0.02 + 0.04 -Aulacoseira alpigena + + + 0Aulacoseira granulata - - + 0A. granulata v. angustissima - + - +Aulacoseira islandica + 0.03 + 0Aulacoseira italica + 0.08 0.04 0A. italica v. tenuissima + 0.02 + 0

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VI VII VIII IX X

2.5 TTT F 1 V S

V VI VII VIII IX X

2JV V

V VI VII VIII IX XFig. 4 The seasonal succession (MayOctober) of phytoplankton as mean, minimum and maximum biomass (wetweight, mg L) in 19811997 in lakes with different trophic status (I, IV, VI, VII, database of FEI).

100

6040200

VEutrophic10806040200

VI

V VIMesotrophic100%80

6040200

10806040200

V VIDystrophic

V VIOligotrophic100%806040200

VI

Fig. 5 The proportionOctober (1 9811 997) in(I, IV, VI, VII,

mg L 4yper-eutrophic

V VI VII VIII IX X

C0.0

a)a)0.

00.0a)Ui

V80

0 600g 40a)

20

VI VII VIII IX X

aa0

V80

o 60Cg 40>, 20

0

800C3-600.0 400

200 V

LcY5n aCrypt. oDin

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1CrYpt. m Dinop. 0 Chrys. C Diat. C Chlor. C Others

Fig. 6 The proportion of different algal groups in phytoplankton (explanation of abbreviations in Fig. 1) in July1981-1997 in lakes of different productivity, determinedas total phytoplankton biomass (wet weight, mgL(l, IV,VI, VII, database of FEI ). The limits, according to the classification of Heinonen (1980) are: < 0.5 mg L1 = oligotrophic, < 1 mg L = oligo-mesotrophic, >12.5 mg L1 =mesotrophic, >2.510mg L1 = eutrophic and>lomg L1= hyper-eutrophic.Spring phytoplankton is dominated by dia

toms, e.g. Rhizosolenia iongiseta, Diatoma tenuisAgardh and Fragiiaria spp., and by cryptomonads, mainly Cryptoinonas spp. (small andmedium), and Rhodomonas iacustris (Table 4,Fig. 5), which are typical for oligotrophic waters(Arvola and Rask 1984, Brettum 1989, Willn1992a). Chrysomonads, especially unidentifiedOchromonadales, are also abundant. Summerphytoplankton is still dominated by chrysomonads, such as Pseudopedinella spp., and Urogiena spp. The cell number of Urogiena, anothermixotrophic genus, can sometimes be very highin oligotrophic lakes (VII). Although Urogiena isoften considered as a typical spring taxa (e.g.Talling 1993), it appeared to occur in this data set

3.5.3 Acidic lakeThe phytoplanktongroup (VII) is equalon average 0.3 mg Lless (SD % = 33 ). Awith eight commoneleven samples. Themon phenomenon fet ai. 1990). Merisimann is clearly acidling and Willn (19water oligotrophic lported by Ilmavirtascribed the dominancation-induced andan acidic, clear wateM. warmingiana whM. tenuissima. Acconostidis (1999), M.ponds but less comm

In terms of biommost important groulakes (VII). Gienodily acidic lakes withtrations. This genustypes (Tables 46).is most abundant inAlgae favouring dysspp., in agreement w(1983) and Brettumakrokomos Ruttnerindifference to acicryptomonads, a verwide ecological specly , Dinobryon diverare very acidic and(VII). However, Elpediforme (Lemm.)

0)

00)C0CCs0.0.00

100%806040200

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Komrek, Monoraphidium dybowskii (Wol.)Hindk & Kom.-Legnerov and Oocystis submarina v. variabilis Skuja as characteristic algae foracidic lakes.3.5.4 Dystrophic lakesThe dystrophic lakes in this study are rather largeand deep (II, IV , VII, database of FBI). Their average phytoplankton biomass, 0.9 mg L1 is threefold compared to that in oligotrophic and acidiclakes, and varies more (SD % = 78). The averagechlorophyll a concentration, 6.2 ig L (SD % =48), is twofold compared to the oligotrophic lakes(Table 3). The variation of biomass is at greatestin MayJune, after which it gradually decreases(Fig. 4) . Lakes may became more dystrophic, asdid Lake Kemijarvi, the recipient for the two reservoirs Lokica and Porttipahta, after their construction. Furthermore, the trophic status of LakeKemijrvi changed slightly towards mesotrophyduring the period of strong water level regulationin the reservoirs. This change was mainly causedby increase of cryptomonads and diatoms.

A total of 519 taxa were observed in 197 samples (Table 3). Diatoms clearly dominated thephytoplankton assemblage at Ca. 40 %. Auiacoseira itaiica, Rhizosolenia iongiseta, Asterioneilaformosa and Tabeiiariaflocculosa, were the maincomponents (Figs. 1, 5, Tables 4-6). A plausiblereason for their occurrence is that the monitoredlakes do not have very high humus content andare rather large in surface area, which enablesmarked turbulence. In the dystrophic, shelteredforest lake, Lake Kalliojrvi, diatoms were observed as abundant only during spring, and flagellated species dominated in the summer months(II). Auiacoseira itaiica has been reported to correlate positively with dystrophy (Pinel-Alloul et

mixotrophic nutrientSalonen and JokinenRoberts and Layborchrysomonads survivcentrations in dystroThe small Chrysococordiformis Naumanlakes and the smallchrysis parva for dys

Green algae, e.g.Botryococcus spp. co10 % to the total biomof Desmidiales origisuch as the large Xanson) Kutzing and HyBrbisson, typical fTikkanen and Willnserved. Cyanophyteslakes (Figs. 1, 5, TabCa. 5 % to the total bboidii (Richter) KomMerismopedia warmthese small-sized cyaportance when consitrophic lakes cyanoflos-aquae, are onlyible greenish flakes oSpring phytoplankchrysomonads, espesquoma but M. aiiorcaudata Ivanov em . Kspp. are also abundanthan 60 % of the totalCryptomonas spp. buCryptomonas spp . aretoms account for 45mass, the dominantiongiseta andAsterioeriensis H.L. Smith is

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26 Lepist Monographs of the Borea

high concentration of nutrients in the hypolimnion (II). A deeper sampling depth than 0 2metres could be more representative due to theoccurrence of this species in deeper parts of thewater column (Eloranta and Rike 1995).3.5.5Mesotrophic lakesIn the moderately shallow mesotrophic lakes (IV,VI, database of FEI), the average total phytoplankton biomass, 2.3 mg L-1 is eightfold that inoligotrophic lakes, and consists mainly of diatoms (46 %) and cyanophytes (17 %) (Figs. 1, 5).The variability of biomass is also higher (SD %87) compared to the less productive lakes. Theaverage chlorophyll a concentration, 8.7 ig L-(SD % = 55), is threefold that in oligotrophiclakes (Table 3). The vernal maximum was not observed in mesotrophic lakes in this study (Fig . 4),due to the late sampling and the varying succession of phytoplankton in the shallow lakes (Tnfonova 1993, Knuuttila et al. 1994).Altogether 556 taxa were observed in 152

samples (Table 3). Furthermore, the diversity ofphytoplankton increased from oligotrophic to mesotrophic lakes (Figs. 1, 5, 6), which is in agreement with previous observations (Brettum 1980,kland 1983). The phytoplankton assemblagewas clearly dominated by diatoms (46 %), e.g. byAcanthoceras zachariasii, Aulacoseira ambigua,A. islandica (Muller) Simonsen, all indicators foreutrophic waters (Jrnefelt 1952, 1956a, Brettum1989, Lepistd 1990) and by A. italica which Brettum (1989) classifies as typical for mesotrophicwaters. However, the abundance of A. italica appeared to increase with increasing trophic degree.Furthermore, Aulacoseira distans is observedmainly in mesotrophic lakes, especially in summer (Tables 4-6). Asterionella formosa, typical

blooms reported inharmful algae of FBa typical cyanophytturn 1989), was onlstudied lakes (IV, Voccurrences belowmesotrophic Lakeberg 1995). Uroglsional fish-like odoearly summer (VI).be suppressed wi(TaIling 1993). Dinvergens, generallyindicators (Jrnefelare frequently obseVI), which is in aEloranta (1989). Sychrysornonad in thJhrnefelt (1952, 19pendix 1) belongs tonyostomum semenhigh quantities espelate summer and inits high photosynthchlorophyll a conduring its maximuabundant, causes(Cronberg et al. 19this alga can clog thwater treatment (Mtally disappears durdestroys the fragileThe spring phyto

diatoms Aulacoseirand medium and la5, Table 4) . The biJuly is caused maindiatoms, especiallyate the summer phy

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fied in 35 samples. The proportion of cyanophytes is on average 40 % and that of diatoms 30% of the total biomass. Cyanophytes belong almost equally to eitherN2 fixing or non-N fixing taxa. Anabaenaflos-aquae (Lyng.) Brbisson(earlier also identified as Anabaena circinalisKtitzing, not Anabaena circinalis Rabenhorst) isthe most abundant cyanophyte in eutrophic lakes,occurring slightly less in hyper-eutrophic lakes.Aphanocapsa reinboldii and Microcystis aeruginosa are common and several other species of thegenus Microcystis are frequently observed in eutrophic lakes. The dominating diatoms, e.g. Aulacoseira ambigua and Acanthoceras zachariasii,both indicators of eutrophy (Lepisto 1990, Rosen-strom and Lepist 1996), clearly prefer eutrophiclakes but are also common in mesotrophic andhyper-eutrophic lakes (Tables 46).

Green algae contribute 10 % to the total average phytoplankton biomass (Tables 46). Thisgroup seldom dominates the biomass, accordingto Mantere and Heinonen (1983). Eutrophic indicators (Jrnefelt 1952, 1956a, Heinonen 1980,Brettum 1989), small in cell size, such asActinastrum hantzschii Lagerheim, Dichotomococcuscurvatus Korshikov and Tetraedron caudatum(Corda) Hanskirg, are numerous. Pediastrumduplex Meyen has many modifications, exemplified by P. duplex v. gracillimum W. & G.S. Westand P. limneticum Thunmark, obviously reflecting differences in environmental conditions, suchas increased concentrations of nutrients. P. duplexoccurred in almost every eutrophic lake sample(VII).

Euglenids, such as Euglena proxima Dangeard, Trachelomonas hispida (Perty) Stein and T.volvocinopsis Svirenko are abundant in eutrophiclakes, and T. volvocina Ehrenberg is typical forthe Lokka reservoir (IV, VI, VII). The large-

nal maximum (Figs.taste and odour in thnumber of diatom celimit (2500 cells! ml)povaara 1971). Diatoters in water treatmenerately nutrient-richinate throughout th1992a), when they beter column during sumthe proportion of cya%, with biomass averphytoplankton is domby cyanophytes, suchAnabaena flos-aquaeDiatoms co-dominatGonyostomum semePediastrum duplex(Table 5). In autummainly of diatom (ceras zachariasii ocAulacoseira alpigenadance.357 Hyper-eutropIn hyper-eutrophic lain which 486 taxa wethe average biomassand the average chlojg L (SD % = 84) (Tmass, consisting mai%), varies strongly d(Fig. 4, Table 3) andsity may decrease (Fthe N2 fixing cyanoN2 fixing cyanophhigher. With increasage summer biomas


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nosa (Ktz.) Kutzing. M. viridis is abundantmainly in hyper-eutrophic lakes (Tables 46).However, Planktothrix agardhii is not among themost typical taxa although it is observed occasionally as a very abundant component of phytoplankton in eutrophic lakes (I, VI, VII). P. agardhii is typical for eutrophic brackish waters (Niemi1973, 1979), and can form pronounced deep layermaxima during suitable conditions (e.g. Watanabe 1979, Lindholm 1992).Medium-sized cryptomonads are typical for

nutrient-rich lakes, which receive sewage (Rosen1981), such as Lake Tuusulanjrvi (I). Dinoflagellates produce by far the largest biomass (0.5mg L1 in hyper- eutrophic lakes, partly due tothe rather large cell size of e.g. Ceratium hirundinella (O.F. Muller) Schrank and Gymnodiniumspp. (VII) (Tables 46). Some dinoflagellates,such as Peridinium bibes Stein and P. willei Huitfeld-Kaas, have been reported as indicators of eutrophy (Jrnefelt 1952, Hutchinson 1967, Brettum1989). In this study P. bibes was observed in eutrophic lakes in summer and in hyper-eutrophiclakes it was moderately abundant in autumn (database of FEI).Spring phytoplankton is dominated by dia

toms, mainly Aulacoseira italica and A. italica v.tenuissima, and by Cryptomonas spp. (medium-sized) and Rhodomonas lacustris. The proportionof cyanophytes is low, less than 10 %, but the average cyanophyta biomass in spring is 0.6 mg L-1In summer the cyanophytes Aphanizomenon,Microcystis and Anabaena dominate the biomasswith a maximum proportion of 72 % in August,and diatoms (maximum 43 %), e.g. Aulacoseiraitalica, A. granulata and the small Stephanodiscus spp. co-dominate. In autumn the abundant cyanophytes, especially Microcystis wesenbergii,M. viridis and Anabaenaflos-aquae, are succeed

in the database. Hstudied lakes themissed due to the la(ca. 20h May). Thdepends on the locaditions and the mDuring more than thmajor methodologiThe decrease of sammetres since 1971 (ted the phytoplanktplankton is concenupper water layershumic lakes (Salonlakes, phytoplanktolayers, and in hummigrate between th(Eloranta and Raikways covered by srecommended a samto 4 5 metres for lal l changes must beris (1986) noted, datification and adequer flagellates are nerect picture of longton abundance.

4.2 AnalysesJuly samples havemethods to those uworkers until the enare considered to gproductivity of inlsummer conditionsJuly-August the przooplankton is ofte


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ods for phytoplankton analysis in fresh waters.The methods include three different levels ofquantitative and qualitative analyses of whichlevel 2, a method without analysis of live samplesand without electron microscopy (SEM andTEM), should be reached in monitoring studies.Unfortunately, the use of living material as an aidto identification is usually not possible in routinemonitoring work. Recommendations for literatureon identification are also given. However, an identification procedure only at the genus level as proposed by Blomqvist and Herlitz (1998) does notprovide sufficient information if qualitativechanges in phytoplankton are considered. The useof the attribute confer (cf.) in front of the speciesname provides more information on the phytoplankton composition togetherwith cell size information, photographs and/or illustrations (IVu).

The counting of taxa observed in low numberscauses statistical error (Willn 1976) but yieldsquantitative observations of individual specieswhich might indicate incipient changes in the water body. The simplified method using enumeration of dominant phytoplankton is complemented with a list of species occurring sparsely but notincluded in the biomass (Willn 1976).

The biomass of phytoplankton estimated aschlorophyll a does not necessarily have a strongcorrelation with biomass on a volume basis (I, IV,V). In this study the best correlation of chlorophyll a with total biomass was obtained in thebrown- water forest lake (II). According to Tolstoy (1979), chlorophyll a concentration generally provides a good measure for phytoplanktonbiomass, although decreasing chlorophyll contentdoes not necessarily signify decreasing phytoplankton biomass. The composition of the algalassemblage affects the chlorophyll a concentration when the chlorophyll a content of diatoms

image analysers, cellpigment analyses usication of species andthe highly advanced to

4.3 Lake groupsThe consideration conton assemblages in dion the classification ovariables, such as totor pH . However, eausually produces sligries. Different physicfactors influence theof the lake groups. Fmade lakes, even wheer (IV, V), do not nectoplankton assemblagter chemistry and mor

4.4 The importancThe processing of thethe oldest being fromland (I, IV , V), hasidentification procedJbrnefelt have been foby his co-workers TNaulapaa, who in tuplankton researchersInstitute and other lithough there are no snent repositories, as pnor photographs of thquite easy due to the cpret the species list


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Table 7. The variation of the average (June-August) phytoplankton biomass (wet weiconcentration (pg L) in lake groups with different total phosphorus concentration (pg(mg L1 Pt). SD % denotes standard deviation percent.Lake type Tot. P Water colour Biomass SD %pg L mg L Pt mg LOligotrophic 6 20 0.3 67Dystrophic 14 70 0.9 78Mesotrophic 18 30 2.3 87Eutrophic 57 90 7.6 100Hyper-eutrophic 65 75 13.2 94Lokka 36 80 3.0 80Porttipahta 19 70 1.0 90in water quality. Phytoplankton assemblages areinfluenced by several factors, of which nutrientconcentrations, water colour and pH are generallyconsidered. Furthermore, turbulence, stratification conditions, light conditions and catchmentproperties as well as the morphology of the lakebasin may influence the phytoplankton assemblage. In this study the quantity and quality ofphytoplankton reflecting the ecological status oflakes of different types are considered. The classification of lakes is based mainly on the averageconcentrations of total phosphorus (Table 7).However, when water colour is used as a factorfor lake classification the abundance of dominating taxa changes although the nutrient concentrations do not markedly change.

In oligotrophic, mainly clear-water lakes, phytoplankton biomass is low and varies only slightly. Small chrysomonads, some of them capableof mixotrophy, cryptomonads, dinoflagellates,diatoms, but also small-celled colonies of cyanophytes, are the main components of the phytoplankton biomass. In dystrophic lakes and in reservoirs phytoplankton biomass is higher than in

dant in summer, acof the total biomassmer and under favorences are possibletheN2 fixing taxaand Aphanizomenon

In eutrophic lakeof mesotrophic lakeerable. Cyanophyteto the total phytopafter July approximmass. The N2 Aphanizomenon domgenus Microcystis istrophic waters arealgae. In hyper-eutrtribute on averageand they become ratcontrast to the sitent trophic status. Incystis dominatesAphanizomenon spptaxon.

The importance


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Effective water protection efforts during thepast two decades appear to have improved thestate of the lakes. Phytoplankton reacts immediately to decreasing waste water load and decreased nutrient concentrations in the water, butthe reaction also continues over a long period.Due to the decrease of nitrogen compounds thenon-N fixing Planktothrix are soon out-competed, and cyanophytes may totally disappear after some years. The internal loading sustains adequate phosphorus concentration for the N2 fixation in water, and the N2 fixing Anabaenaand Aphanizomenon occur alternatively with thenon-N fixing genus Microcystis, which is aneffective competitor for nutrients. Over longerperiods of time, taxa typical for less eutrophicconditions, such as the diatoms Rhizosolenia andTabeilaria and the chrysomonad Dinobryon areobserved. Simultaneously the species diversity ofthe phytoplankton assemblage may increase, andit may change towards smaller species accompanied by decrease in the biomass.The construction, water level regulation, and

later the ageing process have all affected the waterquality and phytoplankton assemblages in the reservoirs Lokka and Porttipahta. In the young reservoirs river phytoplankton with small cyanophyta colonies, the green alga Monoraphidiumcontortum and flagellated and heterotrophic taxawere abundant. The period of strong water levelregulation increased nutrients in the water and water colour also increased due to the organic matter.The high quantity of available silicate benefitteddiatoms regardless of the turbidity of water, andespecially Aulacoseira italica was abundant. Nutrient and organic matter concentrations decreasedduring the ageing process of the reservoirs.However, total phosphorus concentration in the

Lokka reservoir is still twofold that in mesotrophic

The increase of biters as a consequencemotes nuisance in theof algae. Even in mospring maxima of cmay cause taste and oconvenient, especiallyraw water for water win late summer in modbut may increase introphic lakes, and thtoxic Microcystis smum semen increaseslightly eutrophic lakeof dystrophic lakes dthe quantity of cyanop

In general the stathave improved in theof phytoplankton mnumber of observatiocreased. Only in mesplankton biomass app1990s, mainly due toeutrophication procesthis phenomenon. Intotal biomass, includtoms, has strongly dec50 % of cyanophytes aproduce toxic strains.trophic Lake Tuusuladue to the effective reThe estimation of

analysing the chlorophfective method and ptine chlorophyll analytion about the compoformation about watmonitoring of the occmicroscopy, even at a


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tents. Thus the chlorophyll a concentrations inthese lakes are rather low compared to the totalphytoplankton volume. Total phytoplankton volume may be overestimated due to difficulties inestimating the volume of large cyanophyta colonies and also due to errors inmicroscopic analyses.

In long-term monitoring the seasonal and interannual variability of phytoplankton, e.g. due toweather conditions, is often remarkable. The variability appears to increase with increasing trophicdegree. The distinction between changes causedby eutrophication and the natural variability isone of the basic challenges for monitoring.New members in the phytoplankton assemblage might at an early phase predict changes inecological status of a water body. However,progress in the taxonomy of phytoplankton alsocauses the appearance of new species and thedisappearance of some other species during long-term monitoring. Furthermore, in an ideal situation the long-term monitoring based on microscopical analyses should be performed by oneperson or by one group with close cooperationand unbroken tradition. Regularly repeated inter-calibrations between different phytoplankton researchers are therefore important, because otherwise the changes observed in the phytoplanktonassemblage may only reflect changes beside themicroscope, not in the environment.

In conclusion, the results show that phytoplankton reflects the ecological status of lakesand also responds both qualitatively and quantitatively to changes therein. The average phytoplankton biomass increases with increasing eutrophy, as does the variation of biomass. This variation is highest in eutrophic lakes but decreases

slightly in hyper-eulakes different algaabundant as biomasdant as cell numbersare diatoms andbrown-water foresttrophic lakes are altoms, and the averconsisting mainlymoderately low unlakes and especialcyanophytes becomly summer. Finally,important group inmonads co-dominatanophytes dominatblage. Furthermore,tity of cyanophytesthe 1990s.6 YhteenvetoEn tyyppisissh vesiviplanktonyhteiso. Ptyvt kasviplanktonparistossaan, kutenmuutoksia. Kasviplkuttavista tekijdistsuhteita, veden varibvirtaus- ja kerrostuolosuhteet, valuma-morfologia vaikuttamukseen. Tssh tydtonyhteisojen lajikopisten jbrvien ekol

Taulukko. Erityyppisten jarvien keskimaarisen (kesaheinkuu) kasviplanktonbiomasuuksien (tg L) vaihtelu kokonaisfosforin (ig L) ja veden vriluvun (mg L1 Pt) m


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vien luokittelussa on ensisijaisesti kaytetty kokonaisfosforin keskimrisi pitoisuuksia. Kun ye-den varilukua kaytetaan luokittelevana tekijna,valtalajit ja niiden runsausjarjestys muuttuvat,vaikka ravinnepitoisuudet eivt juurikaan muutu.

Vahravinteisten, oligotrofisten, paaosin kirkasvetisten jarvien ryhmassa kasviplanktonbiomassa on aihainen ja sen vaihtelu vhist. Pienikokoiset kultalevt, joista nsa on miksotrofisia,nielulevt, panssarisiimalevt, piilevat muttamys pienisoluiset sinilevayhdyskunnat muodostavat posan kasviplanktonista. Tummavetisissa,eli dystrofisissa jarvissa ja molemmissa tutkituissa tekoaltaissa kasviplanktonin mkr on suurempi kuin oligotrofisissa vesiss ja lajisto koostuulahinn piilevista, poikkeuksena kuitenkin turnmavetiset pienet metsajarvet, ja nielulevist. Miksotrofisia lajeja on runsaasti. Tyypillisia lajejaovat Mallomonas crassisquoma (Chrysophyceae), Rhizosolenia Iongiseta (Diatomophyceae) jaGonyostomum semen (Raphidophyceae).

Mesotrofisista, lievsti rehevoityneista jarvistaosa on suhteellisen kirkasvetisi ja osa melko ruskeavetisi. Piilevt ovat vallitsevina ja nielulevatmuodostavat rnerkittavn osan niden jarvien kasviplanktonista. Sinilevien osuus on keskimarin10 % kesn kasviplanktonbiomassasta, inutta niiden osuus saattaa kasvaa loppukesalla ja suotuisissa olosuhteissa saattaa muodostua massaesiintymia. Phaosa sinilevisth on typensitojia, joistayleisimpia ovat Anabaena flosaquae ja Aphanizomenon spp. Reheviksi, eli eutrofisiksi luokitelluissa vesiss sinilevien osuus biomassasta onkeskimrin 20 %, mutta saattaa heinakuussanousta liki 50 %:iin. Typensitojasuvut Anabaenaja Aphanizonenon ovat valtalajeina, mutta Microcystis -sukuun kuuluvia lajeja on myos runsaasti.Sentristen piilevien ja viherlevien osuus biomassasta on suurimmillaan eutrofisissa jrvissa. Erit

lajikoosturnus saattaaviplanktonlajit ovat ysiltaan joustavia, ja nitelluissa jarviryhmissakinainen kilpailu ja nijoittavat kuitenkin nitoisiinsa, ja niiden mepasuotuisissa olosuhkeuksia, kuten Microsilmalevat, kuten Eugpelkstan reheviss vTehokkaiden vesiesiosta jrvet nyttaisivparin vuosikymmenenteis reagoi jtevesipatuneisiin ravinnepitoiett pitkan ajan kuluesnemisen takia Planktotlemaan ravinteista typsa. Sinilevayhteiso saasesti muutaman vuodelopettamisesta. Sisineedelleen typensidontaadisteita ja typensitojatnon vuorottelevat ei-t-suvun kanssa, joka pkaasti ravinteista. Ajamille vesille tyypillisetja Tabellaria, seka Divat, sarnalla kuin lajistpienikokoisemmaksi ja

Rakentaminen, veikaantyminen ovat nhdan tekoaltaiden vedenyhteisoissa. Nuorissakiplankton, jossa piennat, ja esimerkiksi M-viherleva sek siimaolivat runsaina. Voima


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kin mesotrofisen jarven keskimarista biomassaa. Porttipahdan tekoaltaassa ravinteiden pitoisuudet ovat aihaisempia, mesotrofisille vesilletyypillisi, mutta kasviplanktonin biomassa onalle puolet mesotrofisten j rvien keskimrisestabiomassasta. Aulacoseira italica on edelleen tyypiffinen molemmille tekoaltaille mutta Porttipahdan lajistolle tyypillinen on Rhizosolenia iongiseta. Poikkeuksellisen lmmin kesa saattaa yhaedesauttaa sinilevien massaesiintymien muodostumista Lokan tekoaltaassa.Biologisen tuotannon lisantyminen jrvienvahitellen rehevoityessa ilmenee haitallisina levesiintymin. Alkavan rehevoitymisen myt keviset kultalevien ja piilevien maksimit runsastuvat ja aiheuttavat haju-ja makuhaittoja, jotka ovatkiusallisia etenkin raakavesilhteen kaytetyissapintavesiss. Sinilevt runsastuvat loppukesallamesotrofisissa jrviss mutta erittain rehevisshjrvissjo alkukeslla. Pahimmillaan levaesiintymat ovat myrkyllisten Microcystis-lajien muodostamia. Gonyostomum semen-limalev runsastuu loppukesalla jo lievsti rehevdityneiss ja rvissh. Tummavetisten jarvien lievh rehevityminen ei usda merkittdvdsti sinilevien mardd.

Suomalaisten jdrvien tila kasviplanktonseurannan tulosten perusteella arvioituna ndyttaisi1990-luvulla kohentuneen, vaikka ilmoitukset sinilevaesiintymista ovatkin lisaantyneet. Ainoastaan mesotrofisissa jdrvissd kasviplanktonin indr on 1990-luvulla kasvanut, lahinna piilevien iisaantymisen takia. Kyseessa saattaa olla hidas rehevoityminen eli ns. nuhraantuminen. Erittdinrehevissa jarvissd kasviplanktonin kokonaisbiomassa samoin kuin sinilevien ja piilevien mron selvdsti laskenut 1990-luvulla. Noin 50 % sinilevist on niit, jotka harvemmin muodostavatmyrkyllisia kantoja. Erittain rehevan Tuusulanjarven tila on selvsti parantunut tehokkaiden

pitoisuuden kanssasissdjdrvissd, koskaja muita levia, joidsuuri. Rehevissd jasuurikokoiset sinilefyllin pitoisuus onkasviplanktonbiompi. Biomassa-arvojasuurten sinilevayhdtivirheet, samoinanalysoinnin virhee

Pitkaaikaisessakasvukauden aikainhinna saaolosuhteimattava. Vaihtelu ovammista vesist ontelun erottaminenvaihtelusta on seuplanktonyhteison ham ennustaa jdrvemutta uusien lajielajien hdvidminen pmyds taksonomisissien mukaan tulisijdiden ja tutkimusmdollisimman pitkaatut interkalibroinniuokset. Pahimmillhavaitut rnuutoksetparistdssa, vaan pel

Yhteenvetona vosoittavat kasvipuekologista tiuaa jaettd laadullisesti nitoksiin. Kasvipuanksaja sen vaihtelu kaVaihtelu on suurintkee jonkin verran eharavinteisissa jrv


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ran vhemman, ja vain reheviss jarvissa sinilevtdominoivat kasviplanktonyhteisoa. Tutkimuksenperusteella sinilevien mhr ei nayttaisi lisaantyneen Suomenjhrvissh 1990-luvulla.

AcknowledgementsI started phytoplankton studies on 7 January,1965. Since then my colleagues, Pirkko Kokkonen, Reija Jokipii and Maija Niemela and I haveidentified and counted more than 11 000 phytoplankton samples, both for monitoring and studypurposes. We have had good intercalibration contacts with several phytoplankton researchers bothin Finland and in Nordic Countries. I particularlythank Mrs Ainikki Naulapaa, Phil. Lic. ToiniTikkanen, Dr. Gertrud Cronberg, Professor PerttiEloranta and Dr. Guy Hllfors for the teaching ofspecies identification. Special thanks go to Professor Reino Laaksonen, Professor Ake Niemiand Dr. Pertti Heinonen, who have encouragedme to bring this work to its conclusion. The summary paper was greatly improved due to the comments of Dr. Lauri Arvola. I wish to thank SirkkaVuoristo, who drew the figures in this work andPivi Laaksonen who helped in the typeing of tables. The English language of the summary paperand the articles was revised by Michael Bailey.The summary paper was partly written with support from the Academy of Finland. In addition, Ithank numerous persons in the Finnish Environment Institute and elsewhere not mentionedabove who have helped me in scientific and personal issues.

I dedicate this thesis to my family, especiallyto my three grandchildren Pauli, Heikki and Mikko.

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the eutrophication of lakes and flowing waters,with particular reference to nitrogen and phos

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