Lake Ecology

Lake Ecology

Unit 1: Modules 2/3 Part 6 – ManagementJanuary 2004

Developed by: Axler, Hagley Draft Updated: January 13 , 2004 U1-m2/3part6-s2

Modules 2/3 overview

Goal – Provide a practical introduction to limnology

Time required – Two weeks of lecture (6 lectures) and 2 laboratories

Extensions – Additional material could be used to expand to 3 weeks. We realize that there are far more slides than can possibly be used in two weeks and some topics are covered in more depth than others. Teachers are expected to view them all and use what best suits their purposes.


Modules 2/3 outline

1. Introduction2. Major groups of organisms; metabolism3. Basins and morphometry4. Spatial and temporal variability – basic

physical and chemical patchiness (habitats)5. Major ions and nutrients 6. Management – eutrophication and water

quality


6. Management topics

Trophic status Eutrophication Water quality


Nutrients most limiting to algal growth

Phosphorus Essential for growth PO4

-3 is primary dissolved form

PO4-3 sticks to soil and

sediment particles Usually key nutrient for

triggering excess plant growth

Must be reduced to control eutrophication

1 lb (kg) P can yield 500 lbs (kg) fresh algae

Nitrogen Essential for growth NO3

-, NH4+, and N2 are

primary biological forms NO3

- soluble in water May limit algal growth in

some circumstances More difficult to remove

from wastewater than P Some forms are toxic or

disease-causing to fish and mammals (including humans)


Limiting nutrients – demand versus supply

Nitrogen and phosphorus are typically in extremely short supply in water relative to plant demand

The “Redfield ratio” is the average composition of elements in phytoplankton

Ratio – 100DW:40C:7N:1P


The concept of limiting nutrients

Liebig’s Law of the Minimum (~1840): An organism’s total biomass yield is proportional to the

lowest concentration of nutrient relative to the requirements of that organism (paraphrased).

Lake managers are interested in limiting nutrients because: An increase might change water quality or food

webs. Restoration often requires a strategy for

reducing nutrient loading and predictions of the consequences of specific actions.


Limiting nutrients – a conceptual example

The following set of slides were developed to illustrate more specifically what is meant by “limiting nutrients” in the context of eutrophication studies

This may be appropriate for a lab exercise in which different combinations of N and P are added to lake water

Lake Superior was used as an example because we can see it out our window and because it is the biggest lake in the world and the cleanest of the Laurentian Great Lakes, so it is important to understand


Example – loosely based on Lake Superior

•Dave Hansen

•Minnesota Sea Grant

•Dave Hansen

•Dave Hansen


Conceptual nutrient limitation bioassay – 1

This example is loosely based on Lake Superior

1. Algal composition is approximately:

500 g wet weight : 100 g dry weight : 40 g C : 7 g N : 1 g P

Remember that the ratio of C:N:P is called the “Redfield ratio” and approximates the composition of algae!

40:7:1 by weight100:16:1 by atoms



2. Mid-summer bioavailable water chemistry: Dissolved inorganic carbon (DIC):

~ 10,000 µg C/L (as carbon dioxide and bicarbonate)

Dissolved inorganic nitrogen (DIN): ~ 300 µg N/L (95% as nitrate, with very low

ammonium) Dissoved inorganic phosphorus (ortho-P, DIP):

~ 0.5 µg P/L



3. Assume: Algal biomass = B0 ~ 200 µg C/L (particulate) Algal maximum growth rate ~ 20% per day



4. Run a nutrient enrichment experiment to estimate the limiting nutrient by doubling each nutrient: Set up 4 liter bottles of lake water in triplicate:

Incubate for 1 day and re-measure algal

biomass (Bf)

Control + Carbon+ Nitrogen+ PhosphorusAdd nothing + 0.5 µg P/L + 300 µg N/L + 10,000 µg C/L



5. What happens? After 1 day – algae grow 20% X 200 µg C/L = +

40 µg C/L Apply the “Redfield Ratio” to estimate nutrient

needs

Is there sufficient DIC to support this much growth? Is there sufficient DIN to support this much growth? Is there sufficient DIP to support this much growth?



6. What happened? 40 µg C/L of new growth would require:

40 µg DIC/L + 7 µg DIN/L + 1 µg DIP/L

+0 control treatment: 10,000 µg DIC/L is much more than enough 300 µg DIN/L is more than enough (293 excess) 0.5 µg DIP/L is half of what is needed

Therefore growth is 50% of maximum: = +20 µg C/L





+N treatment: 10,000 µg DIC/L is much more than enough 600 µg DIN/L is more than enough (593 excess) 0.5 µg DIP/L is half of what is needed

Therefore growth is 50% of maximum = +20 µg C/L





+C treatment: 20,000 µg DIC/L is much more than enough 300 µg DIN/L is more than enough (293 excess) 0.5 µg DIP/L is half of what is needed






+P treatment: 10,000 µg DIC/L is much more than enough 300 µg DIN/L is more than enough (293 excess) 1.0 µg DIP/L is just what is needed



An enrichment of only 0.5 µg P/L doubled algal growth

It would take a depletion of 43 µg P/L to deplete the 300ug DIN/L, based on the 7:1 ratio

The DIC is virtually inexhaustible in all lakes. It may “briefly” limit algal growth in

hypereutrophic sewage oxidation ponds The data suggest strong P-limitation for Lake

Superior

100 %

200 %

0 %

+C +P+N+0

Nutrient bioassay – summary and plot


Nutrient limitation bioassay responses•In progress, 10/20/03

Theory

Real data from the epilimnia of pristine northern Minnesota lakes


Halsteds Bay late summer mixing events

What might this mean for phosphorus levels in the water column?

Why?


Medicine Lake– Algal blooms & mixing events - 1

Background:

• Medicine Lake is extremely productive because of historically high nutrient enrichment from its watershed

(go to http://lakeaccess.org/lakedata/lawnfertilizer/mainlawn.htm)

• Major blooms of algae can be detected in the RUSS data set as:

• supersaturated O2 (why ?)

• increased pH (why ?)

• increased chlorophyll-a or turbidity (why ?)


Medicine Lake – Algal blooms & mixing events-2

STRATIFY RE- STRATIFY

MIX MIX

SundaySunday

ThursdayThursday

Friday-Friday-midnightmidnight

SaturdaySaturdayColor = O2

Line = pH


Halsteds Bay – Algal blooms & mixing events- 3

Why did the phosphorus in the bottom water drop so dramatically in August 1999 in Halsteds Bay ?

P levels drop



First, focus on the ice-free season water quality• relatively high epilimnion (surface)TP ~ 75-150 ugP/L• chlor-a (algae ) builds up steadily to levels > 50 ug/L



See how secchi drops as chlorophyll increases ?



Now see how much TP is in the hypolimnion


Halsteds Bay – Algal blooms & mixing events- 7Summary slide without animation


Medicine Lake: Storm mixing events

•This sequence runs from 1-5 from Aug 29-30, 1999

C


Trophic status classification

• This topic will be developed further in Module 22 (Regulations and Compliance Monitoring - Lake Biocriteria)

• Managers need to classify lakes to set water quality standards and prioritize monitoring, research, and restoration $$.

• Lake productivity, as indicated by its production of algal biomass, is a useful classification in regard to water quality issues as well as fisheries management

• Trophic status indices usually assume that nutrient levels (e.g. total-P) control algal biomass (measured by chlorophyll-a) which in turn regulates lake clarity (Secchi disk transparency)


Trophic Status

• Carlson trophic state index (TSI)- most widely used• based on log transformation of Secchi disk values as a measure of algal biomass on a scale from 0 – 110

• 10 units = doubling of algal biomass• TSI’s also developed for chlor-a and total-P based on their relationships to secchi for a set of midwestern lakes

• TSI useful for comparing lakes within a region and for assessing changes in trophic status over time

• Time period: usually summer; often set at June 15 – Sep 15 but it is rarely a good idea to restrict data acquisition without a good reason – especially Volunteer secchi data


Carlson TSI equations

•TSI-S = 60 - 14.41 ln [Secchi disk, m]

•TSI-C = 9.81 ln [Chlor-a, µg/L] + 30.6

•TSI-P = 14.42 ln [TP, µg/L] + 4.15

• Average TSI = [TSI-P + TSI-C + TSI-S] / 3

• If the 3 TSI values are not similar to each other, it is likely that:

• algae may be light- or nitrogen-limited instead of P-limited, or

• secchi is affected by erosional silt rather than by algae, or something else. One should look deeper into the data!

• Note that Dr. Carlson recommended not averaging the 3 values to avoid obscuring important differences


Carlson TSI vs water quality

<40 Oligotrophic; clear water; high hypolimnetic O2 year-round but possible anoxia in the deeper hypolimnion part of year

40-50 Mesotrophic; moderately clear water; possible hypolimnetic anoxia in summer and/or under ice. Fully supportive of all swimmable /aesthetic uses; possible cold-water fishery

50-60 Mildly eutrophic; decreased secchi; anoxic hypolimnion; possible macrophyte “problems”; warm-water fishery; supportive of all swimmable /aesthetic uses but “threatened”

60-70 blue-green algal dominance with scums possible; extensive macrophyte problems; not supportive of all beneficial uses

>70 Heavy blooms and scums in summer likely; dense “weed” beds; hypereutrophic; possible fish kills; fewer plant beds due to high algae; not supportive of many beneficial uses


TSI (Carlson) - graphical

Oligotrophic Mesotrophic Eutrophic Hypereutrophic


What are Ecoregions ?

Areas with similar: Climate Landuse Soils Topography “Potential” natural vegetation

Minnesota has seven major ecoregions Four ecoregions contain most of the lakes Water quality varies greatly from south to north


Minnesota’s Ecoregions


1.6-3.3 ft5-11 ft

1.0-3.3 ft

8-15 ft


Comparison of trophic indicators in Minnesota

TP (ug/L)

Chlor-a(ug/L)

Secchi(m)

TSI(Carlson)

Trophic Status O M E O M E O M E O M E

Standard Criteria <11

11- 24

>24 <3 3- 7>7 >4.0 4.0- 2.2<2.2 <35 40-55 >55

NLF 14 - 27 < 10 2.4 - 4.6 41 - 52

NCHF 23-50 5 - 22 1.5 – 3.2 49 - 66

WCB 65 - 150 30 - 80 0.5 – 1.0 67 - 77

NGP 130 - 250 30 - 55 0.3 – 1.0 67 - 73

O: oligotrophic; M: mesotrophic; E: eutrophic; see slide notes and accompanying slide for Minnesota Ecoregions map and code names; “General” values from Axler et al. 1994.


TSI Trends in Minneapolis, MN area WOW Lakes

Note importance of flagging which TSI is plotted and leaving space for missing years

Solid bars:TSI average for TP, chlor and secchi; striped: secchi only

How would you determine how well the TSI- secchi alone (stripes) predicts average TSI or TSI- chlor ?


Chlor-a - TP and Secchi – TP relationships in MN

Log chlor-a vs Log TP scatterplot for Minnesota’s ecoregion reference lakes (summer mean surface values)

Secchi transparency vs TP for Minnesota’s ecoregion reference lakes (summer mean surface values)

Data from Minnesota Pollution Control Agency Year 2000 Lake Assessment report (www.pca.state.mn.us)

Notice that lake clarity is much more sensitive to

increased phosphorus at the low end of the scale.

Why ?


Ex: Halsteds Bay late summer mixing events

• Run the color mapper from April 1999 through 2002 focusing on storm events in mid August 1999 and 2000

• START with MAP = TEMP and plot =DO to show variable stratification

• Then switch to MAP = DO and PLOT = TEMP to show anoxic events and discuss the release of P from sediments that swamps annual P-inflow from the watershed


Hypolimnion responses to increasing productivity

Trophic Status O2 PO4

-3 NH4+ H2S

Fe+2

(ferrous)

OligotrophicHigh

(mostly)Low Low Absent Absent

MesotrophicLow; partly anoxic

Low High if anoxic

Moderate High if anoxic

AbsentPresent where anoxic

Eutrophic Anoxic High High High High


Hypolimnion responses to anoxia

As the hypolimnion becomes O2 –depleted:

• NH4+ accumulates

• increased organic matter is decomposing

• cannot be converted to NO3- without 2 (bacterial nitrification)

• not much algal uptake (its dark and anoxic)

• Insoluble oxidized Fe+3 (ferric) at sediment surface is reduced to Fe +2

(ferrous) that is soluble; the phosphate adsorbing layer dissolves

• PO4-3 diffusion from the sediments increases dramatically

• Increasing decomposition leads to strong reducing conditions that favor bacterial reduction of sulfate to sulfide - producing rotten egg gas (H2S)

• Mixing adds lots of available N + P to the sunlit zone = ALGAE !!


Eutrophication and water quality


Trophic (feeding metabolism) terminology

Oligotrophic – low nutrients and “productivity;” usually high clarity

Mesotrophic – moderate nutrients, “productivity” and clarity

Eutrophic – high nutrients and “productivity;” low clarity


Eutrophication – Excess fertility leading to excessive plant growth


• excess algae: scums, noxious blue-greens, taste/odor/smell

• excess macrophyte (“weed”) growth- loss of open water; favors exotic species (EWM); sediment destabilization

• loss of clarity (secchi depth); aesthetic loss;

Water Quality Impacts- Eutrophication (some of them)

• O2 depletion; loss of fish habitat

• game fish impacts


• loss of native macrophytes from algal shading; loss of fish & waterfowl habitat and food; reduced shoreline & bottom stabilization, increased erosion

• lower bottom O2: increased sediment nutrient release: loss of fish habitat

• excess organic matter: smothers eggs and bugs

Water Quality Impacts- Eutrophication (…and some more)


Eutrophication – natural vs cultural

Natural filling by mineral and organic sediment – leads to lower V and larger Aw:A0 and A0:V

Lake to wetland conversion

Time scale > 103 years (if at all)

Irreversible

Human-caused from excess nutrient inputs and poor land-use management

Water quality degraded; loss of beneficial uses

Time scale < decades

Reversible loading


Eutrophication – the sad Lake Tahoe story

Data courtesy of C.R. goldman and J.E. Reuter, Tahoe Reesrach Group, U. of California-Davis,

http://www.news.ucdavis.edu/tahoetv/


Other regulators of lake productivity - Grazing

Top-down Model

High rates of nutrient driven algal growth is removed by intense zooplankton grazing pressure (usually cladoceran Daphnids)

Fishless lakes with low zooplankton predation

Lakes where planktivorous fish are regulated by predatory fish (game fish) –usually by intensive control

In these cases, algae are not nutrient limited

management tool = biomanipulation

Bottom-up Model

• Nutrient inputs drive algal growth

• Classic Pyramid

N+P


Potential Top-down effects on food chains

Low Secchi

O2 stress high pH ??

Low Predators

HIGH Planktivores

Low Zoops

HIGH Algae

HIGH Predators

Low Planktivores

HIGH Zoops

Low Algae

Higher Secchi less O2 stress lower pH ??

Smaller Zoops less grazing

= Larger Zoops more grazing =


Biomanipulation – fish management

Fish control • Intensive netting• Rotenone (poison)

• Stock increased #’s of piscivorous fish

• Selective catch or catch restrictions

• Control conditions for fish and zoop growth and survival


Biomanipulation- Summary

Summary:• Considered experimental • Requires complex knowledge of food

web processes (shallow lakes are particularly poorly understood)

• Herbivores may not consume certain blue-greens

• May be more successful in lakes without large-bodied zooplankton

• May require external loading to also be controlled

• Currently considered only a management tool- not a restoration technique


Lake Mendota Biomanipulation Project

Lake Mendota –large, urban, Lake Mendota –large, urban, limnologically “famous” lake limnologically “famous” lake in Madison, WIin Madison, WI

• Eutrophic with blooms of blue-green algae

• Sewage effluents diverted out of basin entirely by 1971

• Continued nonpoint pollution from agricultural and urban runoff

• 1987: attempt to control blooms by a massive stocking of walleye to reduce planktivorous fish

1988 – hot summer causes summerkill of Ciscos, the major planktivore zoops increase and algae decrease for few years Ciscos recover, anglers hammer the walleye, zoops decrease and algae are back


Other lake productivity regulators – light shading

Nutrients aren’t always the whole story Shallow lake research

Aquatic plants vs algae Over a wide range of nutrients there are alternative

stable states of dominance Plants shade phytoplankton creating clearwater Phytoplankton turbidity shades plants and restricts

growth to nearshore Periphyton mats and mucky sediments hinder plant

rooting High densities of grazers regulate periphyton on

leaves


PWC at ~ negative 5 m depth

Water transparency – clear vs turbid state

depth 5 m & secchi >5 m

Water transparency – clear vs turbid state


Water transparency – clear vs turbid state -2Water transparency – clear vs turbid state -2


Shallow lakes vs deeper lakes - “switches”

Usually more productive – higher Aw:Ao ratio Plants vs algae

Natural predominance of macrophytes over algae. Human impacts can switch them from clearwater-plants to turbid water-algae state maintained by

Poor fish management (carp, exotics, …) Inadequate shoreline protection of emergent veg Boat damage Pesticide and nutrient runoff (fish, grazers, plants)

Susceptible to very obnoxious algal blooms Difficult to reestablish clearwater-macrophyte state


Photosynthesis (PPr) & respiration (Rn) effects on routine water quality parameters – examples

Temperature: no effect generally. High rates of respiration can increase the temperature in bottom waters over long periods of time (>decades) but this is unusual and associated with “meromixis”

DO: High rates of photosynthesis (= primary productivity = PPr) produce O2 and can lead to supersaturation (>100%)

EC25: EC increases in the hypolimnion during stratified periods due to mostly to the accumulation of bicarbonate ions (HCO3

- ) from

respired CO2 that dissolves in the water at moderate pH (~6-9). The pH is usually lowered as well.

pH: High rates of PPr increase pH due to the removal of CO2 and HCO3

- from the water (essentially removing carbonic acid);

repiration does the opposite as noted above for EC25.


PPr & Rn effects in relation to density layering

Ice L., MN 6/14/99 Grindstone L., MN 6/20/99

•The line plots are dissolved-O2

•PPr – O2 “bump” •Rn – O2 “dip”

•Rn – O2 decline & anoxia


More about Onondaga in 2003

Here’s DO

Run the color mapper – set EC to 1200-2200 uS/cm and DO to % saturation

• DO > 150% from 0-3m and then <10% down to the bottom !

• pH drops >1 unit from 3 down to 5 m

•EC jumps up and down by 400 uS/cm !


Lake Washington tidbits… Apr- Oct 2002

Apr: high chlor – Is it real ?

Sep: low chlorophyll; low metalimnion DO - Is it real ?

Oct: high chlorophyll; - Is it real ?


Water Quality – What is it ?

Water quality is actually a subjective term that is used to describe the condition of a water body in relation to the needs of humans (beneficial uses in regulatory parlance), or the needs of aquatic organisms

Water quality is not an absolute since different user groups may have different expectations and values

Water quality protection involves both human health and environmental health risk assessments and management

Water quality regulatory standards alone may be met yet the “patient may die”. For this reason biocriteria have become an important new aspect of water resource protection


Water Quality – What’s a high value ?

Protection: environmental health vs human healthBeneficial use: fishable & swimmable vs drinkable

Lake-P : 100 ppb = hyper-eutrophic Cola-P : 1000's ppb = tasty Drinking water-P = no limit (Duluth adds 1000 ppb to

control lead leaching from old pipes) Lake-N : >500 ppb = high Drinking water : OK if < 10,000 ppb Nitrate-N Lake Hg < ~ 3 ppt vs DW < 2,000,000 ppt Lake fish PCB’s< 50 ppb vs Baby food < 2000 ppb


Water quality depends on many factors

Characteristics of the ecoregion Type and size of the watershed Precipitation patterns and surface water

hydrology Groundwater influences Lake size, shape, and retention time Number of people and land uses in the

watershed


Minnesota Ecoregions


1.6-3.3 ft5-11 ft

1.0-3.3 ft

8-15 ft


Other important factors affecting the health and economic sustainability of lake resources

Non-water quality impacts shoreline attached algae & aquatic plants shoreline woody debris & aquatic habitat exotic species (invasive plants & animals) noise pollution light pollution sight pollution

Lake Ecology

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