Science of the Total Environment 438 (2012) 260–270

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Science of the Total Environment

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Influence of fresh water, nutrients and DOC in two submarine-groundwater-fedestuaries on the west of Ireland

Aisling M. Smith ⁎, Rachel R. CaveNational University of Ireland, Galway, Ireland

H I G H L I G H T S

► Adjacent bays along a karst coastline show very different influences of SGD.► SGD is a source of nitrate and DOC but not DIP to both bays, which may act as phosphate sinks.► Nitrate supply to Kinvara is dominated by groundwater input, while adjacent Aughinish Bay is dominated by marine inputs.► Major rainfall events may reduce salinity in Kinvara Bay below guideline limits set in Shellfish Water Quality Regulations.► Kinvara Bay is vulnerable to eutrophication if phosphate inputs increase.

⁎ Corresponding author: Tel.: +353 91494126.E-mail address: aislingmsmith@gmail.com (A.M. Sm

0048-9697/$ – see front matter © 2012 Published by Elhttp://dx.doi.org/10.1016/j.scitotenv.2012.07.094

a b s t r a c t

a r t i c l e i n f o

Article history:Received 2 May 2012Received in revised form 27 July 2012Accepted 29 July 2012Available online 21 September 2012

Keywords:Submarine groundwater dischargeNutrientsCoastal processesDOCGalway Bay

Coastal fresh water sources, which discharge to the sea are expected to be directly influenced by climatechange (e.g. increased frequency of extreme weather events). Sea-level rise and changes in rainfall patterns,changes in demand for drinking water and contamination caused by population and land use change, willalso have an impact. Coastal waters with submarine groundwater discharge are of particular interest asthis fresh water source is very poorly quantified. Two adjacent bays which host shellfish aquaculture sitesalong the coast of Co. Galway in the west of Ireland have been studied to establish the influence of freshwater inputs on nutrients and dissolved organic carbon (DOC) in each bay. Neither bay has riverine inputand both are underlain by the karst limestone of the Burren and are susceptible to submarine groundwaterdischarge. Water and suspended matter samples were collected half hourly over 13 h tidal cycles over severalseasons. Water samples were analysed for nutrients and DOC, while suspended matter was analysed fororganic/inorganic content. Temperature and salinity measurements were recorded during each tidal stationby SBE 37 MicroCAT conductivity/temperature sensors. Long-termmooring datawere used to track freshwaterinput for Kinvara and Aughinish Bays and compare it with rainfall data. Results show that Kinvara Bay is muchmore heavily influenced by fresh water input than Aughinish Bay, and this is a strong source of fixed nitrogento Kinvara Bay. Only during flood events is there a significant input of inorganic nitrogen from fresh water toAughinish Bay, such as in late November 2009. Fresh water input does not appear to be a significant source ofdissolved inorganic phosphate (DIP) to either bay, but is a source of DOC to both bays. C:N ratios of DOC/DONshow a clear distinction between marine and terrestrially derived dissolved organic material.

© 2012 Published by Elsevier B.V.

1. Introduction

Groundwater-fed coastal embayments form a subset of estuarinesystems that have only recently begun to be studied with the samelevel of detail as river-dominated estuaries (Taniguchi et al., 2002;Burnett et al., 2003; UNESCO, 2004). Groundwater which dischargesto coastalwaters, often referred to as submarine groundwater discharge(SGD) is potentially a large but as yet poorly quantified source of freshwater to the oceans (Burnett et al., 2006; Moore, 2010). Groundwater

ith).

sevier B.V.

has the potential to be enriched in nutrients and contaminants(Valiela et al., 1990; Slomp and van Cappellan, 2004; Andersen et al.,2007) compared to river water, due to the absence of photochemicalreactions once it goes underground and also to the often low-oxygenconditions that can prevail due to lack of exposure to the atmosphere.In karst systems, point and diffuse source contaminants can be rapidlytransported throughout the aquifer (days/weeks scale), depending onabstraction rates and the nature of the contaminant, affecting drinkingwater quality and the coastal water it discharges into (Wheater et al.,2007). Due to the often widespread occurrence of seeps along SGDinfluenced coastlines, such contaminants have the potential to detri-mentally impact coastal water in terms of EU shellfish waters and bath-ing water standards, with knock-on effects on tourism.

Hydrometric Area 29

Aran Islands

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1 = Parkmore Quay 2 = New Quay 3 = Microcat 3113 4 = Microcat 3114

5 = Microcat 3115 6 = Castle/Arch 7 = Coranroo 8 = Tarrea Pier

Bell Harbour

Ballyvaughan Bay

(a)

Fig. 1. (a) Location of Aughinish and Kinvara Bays (inset) on the southern coast of GalwayBay, showing the extent of hydrometric area 29with dotted line. Hydrometric area 28 is tothe south, and 30 to thenorth (R. Corrib catchment). Black dots on land showapproximatelocations of rainfall stations, while station locations from research cruise CV10004 inGalway Bay are denoted by ‘x’. (b) Enlarged outline of the bays studied, with variouslocations numbered. Water sampling points are denoted by ‘*’ after the location number.

261A.M. Smith, R.R. Cave / Science of the Total Environment 438 (2012) 260–270

River flow and riverine contaminant loads entering coastal waters innorthern Europe are subject to regular monitoring and reporting re-quirements both under OSPAR (OSPAR, 2006) and under the EU WaterFramework Directive (2000/60/EC). Neal and Heathwaite (2005) high-lighted the importance of groundwater as a source of nutrients in thecontext of the Water Framework Directive (WFD). If a groundwatersource is being exploited for drinkingwater itmust be closelymonitoredunder the Groundwater Directive (80/68/EEC) and if it becomes contam-inated in some way, it may be closed to extraction and efforts made toreduce inputs of contaminants. However, no such monitoring andreporting requirements currently exist for groundwater discharging tocoastal waters where it is not a drinking water source, or has ceaseduse as such.

Many coastal waters in Europe support shellfish aquaculture, andthese waters are regulated under the Shellfish Waters Directive(2006/113/EC). This directive regulates such parameters as salinity(guideline range of 12–38 ppt), faecal coliforms, suspended solidsand dissolved oxygen saturation. Surface waters flowing into marineshellfish waters from land are given an ecological status under theWater Framework Directive, however no classification currently ex-ists for groundwaters discharging to coastal waters. Shellfish watersare given a quality rating under the European Communities Qualityof Shellfish Waters Regulations S.I. no. 55 of 2009 (amendment) andS.I. no. 268 of 2006. Shellfish from Grade A waters can be eaten raw(e.g. oysters) or sent straight to market, whereas shellfish fromGrade B areas have to be either cooked, or depurated, prior to sale.

This paper seeks to establish the influence of fresh water dischargeon salinity, and the relative influence of marine and fresh waterinputs on inorganic nutrients and dissolved organic carbon in a pairof adjacent bays (Aughinish and Kinvara) along the karst limestonecoastline of southern Galway Bay (Fig. 1). These bays are locallyimportant for their aquaculture sites.

Under the European Communities Quality of Shellfish WatersRegulations, which specify which shellfish activities can be carriedout in a given bay, Kinvara is classified as ‘B’ for mussels and oysterswhile Aughinish is classified as ‘B’ for oysters (EPA Water Quality inIreland, 2008, Appendix IV.3). In the S.I. 286 (2006) classification forClarinbridge/Kinvara, one groundwater input to Kinvara is noted,but no classification is given, and no such inputs are noted forAughinish. However the springs at Kinvara have recently been includ-ed in the Irish WFD Groundwater Monitoring Programme as partof the groundwater–surface water interaction monitoring network(EPA, 2011).

1.1. Description of study area

Galway Bay, on the west coast of Ireland, is characterised bysouthwesterly winds and an anti-clockwise residual current direction(Booth, 1975; Fernandes, 1988; Lei, 1995). The Aran Islands provideshelter from oceanic swells in the bay itself. The tidal range for thebay is 3–5 m. Galway Bay can be separated into an inner and outerbay, with the inner bay east of a line drawn due north from BlackHead (Fig. 1a). The outer section is deeper (average 27 m), with fasterflowing water as oceanic water enters via the South Sound andultimately leaves via the North Sound. The inner bay, where boththe inlets in this study are located (Kinvara and Aughinish), is moreshallow (averaging 13 m) with a steady inclining gradient from thewest to east in the inner bay. Island Eddy (Fig. 1b), located justnorth of Kinvara Bay, poses a physical barrier for ebbing water fromKinvara, and creates an anti-clockwise eddy of slow moving waternorth of the mouth of Kinvara Bay.

Kinvara and Aughinish Bays are two adjacent inlets in innerGalway Bay, which host shellfish aquaculture sites. There are norivers discharging into the bays, with all the drainage for the last5 km or so to the sea being underground (Boycott et al., 2003) thoughduring periods of heavy rainfall the water table can rise far enough in

local turloughs for short lengths of surface water runoff to occur. Thegroundwater is a potentially significant source of nutrients to coastalwaters in this region as it drains from a rural catchment, with ground-water quality in the region considered chemically of poor status(Daly, 2009) and groundwater bodies are vulnerable to point anddiffuse source pollution in the region (WFD Ireland, 2009). Dyetracing work indicates that Kinvara Bay is the focal point for a largepart of the underground drainage from the Gort–Kinvara lowlands,which make up most of hydrometric area 29 (Boycott et al., 2003).While there are no major urban centres in the surrounding area,there are small villages that have limited sewage treatment andsome 6000 individual dwellings throughout the approx. 900 km2 ofhydrometric area 29 (Fig. 1a) have septic tanks to store domesticsewage (S.I. 286 of 2006). Poor regulation has resulted in allowingmany of these tanks to be sited in unsuitable areas of very thin soil.The population of the catchment is approximately 23,000, and theurban sewage treatment is of low level. A treatment plant for thetown of Kinvara (pop. 850 in 2009) originally planned for construc-tion in 2006 is now under consideration for construction in 2012, atpresent untreated sewage (286 m3d−1 dry weather flow, EPA,2010) is discharged into the bay 100 m from the pier in Kinvara atthe head of the bay during ebb tides. While across many large ruralcatchments agricultural runoff is the major source of nitrate, weconsider that the daily direct discharge of untreated sewage from

Table 1Sampling dates and long term mooring dates. Long term moorings were downloadedapproximately every 6 weeks on site. Date in/out indicates when instruments weredeployed/retrieved periodically for removal of fouling organisms.

Sampling Dates Autumn Flood Winter Spring Summer

Kinvara(ParkmoreQuay)

18 Oct 09 29 Nov 09 30 Jan 10 21 Mar 10 27 June 10

Aughinish(New Quay)

19 Oct 09 30 Nov 09 31 Jan 10 22 Mar 10 28 June 10

CV10004(Galway Bay)

17/18 Feb 10

Long termmoorings

SBE 3113Kinvara

SBE3114 Augh.inner

SBE3115 Augh.outer

Date in 17 Jun 2009 04 Jun 2009 04 Jun 2009Date out 26 Nov 2010 15 Mar 2010 15 Mar 2010Date in 11 Dec 2010 29 Apr 2010 29 Apr 2010Date out 02 Feb 2011 30 Jun 2010 30 Jun 2010Date in 12 Jul 2010 12 Jul 2010Date out 06 Oct 2010 06 Oct 2010Date in 21 Oct 2010 21 Oct 2010Date out 17 Jan 2011 17 Jan 2011

262 A.M. Smith, R.R. Cave / Science of the Total Environment 438 (2012) 260–270

Kinvara is a significant source of fixed nitrogen to Kinvara Bay.Aughinish has no such urban area discharging into it and there-fore agricultural runoff is likely to be the main source of fixednitrogen.

As fresh water sources other than groundwater discharge to thecoast here are negligible (Drew and Daly, 1993, see also Supplementarymaterials), we deduce that observed changes in salinity in the baysresult from additions of groundwater. During periods of flooding,some temporary short stretches of water appear at the surface as thewater table itself rises, and flow a fewmetres or tens ofmetres overlandto the sea. We consider these to be part of the overall groundwaterdischarge to the sea and therefore do not differentiate them from theSGD. The land to the south and east of Aughinish and Kinvara islow-lying, generally b20 m of maximum elevation, while there ishigher land to the west of Aughinish, reaching a maximum of 240 mbetween Aughinish and Bell Harbour, the next bay to the west.A long-term study by Cave andHenry (2011) estimated thewater avail-able for runoff via submarine groundwater discharge, calculatedthe actual fresh water discharging from Kinvara Bay conduits(the ‘Castle’ and ‘Arch’, the focal points for much of the dischargefrom hydrometric area 29) and examined the relationship betweenthe rainfall in the catchment and the rate of groundwater dischargeinto the bay.

The location of the tidal sampling point in each bay (Aughinish atNew Quay, Kinvara at Parkmore) represents the narrowest point atthe mouth of each bay and we make the assumption that measure-ments made here are representative of water entering and leavingthe bays. In Kinvara Bay, the ‘Castle’ and ‘Arch’ are two intertidalconduit resurgence points (150 m apart) discharging at the head ofKinvara Bay. They are located beside Dunguaire ‘Castle’ (sometimesreferred to as Dungory Castle) and the ruined ‘Arch’ belonging tothe original castle structure (Fig. 1b, location 6*). The Castle resur-gence contributes large volumes of fresh groundwater into KinvaraBay year round, the Arch flow however may decrease/cease duringextended dry periods over the catchment. The sites are also pointsof known saltwater intrusion to the surrounding aquifer. While theCastle and Arch are well known locally they are poorly studied.Cave and Henry (2011) considered that the Castle ‘source water’ isof a different origin than the Arch ‘source water’ owing to the differ-ent chemical signatures of the water sampled.

No similar resurgences are known locally for the neighbouringAughinish Bay, and there are no known surface water inputs. How-ever, Drew (2001) noted that while the main drainage from theGort–Kinvara–Ardrahan region flowed through a large conduit toKinvara Bay, during flood periods some of this water was divertedthrough a conduit, which enters Aughinish Bay.

2. Methods

2.1. Tidal sampling

Tidal sampling was carried out near the mouth of Kinvara Bayat Parkmore Quay and in Aughinish Bay at New Quay (Fig. 1b)half-hourly over full tidal cycles (high tide–low tide–high tide orthe reverse) over several seasons in 2009/2010 (Table 1). Seawatersamples were collected for dissolved inorganic nutrients and dis-solved organic carbon (DOC) analyses, while suspended matterwas sampled and analysed for organic and inorganic contents.Samples were also collected from known intertidal groundwaterdischarge points.

At each tidal station the same pair of Seabird SBE MicroCATsrecorded the temperature and salinity at the surface and bottomof the water column at ten-minute intervals, complementing thelong-term moorings also logging in the bays during the tidal stationsampling events.

2.2. Long-term temperature and salinity moorings

Two Seabird MicroCATs (model number SBE 37-SM RS-485) weremoored in Aughinish Bay (serial number SBE-3114, inner bay,SBE-3115, outer bay), and one in Kinvara Bay (SBE-3113, mid-bay),see Fig. 1b for locations. The MicroCATs were set to log at ten-minute intervals. SBE-3114 and 3115 were deployed on the 4th ofJune 2009 from suspended mixed shellfish and seaweed culturelines in the inner (SBE-3114) and outer (SBE-3115) Aughinish Bay.They remained in Aughinish more or less continuously (see Table 1for dates) until their recovery in January 2011. The instrumentswere lifted periodically for downloading and removal of biofouling.SBE-3113 was deployed in Kinvara on 17th June 2010 on a raft ofshellfish culture lines, in the mid-outer bay of Kinvara at the samesite as used previously by Cave and Henry in 2006/2007 (Cave andHenry, 2011). It was recovered in January 2011, and was also period-ically lifted for downloading and removal of biofouling. The sensordepth was maintained at 1–1.5 m below the water surface, by thebuoyant culture lines or rafts, at all states of the tide. Both bays arerelatively shallow, with 3–5 m of water at low tide around themoorings.

The SBE-37 MicroCAT has a robust external titanium housing andprotective cell guard and is specifically designed for long-durationautonomous moorings. It has an average annual temperature drift of0.002 °C and conductivity drift of 0.003 mS cm−1. The design of theMicroCAT ensures that it is immune to proximity errors and accuracyis unaffected by external fouling making it suitable for use on theproject, see www.seabird.com for more information.

Inter-calibration of the MicroCATs was conducted prior to, andafter use in this project. The difference in salinity measurementsbetween MicroCAT numbers 3114 and 3115 (both in Aughinish)was 0.4 of a salinity unit, and data from this pair of instrumentshave been averaged for presentation of the long-term salinity data.The difference in salinity between MicroCAT-3113 (located inKinvara) and MicroCAT-3114 (Aughinish) was 0.3 of a salinity unit.A further inter-calibration was carried out upon completion of allautonomous data collection at the end of the project. Negligibledrift in salinity of the three MicroCATs used was observed over themooring periods.

2.3. Nutrients

A battery operated electric water pump (Wattera WaSP P2metal-free well pump) with a polycarbonate carrier hose was used

263A.M. Smith, R.R. Cave / Science of the Total Environment 438 (2012) 260–270

to collect water samples from the top 0.5 m of the water column.Both unfiltered and filtered (Whatman cyclopore membrane 0.4 μmfilters) samples were collected for nutrient analysis. Samples werecollected in 15 mL red-cap polypropylene tubes, frozen and stored at−20 °C until analysis. Analysis was carried out for nitrite, nitrateand phosphate using a Lachat QC8000 Flow Injection Analysis (FIA) in-strument. Methods used were those provided by Lachat Instruments—QuikChem® Method 31-107-04-1-E (nitrite (NO2-N) and nitrate(NO3-N), Smith and Bogren, 2001) and QuikChem® Method 31-1115-01-1-I (orthophosphate (P), Ammerman, 2001). Appropriate certifiednutrient standards from Ocean Scientific Ltd. (OSIL) were used togenerate the calibration curve for each method and as referencematerial during analysis. Results for nitrite (NO2-N) plus nitrate(NO3-N) are reported here as total oxidised nitrogen (TOxN). LOD forN is 5 μg L−1 (0.36 μmol L−1) and for P is 1 μg L−1 (0.03 μmol L−1).Note that throughout the text nutrient values are reported in bothgrammes and moles as data from state agencies are generally reportedin grammes.

2.4. Dissolved organic carbon/nitrogen (DOC/DON)

Sixty millilitre, acid-washed, high density polyethylene (HDPE)bottles were used to collect filtered DOC/DON samples in the field,where they were frozen until analysis. Samples were filtered using25 mm 0.4 μm acid washed and rinsed Whatman cyclopore mem-brane filter. Analyses were carried out by the National OceanographyCentre, Southampton, UK, in accordance with method outlined byDickson et al. (2007) in PICES Science report no. 34 using a ShimadzuTOC-VCSH with ASI-V auto sampler and TNM-1 Total Nitrogendetector.

2.5. Suspended particulate matter (SPM)

Pre-washed, ashed and weighed Whatman 47 mm GF/F 0.7 μmfilters were used for the collection of suspended matter. A knownvolume of sample was passed through the filter under low-pressurevacuum, and the filter paper was then rinsed thoroughly with DIwater to remove salt from the filter paper prior to storage. The filterpaper was folded, placed in a sterile bag, and frozen at −20 °Cuntil analysis.

Samples were dried at 60 °C for 24 h, weighed on a Sartoriusmicro-balance to get the sample SPM weight, ashed for 4 h at 450 °Cin a muffle furnace and reweighed on the Sartorius micro-balance toget the PIM (particulate inorganic matter) weight, based on loss onignition. The POM (particulate organic material) value is calculated asthe SPM value minus PIM value. Blanks were used to correct forsampling error, and certified reference micro-balance weights wereused to validate the accuracy of the Sartorius balance prior to andduring each use.

2.6. Rainfall data

Rainfall data for 3 stations in the catchment were obtained fromMet Eireann and combined with rainfall data from NUI Galway'sown station to give daily averages of rainfall over the catchment.Approximate locations of the rainfall stations are shown in Fig. 1a.

3. Results

3.1. Long term salinity data

Two MicroCATs (3114 and 3115) were deployed in Aughinish Baybetween June 2009 and January 2011, while a third MicroCAT (3113)was deployed in Kinvara Bay between June 2010 and January 2011.Kinvara Bay was subject to long term monitoring of temperatureand salinity in 2006/2007 (Cave and Henry, 2011) and was found to

be very strongly influenced by freshwater discharge from SGD.Winter discharge volumes of SGD into Kinvara Bay calculated byCave and Henry (2011) for November 2006–January 2007 rangedfrom 14–31 m3 s−1 (min) to 35–96 m3 s−1 (max). Long term moni-toring in Aughinish Bay was carried out for the first time in 2009/2010 during this project. Results indicate much lower input offresh water to Aughinish than in Kinvara. Fig. 2 shows salinity datafrom Aughinish and Kinvara Bays in June–Aug 2010 (Summer) andfrom mid-December 2010 to mid-January 2011 (Winter), whensimultaneous monitoring was being carried out in the two bays(see Table 1 for sampling dates).

April, May and June 2010 were dry months across the catchment(with 54 dry days over the period and combined rainfall of 193 mm),but July was wet, recording a total of 125 mm of rain for the month(representing 65% of the total rain for the previous 3 months). In earlyJune 2010 the salinity at high tide in both bays was approximately 34(Fig. 2a), however once rainfall increased again in late June the salinityin Kinvara dropped rapidly, with a much smaller (though still clearlyvisible) drop in salinity in Aughinish (Fig. 2a). The salinity in Aughinishrecovered rapidly while the salinity in Kinvara continued to decreaseuntil early August, indicating that Aughinish is only affected by localrainfall events during these months, whereas Kinvara is receivinginput via SGD from the catchment. Average salinity in Aughinish is32.9 over the Winter period (December 2010/January 2011) shown inFig. 2b, while the average salinity in Kinvara for the same period is29.6. The consistent difference in average salinity of the two bays isnot a function of any difference in the instruments, as it is also clearlyseen during the tidal stations where the same MicroCAT is deployedon consecutive days in each bay (Fig. 3). See Discussion and Supple-mentary materials for details of residence time of water in the baysand circulation.

3.2. Tidal station salinity data

Thirteen-hour tidal station sampling was carried out at Kinvaraand Aughinish on consecutive days (Table 1 and Fig. 3), in October2009 (Autumn), November 2009 (flood), January 2010 (Winter),March 2010 (Spring) and June 2010 (Summer). At all seasons it isclearly seen that the salinity in Kinvara Bay is affected by freshwater discharge, as even during dry weather (e.g. late June 2010,Fig. 3d) it is consistently lower than in Aughinish Bay. This is consis-tent with observations that the intertidal discharge of groundwater atDunguaire Castle continues even during prolonged dry spells.

Autumn 2009 surface salinity data over the tidal cycle for bothbays is almost identical to the Spring data and so is omitted fromFig. 3 (see Table A). During long dry spells the salinity of the twobays may be very similar at high tides (Figs. 2a and 3d), when theinfluence of Galway Bay water is greatest, but much fresher water isseen at low tide in Kinvara Bay.

3.3. Nutrients

Kinvara Bay was strongly impacted by fresh water dischargeduring a major flooding event in November 2009 (Fig. 3a), andthroughout the Winter of 2009/2010 (Fig. 3b). Salinity in Aughinishwas also affected during the November 2009 floods, with TOxN(nitrate plus nitrite) showing a strong negative correlation withsalinity (r2=0.91 in Kinvara, 0.71 in Aughinish, Fig. 4a), though theslope of the trendline indicates higher TOxN in the fresh water enter-ing Aughinish Bay. During the Winter period for Kinvara there is alsoa clear negative correlation (r2=0.77) in the TOxN values plottedagainst salinity (Fig. 4b). There was a much smaller impact offresh water in Aughinish in Winter generally, when compared toKinvara (Fig. 2b), and it can be seen that the salinity for the January2010 tidal station is consistently high in Aughinish (Fig. 3b, 4b),

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Fig. 2. Example of salinity from long term moorings in Aughinish and Kinvara Bays in (a) Summer and (b) Winter. The tidal curve for the period is shown above each graph. Theupper salinity trace in each graph is for Aughinish, and the lower trace is for Kinvara. Rainfall is shown at the base of each graph. Arrow in (a) denotes the start of salinity recovery inKinvara Bay, almost 2 weeks after cessation of significant rainfall. It is not clear from the rainfall data why there is a drop in salinity in both bays around 16th December.

264 A.M. Smith, R.R. Cave / Science of the Total Environment 438 (2012) 260–270

with a small range of TOxN values, from ~7.14 to 10.71 μmol L−1

(100–150 μg L−1) (Fig. 4b).Dissolved inorganic phosphate (DIP) shows fairly consistent

concentrations during both the November 2009 flood and the 2010tidal station at Kinvara (Fig. 4c,d), with no salinity effect observed,indicating that fresh water input is not a source for DIP. This is inline with a comprehensive study of phosphate in groundwater byHolman et al. (2010) who found low phosphate in groundwater inthis region. Concentrations of DIP in Aughinish aremore variable duringthe November flood, ranging from 0.1 to 0.5 μmol L−1 (3–16 μg L−1)indicating that DIP may be mobilised, perhaps desorbed fromsuspended sediment, during this period. Like the TOxN, the DIP valuesfor the Winter tidal station in Aughinish (January 2010) show a smallconcentration range, as does the salinity.

Filtered nutrient samples were collected at a grid of 17 stations(at 3 m and bottom depths ranging from 8 to 22 m) in inner GalwayBay during a survey in mid-February 2010 on RV Celtic Voyager(CV10004, Cave and O'Connor, 2010). Average values for TOxNwere 8.8±0.7 μmol L−1 (123±10 μg L−1) and 0.51±0.06 μmol L−1

(16±2 μg L−1) for PO4-P (Fig. 4b,d). The TOxN values from theCV10004 survey can be seen lying along the path of the Kinvaratrendline in Fig. 4b, indicating that the two end-members are freshwater input to Kinvara Bay, and Galway Bay seawater. DIP in Galway

Bay in February 2010 is generally slightly higher than that observed ineither Kinvara or Aughinish Bay in Winter (Fig. 4d), indicating thatthe bays may act as a sink for DIP.

Concentrations of TOxN measured at the two major intertidaldischarge sites in Kinvara in October 2009 were 52.8 μmol L−1

(740 μg L−1, Castle) and 23.6 μmol L−1 (330 μg L−1, Arch). Extrapo-lating the trendline produced in Fig. 4b for the Winter TOxN data forKinvara Bay water to zero salinity, gives a concentration of44.3 μmol L−1 (620 μg L−1) TOxN for the fresh water input. TheCastle discharge site maintains a strong flow year round, and thuslikely draws from a deeper source water (Smyth, 1996; OPW, 1997;Drew and Daly, 1993; Cave and Henry, 2011), whereas the Arch sitesource water appears to be from a shallower source and showsmuch reduced flow during dry periods (also as it is Winter time thetransformation of nutrients should be relatively unchanged as prima-ry production is negligible for the Winter season — thus regression isvalid). Assuming that these two source water ‘types’ are representa-tive of the major sources of fresh water discharging into KinvaraBay, then the fresh water input is approximately 70% Castle ‘type’source water and 30% Arch ‘type’ source water under normal Winterconditions. Spot samples for Nitrate-N taken by the EPA from theCastle site (referred to as ‘Kinvara Springs’ by the EPA) in 2009were 99.6 μmol L−1 (1394 μg L−1) (4th June 2009), 114.4 μmol L−1

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y0

5

Aughinish Kinvara

Aughinish Kinvara

Tid

e m

Tid

e m

Aughinish Kinvara

Tid

e m

Tid

e m

Aughinish Kinvara

(d) Summer, Jun ‘10

Time of day Time of day

Time of day Time of day

(a) Flood, Nov ‘09

(c) Spring, Mar ‘10

(b) Winter, Jan ‘10

Fig. 3. Salinity data for tidal stations. (a) Flood— November ’09, (b) Winter— January ’10, (c) Spring—March ‘10, (d) Summer— June ‘10. Note that the flood tidal station was takenduring neap tide whereas the January tidal station was taken during spring tide of the same Winter season. In each case, the dotted line is the tidal state at the time of sampling.Autumn (October ‘09) data can be found in the Supplementary materials, both stations hover around a salinity of 33.

265A.M. Smith, R.R. Cave / Science of the Total Environment 438 (2012) 260–270

(1606 μg L−1) (29th July 2009) and 64.1 μmol L−1 (898 μg L−1)(11th November 2009). Nitrite-N for these samples was below thedetection limits.

However, extrapolating the trendline produced in Fig. 4a for theNovember flood TOxN data for Kinvara Bay to zero salinity gives aconcentration of 33 μmol L−1 (463 μg L−1) TOxN for the fresh

y = -1.03x + 41.78

R2 = 0.91

y = -2.18x + 74.10

R2 = 0.71

0

5

10

15

20

25

30

15 20 25 30 35Salinity

TO

xN µ

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15 20 25 30 35Salinity

PO

4 µ m

ol L

-1

Spring

Spring

(a) Flood, Nov ‘09

(C) Flood, Nov ‘09

(

(

Fig. 4. Nutrients versus salinity for all seasons in the 2009/10 sampling period where detecthe concentrations observed for the Spring sampling period for both TOxN and PO4, see Ta

water input. This implies that either the Castle source water is dilutedby the November flood waters, reducing its concentration, or the pro-portion of the flow from the Arch source is greatly increased while itsconcentration remains the same (Fig. 4a). Either would dilute theoverall fresh water concentration of TOxN. Field observations indicatethat the Arch flow is heavily influenced by rainfall, as it dries up

y = -1.06x + 44.55

R2 = 0.77

0

5

10

15

20

25

30

15 20 25 30 35

Salinity

0.0

0.1

0.2

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0.7

15 20 25 30 35Salinity

Kinvara

Aughinish

CV10004

Summer

B) Winter, Jan ‘10

D) Winter, Jan ‘10

table levels were measured (with the exception of Autumn ‘09 which closely matchedble A). Legend is the same for each graph.

266 A.M. Smith, R.R. Cave / Science of the Total Environment 438 (2012) 260–270

during periods of dry weather, so it is reasonable to suppose that itwill be more productive during floods. In Aughinish however, duringflood conditions (as seen in November ‘09) a more concentratedsource of TOxN-rich freshwater than either of the Kinvara freshwater sources is being accessed as extrapolation of trendline tozero salinity gives TOxN of 74.1 μmol L−1 (1037 μg L−1) (Fig. 4b).This may include local septic tank overflow water or the fresh watersdischarging into Aughinish Bay may be derived from a differentsource water type than those discharging into Kinvara Bay.

Spring (March 2010) and Autumn (October 2009) TOxN levelsduring tidal sampling were very similar in both bays, and muchlower than in Winter (Fig. 4a, inset showing Spring TOxN) while dur-ing Summer (June 2010) tidal sampling, TOxN was below detectionlimits in both bays. Both Spring tidal station sampling and Summertidal station sampling were carried out following several weeks ofdry and sunny weather. See Table A for Autumn data.

3.4. DOC/DON

DOC samples show a clear negative trend with salinity (Fig. 5),with low salinity DOC samples taken during November 2009 floodshaving 200–300 μmol L−1 of C, while high salinity DOC sampleshave ~50–100 μmol L−1 of C. Only a few samples of DOC from inter-tidal groundwater discharge are available, with Autumn 2009 sam-ples showing high values, ~600 μmol L−1 while Summer samplesshowed lower values of ~100–150 μmol L−1. For comparison, thethree TOC samples taken by the EPA from the Castle in 2009 rangedfrom 290 to 400 μmol L−1of C. Coastal water samples taken in innerGalway Bay in February 2010 show consistent levels of DOC of60–90 μmol L−1 of C at salinities ranging from 29 to 34.5 (Table A).Carbon:nitrogen ratios were calculated for samples where both DOCand DON values were available. C:N ratios (molar) for the freshwatersamples are low (~5) as are Autumn/Winter samples from both bays,while Spring/Summer samples from the bays have higher C:N ratiosof 10–20.

3.5. Chlorophyll

Samples were taken for chlorophyll-a at tidal stations but thelevels detected were generally low (b5 μg L−1, Table A) and noevidence of blooms in either bay was observed. The low level of nutri-ents, often below detection, measured during the Spring/Summer/Autumn months shows that the nutrients supplied from the coast

0

50

100

150

200

250

300

350

400

20 22 24 26

Sa

DO

Cµ

mo

l L-1

TS7 Oct 2009TS13 Nov 200TS17 Jan 201TS19 Mar 201TS25 June 20

Fig. 5. DOC trend with salinity at tidal stations (TS) sampled in 2009/10, including samplesDOC in Galway Bay seawater taken during CV10004 in Feb 2010 were 60–70 μmol L−1 at a

are being utilised but this may be happening in the inner or middleparts of the bay, which were not sampled during this project, withthe bulk of the phytoplankton being removed by the shellfish inmid-bay.

3.6. Suspended particulate matter

Particulate matter was collected to examine the proportion ofparticulate organic matter (POM) in the total suspended particulatematter (SPM) at different seasons. POM averaged 20% of the total ineach of the seasons sampled, in both bays on both flood and ebbtides (Fig. 6a). No SPM samples were taken during the Spring2010 (March) tidal station sampling. The smallest variation in theproportion of POM during a tidal station sampling event was seenin Autumn, and the greatest variation in Summer (Fig. 6a). OverallSPM concentrations were low (5–35 mg L−1) throughout, with thehighest and least variable SPM sampled in Autumn 2009 (October),with lower concentrations but greater variability seen in Winter andSummer 2010 (Fig. 6b). Similar values were observed in both bays.

4. Discussion

The long-term salinity data collected here show that salinity inKinvara is very strongly affected by inputs of freshwater year round,while Aughinish appears to receive significant inputs of fresh wateronly during periods of major floods, such as that which occurred inNovember 2009. At other times, while the mean salinity in Aughinishmay be reduced by high rainfall, it recovers quickly from suchepisodes, while the mean salinity in Kinvara continues its downwardtrend to a much greater extent and over a longer period (sometimesweeks in Kinvara compared to days for Aughinish, e.g. Fig. 2a). Thebulk of the fresh water input to Kinvara is assumed to be fromgroundwater discharge, as there are no visible overland dischargesexcept when the water table rises during heavy floods. This impliesthat the groundwater catchment for Kinvara is much greater thanfor Aughinish, and indeed earlier work indicated that Kinvara maybe the focus for much of the drainage in hydrometric area 29(Fig. 1a, Smyth, 1996; OPW, 1997; Drew and Daly, 1993), for whichrainfall and evaporation data predict a long term average outflow of18–31 m3 s−1 (Cave and Henry, 2011).

The discharge does not always respond directly to rainfall (Fig. 2).Firstly, the full extent of rainfall across the catchment may not becaptured by the limited number of rainfall stations (see Fig 1a).

28 30 32 34

linity

Kinv TS8 Oct 2009 Augh9 Kinv TS14 Nov 2009 Augh0 Kinv TS18 Jan 2010 Augh0 Kinv TS20 Mar 2010 Augh10 Kinv TS26 June 2010 Augh

DOC Castle (S=0) Oct 2009

DOC Castle (S=0) Jun 2010

from Castle groundwater resurgence point, where salinity S=0. Background levels ofsalinity of 34 (see Table A).

0

5

10

15

20

25

30

35

40

AutFlood

Aut Ebb

WinFlood

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SumFlood

SumEbb

SP

M m

g L-

1

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10

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30

40

50

AutFlood

Aut Ebb

WinFlood

WinEbb

SumFlood

SumEbb

% P

OM

Kinvara

Aughinish

(a)

(b)

Fig. 6. Seasonal % POM and SPM from Kinvara and Aughinish on the flooding andebbing tide, n=20–25 for each tidal station. Note that SPM samples were not collectedduring the Spring ‘10 tidal station.

267A.M. Smith, R.R. Cave / Science of the Total Environment 438 (2012) 260–270

Secondly, if the main groundwater system is already full, then anepisode of rainfall in only one part of the catchment may be enoughto overflow parts of the system that are normally inactive, causingoutflow into Kinvara Bay and lowering the salinity. Thirdly, if the sys-tem has had time to empty between significant rainfall events, thenext event may simply refill the system, but not to the point wherea significant amount overflows into the bay. As mentioned above inthe description of the study area, Drew (2001) noted that while themain drainage from the Gort–Kinvara–Ardrahan region flowedthrough a large conduit to Kinvara Bay, during flood periods someof this water was diverted through a conduit, which enters AughinishBay at Coranroo (Fig. 1b, location 7).

The residence time of the water in the bay is influenced by anumber of factors; wind speed and direction along with wave height,phase of tide and volume of freshwater discharging from the head ofthe bay. Outer Kinvara Bay was found to be well mixed vertically aswell as horizontally, getting increasingly stratified toward the headof the bay (O'Toole, 1990). However, the geometry of the area outsidethe mouth of Kinvara Bay makes it likely that some of the ebb waterwill be trapped behind Island Eddy and between there and KilcolganBay, and a portion of this may re-enter Kinvara on the next flood tide.

The high rainfall experienced across the country in November2009 caused severe flooding, particularly in the west of Ireland.There was no long term mooring measuring salinity in Kinvara atthat time (not deployed until June 2010), however during a wetperiod around 16th December 2010, salinities recorded by theMicroCAT located in the middle-outer Kinvara Bay on a rope musselrafts, touched 12 on one low tide (Fig. 2b, though scale on the graphdoes not go low enough to show this). Salinities of 18 were recordedat low tide at the mouth of Kinvara Bay during tidal sampling in lateNovember 2009 (Fig. 3a), a few days after the main rain event, and

reached a low of 17 during the January tidal sampling. Salinities aslow as 15 were observed in December 2006, and again in January2007 by long term moored MicroCATs at the mussel rafts (Cave andHenry, 2011).

The European Communities (Quality of ShellfishWaters) Regulations(2006) guideline values for salinity of shellfish waters are 12 to 38 prac-tical salinity units. Fluctuating salinities cause stress to both musselsand oysters and in the case of mussels, growth rates are depressed asthe animal adjusts to the changing water (Almada-Villela, 1984). Poorgrowth andmortality are a serious economic risk for farmerswhere fluc-tuating salinity and prolonged periods of lowered salinity are a possibil-ity. If climate change should lead to more bouts of unusually heavy orprolonged rainfall, this may lead to Kinvara breaching the legislativeguidelines on salinity for shellfish water designation. For both bays,increased high rainfall events leading tomore frequent significant reduc-tions in salinity are likely to impact the quality and shelf-life of musselsharvested there, with economic consequences for those farming them.

The current EU upper limit of nitrate concentration in waterextracted for human consumption is 50 mg L−1 NO3 (EPA, 2006,11.3 mg L−1 of N in the form of nitrate or ~0.8 mmol L−1). In Ireland,some 23% of all EPA monitored groundwater sources exceeded theguideline value of 25 mg L−1 of NO3 (EPA, 2006, 5.65 mg L−1 of Nin the form of nitrate, ~0.04 mmol L−1). Normal Winter concentra-tions in coastal seawater would not be expected to exceed a fewhundred microgrammes per litre, while Summer concentrations insurface coastal waters may be at or below detection limits, howeverthe EPA sampling in inner Galway Bay in the Summers of 1998 and2000 found median values of 0.009 mg L−1 (0.64 μmol L−1) totaloxidised nitrogen (TOxN) (McGarrigle et al., 2002).

It is clear from Fig. 4 that Kinvara is strongly impacted by inputs offixed nitrogen from SGD, while Aughinish only receives significantinputs of TOxN during flood events. Analyses of the two known inter-tidal sources of groundwater to Kinvara, both in this work and byCave and Henry (2011) show that the water from the Castle resur-gence has concentrations of up to 1 mg L−1 (0.07 mM) of TOxN,whereas the source water for the Arch resurgence tends to haveTOxN concentrations about half those of the Castle. Spot samplestaken by the EPA in 2009/2010 showed Nitrate-N values up to3 mg L−1 (0.21 mM). However, neither groundwater source appearsto be a source of DIP, with concentrations similar to or lower thanbackground concentrations in Galway Bay. Phosphate is known tobe removed from groundwater in limestone areas due to the forma-tion of Ca–P compounds (Cable et al., 2002; Jarvie et al., 2005), andthe subterranean estuary can act as a sink of P from seawater(Slomp and Van Cappellen, 2004). Consequently, a wet Spring,Summer or Autumn will strongly promote phytoplankton growth inKinvara, limited only by the availability of phosphate from seawater,influenced by the bays' retentive circulation pattern. If, however, theSGD should become contaminated with phosphate, then the stagewould be set for eutrophic conditions to develop.

There is an accumulation of nutrients in Winter, both in GalwayBay and in Kinvara and Aughinish (Fig. 4b,d). Galway Bay waterappears to be a source of DIP to the two bays. Despite abundant nutri-ents, low light levels inWinter are preventing a bloom from occurringduring this time.

The Spring tidal sampling data show salinities a little lower inKinvara compared to Aughinish, but with only small variations overthe tidal cycle (salinity change over the tidal cycle is less than0.5 units in each case). This tidal station followed several weeks ofvery dry weather. Nutrients are depleted in both bays (Fig. 4a,c) butinterestingly plotting the nutrient concentrations over the tidalcycle in each bay (Fig. 7a,b) shows clearly that while there is littlechange over the tidal cycle in Kinvara, there is a strong influenceof Galway Bay water on the flooding tide in Aughinish, bringingin both TOxN and DIP. This suggests that there is a strong circulation‐dependent exchange in Kinvara, due to retention of ebb water

268 A.M. Smith, R.R. Cave / Science of the Total Environment 438 (2012) 260–270

outside the mouth of the bay and its return on the next flood tide.Aughinish however, is well flushed out on the ebb tide and thiswater appears to be removed by the residual circulation in GalwayBay. A proportion of the ebb from Aughinish may even enter KinvaraBay on the following tide, as the dominant circulation in Galway Bayis anticlockwise, from south-west to north-east.

During the Summer tidal station the salinities for the two baysmatch closely, and despite this being a spring tide there is little fluc-tuation of salinity at low tide. This indicates that there is a low volumeof fresh water being discharged into Kinvara and little or none intoAughinish. This in turn means no replenishment of nutrients fromland based sources.

The results for DOC indicate that the groundwater discharge is asource of DOC to coastal waters in Autumn/Winter, as it combineshigh discharge with high DOC concentrations, but not in Summer,when both concentrations and discharge are low (Fig. 5). DOC fromfreshwater sources has a low C:N ratio, and appears to accumulatein Autumn/Winter in the bays, while marine-derived DOC with highC:N ratios dominates in spring/summer. Background DOC in GalwayBay in Winter is low (see Table A), consistent with the values seenat high salinities in Aughinish Bay. Extrapolating the data in Fig. 5for Kinvara Bay and including Galway Bay data (actual values can befound in the Supplementary materials) back to a salinity of zerogives a DOC concentration of 448 μmol L−1, while the max DOC

Time of day

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23

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Tid

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(a)

(b)

Fig. 7. Spring ‘10 tidal stations at Kinvara and Aughinish showing concentrations of TOxNand DIP over the tidal cycle. The minor variations in salinity for these tidal stations can beseen in Fig. 3c.

concentration measured in the groundwater at the Castle in Autumn(Oct) is 409 μmol L−1. However two of the highest DOC samplestaken at the lowest salinity values (salinity of ~21) in Fig. 5 were sam-pled during the Nov. flood (TS13), a month later than the samplesfrom the Castle groundwater discharge point, for which no data areavailable during the flood. It is quite likely that at least in the earlystages of the November flood, DOC values were higher than inOctober, as the system got flushed out. DOC data collected during aseparate study from wells near Kinvara in May 2009 (Petrunic et al.,in review) had concentrations up to 408 μmol L−1.

Both bays host shellfish aquaculture sites, which are in commonwith many Irish west coast bays. Mussels and oysters are filter feedersand may be partially responsible for preventing the developmentof eutrophic conditions in Kinvara Bay, by the constant removal ofphytoplankton and other small organic detritus. The shellfish arealso likely responsible for processing much of the effluent cominginto Kinvara Bay, which may be their main food source, particularlyin summer when the population of Kinvara increases due to an influxof tourists, (increasing the effluent load) and when nutrient suppliesfor phytoplankton growth may be low due to the low SGD input indry weather. Not all materials filtered by mussels will be digested,instead some of the particles will be excreted as pseudofaeces,which may be further digested by benthic organisms. Mussels havefiltration rates that can range from 2 to 7 L h−1 per 1 g dry tissueweight (Mohlenberg and Riisgard, 1979). Kinvara has an averagedannual production (2006–2010) of 140 tonnes of mussels and20 tonnes of oysters (BIM, 2012).

Assuming that, at harvest time, the weight per mussel will beapproximately 30 g wet tissue weight (marketable size), giving aminimum/maximum dry tissue weight of 1–2 g (Newell, C., pers.comm.), we can use the following example to examine the effect onthe water column of mussel filtration:

Example. 30 g mussel, maximum dry tissue weight of 2 g, maximumfiltration rate of 7 L h−1 per gramme dry tissue weight, and 3 m tidalexcursion (average neap tide).

This example yields a 12-hour filtration volume of 7.84×105 m3,about 5% of the tidal prism (see Supplementary materials for baysurface area/volume calculations). Mussels in these waters take~18 months to reach marketable size (about 5 cm), so at any timearound 300 tonnes of commercially rope-grownmussels will be pres-ent, filtering 2–10% of the tidal prism volume, in addition to an un-known quantity of wild mussels and other filter feeders. While suchcalculations contain very broad assumptions, including the suppositionthat mussels feed throughout the tidal cycle, this gives some indicationof the potential benefits of commercial shellfish farming on the waterquality of the bay. Kinvara and Aughinish Bays are included in the sub-mission to the OSPAR Commission listing for Marine Protected Areas(MPA) O-IE-0002969, and under the ‘Galway Bay Complex; SpecialArea of Conservation’ under the National Parks and Wildlife Service.The natural filtration process of filter feeders may well be aiding inpreventing eutrophic or anoxic conditions from developing in KinvaraBay.

Aughinish Bay experiences conditions that aremore those of an inletthan an estuary, and except during major floods, its nutrient supply isdriven by conditions in Galway Bay. It has no urban centre to providean effluent source akin to that of Kinvara. Its shellfish population istherefore dependent onGalway Baywater for delivery of its food supply.

Frequency and intensity of rainfall events are likely to alter in awarming world, and both periods of drought and rainfall events cur-rently considered exceptional may increase (IPCC), both of which willaffect groundwater recharge, and therefore discharge, and, whereconnected to the sea will affect SGD. However the effects of climatechange on nitrate concentrations in groundwater are not yet well un-derstood (Stuart et al., 2011). On a shorter time scale, the imminent

269A.M. Smith, R.R. Cave / Science of the Total Environment 438 (2012) 260–270

removal of the quota on milk production in the EU (by 2015) and theIrish government's target of a 50% increase in milk production by2020 (DAFF, 2010), are likely to lead to increased use of fertiliser forgrass production in Ireland, thus increasing nitrate inputs to ground-water, and via both rivers and SGD, to coastal waters.

5. Conclusions

Two adjacent bays on the karst coastline of western Ireland showvery different effects from groundwater input. Year-round inputs ofsignificant volumes of fresh water provide a continuous source ofTOxN to Kinvara, with SGD as the primary source of fresh waterinput. During major floods when overland input is apparent, this isalso primarily from groundwater, as the water table rises and flowsoverland for short stretches to the coast. Fresh water input toAughinish Bay is more episodic, occurring only during periods ofsignificant rainfall in the catchment, and this provides an input ofTOxN to Aughinish Bay mostly during Winter. Fresh water is not asignificant source of DIP to either bay, and both bays may act assinks for DIP brought in from Galway Bay. Fresh water inputs inAutumn and Winter appear to be a source of DOC with low C:Nratio to both bays. If the SGD entering Kinvara Bay was to becomesignificantly contaminated with phosphate then it is then likely thatthis bay might become periodically eutrophic. However the presenceof shellfish aquaculture would help to counteract this. Kinvara Bayexports TOxN and DOC to Galway Bay during wet periods, and wetsummers are likely to extend the phytoplankton blooming periodup to the point of phosphate limitation. If high volumes of freshwater enter the bay in Summer, this may act to flush raw sewageout into Galway Bay, which may adversely affect contiguous coastalareas.

At present, the supply of macronutrients to Aughinish Bay is con-trolled by conditions in Galway Bay. However, if climate change leadsto more flooding events, then the fresh water input and nutrient dy-namics in Aughinish are likely to change. Further, if new pathwaysopen in the submarine groundwater system then there is potentialfor significantly higher inputs of both fresh water, TOxN and DOCto Aughinish Bay. Future regulation of these waters needs to ensurethat contamination by phosphate of groundwater bodies in the coast-al zone is prevented, and monitoring of both bays is recommended tokeep track of changing volumes of freshwater inputs to each, and ofnutrient inputs.

This is the first research demonstrating the impact of groundwa-ter discharge on nutrient and dissolved organic carbon concentra-tions in Irish coastal waters. The karst booklet produced by theGeological Survey of Ireland states that 40% of the island of Ireland isunderlain by limestone, (http://www.gsi.ie/Programmes/Groundwater/Karst+Booklet), with many karst and paleokarst areas around the coast.The Irish National Seabed Survey (INSS) has shown that in many places,the limestone bedrock extends well out to sea. It is likely therefore thatdirect inputs via groundwater are a regionally important component ofthe overall inputs from fresh water to Irish coastal waters.

Supplementary data to this article can be found online athttp://dx.doi.org/10.1016/j.scitotenv.2012.07.094.

Acknowledgements

This research was funded by the Griffiths Geoscience Project(PI Prof. Colin Brown, NUI Galway). The funding for the award wasestablished in Ireland as part of the National Geoscience Programme2007–2013 and the award is administered by the Geological Surveyof Ireland. The authors would like to thank Iarfhlaith Connellan andRainier Kraus who facilitated long-term MicroCAT moorings inAughinish and Kinvara Bays. We would also like to acknowledge theGriffith Summer Bursary students who helped out with fieldwork.

We thank the reviewers for their comments which greatly improvedthe manuscript.

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