Turk J Zool 24 (2000) 315-326 © T†BÜTAK

Benthi-planktivorous Fish-Induced Low Water Quality of Lake Eymir Before Biomanipulation

Meryem BEKLÜOÚLU, S. Levent BURNAK, …zlem ÜNCE Middle East Technical University, Biology, Department, 06531, ,

Received: 28.07.1999

Abstract: In this study, Lake Eymir, Eymir inflow 1, KÝßlak•Ý brook and Eymir outflow were sampled monthly betwen March 1997 and April 1998 for physico-chemical and biological parameters. Within the catchment of the lake, there are small-scale industries, sewage effluent of TEK residence and recreational activities causing high concentrations of dissolved inorganic nitrogen (DIN) and total phosphorus (TP) in the eymir inflow 1 and in KÝßlak•Ý brook. In Lake Eymir although diversion of sewage effluent of the Mogan municipality to Ümrahor valley appeared to have led to a decrease in TP and DIN concentrations compared with those found in 1994, especially the lake TP concentration was still found to be very high (annual mean: 305 µg l-1) this may be due to the high inflowsÕ concentrations. The lake had a high spring increase in chlorophyll-a concentration which remained relatively low throughout the summer (annual mean: 19 µg-1), perhaps due to very low DIN/TP ratios implying possible N-limitation at the very high TP concentration. Moreover, the Secchi depth was also relatively low throughout summer at the low chlorophyll-a concentration possibly due to feeding of the dominant fish tench (Tinca tinca) and carp (Cyprinus carpio) of the lake by stirring up the sediment. The densities of large-bodied zooplankters, Daphnia and Arctodiaptomus remained low in summer following the high spring densities probably due to fish prediation exerted on them by tench and carp. In Lake Eymir, top-down or fish-induced control of the water quality appeared to be the predominant factor,following the reduction of 30% of the tench and carp (biomanipulation), has led to very high Secchi depth and densities of the large grazes. Key Words: Benthivorus fish, biomanipulation, eutrophication, chlorophyll-a planktivorous fish, Secchi depth, total phosphorus.

EymirgšlŸ Biyomanipulasyon …ncesi Benti-Planktivor BalÝk KaynaklÝ DŸßŸk Su Kalitesi …zet: Bu •alÝßmada Mart 1997 ile Nisan 1998 tarihleri arasÝnda Eymir GšlŸÕnde, gšlŸn su kaynaklarÝ Eymir su girdisi 1, KÝßla•Ýk deresi ve gšl •ÝktÝsÝ, fiziksel, kimyasal ve biyolojik parametreleri aylÝk šrneklenmißtir. Gšl havzasÝnda bulunan kŸ•Ÿk šl•ekli endŸstri, TEK LojmanlaarÝ atÝk suyu ve •šp dškŸmŸ gibi faliyetler, gšl su girdilerinden šzellikle Eymir su girdisi 1.Õin, yŸksek toplam fosfat (TP) ve •šzŸnmŸß inorganik azot (DIN) yoÛunluklarÝnÝn kaynaÛÝdÝr. Gšl i•i TP ve DIN yoÛunluklarÝ GšlbaßÝ kasabasÝnÝn atÝklarÝmÝn 1994 Ümrahor vadisine bypas ile verilmesi ile azalmakla birlikte šzellikle gšl TP yoÛunluÛu (yÝllÝk ortalama: 305 µgl-1) •ok yŸksektir. Bu durumda šzellikle Eymir su girdisi 1Õin aßÝrÝ TP miktarÝ ile ilgilidir. Gšlde bahar aylarÝnda klorofil-a artar fakat bu artÝß yaz aylarÝnda dŸßŸktŸr (yÝllÝk ortalama: 19 µgl-1). Bu da dŸßŸk azot yoÛunluÛu ile ilgili olabilir, dŸßŸk DIN/TP oranlarÝ yaz aylarÝnda azotun sÝnÝrlayÝcÝ besin olduÛunu gšstermektedir. Yaz aylarÝnda dŸßŸk klorofil-a yoÛunluÛunda Seki yŸksekliÛide dŸßŸk kalmÝßtÝr. DŸßŸk ÝßÝk ge•irgenliÛide gšlde yoÛun olan ve beslenmeleri ile dip •amurunu karÝßtÝran kadife ve sazanla ilgilidir. AyrÝca, bŸyŸk vŸcŸtlu Daphnia ve Arctodiaptomus yoÛunluklarÝ bahar aylarÝnda yŸksekken yaz aylarÝnda •ok dŸßmŸßtŸr, bu da gšlde baskÝn kadife ve sazan beslenmesi sonucu olabilir. Yaz 1998Õde gšldeki kadife ve sazanÝn % 30Õinin •ÝkarÝlmasÝ (biyomanipulasyon) sonucu Seki yŸksekliÛi ve Daphnia ve Arctodiaptomus yoÛunluklarÝ •ok arttÝ. Bu artÝß da gšldeki biyomanipulasyon šnceki dŸßŸk su kalitesinin kadife ve sazan beslenmesi kaynaklÝ, yukardan-aßaÛÝya kontrol sonucu olduÛunu gšstermektedir. Anahtar SšzcŸkler: Bentivor balÝk, biyomanipŸlasyon, klorofil-a, štrofikasyon, planktivor balÝk, Seki yŸksekliÛi, toplam fosfat.

Introduction rivers, lakes, estuaries and coastal waters (1-8). In lakes, The introduction of excess nutrients (nitrogen and increased growth of aquatic plants and algae are the first phosphorus) from industries, domestic household sewage signals of a moderate level of increase in N and P loading and run off water from the intensive agricultural with into the lakes (5, 7). However, a further increase in heavy application of fertilisers, have lead to worldwide loading of these nutrients has to lead to increased algal water quality deterioration, namely eutrophication of biomass, including potentially toxic blue-green algae, and changes in community structure (4, 5, 7).

315 Benthi-planktivorous Fish-Induced Low Water Quality of Lake Eymir Before Biomanipulation

The consequences of eutrophication can be seen in planktivorous fish stock to increase water clarity to allow both deep and shallow lakes, as alterations in the trophic recolonisation of submerged plants in the littoral zone structure and interactions (9). In deep lakes, an increase through reduction of chlorophyll-a by enhanced grazing in nutrient loading results in an increase in the blue-green pressure of large-bodied grazers (e.g.Daphnia) (6, 8, 10, algae and loss of salmonoid fish, coupled with 22-24). Through removal of benthivorous fish, the deoxygenation of the hypoliminion (10). In shallow lakes, suspended matter related turbid condition is also an increase in nutrient concentration causes the loss of controlled since these fish cause resuspension of sediment submerged plants, leading to a decrease in the ratios of while feeding (20, 21). Therefore, the reduction of the piscivorous to planktivorous stock, and the zooplankton benthivorous fish stock is essential for effective to phytoplankton, and domination of the benthi- biomanipulation. planktivorous fish (e.g. carp, tench, rudd). This leads to a Study site switch to phytoplankton dominated turbid water (7-9, 11). Lake Eymir is an alluvial dam lake that was formed by the damming of the Ümrahor River valley at the beginning A few decades ago, it was strongly argued that food- of this century. Lake Eymir is a shallow lake (area: 1.25 web structure and interactions in the pelagic of lakes km2, Z : 6 m, Z 3.1 m) located 20 km south of max mean were exclusively determined by Òbottom-upÓ forces Ankara (39¡ 57Õ N-32¡ 53Õ) (25). The TEK settlement through availability of nutrients (namely N and P) i.e. with a population of 2500 people, the Police Academy phytoplankton are regulated by nutrients and light and small-scale industries are found within the catchment zooplankton are regulated by phytoplankton (12). of the lake. The water flows to GšlbaßÝ DŸzlŸÛŸ wetland However, it is now evidential that the food-webs may be from , where the TEK Sewage Treatment controlled by Òtop-downÓ, i.e. zooplankton are regulated Works (STW) and industrial zone are located. This water by fish, phytoplankton by zooplankton (9). However, the forms the Eymir inflow 1 that enters the lake at the relative significance of resource and predatory controls southern end. The other inflow to Lake Eymir is KÝßlak•Ý vary along a nutrient gradient (8). The effect of the top- brook that carries water to the lake at the northern end. down control is more significant in shallow eutrophic and The water flows out of the lake to the Ümrahor River that hypertrophic lakes (9). Zooplankton grazing was found is named Eymir outflow. to be very important in the control of phytoplankton in shallow meres in England (11, 13). The loss of the Studies on Lake Eymir are limited compared with Lake macrophytes can collapse the buffering mechanisms, Mogan. Previous studies classified the lake as eutrophic which causes the domination of the plantivorous fish, in (25, 26, 27). These studies recorded very high -1 -1 turn exerting a high predation pressure on zooplankton concentrations of nitrate (20 mg l , 0.5 mg l and 3.5 -1 -1 -1 (13, 14). mg l , respectively) and phosphate (2.47 mg l , 1 mg l and 2.12 mg l-1, respectively). The reduction in the external phosphorus level by diverting the nutrient-rich inflow, is the primary issue to restore the eutrophic lakes (15). In deep lakes, the Materials and Methods external phosphorus reduction theoretically should Water samples were collected at monthly intervals restore the lake (10). It has been observed so (16). between March 1997 and Appril 1998. The lake water However, shallow lakes have shown a long resilience or samples were taken from the deepest point by using a delay in recovery following external phosphorus control hosepipe with lead weights at the end. The major inflows. (15, 17), due to the internal P loading and also resilience Eymir inflow 1 and KÝßlak•Ý brook, and Eymir outflow of the biological structure (domination of benthi- were sampled. Water temperature and dissolved oxygen planktivorus fish, low zooplankton) (3, 8, 18, 19). were measured using a YSI 57 oxygen-meter, with half Benthivorous fish (carp, grass carp) increase the metre increments until the bottom of the lake was phosphorus level in the water column by stirring up the reached. A litre of water was taken from the inflows and sediment while consuming the benthic food items (20, outflow of the lake. Water for chemical analyses was 21). Benthi-planktivorous fish also exert strong predation stored in acid-washed l-1 Pyrex bottles. Soluble reactive pressure on the large zooplankton, e. g. Daphnia, and phosphorus (SRP) and total phosphate phosphorus (TP), lead to high algal biomass and low water clarity (9). and nitrite+nitrate-nitrogen (NO +NO -N) were 2 3 Following the external nutrient control, the next step determined according to Mackereth et al. (28) to is biomanipulation, which is the reduction of benthi- precisions of ±3%, ±8% and ±8%, respectively.

316 M. BEKLÜOÚLU, S. L. BURNAK, …. ÜNCE

Ammonium-nitrogen (NH -N) was determined according 4 Results to Chaney and Morbach (29) to a precision of ±4%. In Lake Eymir, both thermal and oxygen stratification Silicate-silicon was deteremined according to Golterman set in May and continued to the end of August. The et al., (30) to a precision of ± 1-2%. Chlorophyll a was thermocline was found to set around the depth of 4.5m. extracted in acetone, and concentration was calculated Therefore, less than 40% of the total lake water mass from the absorbance reading at 663 (31) to a precision went under thermal stratification. In spring, temperature of ±5%. The caretenoid pigments to chlorophyll-a ratio increased from winter values of 1-3¡C to summer were used to evaluate the state of phytoplankton as maximum of 22-24¡C. Throughout summer, the nitrogen deficiency, healthy phytoplankton, heavily hypolimnetic dissolved oxygen concentrations were near grazed etc. (32). anoxic, and much lower (0-2 mg l-1) than the epilimnetic Zooplankton samples were collected at the same time concentrations (6-8 mg l-1). and depth as for those for chemical analyses. Zooplanton The TP concentrations of the inflows of Lake Eymir samples were collected by vertical tows to the water were higher than that of the in-lake concentrations (305 µ surface using a 45 m mesh-size, nylon plankton net. The µg l-1) (fig.1a). The outflow of Lake Mogan after passing samples were promptly narcotised with chloroform water through Mogan town (annual mean TP: 219 µg l-1) (33) and preserved in a formaldehyde solution to give a receives the TEK STW effluent (annual mean TP: 5413 final concentration of 4%. The samples were normally µg l-1) and forms eymir inflow 1 which had an annual subsampled, using a wide-bore pipette and counted under mean of 474.4 µg l-1 TP. The TP concentration of KÝßlak•Ý a stereo microscope. When samples were subsampled, at brook was also high with an annual mean of 255 µg l-1. least 100 of the most common species were counted The DIN concentrations of inflows were also higher than (34). Animals were identified to species level whenever that of the in-lake (91 µg l-1) (Figure 1b). The annual possible using standard works (35, 36). mean of Eymir inflow 1 was 1480 µg l-1. The outflow of Fish Stock Lake Mogan and the TEK STW effluent, which form the In Lake eymir, the fish stock determination was Eymir inflow 1, had very high DIN concentrations (1351 µ -1 µ -1 carried out in May 1998 using multiple mesh-sized (18, g l and 7880 g l , respectively) (Figure 1b). The DIN 36, 40, 50, 70 mm) gill nets with a length of 100m and concentration of KÝßlak•Ý brook was also high with an µ -1 width of 3.5m. The multiple mesh-sized gill nets were left annual mean of 2559 g l . The eymir outflow had an µ -1 overnight and were collected on the following morning, annual mean of 255 g l that was slightly higher than and the study lasted a week. The stock was determined that of the lake concentration (Figure 1b). according to Bingel (37), calculations as follows: Throughout the study period, TP and SRP A x y concentrations of Lake Eymir showed a similar B= ÐÐÐÐÐÐÐÐÐÐ seasonality in that there was a significant correlation q x a between the SRP and TP concentrations (r: 0.63, p: 0.0183) (Figure 2a). The annual mean ± standard error Where A: the total area of the laki in hectares of TP and SRP concentrations were 305±26 µg l-1 and a: the effective area of the fish net in hectares 202±26 µg l-1, respectively. The concentrations were q: the efficency coefficient of the net, remarkably lower in late winter and early spring in 1998 than that of 1997 (Figure 2a). The SRP concentrations of y: the daily mean of the caught fish in Lake Eymir and Eymir inflow 1 were significantly kilograms. correlated (r: 0.86, p: 0.0004). There was also Aquatic Plants survey significant correlation between the SRP concentration Aquatic plants were surveyed in July 1998. Sampling and Secchi depth (r: 0.57, p: 0.0438). In Lake Eymir the was carried out from a boat using a grapnel. Aquatic annual mean of DIN, which is the sum of nitrate + nitrite (NO +NO -N) and ammonium (NH4-N) concentrations, plants were identified using Haslam et al., (38) and 3 2 -1 AltÝnayar (39). Percentage cover of submerged plants was 91±32 µg l (Figure 2b). Of the two forms of inorganic nitrogen analysed, NO +NO -N was dominant was estimated from a weighed photocopied image of the 3 2 (annual mean: 54±18 µg l-1). The NH -N concentrations, vegetation map. 4 however, were low and undetectable several times in the spring of both years (Figure 2b). NO +NO -N and DIN 3 2

317 Benthi-planktivorous Fish-Induced Low Water Quality of Lake Eymir Before Biomanipulation

a) Mogan out 2 TEK E inflow 1 Kıslakcı Eymir E outflow 2907 30918 2500 4947 TP

) 2000 -1

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months

b) Mogan out 2 TEK E inflow 1 Kıslakcı Eymir E outflow

2500

2000 ) -1

1500

1000 Concentrations (µg I

500

0 MAM JJASOND FMA months

Figure 1. The changes in a) total phopshorus (TP) and b) dissolved inorganic nitrogen (DIN), concentrations of the inflows to Lake Eymir, the lake and the outflow measured March 97 and April 98. concentrations of Eymir inflow 1 and Lake Eymir were Secchi-depth was highest in July 1997 (180 cm) and significantly correlated (r: 0.84; p<0.001 and r: 0.89; relatively low during late summer, winter and early p<0.001, respectively). The annual mean of reactive spring (min: 46 cm in april 1997) (Figure 3b).The silicate-silicon concentration was 3±0.4 µg l-1 in Lake maximum chlorophyll-a concentration in Lake Eymir was Eymir. It was low in autumn and reached a minimum recorded in February 1998 (36.3 µg l-1), which coincided concentration of 0.60 µg l-1 in FebÕ 98 (Figure 3a). The with the minimum concentration of reactive silicate-

318 M. BEKLÜOÚLU, S. L. BURNAK, …. ÜNCE

a) 500.00 SRP

400.00 TP

300.00

200.00 Concentrations (µg/L)

100.00

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MAMJJASONDJFMA

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NO3+NO2-N 400.00 NH4-N

300.00

Concentrations (µg/L) 200.00

100.00

0.00 MA MJJ ASONDJF MA

Sampling Dates

Figure 2. The changes in a) concentrations of soluble reactive phosphorus (SRP) and total phosphorus (TP) and, b) dissolved inorganic nitrogen (DIN) measured in Lake Eymir between March 1997 and April 1998. silicon (Figure 3a). The annual mean of chlorophyll-a The carotenoid pigment to chlorophyll-a ratio is given concentrations was 19±3.5 µg l-1. The chlorophyll-a in Figure 4. The A480/A663 and A430/A410 ratios were concentrations and Secchi-disc depth (Figure 3b) were either>1.3 and <1.2, respectively or > 1.3 and >1.2, inversely correlated but the interaction was not respectively in most of the sampling period indicating that significant (r=-0.36). the phytoplankton was either N-deficient or contaminated

319 Benthi-planktivorous Fish-Induced Low Water Quality of Lake Eymir Before Biomanipulation

a) 7.00 Silicate 40.00

Chl a 6.00 35.00

30.00 5.00 25.00 4.00 20.00 3.00 15.00

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0.00 0.00 MAMJ J A SOND J FMA

b) 200 Secchi 40.00

180 Chl a 35.00 160 30.00 140 25.00 120

100 20.00

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Figure 3. The changes in the concenntrations of a) reactive silicate-silicon and chlorophyll-a and b) Secchi depth and chlorophyll-a measured in Lake Eymir between March 1997 and April 1998. with degradation products via sediment resuspension sphaericus O. F. MŸller and Alona sp. The density of (32). large-sized D. pulex and small-sized Ceriodaphnia sp. were lower than 10 ind.I-1 throughout the study period, Zooplankton especially in summer (Figure 5a). B. longirostiris, In Lake Eymir, the densities of the cladoceran grazers Diaphanosoma sp. C. sphaericus and Alona sp. were were low and the following species were recorded: recorded, however their densities were less than 1 ind. I-1. Daphnia pulex De Geer, Ceriodaphnia sp., Bosmina The densities of Arctodiaptomus p. and Cyclops sp. were longirostiris O. F. MŸller, Diaphanosoma sp. Chydorus abundant in the spring (max: 24 ind. I-1 and 10 ind. I-1,

320 M. BEKLÜOÚLU, S. L. BURNAK, …. ÜNCE

respectively in April 1998), and the densities remained Table 1. The size and the total fish catch in Lake Eymir in May very low in summer (Figure 5b). 1998. Fish Stock The results of the fish stock of Lake Eymir are given Weight (kg) Length (cm) Age Class in Table 1. Pike (Esox lucius L.), tench (Tinca tinca L.), + + carp (Cyprinus carpio L.) and Alburnus escherrichi Tench 6142 20-33 3 and 4 Steinoachner were recorded. The total of 6870 kg fish Carp 698 30-84 with a daily average of 980 kg was caught using 500m Pike 33 25-35 long and 3.5m wide gill nets covering a total effective area of 1.125 ha for seven days. The total fish stock was TOTAL 6873 estimated as 200 000 kg (1600 kg ha-1). The fish stock of Lake Eymir consists of 89% (1420 kg ha-1) tench, 10% (160 kg ha-1) carp and 1 % pike. The tench covering the whole shoreline of the lake like a belt with a population was dominated by 3+ and 4+ age classes. width of 5-10m. Aquatic Plants In Eymir Lake the recorded submerged plants were Discussion Myrophilum spicatum L., Ceratophyllum demersum L., Until 1994, Lake Eymir received the untreated raw Potomageton pectinatus L., Najas marina L. and Najas sp. sewage effluent of the Mogan Municipality that was The distribution of submerged plants was very limited diverted to the outflow (Ümrahor Valley) of Lake Eymir by covering around 5% of the LakeÕs total surface area. The a by-pass channel. Before the effluent diversion, the submerged plants were only confined to the southern and effluent running into Lake Eymir was very rich in DIN and northern end of the lake despite the long littoral shoreline TP (min: 690 µg l-1 and max: 49 mg l-1, and min: 110 µg (5866 m long and 10-15 m width). The dominant l-1 and max:725 mg l-1, respectively) (25). The annual emergent plant is common reed (Phragmites austrialis L.)

1.90 A480/A663 2.60

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1.50 2.00

1.80 1.30 1.60 1.10 1.40

0.90 1.20 1.00 0.70 0.80

0.50 0.60 MAMJJ ASONDJFMA

Figure 4. The changes in carotenoid pigment to chlorophyll-a ratios measured in Lake Eymir between March 1997 and April 1998.

321 Benthi-planktivorous Fish-Induced Low Water Quality of Lake Eymir Before Biomanipulation

mean DIN and TP in-lake concentrations 91 µg l-1 and and neglected that of the hypolimnetic, thus the density 305 µg l-1, respectively) Decreased several-fold compared of the grazers might have been underestimated. Several with before effluent diversion concentrations (183 µg l-1 studies have shown the significance of the diel vertical and 860 µg l-1, respectively) (25). The isolation of lakes migration of the large bodied grazers to the hypolimnion from external phosphorus loading is thus often with a day-time descent and night-time ascent to the considered to be the first step in reversing the surface water to avoid predation (45). undersirable effects of eutrophication (15, 22), and has The lake had a second peak of chlorophyll-a proved to be effective in deep lakes (3,16). Lake Eymir concentration in June then the concentrations remained does not seem to be an exception to this generalisation low in summer 1997, which was not attributable to the with a three-fold decrease in TP concentration. However, grazing pressure exerted by the large grazers whose the present lake TP concentrations were very high densities remained low as well. In Lake Eymir, the especially in summer months, which may be attributed to relatively low summer chlorophyll-a concentrations might internal phosphorus loading which is found to be very have been due to nitrogen limitations. Smith (46) significant in eutrophic lakes after effluent diversion (11, suggested that if the N:P<9-13:1, the phytoplankton 17). The independent measurements of hypolimnetic TP yield was severely nitrogen-limited. The DIN:TP ratios concentrations were slightly higher in Lake Eymir, recorded throughout summer were 0.6-1.7, which however, since the lake is thermaly stratified throughout indicates a strong nitrogen limitation of the algal crop in the summer, the released concentrations might not have the lake. The caretenoid/chlorophyll-a; A480/A663:>1.3 been available to the epilimnion. and A430/410:<1.2 ratios also provided further evidence Following the effluent diversion, there was a decrease for significance of the possible nitrogen limitation (Figure in nutrients from Mogan town to the inflow of the lake, 4). yet the DIN and TP concentrations of the Eymir inflow, The very low summer densities of the large-bodied Which receives the point sources of the TEK STW, were grazers, Daphnia sp. and Arctodiaptomus sp. in the lake still very high (Figure 3a). However, the in-lake appeared to be due to predation pressure exerted by concentrations of these nutrients were much lower than tench (Tinca tinca), whose contribution to the total fish those of the inflow, probably due to the large reed-bed stock of the lake was 89%, and by carp (Cyprinus carpio) 2 (0.2 km : 1/6 of the total lake area) at southern end of to a lower extent. In Lake Eymir, further evidence of the the lake through which the Eymir inflow runs into the significance of fish predation on large-bodied grazers lake. Emergent plants of wetlands are known to act as comes from the tremendous increase in the density of sinks for N and P via denitrification and direct uptake of large-bodied Daphhnia in the lake following the removal phosphorus (40, 41). Denitrification capacity of intact of 30 % of the tench and carp (biomanipulation) wetlands is several-fold higher than that of the undertaken in summer 1998 (Ünce and BeklioÛlu, phosphorus uptake capacity, so that reductions of up to unpublished data). There is also a growing body of 500 kilograms of N and 40 kilograms of P per hectare evidence which shows the significance of have been recorded (40, 41). Therefore, the reed-bed at zooplanktivorous fish predation on large-bodied grazers the southern end of Lake eymir might have acted as a sink in summer (2, 4, 47). In Lake Eymir piscivorous pike for these nutrients though there was no direct measure (Esox lucius) made up a very small portion (1%) of the of the capacity of the wetland for taking up nutrients. fish stock, which appeared to be the main reason for the Lake Eymir had a sharp spring increase in the domination by tench due to less predatory control as well chlorophyll-a in both years (35 µg l-1) that was followed as eutrophic conditions which favour planktivorous fish. by a sharp decrease in the concentration which coincided In Lake Eymir, the summer fish-kill experienced in 1993 with the increase in the density of the large-bodied (the LakeÕs Warden: Personal commun.) due to anoxic grazers, Daphnia and Arctodiaptomus. A sharp early conditions (25), selective high angling pressure on pike, spring increase in chlorophyll-a is very common in (though pike fishing has been banned since May 1998), temperate lakes (7, 42) often followed by a short Ô spring and a very limited littoral zone with submerged plants clearÕ water phase that is attributed to increased density might have been the reasons for low recruitment of pike, of large-bodied grazers (e.g. Daphnia, Diaptomus) (43, leading in turn to the low biomass. However, the 44) as found in Lake Eymir. However, the increase in the dissolved oxygen concentrations have increased and the density of the large bodied grazers in the lake was not as prohibition of pike angling may lead to an increase in the pronounced as found elsewhere (43), probably because fish biomass. our zooplankton data only covered the epilimnetic density

322 M. BEKLÜOÚLU, S. L. BURNAK, …. ÜNCE

Daphnia a) 7.00 Ceriodaphnia

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Figure 5. The changes in densities of a) D. pulex and Ceriodaphnia sp., b) Arcthodiaptomus sp. and Cyclops sp. in Lake Eymir between March 1997 and April 1998.

The poor water clarity of eutrophic lakes is strongly relatively low chlorophyll-a concentrations. Therefore, correlated with phytoplankton biomass (1,2,4,7-9). In the clarity of the water did not appear to be largely Lake Eymir, the water clarity, measured as Secchi depth, determined by phytoplankton biomass. Rather turbid was inversely correlated with the chlorophyll-a water of the lake might be due to the high biomass of concentration, though the interaction was not significant. benthivorous fish, tench and carp. Especially feeding of Throughout summer, the water clarity was low at carp stirs up the sediment and results in resuspension

323 Benthi-planktivorous Fish-Induced Low Water Quality of Lake Eymir Before Biomanipulation

that is strongly correlated with fish total length (20, 21). in the water column (Ünce and BeklioÛlu, unpublished The mean total length of carp was 50 cm in Lake Eymir. data). It appears that top-down control played a The feeding of mainly carp, and tench to a lesser extent significant role in determining the water clarity in Lake might have been very important in determining the water Eymir before biomanipulation and that post- clarity through sediment resuspension. Following the biomanipulation the clear water condition has supported removal of 30% of tench and carp (biomanipulation), this hypothesis. undertaken in summer 1998 (Ünce and BeklioÛlu, unpublished data), the Secchi depth increased to >5m implying that the decrease in suspended matter was Acknowledgement caused by fish feeding. The study was funded by the Research Support Fund In Lake Eymir, following the removal of 30% fish of METU. Thanks to Prof. Dr. Mehmet ‚alÝßkan for (biomanipulation) in summer 1998, the water clarity, providing vehicles, and Sezgin BŸyŸkkußoÛlu, Ülhan GŸven measured as Secchi depth in early summer 1999, and Ahmet Kaya who work in the Fisheries Unit at DSI for increased to >5m owing to very low phytoplankton crop helping with the fish stock estimation. Assitant Prof. Dr. (1-3 µg l-1) due to very high density of large-bodied Ilhami Tuzun, Can O. Tan, Kerem SeveroÛlu and Erdin• grazers (Daphnia) and perhaps reduced suspended matter Ana•oÛlu are also thanked for helping in the field and laboratory.

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