Analysis of the top-down and bottom-up controls on the zooplankton of eutrophic freshwater Yeniçağa Lake (Turkey)

doi:10.21203/rs.3.rs-3338815/v1

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Analysis of the top-down and bottom-up controls on the zooplankton of eutrophic freshwater Yeniçağa Lake (Turkey)

https://doi.org/10.21203/rs.3.rs-3338815/v1

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The objective of this study is to determine the relationship between zooplankton biomass and the factors that affect it and to assess the trophic status in Yeniçağa Lake, Turkey. For this purpose, chlorophyll-a values were used as the bottom-up effect, while planktivorous fish CPUE was used as the top-down effect. Additionally, to understand which nutrient has a stronger impact on chlorophyll-a, total nitrogen, and total phosphorus values were determined. Structural Equation Modeling was used to examine the bottom-up and top-down relationships, and it was understood that Cladocera, Calanoida, and Rotifera groups were negatively controlled by bottom-up effects, while Calanoida is negatively controlled and Cyclopoida and Rotifera are positively controlled by top-down effects. Moreover, it was understood that chlorophyll-a is influenced by both total nitrogen and total phosphorus, with a higher impact of total nitrogen. To determine the trophic status, chlorophyll-a, total phosphorus, Secchi depth, and Carlson TSI Indices obtained from these values were used. According to the findings from these values, the lake can be considered to be at the eutrophic level with a hypertrophic tendency.

Yeniçağa Lake

Zooplankton

Top-down and bottom-up effects

Structural equation modeling

Studies on the biomass of organisms in the pelagic food webs in freshwater ecosystems show that the biomass at the pelagic trophic level is controlled both from the bottom-up by the producers and the top-down by consumers (McQueen et al., 1986). This situation, known as the bottom-up and top-down effect theory, also indicates that top-down effects will be stronger at the top of the food web and weaker at the lower levels, while bottom-up effects will be stronger at the bottom of the food web and weaker at the higher levels (McQueen et al., 1989). In aquatic ecosystems, an increase in piscivorous fish biomass results in a decrease in planktivorous fish biomass. As a result, the decline in plantivorous fish biomass leads to an increase in herbivorous biomass and, consequently, a decrease in phytoplankton biomass (Carpenter et al., 1985). Apart from the biotic factors that control organisms in the food web, abiotic factors such as total nitrogen and total phosphorus can play a crucial role in freshwater ecosystems. The presence of these nutrients can also significantly affect phytoplankton growth rates (Tilman, 1982; Elser et al., 1990).

Zooplankton, which vary in size and food preferences, are a key factor in the pelagic food web, and these organisms are under the control of various biotic/abiotic factors from both the top-down and the bottom-up. These factors may cause different types of effects for various organisms in the zooplankton community. It is known that large zooplanktonic organisms such as Daphnia have decreased numerically and left their place to smaller organisms in lakes where planktivorous fish are abundant (Brooks & Dodson, 1965). Calanoid copepods, which are frequently encountered in zooplankton in freshwater ecosystems, are under more intense pressure from planktivorous fish compared to Cyclopoid copepods (Soto and Hurlbert, 1991a). On the other hand, studies also show that high predator pressure on zooplankton can be caused by low natural mortality in planktivorous fish (Jeppesen et al., 1997). Phytoplankton abundance can be reduced indirectly by the direct consumption of zooplankton as food or indirectly by the nutrients required for the growth of these organisms (Porter, 1977; Sterner, 1990). In addition to the top-down effect of zooplankton on phytoplankton, the increase in phytoplankton can affect zooplankton by lowering the oxygen level of the water and releasing algal toxins (Buskey, 2008). For these reasons, investigating the top-down and bottom-up effects on zooplankton is critical for food web studies.

Studies investigating the interactions of organisms at different stages of the food webs in aquatic ecosystems in Turkey are very limited, and these have been carried out using mesocosm experiments (Özen et al., 2014; Tavşanoğlu et al., 2017). However, no study has been conducted so far that directly examines the top-down or bottom-up effects on any lake ecosystem in Turkey. In Yeniçağa Lake, where the research was conducted, detailed studies have been carried out on the trophic structure of the lake, zooplankton-phytoplankton community, fish community, and heavy metal accumulation in the food web (Saygı & Demirkalp, 2004; Saygı, 2005; Saygı & Yiğit, 2005; Saygı & Atasagun, 2012; Saygı & Yiğit, 2012a,b;lıç & Becer, 2018). Yeniçağa Lake is a eutrophic lake that is under the pressure of pollution caused by the settlement around it, and there are still active commercial fishing activities in the lake (Saygı & Demirkalp, 2004). Fish stocking has been carried out annually in the lake since 2014, and Cyprinus carpio larvae are released into the lake in varying numbers every year (Zengin et al., 2021). It is thought that factors such as increasing trophic status towards hypertrophy due to pollution pressure in Yeniçağa Lake and the predator pressure created by the fish may cause some positive/negative effects on the zooplankton. Therefore, this study examined whether there is a top-down or bottom-up effect among the various factors affecting the zooplankton in Yeniçağa Lake, which has eutrophic freshwater characteristics and a shallow structure. For this purpose, top-down and bottom-up effects on zooplankton were investigated using structural equation modeling. Furthermore, to evaluate the trophic level of the current lake, chlorophyll-a, total phosphorus, Secchi depth and Carlson TSI Indices obtained from these values were used. The top-down effect on the zooplankton in the lake was analyzed based on the seasonally sampled fish in the lake, and the bottom-up effect on the chlorophyll-a. Additionally, the bottom-up effects of total nitrogen and total phosphorus parameters were examined to understand which nutrient had a greater effect on chlorophyll-a. Zooplankton were evaluated in 4 groups as Cladocera, Cyclopoida, Calanoida, and Rotifera, and the differences in the effects on zooplankton among these four groups were interpreted.

Study Area

Yeniçağa Lake is a freshwater lake located in the Western Black Sea region of Turkey, at 40°47' north latitude and 32°01' east longitude. The lake, which has a surface area of 1800 hectares, is located in a 7 X 2.5 km depression formed as a result of tectonic activities in the early Quaternary period, and it is 976 meters above sea level (Erinç et al., 1962). Since its formation, the surface area of the lake appears to have shrunk to nearly three times its current size (Pekcan, 1996). Yeniçağa Lake, which has a shallow structure and eutrophic features, has the status of Wetland of National Importance in Turkey because it hosts numerous bird species and plant taxa (Saygı & Demirkalp, 2004).

Yeniçağa Lake is under great pressure from residential and agricultural areas, and the lake is exposed to industrial, domestic, and agricultural pollution (Saygı & Demirkalp, 2004). The State Hydraulic Works carried out a planned drying operation to fight mosquitoes in the lake in 1965, and the water of the lake was discharged into the Mengen Stream through a channel opened in the north-east (Biçer, 1966). However, in the years when the importance of the ecosystem services of the wetlands was realized, this channel was closed with a barrier, and the water level and surface area in the lake were restored to their former levels. Today, the lake is fed directly by two streams, Kuzuviran Creek in the West, and Deliler Creek in the east. Although there is a treatment plant built by İller Bankası in 1989 in the south-east of the lake, the sewage wastes from the surrounding settlements are usually discharged directly into the streams or the lake without any treatment. Additionally, the sewage load from the medium-sized industrial facilities located on the eastern shore of the lake also contributes to the pollution level in the lake (Saygı & Yiğit, 2012a).

Sampling

Zooplankton, fish, and water samples were obtained simultaneously from three sampling stations on the lake, monthly from December 2021 to November 2022. Among the sampling stations, two were selected in shallower areas close to the shore, while one was selected to be in the deepest part of the lake, and the locations of these sampling stations are shown in Fig. 1. Coastal sampling stations were chosen to be close to the streams that feed the lake.

A Hydro-bios water sampler with a volume of 2 liters was used for sampling from the water column at the sampling stations. To determine the chlorophyll-a values, the water samples were filtered through Whatman GF/C filter papers, and the methanol method was followed in the analysis (Marker, 1994). Total nitrogen and total phosphorus values were determined using 4500 NO2-B, 4500-Norg-B, 4500P-B, and 4500PE standard methods (Greenberg & Clesceri, 1992). Turbidity was measured with the ORION AQ3010 turbidimeter, and Secchi depth was determined with the Hydro-bios Secchi disc. During the study, TSI-Chl-a, TSI-TP, and TSI-SD indices were used to determine the trophic status of the lake. Carlson’s trophic state indices, integrating with Chl-a (TSI-Chl-a), total phosphorus (TSI-TP), and Secchi depth, were calculated according to the equations:

TSI-Chl-a = 9.81 * ln (Chl-a) + 30	(1)
TSI-TP = 14.42 * ln(TP) + 4.15	(2)
TSI-SD = 60–14.41 * ln(SD)	(3)

Mesotrophic state was defined as TSI values between 40 and 50, eutrophic state as those between 50 and 70, and hypertrophic state as those greater than 70 (Carlson, 1977).

Zooplankton samples were collected using the 30 µm pore size Hydro-bios plankton net. Zooplankton samples were taken horizontally and vertically, and concentrated samples were fixed with 4% formaldehyde. Organisms in the Cladocera, Calanoida, Cyclopoida, and Rotifera groups were examined in a 1 ml volume Sedgewick Rafter counting cell and under a Leica DMR microscope. All samples identified in the examinations were counted first, and the total lengths of the samples were measured under the microscope. The dry weight of species in the Cladocera, Calanoida, and Cyclopoida groups was calculated using the length-weight formulas given in Dumont et al., (1975), and Bottrell et al., (1976). To calculate the biomass of the Rotifera group, first the biovolume of the samples was calculated using the formulas given by Ejsmont-Karabin (1998), and then the data were converted to dry weight values using the formulas given in Bottrell et al. (1976). Fish sampling was carried out with the help of fishermen who are members of Yeniçağa Fisheries Cooperative. Fish populations were seasonally sampled using multi-mesh fishing nets (length = 50 m, length = 4 m; knot-to-knot mesh size: 10, 32, 40, 60, 80, 90 mm). The nets were set at dusk at the surface in areas where water depth was < 5 m and removed at dawn with the duration of exposure recorded. Gillnetting provided catch-per-unit-effort (CPUE) estimates of relative densities for all fish species using the formula:

CPUE = (C_N x (A_S/A_N))/t

(4)

where C_N is the nominal catch (kg), A_S is the area of the standard net (100 m²), A_N is the area of the used net (m²) and t is the time of exposure (h) (Tarkan et al., 2012).

Data Analysis

Structural Equation Modeling (SEM) is a statistical analysis that allows the examination of relationships between one or more independent variables and one or more dependent variables. These variables can be either dependent or independent, factors, or measured variables. Compared to other statistical tools such as factor analysis and multivariate regression, SEM can measure the errors of observed variables and simultaneously predict causal relationships between both latent and manifest variables (Ullman & Bentler, 2012; Xiong et al., 2014). Since 2000, SEM has been actively used by ecologists to study the complex interactions found in ecosystems (Fan et al., 2016). One of the most important features that distinguish Structural Equation Modeling from other data modeling methods is its ability to examine path relationships. It is also growing in popularity among biologists as it helps to understand direct and indirect interactions within data (Grace et al., 2010).

In this study, Structural Equation Modeling (SEM) was used to analyze the role of top-down and bottom-up controls in the food web. The analysis examined the effect of top-down and bottom-up control on the zooplankton community. In the model, bottom-up controls revealed the effects of nutrients on phytoplankton and the effect of phytoplankton on zooplankton; top-down controls showed the effect of planktivorous fish on zooplankton. Total nitrogen and total phosphorus concentrations were used to estimate nutrient availability. The effects of phytoplankton were evaluated by the chlorophyll-a concentration. In this model, zooplankton were considered in four different groups: Cladocera, Calanoida, Cyclopoida, and Rotifera.

Before performing the Structural Equation Modeling, a logarithmic transformation was applied to homogenize the variance in the entire dataset. In this study, Structural Equation Modeling was carried out using the R program and the 'lavaan' and 'tidySEM' packages.

In this study, Calanoid copepods were found to be the group that made the largest contribution to the zooplankton biomass in Lake Yeniçağa, while the group that contributed the least was the Cyclopoid copepods. Zooplankton biomass exhibited significant seasonal changes, with the lowest values recorded as 8114 µm/m³ during the summer period, and the highest values observed as 130779 µm/m³ during the winter period. The biomass of Calanoid copepods ranged between 1125–91123 µm/m³, with maximum values observed in the winter and minimum values in the spring. The biomass of the cladocerans ranged between 43-38794 µm/m³, with the lowest biomass values observed in the summer, and the highest biomass values in the spring. Rotifera biomass also displayed significant seasonal variation, with the lowest values recorded as 502 µm/m³ during the winter and the highest values as 39416 µm/m³ in the spring (Table 1).

Table 1

Average biomass of zooplankton and average CPUE of fish measured seasonally in Lake Yeniçağa.
	Spring	Summer	Autumn	Winter
Cladocera Biomass (µg/L)	38794	43	28312.35	33142
Calanoida Biomass (µg/L)	1125	7089	4211.24	91123
Cyclopoida Biomass (µg/L)	14178	121.83	437.82	6012
Rotifera Biomass (µg/L)	39416	858.75	29391.76	502
Zooplankton Biomass (µg/L)	93515	8114.22	62353.18	130779
Cyprinus carpio (kg/ha/h)	55.76	16.91	57.61	-
Squalius cephalus (kg/ha/h)	16.26	5.32	19.03	-
Tinca tinca (kg/ha/h)	1.86	0.28	2.75	-
Carassius gibelio (kg/ha/h)	0,34	-	-	-
Total Fish (kg/ha/h)	74.22	22.51	79.39	-

Fish sampling in Yeniçağa Lake could not be conducted during the winter period due to the complete ice cover on the lake. Among the fish species sampled in the lake, four planktivorous fish species were identified: Cyprinus carpio, Squalius cephalus, Tinca tinca, and Carassius gibelio. Among the species, Cyprinus carpio contributed the most to the fish biomass, while Carassius gibelio contributed the least. The total fish CPUE values in Lake Yeniçağa were calculated between 22.51–79.39 kg/ha/h. The lowest CPUE values were recorded during the summer period. The CPUE values of Cyprinus carpio, which was observed as the most dominant species in the fish community, varied between 16.91–57.61 kg/ha/h (Table 1).

During the sampling period, significant monthly changes were observed in chlorophyll-a, total nitrogen, total phosphorus, and Secchi depth values in Lake Yeniçağa. Chlorophyll-a values in the lake were ranged from 15.1 to 114.4 mg/m³, total nitrogen from 88 to 1987 µm/L, total phosphorus from 141 to 310 µm/L, Secchi depth from 60 to 300 cm, and turbidity from 1.17 to 41.5 ntu, as shown in Table 2. TN:TP ratios in Lake Yeniçağa were calculated to be between 0.60 and 9.24.

Table 2

Chlorophyll-a, total nitrogen, total phosphorus, Secchi depth, turbidity, TN:TP, and Carlson TSI Index values measured in Yeniçağa Lake during the study period.
	December	March	April	May	June	July	August	September	October	November	Mean
Chloropyhll-a (mg/m3)	59.32	32.1	35.92	32.47	15.1	17.27	45.34	114.44	24.87	43.79	42.06
Total Phosphorus (µg/L)	163	177	150	157	223	217	310	147	156	141	184
Total Nitrogen (µg/L)	600	1267	1290	1450	1197	1987	1023	88	167	791	986
Secchi Depth (cm)	68.33	133.33	115	256.67	293.33	300	83.33	60	215	110	163.50
Carlson TSI (Chloropyhll-a)	70.65	64.63	65.73	64.74	57.23	58.55	68.02	77.10	62.12	67.68	65.65
Carlson TSI (Total Phosphorus)	77.63	78.76	76.40	77.03	82.14	81.71	86.87	76.14	76.97	75.48	78.91
Carlson TSI (Secchi Depth)	65.49	55.85	57.99	46.42	44.49	44.17	62.63	67.36	48.97	58.63	55.19
Turbidity (NTU)	8.12	4.12	7.21	1.83	1.17	2.55	6.06	41.53	2.57	6.74	8.19
TN:TP	3.68	7.16	8.60	9.24	5.37	9.16	3.30	0.60	1.07	5.61	5.36

Carlson TSI(TP) Index, calculated from total phosphorus values, varied between 75.48 and 86.87 throughout the year, reaching its lowest values in November and highest values in August, with an annual average of 78.91. The Carlson TSI(CHL) Index, calculated from the chlorophyll-a values, varied between 57.23 and 77.10 throughout the year, with an annual average of 65.65. The month with the lowest values of Carlson TSI(CHL) Index was June, while the month with the highest values was September. Carlson TSI(SD) Index, calculated from Secchi depth, ranged from 44.16 to 67.36, reaching its lowest values in July and highest values in September. The annual average of this value was determined to be 55.19 (Table 2).

The effects of top-down and bottom-up controls on zooplankton in the food web of Yeniçağa Lake were analyzed using Structural Equation Modeling. To test the suitability of the constructed model, commonly used fit statistics such as comparative fit index (CFI) and standardized root mean square residual (srmr) values were calculated (Kline, 2015). In the literature, a CFI value greater than 0.95 and an srmr value less than 0.08 indicate a successful model (Hu & Bentler, 1999; Hooper et al., 2007). In our Structural Equation Modeling, we obtained CFI values of 0.950 and srmr values of 0.058, confirming that the constructed model adequately explained the dataset. All path coefficients used in the analysis were standardized.

Data on how much each variable can be explained by other variables in the Structural Equation Modeling is presented in Table 3. The largest R² value was found to be 0.854 for chlorophyll-a, while the smallest R² value was 0.426 for Cladocera. The results of the Structural Equation Modeling, using planktivorous fish, zooplankton, chlorophyll-a, total nitrogen, and total phosphorus values in Lake Yeniçağa, are schematized in Fig. 2. It was observed that total nitrogen (r = 0.62, p < 0.001) and total phosphorus (r = 0.57, p < 0.001) values exhibited a positive interaction with chlorophyll-a values. The standardized estimation values of the variables examined in the dataset are also presented in Table 4. It was understood that the effect of total nitrogen on chlorophyll-a was higher than that of total phosphorus. The effect of chlorophyll-a parameter on Rotifera (r = -0.48, p < 0.001), Cladocera (r = -0.48, p < 0.001), and Calanoida (r = -0.71, p < 0.001) was found to be negative. However, no significant relationship was found between chlorophyll-a and Cyclopoida (p = 0.15). According to the statistically significant standardized estimation values, the zooplankton group most affected by chlorophyll-a was the calanoids. The analysis results for fish-zooplankton interactions in the food web were statistically insignificant for the Cladocera group (p = 0.30). The effect of planktivorous fish on the zooplankton community was found to have a negative effect on Calanoida (r = -0.74, p = 0.002) and a positive effect on Cyclopoida (r = 0.82, p < 0.001) and Rotifera (r = 0.42, p < 0.002). According to the standardized estimation values, the effect of planktivorous fish on the Cyclopoida group was higher compared to other groups (Table 4).

Table 3

R² values of variables used in Structural Equation Modeling.
	Estimate
Chlorophyll-a	0.854
Rotifera	0.633
Calanoida	0.481
Cyclopoida	0.546
Cladocera	0.426

Table 4

Standardized estimates, standard errors, z-values, and p-values in Structural Equation Modeling
			Standardized Estimate	S.E.	z-value	P
Cladocera	<---	Fish	0.249	0,242	1.027	0.305
Cladocera	<---	Chlorophyll-a	-0.482	0.130	-3.698	***
Calanoida	<---	Fish	-0.747	0.241	-3.096	**
Calanoida	<---	Chlorophyll-a	-0.715	0.160	-4.458	***
Cyclopoida	<---	Fish	0.820	0.115	7.126	***
Cyclopoida	<---	Chlorophyll-a	0.174	0.122	1.423	0.155
Rotifera	<---	Fish	0.421	0.120	3.509	***
Rotifera	<---	Chlorophyll-a	-0.482	0.099	-4.875	***
Chlorophyll-a	<---	Total Nitrogen	0.626	0.065	9.704	***
Chlorophyll-a	<---	Total Phosporus	0.573	0.073	7.848	***

The main reason for eutrophication, one of the most important problems affecting freshwater ecosystems, is the excessive presence of nitrogen and phosphorus elements in the water column (Abell et al., 2011). The reason why these nutrients are associated with eutrophication is that nitrogen and phosphorus ratios have a significant effect on phytoplankton growth. The parameters that need to be examined in order to understand the nitrogen and phosphorus relationship in a lake are the total nitrogen and total phosphorus values in the lake water (Downing & McCauley, 1992). Secchi depth, another trophic level indicator, is inversely proportional to the density of phytoplankton populations in the water because suspended matter scatters and weakens incoming light (Frumin & Gildeeva, 2014). In this study, monthly chlorophyll-a, total phosphorus, and Secchi depth values in Yeniçağa Lake were evaluated using Carlson TSI Indices (Table 2). Carlson TSI Index values calculated based on the total phosphorus results indicated that the lake was at a hypertrophic level. The results calculated from chlorophyll-a showed that the lake had a hypertrophic tendency but at a eutrophic level, while the Secchi depth values showed that the lake was at a eutrophic level. In general, the lake can be considered to be at the eutrophic level with a hypertrophic tendency.

During the study period, Carlson TSI(CHL) and Carlson TSI(TP) values did not show parallel changes. TSI(CHL) results had their lowest values in the summer months when TSI(TP) results reached their highest values. Similarly, in autumn, TSI(TP) values decreased, while TSI(CHL) values increased, indicating a non-parallel relationship. However, TSI(TP) results were generally higher than TSI(CHL) results throughout the year, except for September. TSI(SD) showed quite parallel changes with TSI(CHL). According to Carlson and Simpson (1996), some interpretations can be made about ecosystem functioning based on the relationships between TSI indices calculated from chlorophyll-a, total phosphorus, and Secchi depth values. When the TSI(CHL) value is higher than the TSI(SD) value, the lake is dominated by large-sized cyanobacteria. Conversely, when the TSI(TP) value is higher than the TSI(CHL) value, zooplankton creates grazing pressure on phytoplankton in the food web, suggesting that the limiting nutrient in the lake is something other than phosphorus (Carlson & Simpson, 1996; Saluja & Garg, 2017; Markad et al., 2019). According to the TSI values obtained during the study, the TSI(CHL) value with an annual average of 65.65 is higher than the TSI(SD) with an annual average value of 55.19, and the TSI(TP) value with an annual average of 78.91 is higher than TSI(CHL). In order to better understand the deviations in TSI values, the Trophic Index deviation graph, which is Carlson's two-dimensional approach has been used (Fig. 3). Values below the horizontal axis indicate that chlorophyll-a is not limited by phosphorus, while values above the horizontal axis indicate that it is limited by phosphorus. On the vertical axis, the values to the right of the line represent cases where the light transmittance is higher than the expected chlorophyll index. In this case, it shows that zooplankton uses small-sized phytoplankton as food, and therefore large organisms such as filamentous cyanobacteria become dominant. Values to the left of the vertical axis indicate that water transparency is controlled by factors not dependent on phytoplankton (Carlson & Simpson, 1996). According to the trophic index deviation graph given in Fig. 3, almost all values for Lake Yeniçağa are gathered below the vertical axis and to the right of the horizontal axis. This suggests that phosphorus is not the limiting nutrient for chlorophyll-a in the lake, and phytoplankton is dominated by larger-sized species, with zooplankton exerting grazing pressure on small-sized phytoplankton organisms.

According to Sakamoto (1966), the Total Nitrogen:Total Phosphorus ratio provides information about the limiting element for photosynthetic organisms in an ecosystem. When, the Total Nitrogen:Total Phosphorus ratio is lower than 10, nitrogen is considered the limiting nutrient, and when it is higher than 17, phosphorus is considered the limiting nutrient. Ratios between 10–17 indicate that both nitrogen and phosphorus are limiting nutrients. Total Nitrogen:Total Phosphorus ratios obtained in Yeniçağa Lake indicate that nitrogen is the limiting element for photosynthetic organisms. These results align with our findings in the Carlson Trophic Index deviation graph. Additionally, the effect of total nitrogen and total phosphorus on chlorophyll-a was evaluated using Structural Equation Modeling (SEM). The analysis results showed that bottom-up controls had a positive effect on chlorophyll-a, with nutrients increasing phytoplankton biomass. This finding is consistent with what is commonly observed in freshwater lakes and reservoirs (Smith et al, 1999). The SEM analysis also revealed that the effect of total nitrogen on chlorophyll-a (r = 0.63) is stronger than the effect of total phosphorus (r = 0.57) in Lake Yeniçağa.

The analyses conducted for Yeniçağa Lake revealed that zooplankton was influenced by both top-down and bottom-up controls, and different groups within the zooplankton community responded differently to these effects. The standardized path coefficients obtained from the Structural Equation Model indicated that the top-down control exerted by fish mainly affected the organisms in the Calanoida and Cyclopoida groups. A similar, but weaker, effect was also observed on Rotifera (Fig. 2, Table 3). According to the SEM results, the relationship between fish biomass and zooplankton biomass was statistically significant for all groups except the Cladocera group (p > 0.05). An increase in fish biomass negatively affected the Calanoida group, while positively affecting the biomass of the Cyclopoida and Rotifera groups. No significant relationship was found between the biomass of planktivorous fish in Lake Yeniçağa and the Cladocera group (p = 0.305). The effect of chlorophyll-a on zooplankton was negative, except for the Cyclopoida group, for which no significant relationship could be detected (p = 0.155). The effect of Total Nitrogen and Total Phosphorus on chlorophyll-a in the food web was found to be statistically significant (p < 0.05), and according to the path coefficient results, it was determined that total nitrogen had a higher effect on phytoplankton (r = 0.63 for total nitrogen, r = 0.57 for total phosphorus).

Since a significant relationship could not be determined between the biomass of planktivorous fish and the Cladocera group in Lake Yeniçağa, it is thought that this group is not controlled by top-down effects. It has been reported in the literature that planktivorous fish species exert feeding pressure on Cladocera species with large body sizes (Hrbácek et al., 1961; Brooks & Dodson, 1965; Hall et al., 1976; Lynch, 1980). While there are studies supporting this relationship, there are also some studies where no significance could not be detected (Matsuzaki et al., 2018; Diniz & Moura, 2022). Therefore, it can be said that our analysis results for cladocerans partially contradict the literature. Planktivorous fish species such as Cyprinus carpio, Squalius cephalus, and Tinca tinca are not only dependent on zooplankton, but they are also omnivorous organisms that can feed on phytoplankton, insect larvae, and benthic macro-vertebrates (Rahman et al., 2006; Marković et al., 2007; Giles et al., 1990). Consequently, the food selectivity of the fish may have reduced the top-down pressure on the cladocerans. On the other hand, the feeding habits of fish may also change depending on the turbidity level of the water they are in, and the effect of size-related predation may decrease due to turbidity (Abrahams & Kattenfeld, 1997). Yeniçağa Lake can be classified as a medium turbid lake according to its turbidity values. Studies show that the pigmentation rate of zooplankton is effective in the food selectivity of planktivorous fish in turbid lakes, so transparent organisms with weak pigmentation can avoid the predation pressure of fish in turbid environments (Zaret, 1972; Zaret & Kerfoot, 1975). In the aquatic environment, cladocerans can also be found exhibiting daily vertical or horizontal migration behaviors to escape the predator pressure of fish, and experimental studies have shown that fish signals affect this migration behavior (Dodson, 1988; Ringelberg et al., 1991; Lampert, 1993). Although we have not conducted direct research on these issues, it can be argued that cladoceran organisms in Lake Yeniçağa could escape the predation of planktivorous fish under the influence of these factors.

When bottom-up effects on cladocerans are stronger than top-down effects, large-sized organisms such as Daphnia species may become dominant in the zooplankton community (Korponai et al., 2003). In Yeniçağa Lake, where the bottom-up effects are statistically significant, Daphnia species (Daphnia pulex + Daphnia magna) dominate in terms of numbers and biomass (Sorguç et al., 2022; Sorguç, 2023). The SEM analysis results also indicated a negative bottom-up effect due to chlorophyll-a on the cladocerans (r = -0.48). As the level of eutrophication increases in freshwater ecosystems, there is a decrease in the growth and fertility rates of cladocerans (Gliwicz & Siedlar, 1980; Gliwicz, 1990). Additionally, the increase in eutrophication in lake ecosystems triggers the overgrowth of cyanobacteria, which provide poor nutrition for cladocerans and limit their energy for growth and reproduction, especially in the case of large-sized cladocerans (Porter & McDonough, 1984; Gilbert & Durand, 1990; Havens et al., 2007). It was reported by Kılınç (2003) that the phytoplankton community in Lake Yeniçağa is dominated by large-size filamentous cyanobacteria, and these species, which are dominant in the lake, limit the feeding and reproduction characteristics of Daphnia species. Similarly, there are studies in which cladocerans are negatively affected by phytoplankton through bottom-up control. Elser et al., (1990) stated that the grazing of Daphnia over phytoplankton in a lake at a high eutrophic level decreased considerably compared to a mesotrophic lake, and the increase in the trophic level turned the bottom-up effects on Daphnia into negative.

The top-down effects on the Copepoda group in Lake Yeniçağa exhibited opposite patterns for the two different orders studied within this group. The biomass of calanoid copepods was negatively affected by the presence of fish (r = -0.75), while the biomass of cyclopoid copepods was positively affected (r = 0.82). According to Soto and Hurlbert (1991a), calanoid copepods are more sensitive to top-down effects than cyclopoid copepods. Calanoid copepods may enter into a food competition with cyclopoid copepods by consuming common food groups such as rotifers and protozoa with cyclopid copepods or by feeding on phytoplankton consumed by cyclopoid nauplius, leading to a numerical decrease in the Cyclopoida group (Soto & Hurlbert, 1991a,b). The top-down control of calanoid copepods by planktivorous fish in Yeniçağa Lake may indirectly affect the food competition between calanoid and cyclopoid copepods. Matsuzaki et al. (1998) found a positive relationship between Cyclopoida abundance and the CPUE of fish, while Li et al., (2017) found a negative relationship between the increase in planktivorous fish and calanoid biomass. The results obtained for copepods in Yeniçağa Lake are consistent with the literature.

The bottom-up effect, another effect seen in the Copepoda group, was clearly detected for the order Calanoida (r = -0.71), but no significant relationship was found for the order Cyclopoida (p > 0.05). It is known that calanoids are much more sensitive to eutrophication than cyclopoids and have lower biomass in eutrophic water systems (Gliwicz, 1969; Patalas, 1972; McNaught, 1975; Gannon & Stemberger, 1978; Gulati, 1984; Muck & Lampert, 1984). In our study, SEM results revealed that both top-down (r = -0.75) and bottom-up (r = -0.71) controls acted together on the Calanoida group, and it was understood that these effects were close to each other. The only significant effect in cyclopoid copepods was a positive top-down interaction. It is thought that the reason for this positive effect is the food competition with the calanoids, which decreases as a result of the negative effect exerted by the fish. As an omnivorous species, Acanthodiaptomus denticornis, the only Calanoida species identified in the study area, can feed on rotifers and ciliates (Lair & Hilal, 1992). At the same time, Cyclops strenuus, the most dominant Cyclopoid Copepod species in the lake, has an omnivorous feeding characteristic and feeds on rotifers, other crustaceans, and phytoplankton (Makino & Ban, 1998). For these reasons, it can be assumed that these two species entered into significant food competition.

Increasing the top-down effect of fish predation on Crustacea in the aquatic food web may reduce predation pressure and trigger an increase in both the abundance and biomass of rotifers (Telesh, 1993; Gilbert, 1988; Sanni & Wærvågen, 1990). The results of SEM analysis in Yeniçağa Lake showed a positive top-down control on rotifers from planktivorous fish (r = 0.42). An increase in planktivorous fish biomass also increased Rotifera biomass, and there are studies consistent with our results (Christoffersen et al., 1993; Miracle et al., 2007; Matsuzaki et al., 2018). Predation pressure from planktivorous fish, which negatively controls the calanoids from the top-down, may have indirectly contributed to this increase. Since Acanthodiaptomus denticornis, the only Calanoid species detected in Lake Yeniçağa, is a species that feeds on rotifers (Lair & Hilal, 1992), the decrease in their abundance may have caused a decrease in the predator pressure on the Rotifera. On the other hand, studies in freshwater ecosystems reveal that rotifers are also controlled from the bottom-up, and this effect is higher than top-down control (Yoshida et al., 2003; Du et al., 2015). In bottom-up controls, besides phytoplankton, there is also the effect of food sources such as bacteria, detritus, and Protista at a much higher rate. Due to the organism size of the filamentous algae, the lack of suitable nutrients for the rotifers reduces the filtration rate and creates negative effects on their nutrition and life cycles (Karabin, 1985; Gilbert & Durand, 1990; Rothhaupt, 1991; Weithoff & Walz, 1995). In this respect, it can be thought that the negative interactions we detected between Rotifera and chlorophyll-a are caused by dominant phytoplankton species (Anabaena circinalis, Aulacoseira granulata, Aphanizomenon flos-aquae) in the phytoplankton of Lake Yeniçağa (Kılınç, 2003). According to the SEM analysis results in Yeniçağa Lake, the standardized path coefficient value was calculated as r = -0.48 in the bottom-up interaction of chlorophyll-a. Rotifers can interact positively with phytoplankton with bottom-up control in the food web, but these effects are weaker than interactions with other food sources such as bacteria and Protista (Duncan, 1989; Yoshida et al., 2003; Du et al., 2015). In the zooplankton community, organisms with small body sizes, such as rotifers, may turn to food sources like bacteria and detritus in the absence of suitable phytoplankton nutrients (Bays & Crisman, 1983; Sládeček, 1983; Karabin, 1985). Since these food sources are generally very abundant in eutrophic lakes, it can be hypothesized that in our study, rotifers increase their abundance by turning to these food sources. However, it is not possible to reach a definite judgment since we do not have data on supplementary food sources.

Zooplankton in Yeniçağa Lake, which shows a eutrophic level close to hypertrophic, are affected by both top-down and bottom-up controls, with different levels of exposure among the zooplankton groups. The Calanoida and Rotifera groups are affected by both top-down and bottom-up controls simultaneously, whereas the Cladocera group is influenced by bottom-up control, and the Cyclopoida group is mainly affected by top-down control. The chlorophyll-a values in the lake were affected by the bottom-up controls from both total nitrogen and total phosphorus, but the effect of total nitrogen was stronger. Additionally, nitrogen appears to limit chlorophyll-a levels in the lake.

According to the results of the standardized path coefficients, the summary of bottom-up and top-down controls in the zooplankton community is as follows: a) The Cladocera group was controlled from the bottom up by phytoplankton, b) The Cyclopoida group was controlled from the top-down by planktivorous fish, c) Both top-down and bottom-up effects observed on the Calanoida and Rotifera groups were quite close to each other (r = -0.75 and − 0.71 for Calanoida; r = 0.42 and − 0.48 for Rotifera). For this reason, it was determined that both top-down and bottom-up controls acted together on these two groups.

Author Contributions Bura Uğur Sorguç contributed to the methodology, investigation, writing of original draft preparation, and editing. Fatma Yıldız Demirkalp contributed to the methodology and supervised the whole research work. Yasemin Saygı contributed to the methodology, investigation, writing of original draft preparation and editing, and supervised the research work.

Availability of data and materials Not applicable.

Conflicts of Interest The authors declare no conflict of interest.

Ethics approval and consent to participate Not applicable.

Funding This study conducted with the financial support of Hacettepe University Scientific Research Project Commission (Project no: FHD-2021-19615).

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No competing interests reported.

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Analysis of the top-down and bottom-up controls on the zooplankton of eutrophic freshwater Yeniçağa Lake (Turkey)

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