Multi-proxy palaeoecological responses to water-level fluctuations in three shallow Turkish lakes

Natural or human-induced water-level fluctuations influence the structure and function of shallow lakes, especially in semi-arid to arid climate regions. In order to reliably interpret the effect of water-level changes from sedimentary remains in the absence of historical data, it is crucial to understand the variation in sedimentary proxies in relation to water level measurements. Here, we took advantage of existing water surface elevation data on three large shallow lakes in Turkey to elucidate the impact of lake-level changes on benthic-pelagic primary production over the last 50-100 years. Sub-fossil cladocerans, diatoms, plant remains and pigments were investigated as biological variables; X-ray fluorescence (XRF) and loss on ignition (LOI) analyses were conducted as geochemical-physical variables on a set of Pb-210 and Cs-137 dated cores. Dating of the cores were robust, with the exception of uncertainties in Lake Marmara littoral core due to low unsupported 210Pb activities and high counting errors. Results indicated that Lake Marmara was dominated by benthic species throughout the sediment record, while Lakes Beysehir and Uluabat shifted from a littoral-dominated system to one with increased pelagic species abundance. In all cores there was a stronger response to longer-term (decadal) and pronounced water-level changes than to short-term (annual-biennial) and subtle changes. It was also noted that degree of alteration in proxies differed between lakes, through time and among pelagic-littoral areas, likely emphasising differences in depositional environments and/or resolution of sampling and effects of other stressors such as eutrophication. Our results highlight lake-specific changes associated with water-level fluctuations, difficulties of conducting studies at required resolution in lakes with rather mixed sediment records and complexity of palaeolimnological studies covering recent periods where multiple drivers are in force. They further emphasise the need to include instrumental records when interpreting effects of recent water-level changes from sediment core data in large shallow lakes

Distribution of mollusc assemblages in the surface sediments of the Black Sea, Marmara Sea and Aegean Sea

The Marmara Sea Gateway connects the hypersaline (<36 [per thousand]) Aegean Sea and the low-salinity (∼17-22 [per thousand]) Black Sea through the Straits of Bosphorus and Dardanelles and the landlocked Marmara Sea. This gateway forms a natural laboratory in which to study the effects of climate change, sea-level fluctuations and water-mass exchange between small basins. -- Surface sediment samples were collected from 137 stations across six transects in the Black Sea, the Marmara Sea and the Aegean Sea. At each station grab samples and CTD (conductivity, temperature, depth) measurements were also collected. -- In the Marmara Sea, CTD data revealed the presence of a low salinity surface water mass representing Black Sea outflow and a high salinity deeper water mass, separated by a sharp mixing zone. Across the Southwestern Black Sea shelf the CTD data showed the presence of 3 water masses: (1) a low salinity surface water mass extended down to 10 m; (2) a low salinity, but colder water mass occupies water depth below 40 m; (3) and a higher salinity Mediterranean water occurs between these two water masses. In the Aegean Sea CTD data revealed a relatively low salinity Black Sea outflow water mass at the surface, and high salinity water mass at the bottom separated by a mixing zone. -- The mollusc shells from grab samples were identified using a number of taxonomic keys and analyzed qualitatively and quantitatively in order to investigate the relationship between the community structure and the environment. The mollusc absence and presence data in quadrats was used to delineate seven different mollusc assemblages, with each assemblage representing a distinct set of environmental conditions. Principal component analysis was used to constrain mollusc faunal assemblages and their relationship to environmental variables. Seven hypothetical faunal assemblages and three hypothetical environmental variables were extracted, explaining 73.9 % and 73.8 % of the variance in the faunal and environmental data, respectively. Cross-plots of scores for the major faunal and environmental components revealed empirical relationships between the three oceanographically different seas. The separation of the Black Sea from the Marmara Sea and the Aegean Sea assemblages was found in the cross-plot of Faunal Component 1 versus Environmental Component 1. The separation from the Marmara Sea to the Aegean Sea was found in the cross-plots of Faunal Component 1 versus Environmental Component 3 and Faunal Component 4 versus Environmental Component 3. Mollusc shells were also identified and counted in a ∼8 m long piston core recovered from the SW Black Sea shelf. Visual inspection of the data revealed four distinct faunal assemblages in the core. To test the validity of using the principal components determined from the surface samples to deduce past environments in a core, the same mollusc species found both in the surface samples and core units/subunits were used to calculate faunal scores for each unit/subunit. Principal components scores for units/subunits provided little basis for making strong interpretations about the changing paleoenvironment. Because only five species that were used in the principal component analysis on surface samples were present in the core MAR 02-45P

Sediments, not plants, offer the preferred refuge for Daphnia against fish predation in Mediterranean shallow lakes: an experimental demonstration

1. Different behavioural responses of planktonic animals to their main predators, fish, have been reported from shallow lakes. In north temperate lakes, large-bodied zooplankton may seek refuge from predation among macrophytes, whereas in subtropical lakes, avoidance of macrophytes has been observed. The prevalent behaviour probably depends on the characteristics of the fish community, which in Mediterranean lakes is typically dispersed in both the open water zone and in the littoral, as in temperate lakes, and is dominated by small size classes, as in subtropical lakes

Patterns of microbial food webs in Mediterranean shallow lakes with contrasting nutrient levels and predation pressures

To elucidate the specific and combined effects of bottom-up and top-down control on the microbial community in warm lakes, we sampled microbial community along with physical-chemical and biological variables and performed in situ food web experiments, in 14 Turkish shallow lakes with contrasting nutrient levels and predation pressures. Our field results revealed that differences in microbial communities correlated with differences in zooplankton community structure, temperature (increasing nutrient concentrations, change in zooplankton composition), nutrient concentrations (increasing bacteria and heterotrophic nanoflagellate abundances with increasing nitrogen concentrations and temperature) and macrophyte coverage (ciliates as potential consumers of bacteria and HNF was strongest in macrophyte-dominated lakes). Our in situ experimental study revealed that the zooplankton not only affect the biomass and composition of microbial communities but also alter the microbial structure and trophic relationships. Our results therefore indicate that both bottom-up factors and top-down effects were important for the efficiency of the carbon transfer from bacteria to higher trophic levels in the study lakes. Due to an anticipated increase in eutrophication, temperature and alteration of the classical food web with climate warming, major changes in the microbial community of lakes are, therefore, expected in a warmer future in semi-arid Mediterranean climatic regions

Water level and fish-mediated cascading effects on the microbial community in eutrophic warm shallow lakes: a mesocosm experiment

Information on the effects of water level changes on microbial planktonic communities in lakes is limited but vital for understanding ecosystem dynamics in Mediterranean lakes subjected to major intra- and inter-annual variations in water level. We performed an in situ mesocosm experiment in an eutrophic Turkish lake at two different depths crossed with presence/absence of fish in order to explore the effects of water level variations and the role of top-down regulation at contrasting depths. Strong effects of fish were found on zooplankton, weakening through the food chain to ciliates, HNF and bacterioplankton, whereas the effect of water level variations was overall modest. Presence of fish resulted in lower biomass of zooplankton and higher biomasses of phytoplankton, ciliates and total plankton. The cascading effects of fish were strongest in the shallow mesocosms as evidenced by a lower zooplankton contribution to total plankton biomass and lower zooplankton:ciliate and HNF:bacteria biomass ratios. Our results suggest that a lowering of the water level in warm shallow lakes will enhance the contribution of bacteria, HNF and ciliates to the plankton biomass, likely due to increased density of submerged macrophytes (less phytoplankton); this effect will, however, be less pronounced in the presence of fish

Size-based interactions across trophic levels in food webs of shallow Mediterranean lakes

1. Body size is a key trait of an organism which determines the dynamics of predator-prey interactions. Most empirical studies on the individual size distribution of the aquatic community have focused on the variations in body size of a single trophic level as a response to certain environmental variables or biotic factors. Few studies, however, have evaluated how individual size structure is altered simultaneously across interacting trophic levels and locations. Such comparative examinations of the size distribution in predator and prey communities may bring insight into the strength of the interactions between adjacent trophic levels

Effects of nutrient and water level changes on the composition and size structure of zooplankton communities in shallow lakes under different climatic conditions: a pan-European mesocosm experiment

Lentic ecosystems act as sentinels of climate change, and evidence exists that their sensitivity to warming varies along a latitudinal gradient. We assessed the effects of nutrient and water level variability on zooplankton community composition, taxonomic diversity and size structure in different climate zones by running a standardised controlled 6-months (May to November) experiment in six countries along a European north-south latitudinal temperature gradient. The mesocosms were established with two different depths and nutrient levels. We took monthly zooplankton samples during the study period and pooled a subsample from each sampling to obtain one composite sample per mesocosm. We found a significant effect of temperature on the community composition and size structure of the zooplankton, whereas no effects of water depth or nutrient availability could be traced. The normalised size spectrum became flatter with increasing temperature reflecting higher zooplankton size diversity due to higher abundance of calanoid copepods, but did not differ among depths or nutrient levels. Large-bodied cladocerans such as Daphnia decreased with temperature. Taxonomic diversity was positively related to size diversity, but neither of the two diversity measures demonstrated a clear pattern along the temperature gradient nor with nutrient and water levels. However, genus richness decreased at the warm side of the temperature gradient. Our experiment generally supports recent empirically based findings that a continuing temperature increase may result in lower genus richness and lower abundance of large-sized zooplankton grazers, the latter likely resulting in reduced control of phytoplankton