Seston Fatty Acid Responses to Physicochemical Changes in Subalpine Lake Lunz, Austria

Compound-specific stable isotope analyses of fatty acids indicate feeding zones of zooplankton across the water column of a subalpine lake

Spatial and temporal zooplankton feeding dynamics across the water column of lakes are key for understanding site-specific acquisition of diet sources. During this 6-week lake study, we examined stable carbon (δ13C) and nitrogen (δ15N) isotopes and conducted compound-specific fatty acid (FA) stable isotope analysis (CSIA) of edible seston in the epi-, meta-, and hypolimnion, and zooplankton of Lake Lunz, Austria. We predicted that CSIA of essential FA can discern the foraging grounds of zooplankton more accurately than the commonly used bulk stable isotopes. The δ13C and δ15N values of seston from different lake strata were similar, whereas a dual CSIA approach using stable carbon and hydrogen isotopes of FA (δ13CFA and δ2HFA) provided sufficient isotopic difference in essential FA to discern different lake strata-specific diet sources throughout the study period. We present a CSIA model that suggests strata-specific foraging grounds for different zooplankton groups, indicating higher preference of cladocerans for feeding on epilimnetic diet sources, while calanoid copepods retained more hypolimnetic resources. The CSIA approach thus yields strata-specific information on foraging strategies of different zooplankton taxa and provides more details on the spatial and temporal trophodynamics of planktonic food webs than commonly used bulk stable isotopes.

Dietary fatty acid transfer in pelagic food webs across trophic and climatic differences of Chinese lakes

In eutrophic lake ecosystems, cyanobacteria typically lead to unbalanced phytoplankton community structure and low dietary quality for consumers at higher trophic levels. However, it still remains poorly understood how zooplankton manage to respond to seasonal and spatial differences in lake trophic gradients and temperature factors to retain highly required dietary nutrients from phytoplankton. In this field study, we investigated seston and different size classes of zooplankton of temperate and subtropical large lakes of different trophic conditions in China. We used fatty acids (FA) as dietary nutrients from seston to zooplankton to investigate how eutrophication affects the FA composition of various zooplankton size classes. This study revealed a curvilinear relationship between total phosphorus (TP) and polyunsaturated fatty acids (PUFA) contents of edible phytoplankton ("seston") across 3 seasons and 2 climatic areas. The PUFA content of seston increased until mesotrophic lake conditions (TP: 11-20 μg L-1), after which the dietary provision of PUFA for respective consumers declined. Seston FA, rather than trophic condition or water temperature, primarily predicted changes in zooplankton FA, while this predictive power decreased with zooplankton size. Despite increasing eutrophic lake conditions, LC-PUFA content of the zooplankton consistently increased per unit biomass. The results indicate that the nutritional value of phytoplankton was highest in mesotrophic lakes, and lake zooplankton selectively increased their LC-PUFA retention with body size and/or were able to convert dietary FA endogenously to meet their size-specific FA demands, independent of lake location or time (season) or the measured trophic condition of the lake (from oligo- to eutrophic).

Responses of periphyton communities to abrupt changes in water temperature and velocity, and the relevance of morphology: A mesocosm approach

Sudden instream releases of water from hydropower plants (hydropeaking [HP]) can cause abrupt temperature variations (thermopeaking [TP]), typically on a daily/sub-daily basis. In alpine rivers, hydropeaking and thermopeaking waves usually overlap, which leads to a multiple stressor of flow velocity pulses and temperature alteration. Periphytic communities could give important insights into the effects of combined thermo- and hydropeaking (THP) in stream ecosystems. Thus, the study's first aim was to assess the combined effects of thermo-hydropeaking on structural (composition, biomass) and functional (photosynthesis, enzyme activity) properties of periphyton. The second aim was to assess the interaction between periphytic algae and the heterotrophic communities (bacteria) and determine how biotic and abiotic factors explain the variability of bacterial enzymatic activities in the periphyton. We assessed the effects of repeated cold and warm thermo-hydropeaking for 24 days on periphyton, by manipulating discharge and temperature in six experimental flumes directly fed by an Alpine stream. Our study revealed that THP had structural and functional effects on periphyton in oligotrophic streams, where the effects depending on the direction of the temperature change (cold/warm) and on the morphological setting (pool/riffle). The results showed that even a short-term increase in flow velocity and temperature decrease could induce better growth conditions for diatoms. Additionally, an increase in the interaction between periphytic algae and bacteria during thermo-hydropeaking was also shown, this coupling being more pronounced in pool than in riffle sections. Our results clearly showed that riffle sections develop less periphytic algal biomass and activity and therefore, THP can reduce biomass availability for primary consumers in large areas of impacted streams. These findings highlight the importance of mitigation measures, focusing on establishing heterogeneous stream bed areas, with frequent pool and riffle sequences.