Diversity and distribution of Daphnia across space and time in Danube Delta lakes explained by food quality and abundance

Abstract

Eutrophication of shallow lakes often triggers a series of cascading ecological effects. Among these are shifts in the zooplankton community structure due to phytoplankton changes, or shifts in the fish community reducing size-selective feeding of planktivorous fish. In such conditions larger zooplankton (e.g. Daphnia) can have a selective advantage. Re-oligotrophication can reverse such food web interactions. Europe's largest wetland system (the Danube Delta) went through a period of eutrophication and is now slowly recovering again. However, changes in the Daphnia populations triggered by eutrophication in this system have remained unstudied. We used different sampling strategies to screen 24 lakes (which differ in their ecological state) for the presence of Daphnia as well as for biotic and abiotic parameters potentially explaining Daphnia abundance. Daphnia densities were surprisingly low. We found D. magna ephippia in the sediment, but no D. magna in the water column. Microsatellite analyses on pelagic Daphnia populations confirmed the presence of the Daphnia longispina complex and provided evidence for significant hybridisation events. FluoroProbe data showed that Daphnia was mainly present in lakes with a higher phytoplankton production. Our study provides insights into the spatial and temporal distribution of Daphnia in a very dynamic wetland system.

Sediment underneath charophyte meadows is enriched in viable ephippia and enhances the benthic periphytic biofilm

“…Therefore, the progressive temperature increase as the spring drew on until the sampling date, facilitated a higher development of CV meadows compared to CH, which, moreover, started from the recruitment from the sediment bank, instead of from well-grown cultures as in the case of CV. The 1 cm surficial sediment layer would correspond to approximately one year, based on the maximum sedimentation rates of 0.8-1 cm/year reported by Rodrigo & Alonso-Guille ´n (2013) for the eutrophication period in the Albufera de Vale`ncia, or 0.7 cm/year estimated by Enache et al (2019) for Danube delta wetlands. Moreover, considering that the lagoon was dried out from August 2018, the complete sedimentation of all the material contained in the water column would increase the thickness of the sediment layer.…”

Methane in the Danube Delta: the importance of spatial patterns and diel cycles for atmospheric emission estimates

“…The fluvio-marine delta system accounts for 51 % of the total area (Pavel et al, 2009) in which it sees salt intrusions and through-flow from the Black Sea into the delta. Since the 1970s, the Danube Delta has been subject to eutrophication, with its peak during 1987-1988 (Cristofor et al, 1993;Galatchi and Tudor, 2006;Enache et al, 2019). After a decrease in nutrient loads in the 1990s, due to socioeconomic changes in eastern Europe, a slow decline of nutrient levels was observed (Rîşnoveanu et al, 2004;Pavel et al, 2009); however, more recent levels comparable to those in 1988 were reported (Tudor et al, 2016;Spiridon et al, 2018).…”

“…August showed the largest variability when extracting diel cycles, with an uncertainty range of a factor of 2.27 from 84 ± 38 to 37 ± 33 µmol m −2 h −1 . This greater variability can be linked to higher temperatures, greater stratification, and increased production and organic matter degradation, all leading to potential increases in CH 4 (Duc et al, 2010;Fuchs et al, 2016). However, given diel cycles were not continuously measured throughout the entire system, these values merely illustrate the effect of neglecting or including diel cycle data.…”

Optimized Protocol for Cyanobacterial 16S rRNA Analysis in Danube Delta Lakes

“…flooding's) was registered in LC 4 (Roșu-Puiu) the most eastern complex, forming the maritime delta. A detailed limnological description of the lakes was already done in previous papers (Enache et al 2019, Fontana et al 2018. The sampling campaigns were carried out preliminary in October 2012 (12 lakes) and in spring (May), summer (July) and autumn (September) in 2013 (26 lakes) in order to include the seasonal dynamics of the cyanobacteria communities.…”