Wind and Temperature Drive Zooplankton in Bulgarian Reservoir

Researchers investigating the ecological dynamics of Mandra Reservoir in southeastern Bulgaria have identified specific environmental variables that dictate the distribution of zooplankton communities within this shallow, artificial water body. Published in the journal BioRisk, the study traces the influence of water temperature, wind speed, transparency, and depth on the complex interactions between zooplankton and their habitat. The findings highlight how abiotic factors, particularly wind and temperature, exert a stronger influence on plankton distribution than depth, challenging some traditional assumptions about the role of vertical stratification in polymictic basins. This research contributes to a broader understanding of how climate-driven changes in weather patterns may alter the biological structure of freshwater ecosystems that are increasingly vulnerable to environmental stress.

Mandra Reservoir, part of the protected Mandra-Poda complex, covers an area of 33 square kilometers with a maximum depth of only seven meters. Its shallow nature and exposure to strong winds classify it as a holo-polymictic basin, a system that mixes thoroughly from top to bottom due to wind action. The reservoir is situated along the Via Pontica bird migration route and is protected under European Union directives for the conservation of natural habitats and wild flora and fauna. Previous studies have noted an acceleration of eutrophication in this area, making the monitoring of biological indicators like zooplankton essential for assessing the overall health of the ecosystem and the quality of the water it contains.

To capture the seasonal dynamics of the reservoir, the research team conducted four sampling sessions between February 2020 and January 2021. These sessions were timed to coincide with periods of significant environmental pressure, allowing for a detailed analysis of how zooplankton respond to changing conditions across different seasons. Samples were collected at seven distinct sites, including the mouths of the Fakiyska, Sredetska, Izvorska, and Rusokastrenska rivers, as well as the central part of the reservoir and the dam area. The use of a Garmin Striker 5DV sonar with GPS enabled precise location tracking and depth measurement, while Secchi discs were used to determine water transparency. Wind speed data for the specific days of sampling were obtained from the National Institute of Meteorology and Hydrology in Bulgaria, ensuring that the environmental variables were accurately recorded alongside the biological samples.

The laboratory analysis identified a total of 67 zooplankton taxa, providing a comprehensive snapshot of the community structure. The composition of these communities was dominated by Rotifera, which constituted approximately 48% of the total abundance, followed by Cladocera at 27% and Copepoda at 19%. Protozoa made up a smaller portion, accounting for only 6% of the identified taxa. This distribution reflects the trophic status of the reservoir, with the high prevalence of rotifers often associated with mesotrophic to eutrophic conditions. The presence of specific species, such as Keratella quadrata and Brachionus angularis, further indicated advanced processes of eutrophication, a trend that has been observed in previous studies of this water body.

Quantitative analysis revealed significant fluctuations in zooplankton abundance and biomass across the four sampling periods. In February 2020, abundance was relatively low, with Nauplius and Copepodites-Copepoda being the most numerous taxa. By June, the population surged, with Nauplius reaching a maximum of 172,800 individuals per cubic meter at one of the sampling sites. The biomass of zooplankton also varied dramatically, with the highest values recorded in June and the lowest in other periods. These seasonal peaks and troughs are closely linked to the reproductive cycles of the organisms and the availability of food resources, which are themselves influenced by the physical environment of the reservoir.

Statistical analysis using the Shannon-Weaver index, Simpson’s index of dominance, and Pielou’s evenness index provided further insights into the diversity and stability of the zooplankton communities. The Shannon-Weaver index ranged from 0.52 to 2.37, indicating relatively low diversity, which is often a result of the dominance of a few specific species. The Simpson’s index of dominance was inversely proportional to the diversity index, with higher dominance observed during periods of high abundance of larval stages. The Margalef richness index varied between 1.23 and 4.09, showing that while the number of species remained relatively constant, the distribution of individuals among those species changed significantly in response to environmental conditions.

Canonical Correspondence Analysis (CCA) was employed to determine the specific influence of environmental variables on the abundance and distribution of dominant zooplankton taxa. The analysis revealed that temperature, depth, transparency, and wind were the primary drivers of community structure. The first axis of the CCA, which accounted for 91.45% of the total variance, was positively correlated with depth, transparency, and wind, but negatively correlated with temperature. This suggests that wind and temperature are the most critical factors shaping the zooplankton community in Mandra Reservoir. In contrast, depth showed only a weak positive correlation with the first axis, indicating that it plays a minor role in determining zooplankton distribution in this shallow, wind-mixed basin.

The study also highlighted the role of biotic interactions, particularly fish predation, in shaping the zooplankton community. The presence of small rotifers and the lack of large cladocera and copepods in certain areas suggested significant pressure from planktivorous fish. This predation pressure coincided with the lowest zooplankton biomass observed during the study, indicating that fish spawning and feeding zones have a profound impact on the local ecosystem. Understanding these interactions is crucial for developing effective conservation measures, as the reservoir serves as a key habitat for both aquatic and avian species within the Natura 2000 network.

The findings underscore the importance of zooplankton as a key indicator for monitoring the health of shallow, holo-polymictic water bodies. The dynamic nature of the Mandra Reservoir, with its seasonal variations in temperature and wind, creates a complex environment where zooplankton communities must constantly adapt. The study concludes that while abiotic factors such as wind and temperature are the primary drivers of zooplankton distribution, biotic interactions and the physical structure of the habitat also play significant roles. Future research should focus on the long-term effects of climate change on these systems, as well as the potential impacts of continued eutrophication on the biodiversity and ecological balance of the reservoir.

Subject of Research: Influence of some environmental factors on the distribution of zooplankton complexes in Mandra Reservoir, in Southeastern Bulgaria

Article Title: Influence of some environmental factors on the distribution of zooplankton complexes in Mandra Reservoir, in Southeastern Bulgaria

Article References: Fikovska, E., Kozuharov, D., & Stanachkova, M. (2022). Influence of some environmental factors on the distribution of zooplankton complexes in Mandra Reservoir, in Southeastern Bulgaria. BioRisk, 17, 343-355. https://doi.org/10.3897/biorisk.17.77368

Image Credits: AI Generated

DOI: 10.3897/biorisk.17.77368

Keywords: Influence, some, environmental, factors, distribution, zooplankton, complexes, Mandra, Reservoir, Southeastern, Bulgaria, scientific research