Population dynamics and trophic interactions of zooplankton in the Elbe estuary

Abstract

Estuaries are highly productive transitional zones between freshwater and marine ecosystems, providing essential ecosystem services such as diverse habitats for humans and wildlife, coastal protection, nutrient cycling and carbon sequestration. Their dynamic physical and biochemical gradients challenge estuarine biota, resulting in the dominance of a few species with high plasticity to fluctuating environmental conditions through specialised feeding and reproductive strategies. Climate change and anthropogenic pressures may intensify these stressors on estuarine communities, potentially altering biodiversity and consequently ecosystem functioning. Zooplankton are an important component of the estuarine biota as primary and secondary consumers, playing a key role in food webs by maintaining trophic pathways. Despite their great importance for the ecosystem functioning, little is known about how variable environmental conditions affect the trophic interactions of zooplankton in estuaries (Chapter 1). This is particularly true for the highly modified Elbe estuary, one of the largest estuaries in northwestern Europe, where research on zooplankton population dynamics and trophic interactions is scarce despite recent morphological and biochemical changes in the ecosystem.

This dissertation integrates multiple studies and methodological approaches to assess the zooplankton community structures and their role in the food web of the Elbe estuary across spatial and temporal scales. This research aims to improve our understanding of natural and anthropogenic pressures on ecosystem functioning, offering a valuable basis for ecosystem-based management and conservation. To date, comprehensive studies of zooplankton population dynamics in the Elbe estuary are limited to the 1980s and 1990s, highlighting the need for new, detailed abundance data to enhance our knowledge of their spatio-temporal succession (Chapter 2). We conducted seasonal zooplankton sampling campaigns along the entire salinity gradient of the Elbe estuary and applied redundancy analyses to assess their relationships with the prevailing physico-biochemical conditions. The zooplankton community structure shifted along the salinity gradient from typical freshwater species to coastal taxa, while blooming conditions and turbidity affected population structures based on their feeding characteristics. Overall, we observed a similar zooplankton community structure, but lower abundances compared to previous studies from the 1980s and 1990s. Morphological and hydrological changes, such as in flow velocity, sediment load and oxygen concentrations, may have contributed to the decline in species abundance.

The calanoid copepod Eurytemora affinis is the most dominant species throughout the estuary. Despite its dominance in the zooplankton community, little is known about the life history traits of E. affinis that are important for understanding its development and population maintenance under the estuarine gradients of the Elbe River. To address this, we conducted a detailed investigation of the E. affinis population dynamics by studying its growth, production and mortality rates through bi-weekly stationary sampling in the highly modified port region of the city of Hamburg (Chapter 3). Growth and production rates of E. affinis in this area were often higher than those reported in other estuarine studies, likely due to lower salinity stress and more favourable feeding conditions.

To improve our understanding of the spatio-temporal feeding conditions for zooplankton in the Elbe estuary and their impact on ecosystem trophodynamics, we identified available organic matter sources as potential food sources and examined feeding interactions among dominant species using a stable isotope approach (Chapter 4). We found a diverse mixture of particulate organic matter (POM) from riverine, terrestrial and coastal origins along the salinity gradient. The selected zooplankton taxa primarily derived their carbon source from high quality phytoplankton from the non-dredged freshwater area upstream of the port region, while the lower reaches were characterised by lower quantity and quality of algal food sources due to higher turbidity and intensive remineralisation processes. Selective feeding and food niche partitioning, along with shifts from herbivorous to detrital and heterotrophic food sources (e.g. microzooplankton) allowed species to cope with stressful feeding conditions, especially in winter and in the maximum turbidity zone (MTZ).

In addition, to investigate the impact of the estuarine zooplankton trophodynamics on higher trophic levels, we combined stable isotope and stomach content analyses on the most abundant fish species in the Elbe estuary, the European smelt Osmerus eperlanus (Chapter 5). The study aimed to compare habitat exploitation between juvenile and adult smelt and to identify their feeding preferences to assess the trophodynamic role of estuarine zooplankton in a broader context. While adults may evade unfavourable food conditions by leaving certain areas, juvenile smelt were dependent on the prevailing food supply. We observed a dietary switch from zooplankton to increasing cannibalistic feeding preferences during ontogeny, with the limited food supply playing a key role in the selection of prey organisms. The presence of 15N-enriched juveniles in the MTZ suggested an extended food chain in this area, possibly due to unfavourable environmental conditions.

In the last study, we analysed the spatio-temporal distribution pattern of phytoplankton communities in the Elbe estuary as potential carbon sources for zooplankton, using a combination of flow cytometry and metabarcoding techniques (Chapter 6). Major attention was given to the picophytoplankton, which contributed up to 70% to the total phytoplankton abundance and prevailed year-round in the Elbe estuary. Picophytoplankton may play an important role in sustaining primary production and thus food web structures under conditions of extreme temperatures, high turbidity and intense grazing pressure, which may be challenging for larger phytoplankton.

In summary, our studies have provided valuable new insights into the spatio-temporal dynamics of zooplankton populations and their trophic relationships across different trophic levels in the Elbe estuary (Chapter 7). We were able to classify the Elbe estuary into four distinct zones, thereby providing a comprehensive overview of the food web dynamics based on our findings. We have synthesised these results in a schematic diagram that illustrates our main conclusions (Fig. 7.1).