Stable isotope ecology of fishes in the Elbe estuary

Abstract

Stable isotope analysis of δ13C and δ15N evolved into a favored tool for ecologists enabling them, among others, to identify aquatic food web structures and nutritional pathways from lower to higher trophic levels. Especially the ecological role of aquatic consumers, such as fishes, can be uncovered as stable isotope analysis provides insights into their feeding strategies, migrations, foraging habits and dietary niche overlaps. Estuaries link marine and freshwater ecosystems, forming isotopic imprints with distinct stable isotope signatures along the salinity gradient and enabling them to be excellent study sites that can be analyzed using stable isotopes (chapter 1). In the Elbe estuary European smelt Osmerus eperlanus (Linnaeus, 1758) is the dominating fish species that contributes up to 96% of the overall fish abundance, fulfilling a key function. However, the smelt population has declined dramatically in recent years, leading to a necessity to further improve our knowledge on its habitat exploitation in the estuary (chapter 2). Due to smelt ́s anadromous life cycle juveniles and adults share the same areas and resources during springtime. In our study we conducted stomach content and stable isotope analysis of both life stages, providing information on long-term and short-term feeding preferences and isotopic niche overlaps along the salinity gradient. The dietary preferences changed from smaller to larger prey during ontogeny with a cannibalistic feeding habit, whereby feeding presumably depended on food availability. As we observed both overlapped isotopic niches and prey items (mainly mysids and gammarids), the risk of intraspecific competition in smelt can be discussed. However, as the species richness is overall low in the estuary, but biomass rates are high, we assume minor competition pressure within the smelt population. Along the salinity gradient we observed different isotopic niche widths and overlaps, indicating spatial differences in food web structures. Enriched δ15N values in the maximum turbidity zone of the Elbe estuary revealed an extended food chain in this area that mainly affected juvenile smelt. Based on our findings we conclude that juveniles are exposed to the prevailing food supply and conditions, while adults can leave areas when the environmental circumstances are unfavorable. Various fishes developed different life cycle strategies to exploit estuarine resources, whereby abiotic induced stressors, such as osmoregulation, may hamper their distribution ranges. Ruffe Gymnocephalus cernua (Linnaeus, 1758) and European flounder Platichthys flesus (Linnaeus, 1758) are eponymous for lower regions in temperate European rivers (ruffe-flounder-region). We used stable isotope analysis of δ13C and δ15N of fish data sampled seasonally from spring 2021-2022 to track movements, habitat exploitation, spatial connectivity and potential diet shifts during ontogeny (chapter 3). Our study revealed new insights on fish migrations and habitat use in estuaries. Facultative catadromous flounder use the Elbe estuary especially during early life stages, with juveniles occurring in summer and autumn and migrating upstream of our sampling area. We found two isotopic patterns of flounder throughout the year that indicate two North Sea populations exploiting this area for nursery. The isotope data of ruffe, in contrast, showed residency in the estuary for its entire life cycle, as hardly any marine and riverine isotope signatures were found in the individuals examined. These findings might indicate a distinct brackish water population, while a freshwater population exists further upstream. However, deeper knowledge is needed to confirm our assumption. To uncover trophic interactions on lower trophic levels on a seasonal scale along the salinity gradient of the Elbe estuary, we investigated δ13C and δ15N of particulate organic matter (POM) and dominant meso- and microzooplankton species (chapter 4). High quality carbon (C) was derived from freshwater sources upstream of the city of Hamburg, whereby C sources further downstream were of lower quality (C:N > 8). Overall, the POM δ13C values reflected a mixture of riverine and coastal C that was used by most taxa studied. Calanoid copepods Eurytemora affinis contained a broader range in δ13C than POM, suggesting that this species fed selectively on POM components. Brackish mysid shrimps of Mesopodopsis slabberi showed the least variation in its isotopic values which did not overlap with dietary niches of other taxa. During winter and autumn zooplankton increased carnivorous foraging (enriched δ15N values), especially in the middle section, due to high suspended matter loads and the limitation of high-quality autochthonous phytoplankton. Zooplankton of the Elbe can therefore deal with rapid changing environmental dynamics through their trophic plasticity (e.g., feeding niches partioning, habitat shifting). Some aquatic organisms bind biogenic carbon, e.g., in their shells, bones or exoskeletons, that reflect inorganic carbon from the surrounding environment rather than assimilated carbon derived from their diet, which can be problematic while conducting stable isotope analyses. However, the removal of inorganic carbon (IC) is often neglected in aquatic food web studies. Therefore, we analyzed the effects of acidification versus no acid treatment on both δ13C and δ15N ratios of four common estuarine macroinvertebrates (chapter 5). Our results showed that the necessity of acid treatment prior to stable isotope analysis depends on the species-specific carbonate content. As carbonate is especially high in Gammarus spec. and Palaemon longirostris, we here recommend a removal of IC either by acid or mechanical procedures. In contrast, mysid shrimps showed no strong effect in stable isotope values. We further discussed other options to remove IC based on previous studies as a general protocol on how to proceed is missing. As estuaries are often affected by anthropogenic threats, environmental conditions may change over time, thus shaping fish community compositions, life cycle guilds and abundances. In our 40-year long-term study we compared four seasonal fish monitoring periods from 1980s-2020s (chapter 6). Generally, we observed changes in fish life cycle guilds, especially in marine-estuarine opportunists, anadromous and catadromous species. Overall low fish abundances in the 1980s were potentially induced by poor water quality, due to high concentration of nitrogen compounds, such as NO2 and NH4, that negatively affected fish health and recruitment success. As water quality improved during the 1990s, fish abundances started to increase until 2010. Recently, we observed declining trends in fish stocks again, especially in smelt, flounder, twaite shad and ruffe. Marine-estuarine opportunists, such as herring and whiting, on the other hand increased as they potentially benefit from widening and deepening of the main channel leading to an amplified inflow of saline water. Recent declines in fish abundances were mainly triggered by increased suspended organic matter (SPM) concentrations, reduced river runoff and low oxygen concentrations. We found most significant changes of abiotic parameters and fish stocks in the freshwater and oligohaline areas of the Elbe estuary, where important nursery areas of certain fish species are located. Movements between habitats can lead to certain stress responses in fish and can further affect fish health (chapter 7). Fish gills are in direct connection with the surrounding water, so the microbial composition of gill mucus reflect environmental conditions and represent a major entry for pathogens. We compared gill microbiota using prokaryotic sequencing of resident ruffe and anadromous smelt in space and time to further establish fish health indicators. To estimate migratory behavior, we integrated a stable isotope approach based on δ13C. Ruffe microbiota composition was mainly influenced by spatial drivers with bacterial indicator taxa Verticiella, Polynucleobacter and Candidatur Megaira, reflecting freshwater residency. Anadromous smelt arriving from the adjacent North Sea in autumn showed a stepwise adaptation of the gill microbiome, whereby Luteolibacter became dominant and hampered bacterial dysbiosis in the host. We found an overall small core biome containing ~30 genera that can cope with estuarine conditions. Hypoxia and high nutrient loads lead to dysbiosis states, largely driven by takeover of opportunistic pathogens (Acinetobacter, Shewanella, Aeromonas). In conclusion, we revealed new insights into trophic relationships and habitat exploitation of aquatic consumers in the Elbe estuary on a spatial and temporal scale (chapter 8). Based on our study we were able to prove the efficiency of stable isotope analysis in an estuarine ecosystem to track migration patterns and population dynamics of the most important fish species that has not been investigated before. We found four areas along the salinity gradient that are of crucial relevance in understanding the food web of the Elbe estuary. Especially shallow habitats are important for early fish life stages, such as smelt, flounder and ruffe. Therefore, we conclude that our findings could contribute to future conservation strategies in the Elbe estuary.