Aquatic Food Webs
Secondary production is the least understood process in pelagic and benthic food webs and biogeochemical models. This gap limits reliable prognoses of how these food webs respond to climate change and stressors. New approaches are needed, with functional biodiversity offering a promising path forward.
Functional biodiversity helps aquatic communities cope with stressors like temperature rise, stratification, nitrogen limitation, and eutrophication. The Working Group on Aquatic Food Webs studies functional biodiversity and the biotic and abiotic factors regulating biological production in pelagic and benthic systems using isotope ecology methods. We determine key energy acquisition modes in field samples: phototrophy, chemoautotrophy, and heterotrophy, which includes osmotrophy, phagotrophy, herbivory, omnivory, carnivory, and detritivory. One special focus is on harvestable secondary production (HSP) of mesozooplankton, for which´s quantification nitrogen source quality, new production rates (including N2 fixation), and nitrogen-specific net growth efficiency are examined.
Our recent findings show planktonic food webs adapt to stratification and blooms of N2-fixing cyanobacteria by shifting mesozooplankton feeding from herbivory to carnivory (Loick-Wilde et al., 2019; Weber et al., 2021). This shift has deleterious effects on pelagic fish species, such as cod, which have experienced a significant decline in energy due to trophic lengthening in the central Baltic Sea (Steinkopf et al., 2024). Conversely, a decrease in nutrient loads has been shown to normalize mussel´s trophic mode and enhance benthic mussel productivity in the North Sea (Dippner et al., 2025).
New research from our group highlights nitrogen limitation´s role in microalgae´s trophic mode. Along the aging Amazon River plume, microalgae communities shift from cryptophytes and diatoms to haptophytes and pico-cyanobacteria, with mixotrophy increasing in mature waters, emphasizing heterotrophic strategies in nitrogen-limited environments (Fernández-Carrera et al., in press). Understanding how mixotrophy affects secondary production and other biogeochemical functions in such environments is a next goal.
A comprehensive understanding of nitrogen quality’s impact on phyto-, mixo-, and mesozooplankton trophic modes and secondary production regulation significantly improves prognoses of pelagic and benthic food webs’ responses to climate change and stressors.