Benthic invertebrate food webs in the Baltic Sea : Anthropogenic pressure effects and methodological advancements

Sammanfattning: Trophic interactions are ecologically important, as they structure communities, and globally important for the ecosystem functions that they facilitate. Anthropogenic pressures are altering the structure of food webs, their stability, and resilience to change. Benthic detritivorous food webs are particularly important for their role in nutrient cycling, creating benthic-pelagic links, and provisioning for higher, commercially important trophic levels, such as fish. However, benthic detritivorous food webs have been understudied due to difficulties in sampling and interpreting diet information, and many trophic links are uncertain. Additionally, it is unknown how anthropogenic stresses, such as eutrophication and climate change, impact the structure and function of these food webs. The aim of this thesis is to address how anthropogenic stressors are affecting benthic invertebrate food webs in the Baltic Sea. Additionally, this thesis serves to elucidate some previously understudied trophic links, namely the food sources of generalist detritivorous macrofauna species, by novel molecular techniques (Studies III and IV) alongside traditional food webs assembled from feeding observation literature (Study I) and stable isotopes (Studies II and IV). Results show that benthic macrofaunal food webs have changed, are changing and will change as a result of anthropogenic impacts in the Baltic Sea. Modelling of benthic invertebrate trophic networks demonstrates that the architecture of these networks has changed since the 1980s, primarily in the marine Skagerrak, where we observed a 32 % reduction in species richness (Study I). We also found that network complexity was strongly and positively correlated with species richness and salinity, which gives cause for concern with current rates of biodiversity loss and the decline in Baltic Sea salinity due to climate change. Several benthic detritivore species in the Baltic Sea have shown trophic plasticity and ability to adapt to changes in the environment. The sentinel amphipod species Monoporeia affinis did not show changes in diet with increased organic enrichment, as shown through bulk stable isotopes (Study II). We did, however, find a possible tradeoff between female condition and reproductive success, where females in lower organic matter sediments showed higher body condition (measured through C:N), but produced offspring with lower biomass than females in higher organic matter sediment (Study II). Additionally, the widespread and important bioturbator Baltic clam Macoma balthica showed little change in trophic niche measured through stable isotopes and DNA metabarcoding of phytoplankton in their digestive tract throughout the year (Study IV). This sampling also took place during a major heat wave, with a large bloom of the toxic cyanobacteria Nodularia spumigena. We confirmed by qPCR that M. balthica does indeed consume N. spumigena, but it does not constitute one of the top phytoplankton taxa consumed despite its abundance (Studies III and IV). The diet plasticity of these two important Baltic detritivore species will be vital for maintaining ecosystem functions as climate change exacerbates eutrophication in the Baltic Sea, shifting the phytoplankton communities, and thus organic matter input to the sediments, from a diatom-dominated to a more cyanobacteria-dominated system. As far as we are aware, these are the first studies to investigate the diet of Baltic Sea benthic detritivores with molecular methods (Studies III and IV). This thesis shows that Baltic Sea benthic invertebrate food webs show some resilience to eutrophication and climate change through high diet plasticity of key detritivorous species, but there are also potential tipping points approaching, as shown by the Skagerrak network deterioration. Studies utilizing combinations of trophic network determination methods on benthic detritivores are recommended. Additionally, continued biodiversity monitoring of these species, with added food web monitoring, is needed to ensure future ecosystem functional stability and resilience.