Spatial and temporal variability of organic matter sources and food web structure across benthic habitats in a low diversity system (southern Baltic Sea)

Understanding the effects of basal resources on the functioning of coastal ecosystems is of great interest in ecology since the different composition and availability of food sources directly affect trophic pathways and energy flow in systems. In this study, we determined which basal resources fuel the food webs of four benthic communities from different habitats in the shallow, low diversity Puck Lagoon (Gulf of Gdańsk, southern Baltic Sea) over a full seasonal cycle. Using δ13C and δ15N values, the relative contribution of various potential carbon sources to the diets of dominant consumers were estimated with a mixing model in R (SIAR). The organic matter pool assimilated by macrofauna included primarily suspended particulate matter (SPOM), phytoplankton, and sediment detritus (SOM) that contained degraded vascular plants and macroalgae in areas with high vegetative biomass. Benthic invertebrates fed mostly on food sources available in their habitats and had species-specific carbon and nitrogen isotope compositions. On sediments with little vegetation, two separate trophic pathways included SPOM, phytoplankton, and suspension feeders, while SOM supported deposit feeders and omnivores in a sheltered environment close to a river mouth. On sediments dominated by Stuckenia, SOM and macroalgae supported the benthic food web that was dominated by opportunistic and tolerant omnivores and herbivores. In contrast, the large biomass of benthic vegetation and high SOM quality on Chara/Cladophora sediment induced high trophic diversity with two main trophic pathways, SPOM and phytoplankton supported suspension feeders, and macroalgae and epiphytes promoted grazers and omnivores. Vascular plants were found not to have been consumed by benthic invertebrates or fishes, but they provided suitable substrata for the macroalgae that developed on their blades and roots.