Estimating effects of river runoff, predation, and fish carcasses on intertidal macrobenthic community and food-web structure in a sub-Arctic estuary

Abstract

Coastal benthic organisms play an essential role in many ecosystem functions, such as organic matter utilization and regeneration of nutrients. In northern Norway, coastal ecosystems experience snowmelt-induced increased river runoff in early summer, arrival of poleward migrating predators, and invasive pink salmon runs, potentially affecting macrobenthic communities and food-web structures. This study investigated the impact of river runoff, predation, and carcasses of migratory fish on intertidal benthic macrofauna community and food-web composition in a fjord in northern Norway. Cage experiments were conducted at two sites, one more river-influenced (river site) and the other less river-influenced (marine site), from June (during increased river runoff) until fall (end of September/beginning of October, after increased river runoff) 2023. The total abundance and biomass, measures of biodiversity (taxon richness, Shannon index, evenness), and community structure of the macrobenthos were recorded for community analysis. Additionally, stable isotopes (d13C and d15N) of the five most abundant taxa (Oligochaeta indet., Macoma calcarea, Spionidae indet., Euchone sp., and Chironomidae indet.) were analyzed to identify changes in food-web composition. In fall, total benthic biomass showed an increase (marine site) and a decrease (river site) compared to June, while total abundance, measures of diversity, and community structure remained unchanged. The d13C values increased for several taxa from June to fall, indicating a shift in carbon sources from more terrestrial organic matter sources in June to marine organic matter in fall. The exclusion of predators increased total biomass at both sites and total abundance at the river site. Adding fish to simulate organic input by carcasses of migratory salmon did not influence the macrobenthos total abundance, biomass, or biodiversity, but fish was included to varying degrees into the diet of all taxa (indicated by higher d15N values in fall) at both sites. The study showed that river runoff, as well as predation and fish carcasses, have limited – yet some – effects on the benthic macrofauna community and food web.