Polychaete community responses to two decades of oceanographic change in Isfjorden, Svalbard

Climate-driven Atlantification is rapidly transforming Arctic fjord ecosystems, yet the responses of benthic communities to these changes remain poorly quantified. We examined polychaete assemblages in Isfjorden, Svalbard's largest fjord system, across four climatically distinct years (2002, 2015, 2019, and 2021) spanning two decades of oceanographic transition. Community structure exhibited striking interannual variability, with 2015 emerging as a period of exceptional benthic productivity. Mean abundance peaked at 5266 ± 884 ind. m^-2 (1340-10,350 ind. m^-2), 2.7-fold higher than 2019, while mean biomass reached 101.1 ± 31.4 g m^-2 (maximum: 345.5 g m^-2), 3.7-fold higher than 2019. Core taxa (Lumbrineridae, Maldane sarsi, Leitoscoloplos acutus, Cirratulidae) dominated across all years, but their relative contributions shifted markedly between climatic regimes, with Maldane sarsi comprising 41.5% of biomass in 2015 versus 19.2% in 2002. Redundancy analysis identified sea surface temperature trends as the dominant driver, explaining up to 21.6% of abundance variation, with optimal prediction achieved using 7-year antecedent windows. Generalized linear models revealed divergent response times: longer windows (5-7 years) best predicted abundance, while shorter windows (3-4 years) best predicted biomass, reflecting differential integration of environmental signals into population dynamics versus somatic growth. Our findings demonstrate that polychaete communities function as sensitive, multi-year integrators of climate forcing, providing a powerful tool for detecting and predicting ecosystem responses to ongoing Atlantification.