Impact of multiple drivers on the trophic position, functional diversity, and ecological memory of benthic macrofauna – analysis of 40 years of data using a complex model hierarchy

• Climate regime shifts do not necessarily cause biological regime shifts. • Linear forcing with multiple drivers can cause abrupt biological regime shifts. • Riverine nutrient inputs is modulated by sewage treatment plants and variable precipitation. • Stable isotope values of two macrofaunal species indicated specific feeding modes. • A biological memory of 3 years was due to an internal North Atlantic climate mode. This study analyzed the potential influence of multiple drivers, such as climate variability and regime shifts, on benthic life using long-term data sets describing the abundance, biomass, and stable isotopes (1978–2017) of two benthic species in the southern North Sea, generated from preserved samples. Specifically, changes in nitrogen supply and trophic position were identified by bulk and amino-acid-specific isotope analyses of the native warm–temperate bivalve Fabulina fabula and the native cold–temperate polychaete Magelona spp., which together made up > 60 % of the biomass and abundance of all benthic animals in the original samples. Bulk stable isotopes ratios of total carbon and nitrogen as well as amino-acid specific-isotope ratios were corrected with respect to the preservation method. Statistical downscaling and a scanning t -test were applied to various time series of climate, Rhine and Maas riverine runoff, and local monitoring at the island of Norderney. The scanning t -test identified three regime shifts in the drivers and macrofaunal responses, which allowed four regimes, occurring during the periods 1978–1988, 1989–2000, 2001–2009, and 2010–2017, to be distinguished. Quantitative metrics using a phenotype-based approach were computed for all data and for the four identified regimes, to characterize aspects of the trophic position and functional diversity of the two macrofaunal species. Functional diversity in a single species decreased over time, indicating a normalization of feeding habits and increased productivity under decreasing nutrient loads, a shift in biomass from specialist to generalist, and an increase in stability and resilience after 2000. Based on the de-correlation time, an ecological memory of the system of ∼ 3 years was identified for F. fabula and Magelona spp., attributable to an internal basin mode in climate variability driven by atmosphere–ocean interactions in the North Atlantic.