Long-term shifts in marine ecosystem functioning detected by inverse modeling of the Helgoland Roads time-series

Abstract

Over the past decades, the North Sea ecosystem has been subjected to long-term variations of key natural as well as anthropogenic factors. Decadal trends in eutrophication or of the North Atlantic climate system presumably led to changes in extensive ecosystem variables such as algal biomasses or nutrient concentrations. These are investigated in great detail as part of few monitoring programmes such as the long-term time-series at Helgoland Roads (HR) within in the German Bight. In order to study the more relevant changes in ecosystem functioning as a response to external forcings and to assess the endogenous dynamics of the system, a new combination of modeling methods is proposed. The approach is based on a comprehensive marine food-web model, the European Regional Seas Ecosystem Model (ERSEM) together with an inverse modeling procedure. After integrating the HR time-series together with other environmental data relevant to the German Bight, long-term trends in the calibrated functional characteristics are unequivocally revealed. Apparently, the ecosystem evolves using a small number of distinct regulation modes, as reflected by a sequential timing of transitions in single ERSEM parameters. Typical transition times are estimated to be about 3–6 years. The most prominent shifts are observed for the maximal growth rates of all phytoplankton groups, several diatom growth parameters and for the specific nitrification rate. While for many sites within the North Sea including HR, significant alterations in nutrient and biomass levels were repeatedly reported in the early 1980s, major internal adaptations detected by the inverse method accumulate during the late 1960s and mid-1990s. Hence, one has to differentiate between the long-term evolution of standing stocks and the evolution of ecophysiological traits. First indications of the subtle influence of North Atlantic climate on the southern North Sea ecosystem are discussed. The study also illuminates several limitations together with potential improvements of marine ecosystem models.