Temporal development of coastal ecosystems in the Baltic Sea over the past two decades

201

Climate change in the Baltic Sea region : a summary

Based on the Baltic Earth Assessment Reports of this thematic issue in Earth System Dynamics and recent peer-reviewed literature, current knowledge of the effects of global warming on past and future changes in climate of the Baltic Sea region is summarised and assessed. The study is an update of the Second Assessment of Climate Change (BACC II) published in 2015 and focuses on the atmosphere, land, cryosphere, ocean, sediments, and the terrestrial and marine biosphere. Based on the summaries of the recent knowledge gained in palaeo-, historical, and future regional climate research, we find that the main conclusions from earlier assessments still remain valid. However, new long-term, homogenous observational records, for example, for Scandinavian glacier inventories, sea-level-driven saltwater inflows, so-called Major Baltic Inflows, and phytoplankton species distribution, and new scenario simulations with improved models, for example, for glaciers, lake ice, and marine food web, have become available. In many cases, uncertainties can now be better estimated than before because more models were included in the ensembles, especially for the Baltic Sea. With the help of coupled models, feedbacks between several components of the Earth system have been studied, and multiple driver studies were performed, e.g. projections of the food web that include fisheries, eutrophication, and climate change. New datasets and projections have led to a revised understanding of changes in some variables such as salinity. Furthermore, it has become evident that natural variability, in particular for the ocean on multidecadal timescales, is greater than previously estimated, challenging our ability to detect observed and projected changes in climate. In this context, the first palaeoclimate simulations regionalised for the Baltic Sea region are instructive. Hence, estimated uncertainties for the projections of many variables increased. In addition to the well-known influence of the North Atlantic Oscillation, it was found that also other low-frequency modes of internal variability, such as the Atlantic Multidecadal Variability, have profound effects on the climate of the Baltic Sea region. Challenges were also identified, such as the systematic discrepancy between future cloudiness trends in global and regional models and the difficulty of confidently attributing large observed changes in marine ecosystems to climate change. Finally, we compare our results with other coastal sea assessments, such as the North Sea Region Climate Change Assessment (NOSCCA), and find that the effects of climate change on the Baltic Sea differ from those on the North Sea, since Baltic Sea oceanography and ecosystems are very different from other coastal seas such as the North Sea. While the North Sea dynamics are dominated by tides, the Baltic Sea is characterised by brackish water, a perennial vertical stratification in the southern subbasins, and a seasonal sea ice cover in the northern subbasins.Peer reviewe

Two-way coupling between Ecosim (EwE-F) and a biogeochemical model of the Baltic Sea

The Baltic Sea is heavily affected by eutrophication, with ambitious nutrient load reduction schemes inplace to reduce algal growth and the secondary effects of increased productivity, such as increasedturbidity and the spread of anoxic bottoms. Forcing an Ecosim model of the Central Baltic food web withobserved and predicted productivity trends suggest that changes in primary production channel stronglyinto the pelagic food web, modifying the predation pressure on phytoplankton grazers.To include the feedback of higher trophic levels onto primary producers into simulations, we coupled theFortran version of Ecopath with Ecosim (EwE-F) to the BALTSEM biogeochemical model of the BalticSea. In their coupled code, both models share phytoplankton and detritus components: BALTSEMcalculates phytoplankton growth based on nutrient concentrations and available light; and as a loss term,EwE-F provides predation mortality of phytoplankton that is calculated applying the foraging arena theory.Similarly, EwE-F components produce inputs to BALTSEM sediment and suspended detritus variables,such as non-predation mortality or unassimilated food, and use the BALTSEM detritus variables as a foodresource.Since nutrient turnover within the Baltic pelagic food web is about ten times as large as the riverine inputof nitrogen and phosphorus, small imbalances in simulated nutrient fluxes can generate sources or sinksthat easily reach the magnitude of planned nutrient load reductions. To ensure closed nutrient cycles, wetrack the nutrient intake for each consumer based on fixed prey stoichiometries. We have introducedcompensatory nitrogen and phosphorus fluxes into EwE-F that describe the excretion of nutrients that arenot incorporated into predator growth, and included nutrient limitation functions for EwE-F consumers.Coupling the zero-dimensional EwE-F food web model to the one-dimensional, vertically resolvedBALTSEM biogeochemical model creates vertical fluxes between the BALTSEM depth layers. We willtherefore also discuss strategies to describe the transfer of carbon and nutrients between the food weband the biogeochemical model