Long-term changes in marine ecosystems

Figure 1: Primary production
Map: Hereon/ Ute Daewel

Long-term variations and major changes in ecosystem dynamics occur throughout all trophic levels and have earlier been reported on in a number of studies for both the North Sea and Baltic Sea system. A majority of those studies have been thereby focussing on potential regime shifts (”Changes in marine system function that are relatively abrupt, persistent, occurring at a large spatial scale, observed at different trophic levels and related to climate forcing.“ deYoung et al., 2004). These changes typically coincides with changes in ecosystem biomass and species composition and are thus of economical relevance for the fisheries sector.

By using the three dimensional coupled physical-biogeochemical model ECOSMO II (Daewel and Schrum, 2013) for the North Sea and Baltic Sea (data available through coastDat) we can identify long-term changes in the physical environment and relate those changes to processes that are relevant for long-term ecosystem variability in the area (Figure 1, Daewel and Schrum, 2017). The analysis of a 61 years (1948-2008) long hind cast reveals a quasi-decadal variation on salinity, temperature and current fields in the North Sea in addition to singular events of major changes during restricted time frames. These changes in hydrodynamic variables where found to be associated to changes in ecosystem productivity that are temporally aligned with the timing of reported “regime shifts” in the areas. Especially in the North Sea a correlation analysis between atmospheric forcing and primary production reveals significant correlations for the North Atlantic Oscillation and wind forcing for the central part of the region (see figure), while the Atlantic Multidecadal Oscillation and air temperature are correlated to long-term changes in the southern North Sea frontal areas. Understanding those processes is a prerequisite for reliable projections of ecosystem dynamics under climate change conditions.

Long-term changes in Net Primary Production

Graph: Hereon/ Ute Daewel

Graph: Hereon/ Ute Daewel

Dominant mode of long term changes in simulated net primary production of the North Sea and Baltic Sea from ECOSMO II. The dominant modes were estimated using an Empirical Orthogonal Function analysis (first principal component).