North Sea ecosystem change from swimming crabs to seagulls

A recent increase in sea temperature has established a new ecosystem dynamic regime in the North Sea. Climate-induced changes in decapods have played an important role. Here, we reveal a coincident increase in the abundance of swimming crabs and lesser black-backed gull colonies in the North Sea, both in time and in space. Swimming crabs are an important food source for lesser black-backed gulls during the breeding season. Inhabiting the land, but feeding mainly at sea, lesser black-backed gulls provide a link between marine and terrestrial ecosystems, since the bottom-up influence of allochthonous nutrient input from seabirds to coastal soils can structure the terrestrial food web. We, therefore, suggest that climate-driven changes in trophic interactions in the marine food web may also have ensuing ramifications for the coastal ecology of the North Sea.

Spatial patterns and trends in abundance of larval sandeels in the North Sea: 1950-2005

Early recruitment indices based on larval fish data from the Continuous Plankton Recorder (CPR) have the potential to inform stock assessments of Ammodytes marinus in the North Sea. We evaluate whether the CPR data are reliable for sandeel larvae. Spatially, CPR larval data were comparable with catches by dedicated larval samplers (Gulf and bongo nets) during ICES coordinated surveys in 2004 and 2009. ICES data are also used to explore environmental influences on sandeel distributions. Temporally, CPR data correlate with larval data from plankton surveys off Stonehaven (1999–2005), with sandeel 0-group trawl data at the east Fair Isle ground (since 1984), and with recruitment data (since 1983) for the Dogger Banks stock assessment area. Therefore, CPR data may provide an early recruit index of relative abundance for the Dogger Banks assessment area, where the majority of the commercial catch of A. marinus is taken, and the Wee Bankie area that is particularly important for seabird foraging. While warm conditions may stimulate the production of sandeel larvae, their natural mortality is typically greater, in the Dogger Banks and Wadden Sea areas, when the larvae are hatched in warm years and/or with abundant 1-year-old sandeel that are likely to be cannibalistic.

A long-term chlorophyll dataset reveals regime shift in North Sea phytoplankton biomass unconnected to nutrient levels

During the 1980s, a rapid increase in the Phytoplankton Colour Index (PCI), a semiquantitative visual estimate of algal biomass, was observed in the North Sea as part of a regionwide regime shift. Two new data sets created from the relationship between the PCI and SeaWiFS chlorophyll a (Chl a) quantify differences in the previous and current regimes for both the anthropogenically affected coastal North Sea and the comparatively unaffected open North Sea. The new regime maintains a 13% higher Chl a concentration in the open North Sea and a 21% higher concentration in coastal North Sea waters. However, the current regime has lower total nitrogen and total phosphorus concentrations than the previous regime, although the molar N: P ratio in coastal waters is now well above the Redfield ratio and continually increasing. Besides becoming warmer, North Sea waters are also becoming clearer (i.e., less turbid), thereby allowing the normally light-limited coastal phytoplankton to more effectively utilize lower concentrations of nutrients. Linear regression analyses indicate that winter Secchi depth and sea surface temperature are the most important predictors of coastal Chl a, while Atlantic inflow is the best predictor of open Chl a; nutrient concentrations are not a significant predictor in either model. Thus, despite decreasing nutrient concentrations, Chl a continues to increase, suggesting that climatic variability and water transparency may be more important than nutrient concentrations to phytoplankton production at the scale of this study.