Synchronous ecological regime shifts in the central Baltic and the North Sea in the late 1980s

Abstract

The index of the North Atlantic Oscillation, the dominant mode of climatic variability in the North Atlantic region, changed in the late 1980s (1987–1989) from a negative to a positive phase. This led to regime shifts in the ecology of the North Sea (NS) and the central Baltic Sea (CBS), which involved all trophic levels in the pelagial of these two neighbouring continental shelf seas. Increasing air and sea surface temperatures, which affected critical physical and biological processes, were the main direct and indirect driving forces. After 1987, phytoplankton biomass in both systems increased and the growing season was extended. The composition of phyto- and zooplankton communities in both seas changed conspicuously, e.g. dinoflagellate abundance increased and diatom abundance decreased in the CBS. Key copepod species that are essential in fish diets experienced pronounced changes in biomass. Abundance of Calanus finmarchicus (NS) and Pseudocalanus sp. (CBS) fell to low levels, whereas C. helgolandicus (NS) and Temora longicornis and Acartia spp. (CBS) were persistently abundant. These changes in biomass of different copepod species had dramatic consequences on biomass, fisheries, and landings of key fish species: North Sea cod declined, cod in the CBS remained at low levels, and CBS sprat reached unprecedented high biomass levels resulting in high yields. The synchronous regime shifts in NS and CBS resulted in profound changes in both marine ecosystems. However, the reaction of fish populations to the bottom-up mechanisms caused by the same climatic shift was very different for the three fish stocks.

Effects of Altered Offshore Food Webs on Coastal Ecosystems Emphasize the Need for Cross-Ecosystem Management

“…2a), suggesting a link between declining cod populations and the increase in mesopredators. However, general changes in climatic variability have been proposed to have generated changes in the composition of fish communities during the same time period in the North Sea, with increasing temperatures as a main driver (Alheit et al 2005). Temperature is important for the year-class strength of many fish species.…”

“…Stickleback changes distribution over the ontogeny, and after their first summer the majority migrate offshore (unpublished data). Since the 1980s, the offshore food web in the central Baltic Sea has changed dramatically, indicating a strong basin-wide mesopredator release phase (Alheit et al 2005;Ö sterblom et al 2007;Casini et al 2008;Möllmann et al 2008). Initially, the offshore Baltic Sea cod populations declined by 75% during the 1980s, due to climate induced poor recruitment conditions combined with high fishing pressure ( Fig.…”

The influence of temperature on the development of Baltic Sea sprat (Sprattus sprattus) eggs and yolk sac larvae

“…Alheit et al (2005) compared temperature time-series for the Bornholm Basin from 1970 to 1987 and 1988 to 2003 and found an increase in the spring and autumn surface mixed layer temperature by about 1.5°C. On shorter time scales temperatures in the upper halocline, i.e.…”

How cod shapes its world

“…A number of studies (DFO, 2003;Choi et al, 2004;Frank et al, 2005) have ascribed the lack of recovery of NWA cod following the fishing moratorium from 1993 to changes in environmental conditions (temperature, salinity). Similarly, climate-induced changes in hydrography leading to changes in temperature and salinity have been argued to keep cod from recovering in the Baltic Sea (Alheit et al, 2005;Möllmann et al, 2005Möllmann et al, , 2008. These explanations hold bottom-up (environmental) drivers responsible for the sustenance of the new ecosystem state and imply that heavy fishing is merely a reinforcing mechanism: once environmental conditions again change to the better a fishing moratorium will lead to cod recovery.…”