Long term changes in the North Sea ecosystem
Long term data on the North Sea ecosystem are available for phytoplanktonic, zooplanktonic, berithic, fish, and seabird communities. Temporal changes in these have been examined by numerous researchers over the course of the 20th century, their main objective being to determine how the interannual dynamics of these communities are regulated. This study considers the long term ecosystem dynamics, and the mechanisms behind these dynamics, for the North Sea over the latter half of the 20th century, although it focuses upon the time series operated by the Dove Marine Laboratory, and the Continuous Plankton Recorder (CPR) survey in the central-west North Sea region. A review of the literature suggests that long term changes across the North Sea are ultimately driven by two processes. In the northern, western and central areas of the North Sea, the ecosystem is climatically driven, whilst in the southern and eastern regions, the signal of climate is masked by the large arithropogenic nutrient inputs into these regions. A comparison of the Dove and CPR zooplankton time series for the central-west North Sea area found that although their relative year to year fluctuations were similar, large differences were present in the absolute abundances recorded. Model derived catching efficiencies for the two sampling devices suggested that differences in absolute abundances were mainly due in some zooplankton taxa to a greater degree of active avoidance of the CPR sampling device. Further examination of these two series found that the long term zooplankton trends in the central-west North Sea were dissimilar to those observed for other North Sea regions. Inverse relationships between zooplankton abundance, and the position of the Gulf Stream North Wall, and with air temperatures were also observed. These dissimilar trends and inverse relationships were ultimately found to be due to the presence of an internal predation based mechanism. Cmatic influences were also found to indirectly influence the long term dynamics of the benthos in the central-west North Sea The primary factor influencing interannual variation in benthic abundance was phytoplankton productivity (Le. food), which in turn was related to climatic factors. However, at a second central-west North Sea benthic station, situated within a Nephmps norvegicus fishing ground, constant trawling disturbance of the benthos was the primary factor influencing both benthic abundance and species composition. Over the latter half of the 2O1 century, air temperatures and daily sunshine durations have increased in the central-west North Sea region, alongside changes in climatic proxy variables (e.g. NAO index). However, similar trends were not always observed in the biota. Ultimately, the long term dynamics of taxa, communities and ecosystems may be due to direct or indirect factors, yet interactions between a diversiy of internal and external factors, results in the complex behaviour of biological systems over time. This study shows that, a though the centralwest North Sea ecosystem is climatically driven, similar trends between climate and ecosystem components do not necessarily exist, and nor considenng the high comp exity of the ecosystem, should they be expected.