Effects of increasing atmospheric CO2 on phytoplankton communities and the biological carbon pump

In addition, visual predators are generally less abundant in extremes of low temperature. The appropriate combination of conditions is rarely found in shelf sea waters, and the majority of the overwintering stock resides at depths of 600-2000m in the open ocean, especially in the Norwegian Sea and, it is assumed, in the Labrador/ Irminger Sea in the northwest Atlantic. In the Norwegian Sea, these depths correspond to temperatures less than 1°C. Modelling and field observations have shown that most of the productive populations in shelf seas are sustained by annual recolonisation from these oceanic overwintering stocks. This explains why the long-term trends in abundance in, for example, the North Sea appear to be driven by the changes occurring over the wider northeast Atlantic (Figure 3). So, the issue resolves to what is driving the basin scale changes in C. finmarchicus abundance in the ocean? One theory is that changes in the deep circulation of the North Atlantic and the availability of overwintering habitat in deep water masses are playing a large role. Data from the Faroe-Shetland Channel, which lies between the Faroe Islands and the Shetland Islands off northern Scotland, show that between October and March, dense concentrations of overwintering C. finmarchicus are found in the deep (>600m) cold overflow of Norwegian Sea Deep Water into the Atlantic. This overflow is part of the thermohaline circulation system which ventilates the global oceans. Norwegian Sea Deep Water is generated by deep convection in the Greenland Sea, and as this process has slowed down in concert with the rise in the NAO since the 1960s, so the volume of Deep Water in the Faroe-Shetland Channel has decreased. In consequence, the abundance of overwintering stock available to colonise the North Sea each spring has also declined, thus providing a possible explanation for the Continuous Plankton recorder observation in the North Sea. Are similar relationships responsible for the long-term dynamics in the North Atlantic as a whole? A detailed modelling and observation programme in the North Atlantic basin scale will be getting underway in late 2001 to address this question. The work will be a collaboration between the UK-GLOBEC programme and GLOBEC research programmes in Canada, Iceland, Norway and USA.