| Summary: | Planktonic organisms, representing the lowest trophic levels, are the most likely organisms to respond rapidly to climate change, due to their reduced generation times. It can be hypothesized that they are therefore more prone to exhibiting changes due to hydro-climatic variations. By using data from the Continuous Plankton Recorder survey from the northwest Atlantic (1962-2000). long term trends in both phyto- and zooplankton organisms for the area can be examined. The northwest Atlantic ecosystem is hydrographically complex (due to a series of thermohaline and wind-driven currents forming cold / warm water, surface / deep-water interactions) and economically important (due to the fisheries). The area is subject to large-scale hydro-climatic processes, such as the North Atlantic Oscillation and it is suggested that such processes have the ability to perturb marine ecosystem dynamics. The plankton community of the northwest Atlantic represents a large section of the ecosystem that has been little studied, especially when compared with the work that has taken place on the fisheries in the area. Results from the study have shown that arctic-boreal species of both zoo- and phytoplankton have responded to hydro-climatic changes in the past decade. The calanoid Calanus hypertoreus increased in abundance (and to a lesser extent so did Calanus gtacialis) and occurred over a wider range in the late 1990s than had previously been reported. Phytoplankton Colour, an indication of phytoplankton biomass, increased during the mid to late 1990s. On closer examination of the phytoplankton community, the greatest change was found in the dinofiageltate Ceratium arcticum. This arctic boreal species not only increased in abundance, but its seasonal cycle also changed, with winter blooms becoming more frequent. Over the same time period in the study area the water of the region became more stratified. In addition to the response of endemic plankton taxa, over the past few years a number of invasive species have become more prevalent, notably the diatom Neodenticula seminae and the ciliate Zoothamnium pelagicum. It is suggested that large scale climate changes have altered the hydro-climatic processes of the region, with fresher, colder conditions benefiting arctic-boreal planktonic organisms and either changes in water mass movements or anthropogenic introductions favouring the ongoing prevalence of the Invasive taxa. It is likely that these changes to the lowest trophic levels have had a 'bottom-up' effect on the higher organisms of the local ecosystem.
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