No abstract

Sampling by the Continuous Plankton Recorder (CPR) over the NW Atlantic from 1960 to 2000 has enabled long-term studies of the larger components of the phytoplankton community, highlighting various changes, particularly during the 1990s. Analysis of an index of phytoplankton biomass, the Phytoplankton Colour Index (PCI) has revealed an increase over the past decade, most marked during the winter (December to February) months. Examination of the structure of the community using multiple linear-regression models indicates that the winter phytoplankton community composition has changed markedly in the 1990s compared to the 1960s. One phytoplankter, the dinoflagellate Ceratium arcticum (Cleve), has undergone dramatic changes in abundance during this period, with pronounced large winter blooms and decreased autumnal levels, and its contribution to the Phytoplankton Colour index values has increased significantly. Other dominant species in the phytoplankton community, both diatoms and dinoflagellates, did not show the same variations over the examined time period. It is suggested that the response of C. arcticum is probably a result of previously reported changes in stratification in the NW Atlantic, due to dynamic hydro-climatic (freshening and cooling) events.

During exposure to severe hypoxia (<20 Torr [2.7 x 103 Pal) under both laboratory and field conhtions, prawns Palaemon elegans Rathke frequently exhibited a parbal-emersion response dunng which they moved into shallow water and became quiescent, usually lying on their sides at the water's edge. The Poz at which this behavioural response occurred appeared to be dependent on temperature; at higher temperatures the prawns exhibited this response at higher oxygen tensions. When exposed to anoxic conditions in the laboratory, P. elegans sometimes left the water and emerged onto boulders. Investigations of the physiological advantages of the partial-emersion response demonstrated that the total oxygen content of the blood was significantly higher and the concentration of Llactate lower than that of either fully immersed or totally emersed prawns It appears, therefore, that P. elegans is able to exploit the higher oxygen tensions at the air/water interface and can meet most of its metabolic demands via aerobic metabolism. Although prawns which emerge completely from the water when exposed to anoxia may be able to utilize air as a source of oxygen, they are faced with problems of acid-base disturbance In the blood due to the difficulties of CO2 excretion. In these prawns, the total CO2 content of the blood was significantly higher and the pH lower than that of partially-emersed prawns. By remaining in contact with the water, partially-emersed prawns gain an important respiratory advantage and are able to reduce the problems both of acid-base disturbance and of desiccation during total emersion.