An investigation of factors influencing the spatial and temporal distribution of surface phytoplankton in the English Channel and Bay of Biscay in 2003 and 2004

摘要

Throughout 2003 and 2004 continuous autonomous observations of surfacetemperature, conductivity and chlorophyll fluorescence were recorded on the P&O"Pride of Bilbao" ferry between Portsmouth, UK and Bilbao Spain. Different conditionsover the shelf, slope and deep waters along the route were detected and studied - fromeutrophic harbour waters to the southern Bay of Biscay, which is oligotrophic insummer. During the two years, 21 manned crossings on the ferry provided informationon nutrients, phytoplankton biomass and speciation. Measurements include chlorophylla concentrations (calibration of the fluorimeter is discussed), High Performance LiquidChromatography (HPLC) determined pigments concentrations, phytoplankton speciesabundance (biomass and identification) and nutrients. Data was also available fromsatellite images for estimates of chlorophyll (SeaWIFS), zooplankton abundance fromCPR (Continuous Plankton Recorder) tows, wind speed and direction and irradiance.This has enabled phytoplankton populations to be related to-: (1) nutrient supply (2)grazing pressure (3) fresh water influences, (4) hydrography and (5) climatic conditions.The distributions of hydrographic properties and of plankton were analysed on the basisof four generalised regions (i) well-mixed, (ii) northern summer stratified, and (iii)southern summer stratified on the shelf and (iv) oceanic region in the Bay of Biscaywater. There were differences between the two years in the timing of seasonal changesand in the abundance of phytoplankton species. Chlorophyll a values were generallyhigher in shelf waters compared to oceanic water in both years.The spring phytoplankton bloom reached its peak during March 2003 in the Bay ofBiscay and during April on the continental shelf with maximum chlorophyll values of 2-4 mg m-3. Whereas, the strong SW wind in Bay of Biscay in winter 2004 may havedelayed growth in this region. In the shelf regions in 2004 low salinity values offwestern France and high solar irradiance in the north are likely to have led to earlierphytoplankton biomass than in 2003. During early summer, the coccolithophore,Emiliania huxlyei (1000 cells ml-1) was widespread in northern stratified regions, moreso in 2003 than in 2004. In the summer of 2003 an exceptional dinoflagellate bloomoccurred in the western English Channel. The bloom was composed of a monspecificsurface population of Karenia mikimotoi, giving cell densities up to 8000 cells ml-1 andchlorophyll a concentrations up to 70 mg m-3. Development of this dinoflagellate bloomin the western English Channel could be explained in terms of physical stability, andiilow wind speed together with sufficient light and a supply of inorganic nutrientsfavouring growth of the cells. By contrast, in 2004, the abundance of diatoms washigher than 2003 and K. mikimotoi was common but not at bloom levels (chlorophyll a~4.0 mg m-3). A mixed diatom-dinoflagellate community was the dominant the finalstage in the succession, as the summer thermocline was less well developed. Thephytoplankton biomass and composition in 2003 matches the classical model ofphytoplankton seasonal succession in temperate waters (Margalef, 1978; Smayda, 1980)but this was not obvious in 2004.In general, the FerryBox system on the Pride of Bilbao in 2003 and 2004 was successfuland improved understanding of the relationship between the phytoplankton populationand hydrographic regimes in 2003 and 2004 between Portsmouth and Bilbao.Improvements in future might include continuous observations of oxygen and nutrientsand more work can be done to link FerryBox data, satellite and CPR basedobservations.