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

Abstract

Throughout 2003 and 2004 continuous autonomous observations of surface
temperature, conductivity and chlorophyll fluorescence were recorded on the P&O
"Pride of Bilbao" ferry between Portsmouth, UK and Bilbao Spain. Different conditions
over the shelf, slope and deep waters along the route were detected and studied - from
eutrophic harbour waters to the southern Bay of Biscay, which is oligotrophic in
summer. During the two years, 21 manned crossings on the ferry provided information
on nutrients, phytoplankton biomass and speciation. Measurements include chlorophyll
a concentrations (calibration of the fluorimeter is discussed), High Performance Liquid
Chromatography (HPLC) determined pigments concentrations, phytoplankton species
abundance (biomass and identification) and nutrients. Data was also available from
satellite images for estimates of chlorophyll (SeaWIFS), zooplankton abundance from
CPR (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 basis
of 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 Biscay
water. There were differences between the two years in the timing of seasonal changes
and in the abundance of phytoplankton species. Chlorophyll a values were generally
higher in shelf waters compared to oceanic water in both years.
The spring phytoplankton bloom reached its peak during March 2003 in the Bay of
Biscay 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 have
delayed growth in this region. In the shelf regions in 2004 low salinity values off
western France and high solar irradiance in the north are likely to have led to earlier
phytoplankton biomass than in 2003. During early summer, the coccolithophore,
Emiliania huxlyei (>1000 cells ml-1) was widespread in northern stratified regions, more
so in 2003 than in 2004. In the summer of 2003 an exceptional dinoflagellate bloom
occurred in the western English Channel. The bloom was composed of a monspecific
surface population of Karenia mikimotoi, giving cell densities up to 8000 cells ml-1 and
chlorophyll a concentrations up to 70 mg m-3. Development of this dinoflagellate bloom
in the western English Channel could be explained in terms of physical stability, and
ii
low wind speed together with sufficient light and a supply of inorganic nutrients
favouring growth of the cells. By contrast, in 2004, the abundance of diatoms was
higher 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 final
stage in the succession, as the summer thermocline was less well developed. The
phytoplankton biomass and composition in 2003 matches the classical model of
phytoplankton 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 successful
and improved understanding of the relationship between the phytoplankton population
and hydrographic regimes in 2003 and 2004 between Portsmouth and Bilbao.
Improvements in future might include continuous observations of oxygen and nutrients
and more work can be done to link FerryBox data, satellite and CPR based
observations.

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