Seasonal and spatial variations in species composition, abundance, biomass and primary production of phytoplankton in Southampton Water, U.K.

Abstract

The seasonal and spatial variations of species composition, abundance, biomass and primary production of phytoplankton in Southampton Water were investigated. The study included two contrasting segments of Southampton Water, NW Netley (mid-estuary) and Calshot (mouth of the estuary). The first phase of the research project involved detailed enumeration of the species present in the phytoplankton samples collected from the two sites at monthly intervals during winter and autumn and 3 to 5 times a month during spring and summer in 1988. The phytoplankton community of Southampton Water showed differences in species composition and dominance between periods of cold turbulent water and warm, relatively calm water column. The winter-spring phytoplankton communities consisted mainly of diatoms at both sites while in summer dinoflagellates and Mesodinium rubrum dominated at NW Netley. The biomass indicator, chlorphyll a showed considerable seasonal and spatial (both vertical and horizontal) variations in the estuary. Chlorophyll a varied from a low winter value of < 2 jig 1"' to the summer maximum of 73 /xg I"1. In the second phase of the study, seasonal changes in the contribution of four size fractions ( > 10, 3-10, 1-3 and 0.2-1 fim ) to the total phytoplankton chlorophyll a and primary production were investigated during 1990. The present observation indicates that for most of the annual cycle of phytoplankton, the microplankton (> 10 /un), which was dominated primarily by species of > 20 /iin dimensions, is the most important fraction. This size fraction accounted, on average, for 66 and 67 of the productivity and 63 and 66 % of chlorophyll a at NW Netley and Calshot respectively. The seasonal changes in the relative contribution of the four fractions to the total primary production in Southampton Water were mainly a function of the variation in biomass rather than physiological differences related to size. A one-day investigation into the size distribution of chlorophyll a relative to salinity showed the existence of considerable horizontal variations. Concentration of total chlorphyl) a and chlorophyll a in the > 10 fim fraction increased with an increase in salinity while chlorophyll a in the lower size fractions was indirectly related to salinity. Although the factors that are responsible for the seasonal changes in the biomass and primary production of phytoplankton in Southampton Water are diverse, light and temperature seem to be of overriding importance. In the third phase of the present study, the growth responses of phytoplankton dominant in Southampton Water to different light and temperature regimes were investigated in the laboratory with a view to generate data sets that may help explain the sequence of species succession observed in the field. The results demonstrate that the success of S. costatum and T. rotula as dominant species of the winter-spring phytoplankton blooms resulted from their adaptation to low light and temperature conditions. Their higher growth rates and shorter lag phases enable them to outcompete other diatoms and dinoflagellates and develop into blooms. Finally, comparison of the present study area with many other estuarine environments were made and possible factors influencing phytoplankton biomass and primary production discussed.

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