Caccolithopliorids are important members of phytoplankton communities in most parts of the world's oceans. The formation, release and flux of the calcite scales (coccoliths) of these organisms over millions of years, has led to sedimentary deposits of great geological significance. The reporting of immense blooms of coccolithophorids by satellite photography and their passible involvement in atmospheric sulphur emissions has emphasised the necessity for further information on their distribution and ecology. During an extensive sampling programme in the N.E. Atlantic, water samples were collected throughout the photic zone at 30 stations. Scanning electron microscopy of filtered water samples enabled coccolithophorid family and species distributions to be compiled. It was found that certain species assemblages could be assigned to distinct biogeographic zones and that these zones closely conformed to the movements of the N. Atlantic surface currents. However, at the Azores Front it was shown that a transitional region existed between the cold and warm water masses, where the flora of each zone could be found. Detailed studies of coccolithophorid vertical distribution has shown that most species can be assigned to discrete depth bands within the euphotic zone. Furthermore, it was noticed that in two species which have no depth preference, the degree of calcification increased with depth. One of these species, E. huxleyi, displayed two ecomorphotypic variants differing in their calcification, size and morphology. During two east to west transects it was noticed that the halocaccolitiiophorid numbers increased towards the Mid-Atlantic Ridge. A number of new species are described and current taxonomic problems reviewed.

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Plankton communities are influenced, in part, by water exchange with adjacent estuarine and oceanic ecosystems. Reduced advective transport through tidal passes or with adjacent bay systems can affect chemical processes and biological interactions, such as nutrient cycling, phytoplankton abundance and productivity, community respiration, and zooplankton biovolume. The most threatened estuarine ecosystems are shallow, bar-built, microtidal estuaries with small water volumes and restricted connections through tidal passes and other water exchange points. This research explored spatio-temporal trends in plankton communities and the physicochemical environment in Mesquite Bay, Texas a microtidal, bar-built, subtropical estuary in the Gulf of Mexico. This research couples sampling at fixedstations for multiple physical and biological parameters with high-resolution spatial mapping of physicochemical parameters. Spatial trends were less in magnitude and affected fewer parameters in fixed station and spatial data. Two dimensional ordination plots indicated spatial heterogeneity with a more pronounced temporal trend affecting parameters including temperature, salinity as a function of inflow timing, and seasonal wind direction affecting primary production and zooplankton biovolume. Temperature was positively correlated with gross production and respiration rates during spring and late summer with sporadic positive and negative correlations with phytoplankton biomass. The timing and magnitude of freshwater inflow affected various physicochemical and biological parameters. Higher than 71-year inflow rates resulted in low salinity system wide, with spatial heterogeneity increasing over the course of the study, which was confirmed by spatial maps. Additionally, high inflow rates led to two periods of increased inorganic nutrients and dissolved organic matter. Low salinity periods coincided with persistence of higher turbidity, likely because of decreased sediment flocculation. Gross production was low at this time, and likely from light limitation. Additionally, wind magnitude and direction created spatial heterogeneity in turbidity levels and phytoplankton biomass. Zooplankton biovolume was highest during spring and late summer with high species diversity in total rotifers. Copepod biovolume and phytoplankton biomass were positively correlated. Other zooplankton taxonomic groups exhibited variable correlations with phytoplankton biomass and other taxonomic groups. Further long-term studies are needed to determine interactions of various components of trophic food-webs and account for interannual variability in all system parameters.