I investigated spatial and temporal variability in benthic and pelagic (phytoplankton) microalgal assemblages in 2 harbours and a coastal embayment off the east coast of the North Island of New Zealand in late spring, summer and autumn. For the first time pelagic microalgal assemblages were sampled just above the sediment in the layer from which suspension feeders are likely to feed (~10 cm). Benthic microalgal assemblages were also assessed within the sediments. Strong links were observed between pelagic and benthic microalgal populations, with the amount of benthic microalgae found in suspension correlated with current speed and turbidity. Conversely, a number of species found within the benthic counts were tychopelagic species whose life habit is to rely on currents which re-suspend them from the benthic to pelagic environment. In late spring, pelagic water samples from 1 harbour and the embayment it opened into were dominated by dinoflagellate taxa, while diatoms dominated the second harbour location. In autumn, all the locations were dominated by diatom taxa but the 2 harbours were most similar in community composition. In summer, local conditions favoured the development of different microalgal populations in each location. Both pelagic and benthic microalgal assemblages generally decreased in similarity as spatial scale increased, with populations being most similar within a sampling site and least similar between locations. Benthic microalgae assemblages were more variable than pelagic assemblages within and between sampling sites at the locations, but had a greater similarity between locations (40 to 64%). Our results provide a new insight into the scales of variability of microalgal populations within and between different nearshore environments, and identify some of the forcing factors that drive changes in microalgal composition. By sampling microalgal populations just above the sediment we have begun to characterise the microalgal communities that sustain suspension feeders.
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