Natural phytoplankton assemblages from the Patuxent River estuary were maintained in outdoor, large-volume, continuous cultures and subjected to nutrient enrichment in a series of experiments over an 18-mo period. Nitrogen (either ammonium or nitrate) enrichment during summer and fall produced large changes in dominant species and patterns of species successions, yielding an assemblage much different from unenriched controls: strong, continued dominance by small centric diatoms at the expense of predominant flagellate species. Succession of dominant species occurred very rapidly, within 2–4 d after enrichment began. Increases in cell densities and the changes in successional patterns were caused by increased diatom growth rates rather than reduced growth rates of other dominant species. During winter and spring, N enrichment did not cause a response in the phytoplankton assemblage. Twice during winter experiments, assemblages exhibited a weak, delayed response to P; however, P enrichment did not affect individual species' growth rates or species succession at any time of year. Thus, further enrichment can affect phytoplankton growth and community structure, even in nutrient-enriched estuaries.

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Long-term experiments performed with large-volume continuous cultures of natural phytoplankton assemblages exposed to low levels of arsenate (1 to 10 X ambient concentrations) have shown that arsenate is differentially inhibiting to some phytoplankton species. This observed variance in sensitivity is sufficient to cause a marked change in species composition and succession of dominant species in arsenic-treated assemblages with a potential for impact upon carbon transfer between trophic levels, even though estimates of community biomass and productivity indicate no change.