Complex interactions between nutrient enrichment and zooplankton in regulating estuarine phytoplankton assemblages: Microcosm experiments informed by an environmental dataset

• Effects of grazers and nutrients on phytoplankton were investigated using microcosms.
• Microcosm design was informed by results of a multiyear monitoring study.
• Nutrients had a greater impact on phytoplankton when mesozooplankton were removed.
• Enrichment with iron alone resulted in significant increases in diatoms.
• Enrichment with iron and Si:N < 1 resulted in significant increases in dinoflagellates.

The interactive effects of nutrients and zooplankton grazing on phytoplankton assemblage composition and bloom formation are not well understood, especially in coastal ecosystems. Therefore, the objective of this study was to evaluate phytoplankton assemblage responses to changing environmental conditions in Raritan Bay, an estuary between the states of New York and New Jersey with a long history of cultural eutrophication and harmful algal blooms (HABs). Environmental monitoring of water quality and plankton species composition (monthly data collected from April 2010–April 2013), multivariate ordination techniques, and microcosm experiments were integrated to achieve this objective. Multivariate analysis of the monitoring dataset led to the generation of a number of hypotheses regarding phytoplankton composition, individual nuisance species, environmental factors, and zooplankton composition. The field observations were then supplemented with a series of controlled microcosm experiments designed to test those hypotheses. In particular, the effects and interaction of varying mesozooplankton abundance (sieved, unsieved and enriched) and enrichment with two nutrients (nitrate and dissolved iron) on spring and summer phytoplankton assemblages were evaluated. The environmental monitoring data and the results of the microcosm experiments both indicate that Si:N ratios are important factors governing phytoplankton dynamics in Raritan Bay and that effects of nutrient enrichment on phytoplankton species composition are magnified when mesozooplankton abundance is low. The microcosm experiments, however, led to unexpected results regarding the influence of iron on phytoplankton assemblages. Iron did not emerge as one of the environmental parameters related to spring and summer phytoplankton species composition in the multivariate analysis of field data. Yet, enrichment with iron in microcosms resulted in significant increases in diatoms, and enrichment with both iron and nitrate resulted in significant increases in dinoflagellates and HAB taxa, including Heterocapsa triquetra (Ehrenberg) Stein and Dinophysis spp. This finding suggests that small pulses of iron, in combination with low Si:N ratios (< 1) and low mesozooplankton abundance, would shift phytoplankton assemblages in Raritan Bay toward greater dinoflagellate dominance.