Zooplankton Community Grazing Impact on a Toxic Bloom of Alexandrium fundyense in the Nauset Marsh System, Cape Cod, Massachusetts, USA.

Embayments and salt ponds along the coast of Massachusetts can host localized blooms of the toxic dinoflagellate Alexandrium fundyense. One such system, exhibiting a long history of toxicity and annual closures of shellfish beds, is the Nauset Marsh System (NMS) on Cape Cod. In order measure net growth rates of natural A. fundyense populations in the NMS during spring 2012, incubation experiments were conducted on seawater samples from two salt ponds within the NMS (Salt Pond and Mill Pond). Seawater samples containing natural populations of grazers and A. fundyense were incubated at ambient temperatures. Concentrations of A. fundyense after incubations were compared to initial abundances to determine net increases from population growth, or decreases presumed to be primarily due to grazing losses. Abundances of both microzooplankton (ciliates, rotifers, copepod nauplii and heterotrophic dinoflagellates) and mesozooplankton (copepodites and adult copepods, marine cladocerans, and meroplankton) grazers were also determined. This study documented net growth rates that were highly variable throughout the bloom, calculated from weekly bloom cell counts from the start of sampling to bloom peak in both ponds (Mill Pond range = 0.12 - 0.46 d-1; Salt Pond range = -0.02 - 0.44 d-1). Microzooplankton grazers that were observed with ingested A. fundyense cells included polychaete larvae, rotifers, tintinnids, and heterotrophic dinoflagellates of the genera Polykrikos and Gymnodinium. Significant A. fundyense net growth was observed in two incubation experiments, and only a single experiment exhibited significant population losses. For the majority of experiments, due to high variability in data, net changes in A. fundyense abundance were not significant after the 24-hr incubations. However, experimental net growth rates through bloom peak were not statistically distinguishable from estimated long-term average net growth rates of natural populations in each pond (Mill Pond = 0.27 d-1 and Salt Pond = 0.20 d-1), which led to peak bloom concentrations on the order of 106 cells l-1 in both ponds. Experimental net growth rates from the incubations underestimated the observed natural net growth rates at several time intervals prior to bloom peak, which may indicate that natural populations experienced additional sources of vegetative cells or periods of reduced losses that the 24-hr incubation experiments did not capture, or that the experimental procedure introduced containment artifacts.