Zooplankton community grazing impact on a toxic bloom of Alexandrium fundyense in the Nauset Marsh System, Cape Cod, Massachusetts, USA

Abstract

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.

Rapid growth and concerted sexual transitions by a bloom of the harmful dinoflagellate Alexandrium fundyense (Dinophyceae)

“…Assuming all of these transformed to new cysts, average deposition on the pond's bottom would be approximately 15,000 cysts cm 2 , comparable to the cyst deposition observed after the Salt Pond bloom in 2013, a year when peak cell concentrations again were > 10 6 cells L −1 (Brosnahan, Fischer, and Anderson unpubl.). Because the bloom's rate of decline during its zygotic phase (−1.0 d −1 ) far exceeded estimates of overall losses due to grazing and parasitism (approximately −0.3 d −1 ; Petitpas et al ), encystment was likely the dominant factor leading to bloom termination. We are unable to confirm this conclusion, however, because no comparable time series of cyst abundance during the 2012 bloom exists.…”

“…Our approach to the third challenge was made possible through the development of a cable-free raft that supported the IFCB deployment at Salt Pond, a site that retains A. fundyense blooms and can be adequately characterized by sampling at a single location and depth. Deployment at Salt Pond also enabled comparison to weekly surveys of A. fundyense that were ongoing through other parallel projects within the NMS (Velo-Su arez et al 2013;Ralston et al 2014Ralston et al , 2015Petitpas et al 2015).…”

“…Concurrent projects also quantitatively evaluated losses due to parasitism by Amoebophrya (Velo‐Suárez et al ) and grazing by diverse species (Petitpas et al ), enabling comparisons between the roles of these losses and sexual fusion as contributors to the A. fundyense bloom's decline.…”

Alexandrium catenella cyst dynamics in a coastal embayment : temperature dependence of dormancy, germination, and bloom Initiation

“…Daily loss rates are estimated to alternate between approximately 0.1 d -1 and 0.3 d -1 during neap and spring tide cycles, respectively, and there is interannual variability depending on the stratification and swimming behavior . Specific loss rates due to grazing are unknown but their impact has not been shown to be a major factor in bloom development or decline (Petitpas et al 2015;. Unlike the model of , this study's model begins many months earlier in the year when loss rates have not been estimated.…”

“…Finally, perhaps the greatest uncertainty is the lack of understanding of grazing rates and the environmental factors or population dynamics that control them (Petitpas et al 2015). The work of demonstrated that the net accumulation rates during the bloom were similar to the upper limit of laboratory growth rates of A. catenella.…”