Suspended Alexandrium Spp. Hypnozygote Cysts in the Gulf of Maine

Hypnozygote cysts are a known stage in the life cycle of Alexandrium spp. Negatively buoyant cysts purportedly fall ,to the benthos where they undergo mandatory quiescence until their endogenous clock makes germination possible. If oxygen is present and the endogenous clock allows, germination will occur at a rate proportional to light and temperature. Offshore in the Gulf of Maine, where Alexandrium blooms are well documented, the paradigm of benthic cysts is problematic. Sediment surveys have found wide distribution of cysts, with highest cyst concentrations below 100 m in areas where deposition is favored. Germination at these depths is likely slow due to light inhibition; negative effects of burial on germination rate, and losses of cells during the transit from benthos to photic zone also present problems. The purpose of this study was to determine if cysts are suspended in the water column where they would be better positioned to initiate springtime Alexandrium populations. During cruises in February, April, and June of 2000, thirty liter samples were taken from the near-bottom, the top of the bottom nepheloid layer, and near surface, concentrated and stained with primulin for examination using epifluorescence microscopy. Suspended cysts were found widely distributed throughout the Gulf of Maine and Bay of Fundy. Generally, cysts were more abundant in near-bottom and top of the bottom nepheloid layer samples than in the surface.\u27 Alexandrium hypnozygote cysts were most abundant in February and least abundant in April. As a first order assessment of the potential for planktonic cysts relative to benthic cysts to initiate springtime populations of Alexandrium vegetative cells, the following rough calculation was performed using as an example the data from the Bay of Fundy in February. In February planktonic cysts numbered on the order of lo2 cysts m in surface waters in the Bay of Fundy; near bottom cyst concentrations were between lo2 - lo3 cysts m . Roughly integrating total suspended cysts yields lo4 cysts m-2. Cysts in the Bay of Fundy sediments were found at concentrations of lo3 cysts ~ m - ~ . Conservatively, only the uppermost 1 rnm layer of cysts will be able to successfully geminate, which equals lo2 cysts cm-2 or lo6 cysts m-2. Studies have shown that only 10 % of benthic cysts germinate and that light enhances germination rate 10-fold, which lowers the benthic estimate to lo4 cysts m-2, the same concentration as found in the water column. The purpose of this study was primarily to determine the presence or absence of Alexandriurn hypnozygote cysts in the water column in the Gulf of Maine. The data, while significant in that they show the presence of potentially important cyst in the water, do not lend themselves well to robust statistical analysis. However, this study provides evidence that suspended cysts likely contribute significantly to spring Alexandrium populations. Suspended cysts should be considered in future investigations and computer modeling designed to predict Alexandrium caused paralytic shellfish poisoning events