Benthic nepheloid layers in the Gulf of Maine and Alexandrium cyst inventories.

Cysts residing in benthic nepheloid layers (BNLs) documented in the Gulf of Maine have been proposed as a possible source of inoculum for annual blooms of a toxic dinoflagellate in the region. Herein we present a spatially extensive data set of the distribution and thickness of benthic nepheloid layers in the Gulf of Maine and the abundance and inventories of suspended Alexandrium fundyense cysts within these near-bottom layers. BNLs are pervasive throughout the gulf and adjacent Bay of Fundy with maximum layer thicknesses of 50-60 m observed. Mean BNL thickness is 30 m in the eastern gulf and Bay of Fundy, and 20 m in the western gulf. Cyst densities in the near-bottom particle resuspension layers varied by three orders of magnitude across the gulf with maxima of 105 cysts m-3. An important interconnection of elevated BNL cyst densities is observed between the Bay of Fundy, the Maine Coastal Current and the south-central region of the gulf. BNL cyst inventories estimated for the eastern and western gulf are each on the order of 1015 cysts, whereas the BNL inventory in the Bay of Fundy is on the order of 1016 . Although BNL cyst inventories in the eastern and western gulf are 1-2 orders of magnitude smaller than the abundance of cysts in the upper 1 cm of sediment in those regions, BNL and sediment-bound cyst inventories are comparable in the Bay of Fundy. The existence of widespread BNLs containing substantial cyst inventories indicates that these near-bottom layers represent an important source of germinating A. fundyense cysts in the region.

Spatial and temporal variability of Alexandrium cyst fluxes in the Gulf of Maine: Relationship to seasonal particle export and resuspension.

Quantification of Alexandrium cyst fluxes through the Gulf of Maine water column is central to understanding the linkage between the source and fate of annual Alexandrium blooms in the offshore waters. These blooms often lead to paralytic shellfish poisoning (PSP) and extensive closures of shellfish beds. We report here on time-series sediment trap deployments completed at four offshore locations in the gulf between 2005 and 2010 as components of two ECOHAB-GOM field programs. Data presented documents the substantial spatial and temporal fluctuations in Alexandrium fundyense cyst fluxes in the gulf. Cyst delivery out of the euphotic zone peaked primarily between July and August following annual spring-summer Alexandrium blooms and was greatest in the western gulf. At all sites, cyst flux maxima to the subsurface waters were rarely coincident with seasonal peaks in the total mass export of particulate material indicating that cyst delivery was primarily via individually sinking cysts. Where persistent benthic nepheloid layers (BNLs) exist, significant sediment resuspension input of cysts to the near-bottom water column was evidenced by deep cyst fluxes that were up to several orders of magnitude greater than that measured above the BNL. The largest cyst fluxes in the BNL were observed in the eastern gulf, suggesting greater resuspension energy and BNL cyst inventories in this region. Temporal similarities between peak cyst export out of the upper ocean and peak cyst fluxes in the BNL were observed and document the contribution of seasonal, newly formed cysts to the BNL. The data however also suggest that many Alexandrium cells comprising the massive, short-lived blooms do not transition into cysts. Time-series flow measurements and a simple 1D model demonstrate that the BNL cyst fluxes reflect the combined effects of tidal energy-maintained resuspension, deposition, and input of cysts from the overlying water column.

A red tide of Alexandrium fundyense in the Gulf of Maine.

In early July 2009, an unusually high concentration of the toxic dinoflagellate Alexandrium fundyense occurred in the western Gulf of Maine, causing surface waters to appear reddish brown to the human eye. The discolored water appeared to be the southern terminus of a large-scale event that caused shellfish toxicity along the entire coast of Maine to the Canadian border. Rapid-response shipboard sampling efforts together with satellite data suggest the water discoloration in the western Gulf of Maine was a highly ephemeral feature of less than two weeks in duration. Flow cytometric analysis of surface samples from the red water indicated the population was undergoing sexual reproduction. Cyst fluxes downstream of the discolored water were the highest ever measured in the Gulf of Maine, and a large deposit of new cysts was observed that fall. Although the mechanisms causing this event remain unknown, its timing coincided with an anomalous period of downwelling-favorable winds that could have played a role in aggregating upward-swimming cells. Regardless of the underlying causes, this event highlights the importance of short-term episodic phenomena on regional population dynamics of A. fundyense.