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May 4, 2008 - Oceanus Update

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A near-real-time quasi-operational forecast of A. fundyense cells in the Gulf of Maine.

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A near-real-time quasi-operational forecast of A. fundyense cells in the Gulf of Maine.
(R. He, NCSU)

Alexandrium surface live counts. R/V Oceanus April 28 - May 5, 2008

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Alexandrium surface live counts. R/V Oceanus April 28 - May 5, 2008
((D. Anderson & D. McGillicuddy laboratories, Woods Hole Oceanographic Institution))

Timely completion of the Georges Bank survey conducted on voyage #445 of R/V Oceanus allowed for some sampling in the western Gulf of Maine.  The near-real-time quasi-operational A. fundyense forecast was received at sea, indicating that the strong southeast winds early in the week reduced cell concentrations (presumably by vertical mixing) and arrested the along-shore movement of the bloom (see map, upper right).  It was therefore decided to begin the WGOM sampling with the Casco Bay transect, just upstream of where the model indicates the highest cell concentrations reside.

Live counts from the western Gulf of Maine survey indicate significant cell concentrations in the Casco Bay and Saco Bay transects (see map, lower right).  In the Casco Bay transect, the peak concentration of 112 cells l-1 was observed just offshore of a very strong river plume signature that extended 30 km offshore.   Cell concentrations were considerably higher in the Saco Bay transect, where the peak concentration of 882 cells l-1 was observed.  Alexandrium concentrations decreased to below 100 cells l-1 in the Cape Ann transect, and below 30 cells l-1 in the Boston transect.  The volume of fresh water is very large in the Boston transect, with the outcrop of the 32 ppt isohaline nearly 70km offshore.

These findings are in stark contrast with those from 2007, where surface waters of the western Gulf of Maine were virtually devoid of Alexandrium cells in mid-to-late May (later in the bloom season). Live counts from EN437 (May 17-31 2007) were almost all zeros, with Alexandrium detected at only a few isolated stations.  The peak concentration of 84 cells l-1 was observed at the innermost station of the Saco Bay transect.

Surface observations in the western Gulf of Maine during the late April / early May time period during the ECOHAB-GOM era indicate peak cell concentrations ranging from zero to approximately 200 cells l-1.  In comparing the live counts from the present cruise with those published observations, one must bear in mind the methodological differences: the former are difficult to distinguish A. fundyense from other species that are morphologically similar, whereas the latter are more specific.  Nevertheless, the initial results of OC445 indicate peak surface cell concentrations in the western Gulf that are higher than prior observations at this very early stage of the bloom season.  Our findings are thus not inconsistent with the hypothesis that the 2008 bloom will be anomalously large.

Qualitatively, the observations agree with the model predictions for May 1 insofar as (1) cells are present from Casco Bay down to Cape Ann; (2) peak concentrations occur offshore of Saco Bay; (3) cells are virtually absent from Massachusetts Bay.  Modeled concentrations are somewhat higher than observed, but the patterns are similar.

I'd like to close with highest compliments to the science party and crew of the R/V Oceanus.  Thanks to their superb efforts, a very large area of ocean was sampled in only eight days.  It has been a privilege to work with such a talented and dedicated group of individuals.

Dennis McGillicuddy
Chief Scientist

Last updated: July 1, 2019

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