Blooms of the toxic dinoflagellate Alexandrium fundyense in the Gulf of Maine : investigations using physical-biological model

Abstract

Blooms of the toxic dinoflagellate Alexandriumfundyense are annually recurrent in the western Gulf of Maine (WGOM) and pose a serious economic and public health threat. Transitions between and vital rates within the life stages of A. fundyense are influenced by diverse environmental factors, and these biological dynamics combine with energetic physical motions to yield complex bloom patterns. In this thesis, a biological model of the A. fundyense life cycle developed from laboratory and field data is combined with a circulation model to test hypotheses concerning the factors governing A. fundyense blooms in the springs of 1993 and 1994. There is considerable uncertainty with the biological dynamics, and several biological model structures are tested against the 1993 observations. Maximum likelihood theory is used to evaluate the statistical significance of changes in model/data fit between structures. Biological formulations that do not include either nitrogen limitation or mortality overestimate observed cell abundances and are rejected. However, formulations using a wide range of mortality and nitrogen dependence, including the exclusion of one or the other, were able to match observed bloom timing and magnitude and could not be statistically differentiated. These simulations suggest that cysts germinating offshore of Casco Bay provide a plausible source of cells for the blooms, although cell inputs from the eastern Gulf of Maine gain importance late in the spring and in the northeast portion of the study area. Low net growth rates exert a notable yet non-dominant influence on the modeled bloom magnitude. When simulations tuned to 1993 were applied to 1994 the degree of model/data fit is maintained only for those simulations including nitrogen dependence. The model suggests that differences in toxicity between the two years result from variability in the wind and its influence on the along and cross-shore transport of cells. Extended simulations generally predict a proliferation of A. fundyense abundance in mid-June within areas of retentive circulation such as Cape Cod Bay. This proliferation is not observed, and better resolution of the losses and limitations acting on A. fundyense is needed at this stage of the bloom.