Bioturbation, germination and deposition of Alexandrium fundyense cysts in the Gulf of Maine

Abstract

Like many other dinoflagellate species, Alexandrium fundyense possesses a benthic resting cyst which enables long-term persistence and annual blooms of this species in the Gulf of Maine. The size and extent of these harmful algal blooms are associated with high cyst concentrations in the top 1cm of sediment. Despite the importance of this resting stage in the life history of A. fundyense, little work has been done on bioturbation of cysts in the deep-water cyst beds of the western Gulf of Maine. Our work intensively examined one site within a major regional “seedbed” from February 2003 until August 2005, a time span that included an extraordinarily large bloom of A. fundyense in 2005.Over the course of 2 years we collected samples for benthic infauna and cyst profiles down to a depth of 30cm. We also measured sediment porosity, organic carbon, 210Pb, and porewater dissolved oxygen. On several dates we measured depth profiles of cyst autofluorescence. Profiles of cysts revealed large subsurface maxima peaking between 10 and 15cm depth with cyst concentrations declining strongly toward the sediment surface. On one sampling date (August 2004) we observed a cyst concentration peak at the sediment surface. Using these data we constructed a mechanistic model of cyst bioturbation, mortality, germination, and deposition. Modeled bioturbation was calibrated using 210Pb and modeled cyst profiles were compared to measured profiles. Model runs with constant and interannually-varying rates of cyst deposition produced similar time-averaged cyst profiles. Results indicate that the deeper portions of cyst profiles are determined primarily by bioturbation, germination and cyst mortality and less so by interannual variation in cyst depositional history. This is due to the relatively low sedimentation rate at the study site compared to the rate of bioturbation, and the fact that the number of cysts deposited each year tends to be a small fraction of the total inventory. Seasonal and interannual variation in cyst deposition strongly influenced concentrations of cysts in the top few millimeters of the sediment, however. When cyst deposition rates are low, bioturbation and germination are sufficient to rapidly deplete cysts in this surface layer, leaving relatively few cysts within the sediment depth range that allows germination. But, bioturbation is not rapid enough to homogenize surface sediments within one year. As a result, cyst deposition results in a concentration peak at the sediment surface that persists to fuel germination the following year. Because of this phenomenon, the model predicted that years with high rates of germination follow years with large levels of cyst deposition. Over longer time scales, bioturbation transports cysts from the sediment surface to depth, which, along with germination, creates a persistent subsurface maximum in cyst concentration. Bioturbation also serves to maintain persistence of A. fundyense by transporting older cysts from these deeper layers to the sediment surface where they can germinate. Since high concentrations of cysts near the sediment surface indicate large numbers of cysts deposited during the previous year, if enhanced germination leads to large blooms of A. fundyense, these blooms are predicted to occur in years following large cyst deposition events.