The first documentation of a dinoflagellate bloom localized to South Carolina estuaries occurred in spring 1998 (Lewitus et al. 2001).  The bloom was characterized as a “red tide” in that the water was discolored (rust to red-orange) and the phytoplankton community was monospecific.  From spring through summer of 1998, similar blooms recurred at several estuaries from Georgetown to Hilton Head (over 100 miles apart).  In each successive year since 1998, observations of these blooms have increased in frequency, and have extended to several tidal creek systems in SC (Lewitus and Holland 2003).  Furthermore, these blooms have been associated with shellfish mortality events, and in bioassay experiments, physiological stress in oysters exposed to bloom waters has been demonstrated (link to Chuck blerb).  These blooms are widespread, reach densities high enough to discolor water (commonly > 100,000 cell ml^-1), and appear throughout the spring and summer.

Lewitus et al. (2001) found that Kryptoperidinium bloom initiation in North Inlet estuary followed rain-driven runoff events carrying dissolved organic material (DOM)-rich loads (Figure below).  Over the bloom periods, Kryptoperidinium sp. abundance varied inversely with DOC, DON, and DOP concentrations, and positively with dissolved inorganic carbon concentrations, suggesting high respiratory activity.  A box model comparing DOC, DIC, and salinity patterns indicated that variability in C flux could not be attributed solely to tidal exchange, arguing for control by water column biological processing (Lewitus et al. unpub. data).  It is unknown to what extent Kryptoperidinium contributed to heterotrophic activity during those periods, but the association of bloom initiation with forest-derived DOM loads suggests that bloom development was related to the dinoflagellate’s ability to use DOM.

An association between dinoflagellate blooms and rain-driven runoff of humic-rich water has been repeatedly demonstrated.  The link between DOM loading and dinoflagellate blooms can be explained in several different ways, including potential roles of DOM in enhancing trace metal availability (i.e. chelation) or in supplementing metabolic requirements; i.e. direct uptake of DOM or indirect uptake of DOM catabolic products (e.g. from degradation by bacteria).  Carlsson et al. (1999) concluded that the association between organic loading and Prorocentrum minimum blooms could best be explained by growth stimulation through direct uptake of humic matter-derived DON.  Also, Legrand and Carlsson (1998) using Alexandrium catanella cultures, and Willis (2001) using SC Kryptoperidinium bloom populations demonstrated high uptake rates of fluorescently-labeled high molecular weight dextran, a polysaccharide. Both studies indicated a preference for high molecular weight (e.g. colloidal) over low molecular weight dextran, and Willis (2001) demonstrated that dextran was taken up phagocytotically.  The use of high molecular weight DOM has been proposed as a “niche” for  phototrophic and heterotrophic flagellates in direct competition with bacteria.

We hypothesize that high molecular weight DOM is stimulatory to SC Kryptoperidinium growth, and that bloom formation is favored and sustained by this heterotrophic pathway.  We are evaluating the link between organic loading and Kryptoperidinium bloom dynamics in several ways, emphasizing nutrient (inorganic and organic) enrichment responses and uptake capabilities, and tracer studies assessing photosynthetic and heterotrophic capabilities. Experiments include the analysis of N uptake kinetics of Kryptoperidinium blooms for  inorganic and organic N, using among other substrates, ¹⁵N-labeled humic material derived from Spartina plants (in collaboration with D. Bronk and J. See, VIMS).