On 29 April, 2003, a massive bloom of Heterosigma akashiwo occurred in Bulls Bay that extended from inside the Bay to 4-5 miles offshore (> 80 square miles).  H. akashiwo abundance was measured at ~10⁵ cell ml^-1, and the bloom was associated with a fish kill.  “Reference” oysters experienced physiological stress (increased % lysosomal destabilization) when exposed to bloom waters (Link to Keppler).  The areal extent of this bloom exceeded any previously documented from SC waters, and marked only the second harmful algal bloom documented off the coast of SC. The first documentation of an offshore bloom was a Karenia brevis bloom that was transported with the Gulf Stream from the Gulf of Mexico to the continental shelf waters off North Carolina and then southward to SC nearshore waters. However, the H. akashiwo bloom was not transported with the Gulf Stream based on evaluations of satellite imagery, and may have developed from a significant cyst bed in the Bulls Bay area. Water temperature and salinity were ideal for the germination of H. akashiwo cysts. Imai and Itaku (1999) noted that the rate of germination of H. akashiwo cysts increased at 15ºC, and was maintained at a high level up to 25ºC. Water temperatures during the Bulls Bay bloom were 22.7ºC. Hershberger et al. (1997) reported that Heterosigma cells concentrate in surface regions of lesser salinity water. An increase of freshwater input from rain events and the release of freshwater from the Santee River may have provided a freshwater lens ideal for the aggregation of H. akashiwo cells. Surface salinity measurements during the bloom were 21.9 ppt, significantly less than the average salinities for the area and within the optimal range for growth of a SC isolate of H. akashiwo (S. Chambliss, unpub. data).

Fig. 1 H.. akashiwo bloom, Bulls Bay, SC, 28 April 2003 (Sea WiFS, M. Kahru)	Fig. 2 H. akashiwo bloom, Bulls Bay, Sc, 29 April 2003 (D. Griffin)

In response to the 29 April 2003 H. akashiwo bloom event, a survey was conducted to evaluate the distribution of H. akashiwo cysts in the sediments from inshore tidal creeks and the offshore waters of Bulls Bay. In addition to sediment sample collections, whole water samples were collected for screening via light microscopy and real-time PCR. Results from the screening of 41 inshore tidal creek and 20 offshore sediment and water samples suggests that a significant cyst bed is present throughout the Bulls Bay area, and confirms the widespread distribution of H. akashiwo in the water (Figures below). H. akashiwo was identified via real-time PCR in 36% of the sediment samples collected, and 75% of the water samples. The post-bloom evaluation of Bulls Bay inland and offshore sediment samples using the real-time PCR assay suggests that a significant H. akashiwo cyst bed may be present in the area, and should be monitored in the likelihood that additional blooms may occur in this area.

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