Stormwater detention ponds are a common best management practice (BMP) to buffer flooding and nonpoint source (NPS) impacts between housing or recreational developments and local estuaries.  However, whereas nuisance blooms, one category of HAB that causes water discoloration and odors, are a well-known byproduct of detention ponds, blooms that can produce toxins are rarely reported from these waters.  The Algal Ecology Laboratory sampling plan extended to detention ponds when a Pfiesteria-related fish kill in a Hilton Head subdivision pond was followed by dense blooms of several other potentially toxic species in this and Kiawah ponds throughout the spring of 2001.  Over the last 3 years, we have found an unprecedented density and prevalence of HABs in SC brackish detention ponds (Kempton et al. 2002, Lewitus and Holland 2003, Lewitus 2003, Lewitus et al. 2003, subm.).  All of the blooms have precedence for toxicity and causing fish kills, either through toxin production or indirectly through oxygen depletion. Located primarily in housing developments and golf courses, these ponds are marked by close human interaction, including recreational activities such as fishing, crabbing, swimming, boating, and golfing.   Because these HABs are consistently associated with high inorganic and organic nutrients (Lewitus et al. 2003, subm.), we hypothesize that pond eutrophication through NPS loading is a causative factor in HAB formation and maintenance.  These brackish ponds are estuarine systems (lagoons) and exchange water with tidal creeks, and therefore exchange harmful algae, their cysts, and possibly their toxins.  These eutrophic ponds are apparently optimal systems for the proliferation of the observed harmful species, which have the potential to impact natural resources not only within the ponds but in the tidal creeks and open estuaries.

One of the more common HAB groups in these ponds is the toxic Pfiesteria complex (P. piscicida, P. shumwayae).  These species have been documented frequently in several brackish ponds, sometimes in high numbers (e.g. 1.7 X 10⁴ cell ml^-1 on 16 April 2003 in a Kiawah Island pond)..  Using direct real-time PCR, Pfiesteria piscicida was detected in sediments from 34 of 55 detention ponds surveyed in Kiawah on 4 April 2003 (Click on figure at left).  The likelihood that these were cysts has implications to the effects of dredging on increasing the distribution of this species.

           

    Other common bloom-formers in SC brackish ponds are the raphidophytes. From 2001-2002, raphidophytes were found in 47% of 538 total samples (27 of 40 ponds sampled) collected during routine monitoring and fish kill response efforts. Four raphidophyte species were identified, Heterosigma akashiwo, Chattonella subsalsa,C. verruculosa, and Fibrocapsa japonica (Figure below).  Although salinity and temperature ranges generally overlapped, F. japonica was not found in waters < 10‰ or < 22.6^oC, and H. akashiwo did not occur in waters > 30‰. Samples with high raphidophyte abundances were common (e.g. > 10³ cell ml^-1 in 49 samples, encompassing 12 different ponds).  Maximum abundance for each species exceeded 10⁴ cell ml^-1, and raphidophytes frequently dominated phytoplankton community biomass.  PO₄ concentrations were relatively high (mean 9.7 ± 7.1 μM; mean DIN:DIP was 0.72 ± 0.98), and are likely a function of fertilizer and/or sewage inputs.  Maintenance of high raphidophyte biomass under these conditions would presumably be limited by N supply, and may depend on replenishment from the same non-point source loads or groundwater flux.  Mean DOC and DON concentrations were markedly high (1213 ± 584 and 60 ± 20 μM, respectively).

 Although brevetoxin or brevetoxin-like substances were detected from raphidophyte blooms based on the ELISA assay results (Lewitus and Holland 2003), these levels were generally low and not measured in association with fish kills. However, information is needed on the sublethal effects of raphidophyte-produced toxin on fish (or humans), especially considering that raphidophyte blooms can persist for lengthy periods in these ponds.  The association of pond fish kill events with raphidophyte blooms on several occasions (Lewitus and Holland 2003, Lewitus et al. 2003) warrants study of the toxic potential of these HABs, whether by brevetoxin production or other mechanisms.