The hard clam Mercenaria mercenaria
is a common inhabitant of estuarine sediments and an important aquaculture species in the U.S. In coastal environments, high nutrient inputs lead to increased rates of phytoplankton production followed by increased organic material availability. Some of this phytoplankton will die and sink to the bottom where it is consumed by bacteria. As this organic material is decomposed, the increased bacterial respiration rates can translate to reduced oxygen availability, particularly in benthic waters and sediments. Sediments are especially prone to oxygen limitation because of reduced diffusion rates, and, as a result, deeper sediments often completely lack oxygen. Under these conditions, other types of bacteria decompose the organic matter and, in doing so, produce hydrogen sulfide. Sulfide is responsible for the black sediments found in many estuaries and is the source of the “rotten-egg” smell. Therefore, although proximity to coastal sources of nutrient helps to ensure an abundant food source for hard clams, it also exposes them to hydrogen sulfide. The combined effects of low oxygen concentrations and hydrogen sulfide are known to reduce the growth and survival of many bivalve species. However, the direct impact of sulfide on growth and survival of hard clams has not been examined.
This study integrated field surveys and laboratory experiments to determine the 1) hydrogen sulfide exposure levels of hard clams and relationships among sediment hydrogen sulfide and various environmental parameters in the Suwannee River estuary, and 2) effect of hydrogen sulfide on the survival of two size classes of hard clams. Sediment porewater samples were collected at 11 sites, both inside and outside of shellfish aquaculture leases in Levy and Dixie Counties. In the lab, two sizes of clams were challenged with both sulfide and hypoxia (low oxygen).
Hydrogen sulfide was found in sediment pore water near and within hard clam high density lease areas (HDLAs). Porewater sampled from clam lease sites had mean sulfide levels up to 110 μmol/L. The Derricks, Gulf Jackson, and Horseshoe Beach HDLAs were found to have significantly higher sediment pore water hydrogen sulfide concentrations than the Pine Island and Pelican Reef HDLAs. Non-lease sites had higher levels, with mean sulfide levels up to 300μmol/L. Sulfide levels varied significantly with temperature, with highest levels occurring in the warm months of August and September. Unexpectedly, hydrogen sulfide concentrations did not vary predictably with sediment organic matter content, sediment grain size, or most water quality parameters; but it did vary predictably with salinity at some HDLAs. Mean hydrogen sulfide concentrations peaked during late August and early September and remained relatively low and constant during the remainder of the sampling period. In laboratory experiments, the survival of hard clam nursery seed (4-6mm) and growout-sized seed (12-15mm) was reduced over normoxia conditions (normal oxygen) when exposed to 0.10-0.35 mM hydrogen sulfide. Addition of the antibiotic chloramphenicol to limit growth of anaerobic bacteria tended to increase hard clam survivorship. In summary, hydrogen sulfide occurs in the sediments of Florida’s west coast clam aquaculture areas at concentrations capable of reducing hard clam survivor. The reduced survival of seed clams exposed to hydrogen sulfide in the laboratory and the beneficial effect of antibiotic suggests that hydrogen sulfide and accompanying bacterial growth play a role in hard clam mortality during the field growout process. Hydrogen sulfide was found to vary substantially between different HDLAs and between different times of the year. However, predicting which HDLAs and which leases within an HDLA are most at risk for hydrogen sulfide toxicity will require sampling of sediment pore water at specific planting locations and planting times.
A presentation on project results made at a state meeting and final report can be viewed below in PDF
- Derk Bergquist, South Carolina Department of Natural Resources
- David Julian, University of Florida, Biological Sciences Department
- Shirley Baker, University of Florida IFAS Fisheries and Aquatic Sciences
Florida Sea Grant Program Development, 2003