Toxic Sulfide

Toxic Sulfide Concentrations in the Sediments and Water Column of the Suwannee River Estuary and Its Influence on Hard Clam Survival

Investigators:
Dr. Derk C. Bergquist, University of Florida, Department of Fisheries and Aquatic Sciences
Dr. Shirley M. Baker, University of Florida, Department of Fisheries and Aquatic Sciences
Dr. David Julian, University of Florida, Department of Zoology
Joanna Joyner, University of Florida, Department of Zoology
Carla Beals, University of Florida, Department of Fisheries and Aquatic Sciences

Funding:
Florida Sea Grant College Program

Time Period:
May 2003 - October 2003

Background:
The hard clam is a common inhabitant of estuarine sediments and an important US fishery species. Currently, over 1,600 acres of state-owned, submerged land on the Atlantic and Gulf coasts of Florida are utilized as field nurseries for clam aquaculture, and much of this land is located within nutrient-rich coastal environments. In these environments, high nutrient inputs lead to increased rates of primary production followed by increased organic material availability. 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. In environments lacking sufficient oxygen to support aerobic respiration, bacteria utilize alternate oxygen sources, such as nitrate and sulfate, and make by-products such as nitrogen gas and hydrogen sulfide, respectively. Of these byproducts, hydrogen sulfide (H₂S) is particularly important from a biological perspective because it is toxic to animal tissues in very small concentrations.

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 shellfish species, including various clams such as the coot clam Mulinia lateralis, the Baltic clam Macoma balthica, and the surf clam Donax serra. However, the direct impact of sediment sulfide on growth and survival of the commercially important hard clam Mercenaria mercenaria has never been examined.

Objectives:
This study integrated field surveys and laboratory experiments to determine 1) the hydrogen sulfide exposure levels of hard clams and the relationships among sediment hydrogen sulfide and various environmental parameters in the Suwannee River estuary and 2) the effect of hydrogen sulfide on the survival of two size classes of hard clams used in field aquaculture facilities.

Results:
Hydrogen sulfide was found in sediment pore water near and within hard clam high density lease areas (HDLA’s) at concentrations averaging 0.079mM and as high as 0.567mM. The Derricks, Gulf Jackson and Horseshoe Beach HDLA’s were found to have significantly higher sediment pore water hydrogen sulfide concentrations than the Pine Island and Pelican Reef HDLA’s. (These lease areas are located in Levy and Dixie Counties.) 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 HDLA’s. 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 survivorship of hard clam nursery seed (4-6mm) and grow-out seed (12-15mm) was reduced over normal conditions (“Normoxia”) when exposed to 0.10-0.35 mM hydrogen sulfide (see figure). Addition of the antibiotic chloramphenicol to limit growth of anaerobic bacteria tended to increase hard clam survivorship.

Conclusions:
Hydrogen sulfide occurs in the sediments of Florida’s west coast hard clam aquaculture areas at concentrations capable of reducing hard clam survivorship. The reduced survivorship 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 aquaculture process. Hydrogen sulfide was found to vary substantially between different HDLA’s and between different times of the year. However, predicting which HDLA’s and which lease areas within an HDLA are most at risk and when they are most at risk for hydrogen sulfide toxicity will require sampling of sediment pore water at specific planting locations and planting times. For a complete summary of this project’s findings, see the full report.

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