AN EVALUATION OF THE TOXICITY AND SUBLETHAL EFFECTS OF ATRAZINE ON THE PHYSIOLOGY AND GROWTH PHASES OF THE AQUATIC MACROPHYTE VALLISNERIA AMERICANA L. (HERBICIDE, ESTUARINE, PHOTOSYNTHESIS, COMPUTER, CHESAPEAKE)

Sublethal effects of an herbicide on the life cycle of a submersed aquatic macrophyte were investigated. A unique microcosm was designed to accommodate long-term culturing of submersed macrophytes in the laboratory and to provide a medium for testing of toxic substances. The Gilson Differential Respirometer was modified for short-term toxicity testing. A computer program was written for the reduction of data obtained from the Respirometer (appendix). Atrazine was utilized to assess the sublethal effects of a commonly used herbicide on a submersed aquatic plant, Vallisneria americana, one species which has experienced decline in distribution in the Chesapeake Bay. The results of this research indicate that all phenophases of V. americana can be duplicated in these microcosms. V. americana exhibited a significant attrition in the life-cycle completion when exposed to sublethal concentrations of Atrazine. Every stage in the life cycle of V. americana was affected. Leaf growth and whole plant biomass were significantly affected at all times during the growing season at concentrations above 8 ppb. Sexual reproduction was affected by either a lack of flower production (female) of deterioration of the flower (male) above 16 ppb. The ability of V. americana to over-winter as underground tubers was also significantly reduced to 4 ppb and above. Tuber number was reduced by 60% and tuber biomass (stored food) was reduced by 27% at 16 ppb. Other research has documented ephemeral pulses of these sublethal concentrations of Atrazine in the headwaters of tributaries to the Chesapeake Bay, where V. americana commonly occurs. The occurrence of this herbicide in these tributaries in the author's estimation is largely due to misuse of this herbicide and farming practices which lend themselves to soil erosion into nearby bodies of water. The results of this research indicate that Atrazine is as important as any other factor in contributing to the decline of V. americana.