Effects of Submersed Macrophytes on Dissolved Oxygen, pH, and Temperature under Different Conditions of Wind, Tide, and Bed Structure

ABSTRACT

Seasonal data on diurnal dissolved-oxygen concentration (DO), pH, temperature and chlorophyll-a were collected and species composition and vertical structure of macrophyte beds were analyzed in the tidal Potomac River during the 1987 growing season. The relationships among these variables and physical and climatic factors were analyzed. Elevated surface temperatures, DO and pH were found in macrophyte beds in June and August; surface temperatures were also elevated in the dense Hydrilla verticillata dominated bed in October-November after senescence had begun. Bottom DO, pH and temperature were lower than surface values. Bottom temperatures in vegetated sites were highly variable compared with bottom temperatures in unvegetated sites.

Tide, wind, vegetative structure and available sunshine interacted in a complex fashion to control the magnitude of the diurnal DO, pH and temperature and the stratification of DO, pH and temperature with depth in vegetated sites. Fluctuations in DO and pH at unvegetated sites were the result of phytoplankton photosynthesis and proximity to beds of vegetation as well as wind and available sunshine. The tidal cycle drove the daily exchange of water between macrophyte beds and adjacent water.