The relationship of physical factors and biological response in coastal seagrass meadows

Abstract

Continuous, long-term studies of coastal grassbed assemblages in the N.E. Gulf of Mexico indicate complex relationships between physical controlling factors and biological response. Such seagrass systems are physically unstable over short periods. Seasonal ranges of temperature, salinity, and natural water quality conditions are considerable with periodic, recurrent “catastrophic” events such as floods and cold winters. These factors control the distribution and productivity of the seagrasses and algae which constitute the habitat and organic substrate for diverse assemblages of organisms. In addition, the benthic plants mediate predator-prey relationships and competitive interactions. Despite the physical instability, timed sequences of distinct ontogenetic feeding populations are generally stable from year to year as are other population and community characteristics. Thus, physical processes determine overall habitat conditions and productivity cycles whereas biological processes such as predation and competition define specific community relationships. However, seemingly minor changes in the physical environment due to anthropogenous activities can lead to major reorganization of the biological system; the observed biological stability of the seagrass beds can be ephemeral if important habitat features are altered in a way that exceeds the adaptive response of the system. Concepts are discussed which relate observed sequences of ontogenetic feeding units to food web patterns and geographic differences of population-niche relationships from one estuary to another.