Vegetative characteristics of three low-lying Florida coastal rivers in relation to flow, light, salinity and nutrients

The Chassahowitzka, Homosassa and Crystal rivers along the central Gulf coast of Florida were studied from 1998 to 2000 to identify factors controlling the abundance and distribution of submersed aquatic vegetation (SAV). Each of these three low-lying coastal rivers are spring-fed and exhibit low to moderate absolute flow rates (flows in either direction because of tidal influences, 0.06-0.46 m s(-1)) with only 14% of the stations sampled for SAV having flow rates in excess of 0.25 m s(-1). At those stations where flow rates exceeded 0.25 m s(-1), the substrate was generally comprised of exposed limestone outcroppings and did not provide a favorable habitat for either submersed macrophytes or macroalgae. The remaining sampling stations, where flow rates were less than 0.25 m s(-1), had suitable substrates (e.g. mud, mud/sand, and sand) for the colonization and subsequent growth of SAV. Light availability and salinity were determined to be major factors affecting the distribution and abundance of SAV. Sampling stations, where the percent of incident light at the surface reaching the substrate was less than 10%, had little or no SAV biomass. Low SAV biomass was also linked to sites where annual average salinities exceeded 3.5parts per thousand. Nutrient loads and nutrient concentrations accounted for little variance in SAV biomass after accounting for flow and related substrate type, light and salinity. These latter factors control the distribution and abundance of SAV in these three Florida coastal rivers.