Factors controlling seagrass growth in a gulf coastal system: Water and sediment quality and light

Abstract

A combined field descriptive/experimental and laboratory experimental study was carried out to determine the relationships of water quality, qualitative and quantitative light factors and sediment characteristics in the definition of the distribution of submerged aquatic vegetation (SAV) offshore of two streams, one polluted and one natural, that drain into the northeastern Gulf of Mexico. Release of pulp mill effluents into a small drainage system were associated with increased loading of dissolved organic carbon (DOC), water color and nutrients to offshore areas relative to an unpolluted reference system. This loading resulted in changes of water quality factors and light transmission characteristics in the receiving Gulf area. Sediments in affected offshore areas were characterized by increased silt/clay fractions and altered particle size relative to reference sites. Based on the field data, the best (statistically significant) predictors of SAV distribution were photic depths, qualitative aspects of wave length distributions, water quality factors (color, DOC and chlorophyll a) and sediment characteristics. Photic depths were good predictors of SAV distribution, with depth as an important modifying factor. Mesocosm experiments showed that pulp mill effluent in direct contact with Thalassia testudinum and Syringodium filiforme had significant adverse impacts on growth at relatively low concentrations (1–2%) of effluent. Light mesocosm experiments indicated that light levels in inshore Fenholloway areas were associated with reduced growth of Halodule wrightii, S. filiforme and T. testudinum. Field transfer experiments showed that altered sediment and water quality in inshore polluted areas induced significant adverse effects on growth indices of all three species. By comparing the field and experimental results, a hierarchy of habitat requirements for the subject seagrass species was determined. Salinity, temperature and depth restraints are important habitat variables that control seagrass growth; when such variables are not limiting, light, sediment and nutrient characteristics become important in the determination of the distribution of seagrasses in coastal areas.