Seagrass species composition and distribution trends in relation to salinity fluctuations in Charlotte Harbor

Seagrass species composition and distribution reflect environmental changes, making these measures potentially useful estuarine indicators. An annual seagrass transect and quadrat monitoring survey program including 50 locations in Charlotte Harbor, Florida, began in 1999. This six-year data set was analyzed in conjunction with a monthly water quality monitoring program covering the same time period to examine trends in seagrass species composition and distribution. Analyses of the maximum depth of seagrass distribution for each transect did not indicate any large-scale changes in seagrass depth distribution. This suggests a stable overall area of seagrass distribution in the Charlotte Harbor area during the study period. However, abundance of Halodule wrightii and Thalassia testudinum has significantly declined, along with the overall frequency of seagrass occurrence among quadrats. Finally, the distribution of the three dominant seagrass species, H. wrightii, T. testudinum, and Syringodium filiforme, appear to be influenced by low, wet-season salinity and high variation. This study highlights the value of research into seagrass species abundance and distribution on a meter-to-meter scale to recognize the effects of water quality or environmental variables such as salinity on a small scale, prior to large scale loss.