Factors Affecting Changes in Seagrass Distribution and Diversity Patterns in the Indian River Lagoon Complex Between 1940 and 1992

Existing and new data were synthesized to analyze distribution patterns of seagrasses throughout the Indian River Lagoon complex from Ponce Inlet to Jupiter Inlet. Changes in abundance, distribution, and depth of occurrence between the 1940s and 1992 were evaluated. Potentially available habitat was estimated from bathymetric maps to estimate the extent of seagrass colonization of available habitat and the potential for increasing seagrass acreage by management programs. Seagrass abundance was estimated to be 11% less in 1992 than in the 1970s and 16% less than in 1986 for the entire complex. In particular, decreases in abundance appeared in the Indian River Lagoon north of Vero Beach. Evaluation of depth of occurrence indicated that the current (1992) maximum depth of seagrass beds is as much as 50% less than it was in 1943 in this portion of the Lagoon. Consistent decreases in depth were not found in Mosquito Lagoon, Banana River, or the Indian River Lagoon south of Ft. Pierce. Species composition patterns seem to follow patterns based on shallow water and salinity. Seagrass distribution in the lagoon may affect biodiversity patterns in other groups of plants and animals. Responses to short-term events and local water quality may also affect seagrass distribution. These factors must also be considered when assessing trends and in developing management strategies for the Indian River Lagoon. The Indian River Lagoon has the greatest seagrass diversity of any United States estuary, with a complex pattern of species assemblages that may depend on water clarity, salinity, temperature, or other factors. It is suggested that alteration of any of these, such as water clarity, may upset this diversity and balance of seagrass species in the Indian River Lagoon.