Smithsonian Marine Station at Fort Pierce

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A lagoon-wide seagrass monitoring program led by scientists from the St. Johns River Water Management District is responsible for monitoring long-term trends in IRL seagrass population status, health, and coverage trends. Data from this program and associated water quality monitoring programs indicates that the amount of light reaching the benthic habitat is the primary factor limiting seagrass coverage in the lagoon. Water column turbidity, chlorophyll a concentration, and color are the principal determinants of water clarity in the lagoon, with turbidity being by far the most important (Christian and Shang 2003).

Comparison with historic data and aerial photographs from the 1940s reveals that there has been a significant decline in IRL seagrass coverage over the years. Seagrass biologists estimated that total acreage in the early 1990s was estimated to be approximately 80% of the acreage 60 years earlier. The actual extent of seagrass loss in specific portions of the estuary has been highly variable, however, and this lagoon-wide snapshot doesn't tell the whole story. In some areas, once expansive seagrass meadows had almost entirely disappeared.

In general, the areas of the IRL with the healthiest seagrass beds are adjacent to relatively undeveloped watersheds or near ocean inlets. Conversely, areas exhibiting the greatest seagrass loss are typically located near extensively developed shorelines and watersheds.

On the positive side, the last few years have seen measurable recovery of seagrass habitat in the lagoon, with expanded acreage in many segments. Some of this rebound is believed to be the result of a favorable spring-summer growing season climate the last few years in which lower-than-average rainfall has kept the lagoon water clear and at salinities that are optimum for seagrass growth. If we start to again see more typical wet season patterns in coming years, some of the recent gains in seagrass acreage may be reversed (Virnstein et al. 2007, Morris and Virnstein 2008).

Nevertheless, interventions in the form of proactive natural resource and human footprint management have also played an important part in allowing seagrass habitat recovery in the IRL. These interventions encompass all of the projects discussed here, including elimination of wastewater treatment plant discharges, management of the volume and timing of freshwater discharge from large drainage networks, muck removal, and installation of stormwater management structures (SJRWMD 2007).

References:

Christian D and YP Sheng. 2003. Relative influence of various water quality parameters on light attenuation in Indian River Lagoon. Estuarine, Coastal and Shelf Science 57: 961-971.

Morris LJ and RW Virnstein. 2008. The demise and recovery of seagrass in the northern Indian River Lagoon, Florida. Estuaries and Coasts 27:915-922.

Saint Johns River Water Management District (SJRWMD). 2007. Indian River Lagoon, An Introduction to a Natural Treasure. Saint Johns River Water Management District. 36 p.

Virnstein RW, Steward JS, and LJ Morris. 2007. Seagrass Coverage Trends in the Indian River Lagoon System. Florida Scientist 70:397-404.

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