Document Type

Statistical Report

Publication Date

2000

Abstract

The Tampa Bay Estuary Program (TBEP) has selected seagrass restoration target depths for each major bay segment at which adequate light conditions (20.5% of subsurface PAR irradiance) shall be maintained to ensure seagrass growth and the long-term Tampa Bay seagrass restoration goal of 15,400 ha. To evaluate the progress towards the goal, information on today’s seagrass depth distribution is needed. Specifically, a need exists to accurately determine the water depth at the deep edge of the meadows for each seagrass species in different sections of the bay. A relatively simple technique that provides elevation measurements, related to the mean tide level (MTL), of Tampa Bay seagrass meadows is described and evaluated. The technique uses mapping grade differential Global Positioning System (GPS) carrier-phase processing equipment that is currently owned by several TBEP partners. The elevation of a specific seagrass location is determined by placing one GPS instrument as a base station at a surveyed benchmark with a known elevation above MTL and a second instrument at the seagrass site to be surveyed. Tests of measurement errors indicate that the technique yields elevation measurements with an error that is less than ±10 cm for survey sites located up to 10 km from bench mark sites. Field evaluations of the technique that included measurements in the four major bay segments and the deep edge of the three major Tampa Bay seagrass species, Halodule wrightii, Thalassia testudinum, and Syringodium filiforme, were conducted at ten Tampa Bay seagrass study sites. The depth of the measured deep edges ranged from about -0.30 m MTL for H. wrightii meadows in the upper section of Hillsborough Bay to near -2.0 m MTL for S. filiforme meadows on the southwestern side of Middle Tampa Bay. All sites surveyed had deep edge elevations shallower than the TBEP seagrass restoration target depth for the respective bay segment. The estimated average percent of subsurface incident light available at the deep edges of the surveyed seagrass meadow ranged from 59.8% to 28.9% for H. wrightii, from 19.0% to 16.9% for T. testudinum, and from 16.7% to16.2% for S. filiforme. The differential GPS carrier-phase processing technique was field practicable and measured seagrass elevations with acceptable quality. The field measurements provided an important first step in understanding the current depth distribution of the major Tampa Bay seagrass species. However, many more elevation measurements should be obtained to yield a more complete understanding of the seagrass depth distribution in the bay.

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