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Gulf of Mexico Estuaries Integrated Science |
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Tampa Bay Pilot Study 2002 Poster Series |
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| Abstract | Introduction | Methods1 | Methods2| Results1 | Results2 | Results3 | Discussion1 | Discussion2 | Discussion3 | Future| Acknowledgments |
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| Determination of Water Depth at the Deep Edge of Seagrass Meadows in Tampa Bay Using GPS Carrier Phase Processing |
| J.O.R. Johansson, K.B. Hennenfent and J.J. Pacowta. (City of Tampa, Bay Study Group) |
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Light Availability
Light attenuation measurements of the water column directly above the deep edges of seagrass meadows in Tampa Bay are scarce. However, photosynthetically active radiation (PAR) measurements are often collected at deeper Tampa Bay sites during routine water quality monitoring. Light attenuation at the seagrass survey sites was, therefore, estimated from the deep site light data. Monthly Secchi disk depths for the period 1994-1999 collected near the seagrass elevation survey sites were converted to diffuse attenuation coefficient for PAR (KdPAR) values using bay segment specific factors derived from concurrent Secchi disk depth and PAR measurements at deep sites for the same six year period.
The percentage of subsurface light (PAR) remaining at the sediment surface at the deep edge of the seagrass meadows can be estimated from KdPAR and the seagrass elevation measurements using:
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IZ = IO * e -kz
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where IZ is the incident light (PAR) at depth z expressed as mMTL, IO is the incident light (PAR) just below the water surface, and k is the diffuse attenuation coefficient.
The estimated average percent of subsurface incident light available at the deep edges of the seagrass meadows over the six year period are shown below. The available light at the deep edges of H. wrightii meadows in all four bay segments ranged from 59.8 to 28.9% of subsurface incident light and was substantially above the adopted TBEP seagrass restoration light target of 20.5%. The available light at the deep edges of S. filiforme ranged from 31.4 to 16.0%. The relatively deep S. filiforme meadows at Coquina Key and Coffeepot Bayou in Middle Tampa Bay received less light than the target level, 16.7 and 16.2%, respectively. Available light at the deep edges of T. testudinum meadows also appeared to be lower than the target. The limited light availability measurements of this species to date ranged from 19.0 to 16.9%.
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| Light avalability at the deep edge of Tampa Bay seagrass species Halodule Syrngodium Thalassa. |
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The average percent of subsurface incident light available at the deep edges of the seagrass meadows may not correspond to the minimum light requirement for maintaining sustained growth of the different Tampa Bay seagrass species. Determination of minimum light requirements was beyond the scope of this study. Additional work is required to resolve uncertainties about extrapolating light availability data to seagrass light requirements. These uncertainties include, but are not limited to:
- Light attenuation of the water column over the seagrass meadows may be different than that estimated from deep water data.
- The time period selected for calculating the average light attenuation may not properly reflect the lag-time of seagrass growth response to changes in light availability.
- Seasonal light availability, specifically during the active seagrass growing season, may be more appropriate for estimating minimum seagrass light requirements than annual averaged values.
- Epiphytic growth on the seagrass blades may have caused additional reductions in light availability.
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