http://gulfsci.usgs.gov/tampabay/conf2002/sea_johan2/index.html
U. S. Geological Survey
Florida Integrated Science Center, Center for Coastal & Watershed Studies
600 4th Street South, St. Petersburg, FL 33701, Phone: 727-803-8747
Gulf of Mexico Estuaries Integrated Science
Tampa Bay Pilot Study
2nd Annual Science Conference, St. Petersburg, FL, Sept. 19, 2002
Poster Presentations: Seagrass Section
Task Leader: Kimberly Yates, Email: kyates@usgs.gov
Poster Title: Determination of Water Depth at the Deep Edge of Seagrass Meadows in Tampa Bay Using GPS Carrier Phase Processing
by J.O.R. Johansson, K.B. Hennenfent and J.J. Pacowta. (City of Tampa, Bay Study Group)
Email: roger.johansson@ci.tampa.fl.us
Abstract
The Tampa Bay Estuary Program has selected seagrass restoration target depths for each major bay segment at which adequate light conditions (20.5% of subsurface irradiance) shall be maintained to ensure seagrass growth and the long-term Tampa Bay seagrass goal of 15,400ha. To evaluate the progress of the seagrass restoration effort, field investigations of accurate water depths at the deep edges of Tampa Bay seagrass meadows are now underway using differential GPS carrier-phase processing.
One GPS instrument serves as a base station at a surveyed benchmark with a known altitude above the mean sea level and a second instrument is towed across the seagrass meadows. The technique yields accurate depth measurements of approximately +/- 5cm of survey sites located up to 10km from bench mark sites. The depth of the measured deep edges ranged from about -0.30mMTL for Halodule wrightii meadows in the upper section of Hillsborough Bay to near -2.0mMTL for Syringodium filiforme meadows on the southwestern side of Middle Tampa Bay. 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 31.4 to 16.0% for S. filiforme and from 20.9 to 16.9% for Thalassia testudinum.
The differential GPS carrier-phase processing technique was field practicable and measured seagrass elevations with acceptable accuracy. The field measurements provided an important first-step in understanding the current depth distribution of the major Tampa Bay seagrass species.
Introduction
The Tampa Bay Estuary Program (TBEP) has adopted a long-term Tampa Bay seagrass restoration goal of 15,400ha, which is approximately 95% of the estimated Tampa Bay seagrass cover present in 1950. Protection of the 10,400ha existing in 1994 and the restoration of an additional 5,000ha will be accomplished primarily through management of external nitrogen loadings and bay water quality (Johansson and Greening 2000).
Estimates of the 1950 seagrass depth distribution were used to develop bay segment specific seagrass target depths, expressed as mean tidal level (MTL), for Tampa Bay. The adopted approximate target depths were: -1.0m for Hillsborough Bay, -1.9m for Old Tampa Bay, -1.6 to -2.4m for Middle Tampa Bay (depending on sub-segment), and -2.5m for Lower Tampa Bay.
The estimated 1950 Tampa Bay seagrass depth distribution was important for the development of the TBEP seagrass restoration and protection goal. Likewise, information on today=s seagrass depth distribution is needed to evaluate the progress of the seagrass restoration process. Further, the present seagrass depth information combined with light attenuation data from routinely conducted water quality monitoring programs, could be used to calculate the percentage subsurface irradiance available for different seagrass species found in the different bay segments and potentially also be used to estimate specific seagrass species light requirements. Finally, these elevation measurements would also complement the cooperative Tampa Bay permanent seagrass monitoring program by providing detailed seagrass elevations and transect profiles (see Avery et al. this poster session).
Methods:
Determination of Measurement Errors
Trimble specifications for the GPS Pathfinder PRO XR system with carrier-phase post-processing report the accuracy of position determinations as approximately 5cm+5ppm with 30min of satellite tracking (see figure below). The 5ppm error is caused by the distance between the base and the rover stations (baseline) and equals 0.5cm of error for each kilometer of separation. Additionally, the vertical error for carrier-phase processing solutions is similar to the horizontal error. The accuracy of the measurements can be further improved by at least 45min of of satellite tracking and post-processing the data using the Trimble Centimeter Option software.
Tests were conducted on the roof of the City of Tampa Bay Study Group (COT) laboratory to determine the vertical measurement performance of the PRO XR system both with and without using the Centimeter Option (see photograph below). Two PRO XR instruments were placed on the roof at a location with a known MTL elevation. The two antennas were located at near identical elevations and separated less than 1.0m horizontally. One instrument was used as a base station and the other as a rover station. The instruments were configured to the Trimble recommendations and the time interval between measurements was 5s.
Measurements of Seagrass Elevations and Transect Profiles
Numerous seagrass elevation measurements and seagrass transect profiles have been performed in Tampa Bay since October 1999 (see map). Specific deep edge measurements were conducted at ten general areas in the four major bay segments. Seagrass species investigated were: H. wrightii, T. testudinum, and S. filiforme. Most study areas were located at, or close to, an established Tampa Bay fixed seagrass transect and included different seagrass species when present. Further, the depth profile of 20 of the 62 seagrass transects included in the annual interagency seagrass transect monitoring program have been surveyed to this date. These profiles start at the shoreline and extend across the shallow sandbar to approximately the 2m depth contour.
Prior to conducting the elevation measurements at seagrass areas and transects, suitable bench marks had to be located within 10km of the survey sites. NOS tidal bench marks were the primary type used.
Typical set-ups of the GPS instruments for field elevation measurements are shown below. The base station antenna was placed vertically above the bench mark and the rover station antenna was placed, either on a tripod vertically above the seagrass edge (static technique), or as shown here for transect profiling, on a pole attached to a benthic sled (Aon-the-fly” technique). Recorded GPS satellite data was post-processed using the Trimble phase processor software. The software calculated the relative elevation difference between the two antennas (D). Since the MTL elevation of the bench mark (B) was known and the antenna heights (A and C) had been measured in the field, the MTL elevation of the survey site (X) could easily be calculated.
Results:
Measurement Errors
Results from one of several tests conducted on the roof of the laboratory to determine elevation measurement errors of the PRO XR system with the Centimeter Option processor are shown to the right. Measurements were collected every 5s over approximately 1.5h. The true elevation difference between the two instruments was 0m. Measured differences ranged between +1.9 to -2.5cm. The average difference of the 934 measurements was 0.2cm (STD 1.0cm). It should be noted that most of the deep edge seagrass elevation data were processed using an earlier version of Trimble software that gave larger errors.
The baseline error during these tests was near zero since the two antennas were only separated by less than 1.0m. However, the potential baseline error must be considered during field measurements. The baseline distance should, therefore, be kept as short as possible and not exceed 10km.
Seagrass Elevations
Results of seagrass elevation measurements using the static technique are shown in the table below. The shallowest deep edge of H. wrightii meadows was found in the upper section of Hillsborough Bay (-0.30 to -0.34mMTL). Deep edge depths were intermediate in lower Hillsborough Bay and the north-eastern area of Middle Tampa Bay (-0.48 to -0.58mMTL). The deepest H. wrightii surveyed was found at similar depths at Big Island in Old Tampa Bay and at Port Manatee in Lower Tampa Bay (-0.71 to -0.76mMTL).
Deep edges of T. testudinum meadows were found at similar depths at Bel Mar Shores in eastern Old Tampa Bay and at Port Manatee. Depths for these edges ranged from -1.53mMTL at Bel Mar Shores to -1.73mMTL at Port Manatee. Isolated patches of T. testudinum located on the shallow sandbar at Picnic Island and the Wolf Branch area were found at considerably shallower elevations (-0.53 to -0.90mMTL).
Deep edges of S. filiforme meadows were also found at similar depths at the sites in eastern Old Tampa Bay and at Port Manatee. Depths for these edges ranged from -1.19 to -1.46mMTL. However, the deepest S. filiforme edges were measured at the two sites on the western side of Middle Tampa Bay. At Coffeepot Bayou the deep edge was found between -1.79 and -1.81mMTL and at Coquina Key between -1.93 and -1.96mMTL. The latter depths were the deepest seagrass elevations measured in this study.
Technique Evaluation
Results from tests of measurement errors indicate that the technique using PRO XR instruments and the Centimeter Option Phase Processor software will yield elevation measurements with an error approximating +/- 5cm for survey sites located up to10km from bench mark sites. The 5s minimum time interval between measurements will allow for 720 elevation measurements per hour.
Field evaluations, which included both static and Aon-the-fly@ elevation measurements, found the the GPS carrier-phase processing technique to be practical. Further, excellent replications of elevations were obtained when several measurements were taken in the same general area using different bench marks.
Transect Profiles
Most of the depth profile information collected to date using the Aon-the-fly@ benthic sled technique has not yet been fully analyzed. Two examples of profiles are shown below: the Kitchen and Wolf Branch areas in Hillsborough Bay and Middle Tampa Bay, respectively.
The transect located in the Kitchen ranges from shore to 1350m offshore. The section of the transect from approximately 200 to 620m contains the main H. wrightii meadow. The majority of the seagrass on this transect is, therefore, found between -0.25 and -0.50mMTL depths. The elevation of the deep edge of the meadow (-0.50mMTL) agrees with the earlier elevation studies conducted using the static technique (see Johansson 2000).
The Wolf Branch transect ranges from shore to approximately 1200m offshore. The 100 to 360m section of the transect contains the main H. wrightii meadow. On this transect the main seagrass meadow is found between -0.25 and -0.55mMTL depths, which is almost identical to the Kitchen meadow.
Discussions:
Seagrass Elevations
Deep edge elevations of the measured seagrass meadows ranged from -0.30mMTL for H. wrightii in the upper portion of Hillsborough Bay to -1.96mMTL for S. filiforme in southwestern Middle Tampa Bay. Further, all sites visited in the present study had deep edge elevations shallower than the TBEP seagrass restoration target depth for the respective bay segment. The greatest deviation from the target depth was found at the Long Branch and Big Island sites in western Old Tampa Bay, where the deep edges of the H. wrightii meadows were about 1.20m shallower than the -1.9mMTL target depth.. The least deviation was found at three sites: the H. wrightii meadow in the Kitchen in southeastern Hillsborough Bay, the T. testudinum meadow at Bel Mar Shores in eastern Old Tampa Bay, and the S. filiforme meadow at Coquina Key in southwestern Middle Tampa Bay. These three areas had deep edges that were approximately 0.50m shallower than the respective bay segment targets.
Additional elevation measurements in Lower Tampa Bay may find deeper seagrass edges than those measured at Port Manatee. Dixon (2000) conducted light requirement studies on T. testudinum sites in Lower Tampa Bay that ranged in depth from -1.98 to -2.37mMTL. These depths, which were estimated from sea surface observations, are approximately 0.3 to 0.6m deeper than the T. testudinum meadows surveyed at Port Manatee.
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:
IZ = IO * e -kz
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%.
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:
Recommendations for Future Studies
Recently, seagrass recovery has stagnated in several areas of Tampa Bay, despite ambient water quality and light availability conditions that appear adequate to support continued seagrass expansion. For example, the deep edges of the H. wrightii meadows in southeastern Hillsborough Bay and in eastern Middle Tampa Bay were estimated to receive an average 44 and 57% of the incident light, respectively. These light levels are considerably greater than the 20.5% light target adopted by the TBEP, however, no significant expansion of these meadows into deeper water has occurred over the last five years.
Many factors may limit seagrass expansion in Tampa Bay in addition to water quality. Lewis et al. (1985) discussed the importance of an offshore unvegetated sand bar, that separates the main seagrass meadow from the open bay waters, to protect the seagrass meadow by reducing wave impacts from storms and ship traffic. Destabilization and the ultimate loss of the bar may result in the shoreward migration of the seagrass meadow. However, studies examining the dynamics of the shallow sand bars and their interaction with the development of seagrass meadows are lacking for Tampa Bay.
Additional elevation measurements are recommended to learn more about the seagrass depth distribution and the dynamics of the shallow sand bars in Tampa Bay. The GPS carrier-phase processing technique could be used to accurately and quickly determine the transect depth profiles of the 62 bay-wide seagrass monitoring transects (see Avery and Johansson 2001). Further, the depth distribution of the different seagrass species found on each transect could easily be determined during the profile measurements.
Periodically conducted seagrass and transect elevation measurements will provide important information to complement the biennial high altitude aerial seagrass photography conducted by Southwest Florida Water Management District and the yearly cooperative Tampa Bay seagrass transect monitoring program. Combined, the three programs would become a powerful tool for evaluating the progress of the Tampa Bay water quality and seagrass restoration effort.
Literature Cited
Printed by The City of Tampa, Department of Sanitary Sewers, Design Division
Captions:
(Under bar graph) Performance of the Trimble Phase Processor v.2 software with the GPS Pathfinder PRO XR system according to Trimble (1997). Figure modified from Trimble (1997). Horizontal errors are shown, however, Trimble (1997) states that vertical and horizontal errors are similar for phase processed solutions.
(Under equipment) Trimble Pathfinder PRO XR instruments located on the roof of the City of Tampa Bay Study Group laboratory during tests of vertical measurement errors.
(Under cartoon) Schematic of typical GPS stations set-up during “on-the fly” elevation measurements.
A = Base station antenna height.
B = Bench mark elevation above MTL tidal stage.
C = Rover station antenna height.
D = Relative elevation difference between base station and rover station antennas.
X = Calculated elevation of the deep seagrass edge.
(Under boat) A benthic sled equipped with the rover station antenna is towed along the transect for “on-the-fly” measurements of seagrass transect elevation profiles.
(Under tbay Map) Locations of seagrass elevation survey sites in Tampa Bay. Also shown are major bay segments (HB=Hillsborough Bay; OTB=Old Tampa Bay; MTB=Middle Tampa Bay [including sub-segments]; and LTB=Lower Tampa Bay).
(Under green bargraph) Light avalability at the deep edge of Tampa Bay seagrass species Halodule Syrngodium Thalassa
(Over Table) Results of GPS seagrass elevation measurements conducted for different seagrass species in different sections of Tampa Bay. Elevation expressed as mean tie level (MTL)
(Kitchen)Oct. 26, 1999: Depth distribution of the main H. wrightii meadow along the transect located in the Kitchen in Hillsborough Bay.
(Wolf Branch) Dec. 8, 1999: Depth distribution of the main H. wrightii meadow along the transect located at Wolf Branch in Middle Tampa Bay.
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