Geospatial Video Monitoring of Nearshore Benthic Habitats of Western Biscayne Bay (Florida) Using the Shallow-Water Positioning System (SWaPS)

Transcription

1 Journal of Coastal Research 24 1A West Palm Beach, Florida I January 2008 Geospatial Video Monitoring of Nearshore Benthic Habitats of Western Biscayne Bay (Florida) Using the Shallow-Water Positioning System (SWaPS) Diego Lirmanl, Gregory Deangelo", Joseph E. Serafy*, Amit Hazra 1, Destiny Smith Hazra 1, and Alexandra Brownft tuniversity of Miami Rosenstiel School of Marine and Atmospheric Science 4600 Rickenbacker Causeway Miami, FL 33149, U.S.A. dlirman@rsmas.miami.edu *NOAA/National Geodetic Survey 1315 East-West Highway Silver Spring, MD U.S.A. ýnoaa/national Marine Fisheries Service 75 Virginia Beach Drive Miami, FL , U.S.A. ttuniversity of Miami Department of Marine Science, Coral Gables, FL 33124, U.S.A. e#11lll116, ABSTRACT LIRMAN, D.; DEANGELO, G.; SERAFY, J.E.; HAZRA, A.; HAZRA, D.S.; and BROWN, A., Geospatial video monitoring of nearshore benthic habitats of western Biscayne Bay (Florida) using the shallow-water positioning system (SWaPS). Journal of Coastal Research, 24(1A), West Palm Beach (Florida), ISSN The nearshore habitats of western Biscayne Bay, a shallow lagoon adjacent to the city of Miami, are influenced by salinity fluctuations caused by freshwater discharges from canals. Benthic communities in these susceptible littoral habitats have been underrepresented in monitoring programs because of the difficulties associated with boat access. In the present study, we implement a geospatial video-based survey technique, the shallow-water positioning system (SWaPS), to document the abundance and distribution of benthic organisms in these shallow habitats. Mounted on a shallow-draft vessel, SWaPS integrates a global positioning system receiver with a video camera such that each video frame recorded is stamped with position information, date, water depth, heading, and pitch and roll. The georeferenced digital frames collected can be easily analyzed to document patterns of abundance and distribution of submerged aquatic vegetation and other benthic organisms. The field surveys conducted using SWaPS showed that species distributions are influenced by their respective tolerances to salinity patterns. Seagrass species with relatively high tolerance for low, variable salinity (e.g., Halodule wrightii and Ruppia maritima) tend to have high abundance only in areas influenced directly by canal discharges, while species with relatively limited tolerance for low salinity (e.g., Thalassia testudinum) tend to increase in abundance with increasing distance from the mouths of canals. The use of video-based surveys with high spatial precision facilitates rapid, cost-effective, and repeatable monitoring of shallow marine benthic communities. The most attractive features of this system are (1) the ability to cover large areas rapidly without divers and (2) the ability to return to precise locations without establishing permanent markers (e.g., stakes). Moreover, the georeferenced digital images collected with SWaPS are a valuable permanent visual archive that can provide the baseline information needed to evaluate long-term patterns of change in environments such as western Biscayne Bay that are subject to increasing pressure from human activities. Geospatial video surveys, salinity patterns, seagrass distribution, Everglades resto- ADDITIONAL INDEX WORDS: ration. INTRODUCTION Bay and estuarine systems are among the world's most productive ecosystems, providing essential habitat for wildlife and both commercial and recreational fisheries resources. In the United States, a disproportionately large percentage of the population lives along coastal areas, placing an increasing level of environmental pressure on these critical habitats. Locally, rapid levels of coastal development and exploitation have resulted in habitat losses and significant reductions in productivity (NRC, 2000). The coastal bays of South Florida are uniquely productive and diverse natural resources located just downstream of the Everglades and an upland drainage basin that has been highly modified by human activities. DOI: / received 17 December 2004; accepted in revision 12 January Within the coastal lagoons of South Florida, benthic habitats dominated by seagrasses and macroalgae play a key role by providing food and shelter for fish and invertebrates (SERAFY et al., 1997) and influencing sedimentation and water clarity patterns (see FOURQUREAN et al., 2002, and references therein; WALKER et al., 2001). Because of their ecological role, seagrasses have been proposed as one of the best indicators of the status and change in condition of Florida's coastal hydroscape (DURAKO, HALL, and MERELLO, 2002). In the present study, we describe the application of a newly developed video-based survey methodology that is especially well suited for conducting georeferenced surveys of the shallowest benthic habitats of coastal lagoons, which have been largely neglected primarily because of logistic problems associated with boat access. These habitats support healthy seagrass and hardbottom communities

2 136 Lirman et al. 136 Lirman et al. (LIRMAN et al., 2003; LIRMAN and CROPPER, 2003) and are critical nursery habitats for pink shrimp, Farfantepenaeus duoraraum, and fishes such as gray snapper, Lutjanus griseus, and spotted seatrout, Cynoscion nebulosus (AULT et al., 1999a, 1999b; DIAZ, 2001; SERAFY et al., 1997, 2001). However, because of their location at the land-sea interface, these living resources are especially vulnerable to changes in freshwater flows from upland sources. The hydrology of the Everglades drainage basin has been severely modified over the last 70 years by the construction of the Central and Southern Florida Project, an extensive water management system comprised of levees, canals, control structures, and pumping stations that have modified the natural timing, quantity, and quality of freshwater flows across the landscape (BROWDER and OGDEN, 1999; McIvoR, LEY, and BJORK, 1994). In Florida Bay and Biscayne Bay, the modifications to the natural drainage system have resulted in (1) a reduction in the total amount of freshwater reaching the bays and (2) a shift from overland and groundwater sources of freshwater to highly concentrated releases from canals (LIGHT and DINEEN, 1994; SKLAR et al., 2002). These modifications have caused adverse ecological changes that include (1) the mass mortality of seagrasses within Florida Bay (DuRAKO, 1994; ROBBLEE et al., 1991; ZIEMAN, FOURQUR- EAN, and IVERSON, 1989; ZIEMAN, FOURQUREAN, and FRON- KOVICH, 1999); (2) significant declines in pink shrimp catches (BROWDER, 1985; BROWDER et al., 1999); and (3) the mortality of marine sponges (BUTLER et al., 1995). In response to these patterns of decline, the Comprehensive Everglades Restoration Plan (CERP) has been designed to restore, through a series of large-scale engineering projects, some of the lost natural hydrologic characteristics of the South Florida regional ecosystem. One of the explicit goals of CERP is the modification of freshwater delivery patterns into coastal bays to recover estuarine conditions that once prevailed along littoral environments (BROWDER and WANLESS, 2001; DAVIS and OGDEN, 1994; STEINMAN et al., 2002). Increased freshwater inflows through canals, tidal creeks, and marshes are expected to lower salinities at the point of discharge and expand areas of mesohaline conditions with largely unknown ecological effects on the organisms found there (BROWDER and WANLESS, 2001). Considering the potential future impacts of CERP projects, it is crucial to document present-day patterns in abundance, distribution, and condition of benthic communities at nearshore habitats likely to be affected by CERP activities. Previous monitoring, modeling, and experimental studies have documented the significant influence of salinity patterns on the abundance, distribution, diversity, growth, and mortality of submerged aquatic vegetation (SAV), especially seagrasses within coastal lagoons and estuaries (e.g, FOUR- QUREAN et al., 2003; LIRMAN and CROPPER, 2003; MCMIL- LAN, 1974; QUAMMEN and ONUF, 1993; ZIEMAN, FOURQUR- EAN, and IVERSON, 1989). In Florida Bay, areas with low and variable salinity are typically dominated by mixed communities of Halodule wrightii and Ruppia maritima, two seagrass species with high tolerance to low-salinity patterns (FOURQUREAN et al., 2002; ZIEMAN, FOURQUREAN, and IVER- SON, 1989), while habitats with higher, more stable salinity patterns are dominated by Thalassia testudinum, or Syringodium filiforme in deeper habitats (HALL et al., 1999; ZIE- MAN, FOURQUREAN, and IVERSON, 1989; ZIEMAN, FOURQUR- EAN, and FRANKOVICH, 1999). However, limited information is available to evaluate whether present-day seagrass abundance and distribution patterns are influenced by salinity patterns in Biscayne Bay. In this study, we describe the use of the shallow-water positioning system (SWaPS), a geospatial video-based survey methodology, to (1) evaluate whether distinct patterns in abundance and distribution of submerged aquatic vegetation can be observed along the littoral zone of western Biscayne Bay in relation to patterns in salinity created by the inflow of freshwater from upland sources and (2) provide a spatially explicit baseline database on the abundance and distribution of benthic organisms against which the effects of future watershed restoration activities may be discerned. Study Area METHODS Biscayne Bay is a shallow subtropical lagoon adjacent to the city of Miami (Figure 1A). The location of Biscayne Bay along a highly populated, rapidly growing urban center and directly downstream of CERP activities on the watershed makes this important natural resource especially vulnerable to human activities and changes in water quality and flow (SERAFY et al., 2001; LIRMAN et al., 2003). Salinity fields within Biscayne Bay are influenced by precipitation; freshwater inputs from land, canal, and groundwater sources; and tidal influx of oceanic water. Presently, freshwater from upland sources is carried into western Biscayne Bay primarily through control structures managed to meet agricultural, municipal, and flood-control needs and objectives (WANG, Luo, and AULT, 2003). The spatial and temporal distributions of these influences delineate regions with distinct salinity characteristics. Areas with low and variable salinity are found along the western margin due to freshwater inflows from canals, groundwater sources, and surface runoff, while higher, more stable salinities are found where oceanic influences prevail (ALLEMAN, 1995; MEEDER et al., 1997; SERAFY, FAUNCE, and LORENZ, 2003; WANG, Luo, and AULT, 2003). The lowest mean salinity values as well as the greatest salinity variability can be observed in the area between Black Point and north of Turkey Point, where four canals discharge directly into the bay (AULT et al., 1999b; WANG, Luo, and AULT, 2003) (Figure 1A). The Shallow-Water Positioning System The shallow-water positioning system, developed by scientists from the National Oceanographic and Atmospheric Administration's National Geodetic Survey, uses a global positioning system (GPS) receiver integrated with a video camera and installed on a shallow-draft vessel (14-ft Carolina skiff) (Figure 1B). The GPS receiver is centered over a gimballed digital video camera that is suspended over a Plexiglas enclosure that provides a looking view of the bottom. A static GPS base station is established in the vicinity of SWaPS op- Journal of Coastal Research, Vol. 24, No. IA (Supplement), 2008

3 Geospatial Video Surveys of Benthic Marine Communities 137 Figure 1. (A) Map of South Florida and the study area showing the location of the water management canals in white and the location of the salinity probes (stars) used to measure salinity patterns (see Figures 3B and 5A for values). (B) Photograph of the shallow-draft vessel used during our surveys. (C) Example of the digital frames obtained during the surveys using the shallow-water positioning system (SWaPS). erations and serves to track the detectable GPS satellites in synchrony with the mobile GPS receiver onboard the SWaPS survey vessel. Both receivers record the GPS LI and L2 carrier phases and code ranges every second during operations. After each survey period, both data files are postprocessed using the software program KINPOS as described in MADER (1996). The position of the base station is accurately determined using OPUS, a GPS processing service created by the National Geodetic Survey (see OPUS). The code ranges are used in differential mode to locate the position of the SWaPS platform. The data collected by the base station are relayed via radio modem to the SWaPS survey platform where the data can be processed in real time as described above. Each video frame recorded is stamped with time, date, water depth, heading, pitch, and roll (Figure IC). The time code is used to retrieve the precise location of each frame based on the location of the vessel with respect to the base station. Although not used in the present study, water depth, pitch, roll, camera specifications, and lens specifications can be used to obtain density and size estimates for organisms and/or for other benthic features. The video surveys conducted with SWaPS provide a continuous digital video track of the bottom. The data are archived in video format and by grabbing frames at a rate of one frame per second and storing these as digital still images. A geospatial information system (GIS) is used subsequently to link the geospatial (locations) and thematic (descriptive) data to their respective images. Spatial Precision of SWaPS While high spatial precision can be achieved on land using a GPS base station, field conditions (e.g., winds, currents, wave action) can often limit the ability to return to the same Journal of Coastal Research, Vol. 24, No. 1A (Supplement), 2008

4 138 Lirman et al. 138 Lirman et al. location in water. The spatial precision (i.e., the ability of the system to return to a given location after a position has been established) of SWaPS was tested using ceramic tiles (15 cm x 15 cm). The location of each tile on the bottom was determined by maneuvering the boat over the tiles and capturing each one at the center of the video frame. The following day, the position of each tile was retrieved from the video and entered as waypoints into a real-time kinematic GPS-navigation software package. With the GPS unit as a guide, the boat was repositioned over each waypoint. If any portion of the tile could be seen on the video screen, the tile was counted as a hit. If the tile was not seen on the screen, the distance between the position of the boat and the position of the tile was estimated to determine the extent of the miss. Field tests showed that SWaPS can provide submeter precision consistently in shallow coastal areas. Out of the 30 tiles deployed, 24 (80%) were relocated within the video frame using only the positions recorded during deployment. At the depths at which the tiles were deployed ( cm), the video frames covered a maximum width of approximately 60 cm at the bottom. Therefore, the minimum spatial precision for the relocation of these tiles was 50 cm, the maximum distance between the center of the frame where the tile was originally positioned during deployment and the center of a tile for which only a small portion is showing in the video frame during recovery. For the tiles that were not relocated within the video screen (20%), the mean distance to the center of the frame was 75 cm (SD = + 12). Field Surveys In February-April of 2003, field surveys were conducted in western Biscayne Bay following the shoreline at a distance of <300 m from shore at depths of cm. A subset of stations (n = 130) was sampled at -300-m intervals from the continuous video data set collected along the survey path (Figure 2). For each survey location (i.e., a transect <25 m along the survey track), 10 nonoverlapping frames were chosen at random from the image library. This approach was chosen to be consistent with the methods used by existing long-term SAV monitoring programs in the region, where multiple sites are sampled visually using 0.25-M 2 quadrats (FOURQUREAN et al., 2002). For each georeferenced digital image (i.e., the sample unit for that site), community type, species observed, and abundance (percentage cover) were recorded from a computer monitor. The contrast and brightness of each image were adjusted to improve classification. Percentage cover was determined as the fraction of each frame occupied by each taxon and the values recorded for each frame were averaged by site (n = 10 frames per site). The hypothesized direct influence that freshwater discharge from canals may have on the abundance and distribution of benthic organisms was tested by conducting detailed SWaPS surveys in the immediate vicinity of two canals, Military Canal and Mowry Canal (Figure 3A), in August These canals were chosen because they are in close proximity to each other (2.2 km) but differ significantly in the amount of freshwater they deliver to western Biscayne Bay ( These areas were sampled in a regular grid pattern following parallel tracks centered on the canal structures (Figures 3C, 3F). Survey tracks (n = 8) were separated by a distance of 75 m, and sites along each track were surveyed at 75-m intervals using preloaded GPS tracks (n = 6 sites per track). For each survey location (i.e, a transect of m along the survey track), 10 nonoverlapping frames were chosen at random from the image library and analyzed using the methods previously described. The benthic coverage data obtained were averaged for each site and used to develop percentage cover surface contours using ArcView's spatial analyst using an inverse distance weighted interpolation procedure. The distance between each survey point and the discharge point of each canal (i.e., the point at which the canal structure enters the bay) was used to test the hypothesis that distance to canals influences the abundance and distribution of seagrasses using regression analysis. Calibration between SWaPS and Visual Surveys Because other benthic research programs in the region use visual surveys performed by divers to estimate abundance and distribution of benthic organisms (FOURQUREAN et al., 2002), it is important to compare the values obtained using SWaPS with those obtained by trained observers at the same sites. To provide this calibration, a subset of sites (n = 22) with different characteristics (e.g., seagrass-dominated, algaldominated, and mixed communities) was surveyed using both methods, and benthic attributes and time and effort estimates were compared. Trained observers followed the methods outlined by FOURQUREAN et al. (2002) to estimate the percentage cover of benthic organisms from 10 haphazardly placed PVC quadrats (0.25 M 2 ) along a m transect. The same area was surveyed using SWaPS, and the data were analyzed as described above. The estimates of the mean percent cover for each seagrass species and macroalgal group were compared between survey methods for each site individually as well as among all sites (i.e., pooled data) using a Wilcoxon test due to nonconformity to the normality assumption required for parametric tests. Salinity Patterns Salinity patterns in the shallowest habitats of western Biscayne Bay were measured using miniature sensors (1.5 X 5.0 cm) developed by Star-Oddi (see deployed between March and September Salinity records for these habitats are generally lacking because of problems associated with boat access and because most water quality instruments available are too large to be deployed in extremely shallow habitats. The probes were positioned at a distance of <50 m from shore in two areas influenced by canal flows (Military and Mowry canals), as well as in the Turkey Point area where no canals are found (Figures 1A, 3A). Data were collected at 30-minute intervals. In addition, the amount of freshwater discharged through Military and Mowry canals as well as the precipitation values in the vicinity of these canals were obtained from historical records from the South Florida Water Management District ( The daily canal discharge (cubic feet/s) and precipitation values were added to obtain monthly values, Journal of Coastal Research, Vol. 24, No. 1A (Supplement), 2008

5 Geospatial Video Surveys of Benthic Marine Communities 139 Figure 2. Abundance of seagrasses (mean percentage cover) in western Biscayne Bay in February-April (A) Thalassia testudinum, (B) Halodule wrightii, (C) Syringodium filiforme, (D) Ruppia maritima. Halodule Thalassia I 1) e? 777/ - Nicaii % Cover I *0-10 Thalassia -"" l40-60 l I/ A" APR MAY JUN JUL AUG SEP 250 m r Figure 3. Abundance and distribution of seagrasses in the vicinity of water management canals that carry freshwater from upland sources into western Biscayne Bay in August (A) Map of the study area with the location of Military and Mowry canals, (B) Mean daily salinity recorded in 2005 using salinity loggers placed at a distance of -50 m from the point of freshwater discharge. Data collected at 30-minute intervals were averaged to obtain the daily means. The arrow in this panel shows a rapid drop in salinity (i.e., over a 2-day period) observed in June 2005 when >8 inches of rain were recorded. (C, F) Aerial photographs of the study area showing the location of the survey points. (D, E, G, H) Percentage cover contours for the dominant seagrass species obtained using data from 48 survey points. Journal of Coastal Research, Vol. 24, No. IA (Supplement), 2008

6 140 Lirman et al. 140 Lirman et ol. Figure 4. Percentage cover of (A) Thalassia testudinum and (B) Halodule wrightii in the region between Black Point and Turkey Point, an area with low and variable salinity because of the inflow of freshwater from water management canals. and monthly values were averaged for the period of , providing a 10-year mean. Monthly values for precipitation and canal flows were also obtained for the period March-September Field Surveys RESULTS The skiff used by SWaPS performed well in the shallow environment of western Biscayne Bay, and approximately 35 km of littoral habitats at depths <75 cm were easily surveyed. Moreover, the shallow draft of the vessel allowed us to survey efficiently even the shallowest habitats (<40 cm) that are hard to access using most other platforms. Depending on the depth of the habitats, survey speeds of <1 knot provide the best image quality during continuous surveys. The average time required to collect georeferenced video along a 25-m transect is 2-3 minutes and, depending on the spacing of the survey locations, a large number of sites can be easily surveyed in a short period of time. In applications such as the canal surveys where sites are closely spaced, up to 100 sites can be surveyed in a day. Three main benthic community types were documented in western Biscayne Bay: (1) seagrass communities composed of one or more of four seagrass species (T testudinum, H. wrightii, S. filiforme, R. maritima); (2) macroalgal communities with attached and/or drift components; and (3) hardbottom communities composed of sponges, soft corals, and hard corals. While these were the three main categories, mixed benthic communities composed of organisms from two or more of these broad categories were commonly observed. The most abundant seagrass species, T testudinum, was found throughout the study area (82% of sites [107 of 130]; mean percentage cover [SD] = 43.0 [38.9]). The lowest abundance of this species was recorded in the areas south of Black Point and directly opposite Military and Mowry canals (Figures 2A, 4A). In contrast, H. wrightii (18% of sites; mean percentage cover = 6.0 [17.9]), S. filiforme (9% of sites; mean percentage cover = 4.8 [16.9]), and R. maritima (12% of sites; mean percentage cover = 0.6 [3.7]) had lower overall abundance and restricted spatial distributions (Figure 2B). Syringodium filiforme was restricted to the northern section of the survey area, while R. maritima was restricted to the southern region in areas directly influenced by freshwater inflows from canals. The highest abundance of R. maritima was documented in the vicinity of Black Point and Military Canal. Similarly, the distribution of H. wrightii was associated with areas of relatively high freshwater inflow reaching high abundance only in the immediate vicinity of the Coral Gables Canal, Snapper Creek Canal, Princeton Canal, Military Canal, and Mowry Canal (Figures 2B, 4B). Finally, only a few sites in western Biscayne Bay were completely devoid of submerged aquatic vegetation. In fact, only 2% of sites had no seagrass biomass present and 10% of sites had no macroalgal biomass present. Attached and drift macroalgae are important components of the benthic communities of western Biscayne Bay and were found throughout the area at 47% and 70% of sites respectively. The main components of the attached macroalgal group included Halimeda spp., Caulerpa spp., Penicitlus spp., Batophora spp., and Acetabularia sp., while Laurencia spp., Chondria spp., and Dictyota spp. were the most abundant Journal of Coastal Research, Vol. 24, No. 1A (Supplement), 2008

7 Geospatial Video Surveys of Benthic Marine Communities 141 components of the drift macroalgal group. Sponge-dominated hardbottom communities were only found at two sites in the northern region of the study area. Only three seagrass species, T testudinum, H. wrightii, and R. maritima, were found in the vicinity of Military and Mowry canals in August The benthic habitats near both canals were dominated by H. wrightii and mats of the drift alga Laurencia (Figures 3D, 3G). The low abundance (<3% maximum cover) and distribution (present at only 21% of sites) of R. maritima precluded meaningful statistical analyses. At Military Canal, the abundance of both H. wrightii and T testudinum was significantly influenced by distance to the canal discharge point (linear regression, p < 0.01). The abundance of H. wrightii decreased with distance to the canal discharge point, and the opposite pattern was documented for T testudinum (Figures 3D, 3E). At Mowry Canal, only the abundance T testudinum decreased significantly with distance to the canal (p < 0.01), while no significant spatial patterns with respect to canal influences were observed for H. wrightii (Figures 3G, 3H). Only six stations (12.5%) were completely devoid of seagrass in the vicinity of Mowry Canal, while no stations devoid of seagrass biomass were observed in the vicinity of Military Canal. Calibration between SWaPS and Visual Surveys Twenty-two sites were surveyed visually by trained observers in situ and by SWaPS. The time it took a trained observer to collect percentage cover data from ten 0.25-iM 2 quadrats in the field ranged from 10 to 20 minutes, depending on the composition of the benthic community. Using SWaPS, video was collected over a 2-3-minute period. The time required to score on a computer monitor the 10 digital frames chosen from each site ranged from 5 to 12 minutes. When data were pooled among all sites, no significant differences in the percentage cover of T testudinum, H. wrightii, S. filiforme, R. maritima, drift macroalgae, and attached macroalgae were found between survey methods (Wilcoxon test, p > 0.05). When data were analyzed for each site separately, no significant differences in the percentage cover of T testudinurn were found for any of the sites. For the other species or groups, no significant differences were found for H. wrightii at 18 sites (82% of sites), S. filiforme at 86% of sites, R. maritima at 77% of sites, drift macroalgae at 77% of sites, and attached macroalgae at 73% of sites. Salinity Patterns The data collected at three locations revealed the influence of precipitation and canal outflow on the salinity patterns of western Biscayne Bay as well as the wide fluctuation in salinity that can be caused by precipitation and canal releases. During the dry season (March-May), mean salinity was higher at all three sites compared to the values obtained during the wet season (June-September) (Figure 5A). Moreover, the areas in the vicinity of Military and Mowry canals had consistently lower mean monthly salinity compared to the Turkey Point area, where no canal discharge is present (Figure 5A). Similarly, the variability in salinity (expressed as the coefficient of variation [CVD) increased with decreasing salinity in the wet season, and canal-influenced areas had a higher degree of variability compared to non-canal-influenced areas (with the exception of September, when a higher CV was obtained for the Turkey Point area compared to the Military Canal area) (Figure 5B). These patterns were accentuated in the wet season when both precipitation and canal discharges increase (Figure 5C, 5D). While both canal-influenced areas had lower salinity than the non-canal-influenced area, mean salinity in the vicinity of Mowry Canal was consistently lower than the mean salinity recorded in the vicinity of Military Canal (with the exception of May when this pattern was reversed). Finally, the high-resolution data collected in the immediate vicinity of canals show that these areas can experience drastic fluctuations in salinity, especially during South Florida's wet season when mean daily salinity can drop by as much as 35 psu over a 2-day period and remain <5 psu for >7 days (Figure 5B). The salinity patterns documented can be explained by the canal-specific and seasonal differences in freshwater flow. Canal discharge rates were higher in the wet, rainy season than in the dry season. Moreover, discharge values were considerably higher for Mowry Canal compared to Military Canal throughout the year. The only time when canal flow values were higher at Mowry Canal in 2005 was in May (Mowry Canal = 67 cubic feet/s, Military Canal = 32 cubic feetds), which corresponds with the period when monthly mean salinity was lower in the vicinity of Mowry Canal (Figure 5A). Finally, the 10-year record of precipitation correlates with the observed salinity patterns, with a marked increase in precipitation in June at the onset of the rainy season that is captured in the drop in salinity at all three sites at this time (Figures 5A, 5D). The precipitation values recorded for 2005 were within the 95% confidence interval recorded for the 10- year precipitation mean (Figure 5D). DISCUSSION The abundance and distribution patterns recorded with SWaPS are consistent with previous reports that showed that the large-scale abundance, diversity, and spatial distribution of benthic organisms are often correlated with salinity patterns. The high abundance of Halodule wrightii and Ruppia maritima in areas of freshwater discharge from canals corresponds well with their reported tolerance for low and variable salinity (BIRD et al., 1993; McMAHAN, 1968; McMIL- LAN, 1974). Halodule wrightii is commonly regarded as an early successional species able to survive where other seagrass species would be removed by disturbance and remain dominant under fluctuating conditions (FOURQUREAN et al., 1995; MONTAGUE and LEY, 1993), while R. maritima is a species that thrives under low-salinity conditions (KANTRUD, 1991). The distribution of S. filiforme, restricted to the northern portion of the study area, is also consistent with studies that suggest that this species can only sustain high productivity in well-flushed areas (FOURQUREAN et al., 2002; ZIE- MAN, FOURUREAN, and FRANKOVICH, 1999) as in the northern section of the study region where oceanic influences prevail (WANe,, Luo, and AULT, 2003). The detailed surveys conducted in the vicinity of two canals Journal of Coastal Research, Vol. 24, No. 1A (Supplement), 2008

8 142 Lirman et at. 142 Lirman et at. [] Military Canal E Mowry Canal E] Turkey Point C. U S e S C Cu C U Z, 60 E 40 U C C C cu C- U MAR APR MAY JUN JUL AUG SEP JAN FEB 'MAR' APR' MAY' JUN 'JUL 'AUG' SEP 'OCr NOV DE_C Figure 5. Salinity patterns, canal discharge rates, and precipitation patterns for selected locations in western Biscayne Bay. (A) Mean monthly salinity values (-- 1 SD). (B) Salinity variability (coefficient of variation, CV). Data for A and B were collected using miniature loggers deployed in Values were collected at 30-minute intervals and used to calculate daily and then monthly averages. Monthly SD and CV were calculated using daily averages. (C) Mean monthly canal discharge patterns (-_ 1 SD). (D) Mean monthly precipitation values (t 1 SD). Data for C and D were obtained from the South Florida Water Management District. Daily discharge and precipitation data were added to obtain a monthly total, and these values were averaged for the period between 1995 and 2004 to obtain 10-year means. The diamonds in D show precipitation values for emphasized the localized correlation between salinity patterns (determined by precipitation and canal flow rates) and seagrass abundance and distribution. The decreasing abundance of H. wrightii with increasing distance from the mouth of Mowry Canal is consistent with experimental data that show optimal growth for this species at the low end of the salinity spectrum (LIRmAN and CROPPER, 2003). The most abundant seagrass species, T testudinum, was found throughout western Biscayne Bay, but an increase in abundance with increasing distance from the point of freshwater discharge was still apparent. Similarly, MEEDER et al. (1997) documented a negative relationship between freshwater seepage from groundwater sources and abundance of T testudinum in western Biscayne Bay. While this study focused on salinity influences, there are clearly other, highly intercorrelated, factors such as hydrodynamic regime, substrate characteristics, sedimentation, light, temperature, grazing, nutrients, diseases, and competition that can also influence recruitment, growth, and survivorship patterns of SAV. These factors need to be considered in future studies if we are to fully understand the distribution patterns of benthic organisms. Salinity, however, is the driving factor that is potentially under the greatest management control in South Florida (FOURQUREAN et al., 2003). In fact, the ability to control salinity patterns through modifications in freshwater deliveries forms the basis of the restoration targets for Biscayne Bay. One of the hypotheses of the Comprehensive Everglades Restoration Plan is that the restoration of favorable flow and salinity regimes will extend the range of Halodule seaward, reduce the region of Thalassia dominance, and increase both Halodule and Ruppia cover into areas presently devoid of seagrass (USACE and SFWMD, 1999). An important finding of our video surveys is that only a few sites were completely devoid of submerged aquatic vegetation at the present time. Moreover, sites devoid of SAV were only found at the mouths of canals with high Journal of Coastal Research, Vol. 24, No. 1A (Supplement), 2008

9 Geospatial Video Surveys of Benthic Marine Communities 143 rates of discharge such as Mowry Canal, indicating that the detrimental impacts of point discharges of freshwater are spatially restricted and species specific. This information provides useful scientific input into the restoration process and suggests that if additional freshwater is to be directed into Biscayne Bay, significant efforts need to be undertaken to modify the existing delivery patterns away from point discharges such as canals toward a more natural overland flow pattern to avoid the expansion of areas devoid of SAV. The proposed Biscayne Bay Coastal Wetlands project, a component of CERP, was specially developed to rehydrate wetlands and reduce point sources of freshwater discharge through the use of a spreader system aimed at restoring sheetflow patterns in southern Biscayne Bay (CERP, 2005). Future surveys will be needed as the proposed changes are implemented to fully evaluate their impacts on the extent and condition of SAV in western Biscayne Bay. The SWaPS platform was shown to be well suited for monitoring benthic communities in shallow coastal environments such as western Biscayne Bay where a large number of sites were surveyed rapidly, providing an extensive, spatially explicit dataset directly comparable to the data collected in situ by trained observers. Video-based surveys with position information have been used elsewhere to document the largescale patterns of abundance and distribution of coral reef organisms (RIEGi, et al., 2005) and macroalgae (RIEGL, KOR- RUBEL, and MARTIN, 2001), but the present study represents the first time in which such a system was used to survey extremely shallow (<1 m in depth) marine environments. The SWaPS methodology presented here provides a relatively fast, spatially precise, moderate cost (startup costs are commonly less than $25,000) alternative to diver-based surveys that can be especially useful when (1) a large number of closely spaced sites need to be surveyed rapidly; (2) field time is limited, as is often the case prior to the onset of an acute disturbance (e.g., hurricane, dredging project); (3) the availability of trained field personnel is limited; (4) resources need to be precisely mapped; (5) a permanent visual archive of the extent and condition of benthic resources is needed; and (6) the same locations require surveying repeatedly without establishing permanent markers. Field surveys using SWaPS can be easily conducted by a single boat operator without any scientific training, thereby removing the need for specialized field personnel and reducing the cost of field operations. Moreover, the collection of the benthic information is shifted from the field to the lab, where time and personnel constraints are generally lower. The main limitations of the video-based methodology are those imposed by water clarity and sea conditions. Low visibility and rough sea conditions limit the quality of the images collected and significantly hinder species identification. Similarly, video-based surveys may not be adequate when a high taxonomic resolution of benthic organisms is needed. For taxa like macroalgae that are often small and exhibit subtle morphological differences among species, a trained observer may need to get close to the substrate to be able to detect rare species and tell similar species apart. However, it is expected that future developments in video technology will provide improvements in image and taxonomic resolution without significant increases in the cost of the equipment. Because of the high spatial precision provided by SWaPS, surveys with this system can also be used effectively to document damage and recovery patterns in shallow habitats. One of the main sources of disturbance for seagrass and hardbottom communities in shallow coastal lagoons is the physical damage caused by boat groundings, boat propellers, and shrimp-trawling activities (AuLT et al., 1997; SARGENT et al., 1995). With the aid of SWaPS, individual seagrass scars can be identified unequivocally, and both the area damaged and the rate of scar closure can be quantified directly from the video frames taking into account pitch, roll, depth, and camera/lens information. Similarly, the damage caused by rollerframe trawls can be assessed and the removal of organisms like sponges and soft corals can be documented if surveys are conducted prior to and after these activities. Finally, stable isotope, gut contents, and visual survey studies indicate that many adult and subadult fishes that occupy mangrove shoreline habitats by day derive their nutrition by foraging nocturnally in nearby seagrass beds (HARRIGAN, ZIEMAN, and MACKO, 1989; KlECKBUScH et al., 2004; ROOKER and DEN- NIS, 1991; SERAFY, FAUNCE, and LORENZ, 2003). The SWaPS also has utility, therefore, in the design and analysis of fish population studies that aim to understand the linkages between shoreline fishes, their smaller fish and invertebrate prey base, and the nearshore seagrass communities that support them. ACKNOWLEDGMENTS This research was funded by NOAA's National Geodetic Survey and the U.S. Department of the Interior's Critical Ecosystem Studies Initiative (Award Q528404CESI). Research activities were conducted under permit BISC SCI We are grateful for the support and guidance provided by S. Bellmund and R. Curry (Biscayne National Park), and J. Browder and C. Rivero (NOAA/National Marine Fisheries Service). R. Clausing and J. Herlan provided assistance in the field. We appreciate the helpful comments of two anonymous reviewers. LITERATURE CITED ALLEMAN, R.W., An Update to the Surface Water Improvement and Management Plan for Biscayne Bay. West Palm Beach, Florida: South Florida Water Management District. AULT, J.; DIAz, G.A.; SMITH, S.G.; Luo, J., and SERAFY, J.E., 1999a. A spatial dynamic multistock production model. North American Journal of Fisheries Management, 19, AULT, J.; Luo, J.; SMITH, S.G.; SERAFY, J. E.; WANG, J.D.; HUM- STON, R., and DiAz, G.A., 1999b. A spatial dynamic multistock production model. Canadian Journal of Fisheries and Aquatic Science, 56, AULT, J.S.; SERAFY, J.E.; DIRESTA, D., and DANDELSKI, J., Impacts of commercial fishing on key habitats within Biscayne National Park. Homestead, Florida: Biscayne National Park Report, 80p. BImD, K.T.; CODY, B.R.; JEWETT-SMITH, J., and KANE, M.E., Salinity effects on Ruppia maritima L. cultured in vitro. Botanica Marina, 36, BROWDER, J.A., Relationship between pink shrimp production on the Tortugas Grounds and water flow patterns in the Florida Everglades. Bulletin of Marine Science, 37, Journal of Coastal Research, Vol. 24, No. IA (Supplement), 2008

10 144 Lirman et al. 144 Lirman et al. BROWDER, J.A. and OGDEN, J.C., The natural South Florida system II: Predrainage ecology. Urban Ecosystems, 3, BROWDER, J.A.; RESTREPO, V.R.; RICE, J.K.; ROBBLEE, M.B., and ZEIN-ELDIN, Z., Environmental influences on potential recruitment of pink shrimp, Farfantepenaeus duorarum, from Florida Bay nursery grounds. Estuaries, 22, BROWDER, J.A. and WANLESS, H.R., Survey Team Final Reports. Miami, Florida: Biscayne Bay Partnership Initiative. pp BUTLER IV, M.J.; HUNT, J.H.; HERRNKIND, W.F.; CHILDRESS, M.J.; BERTELSEN, R.; SHARP, W.; MATTHEWS, T.; FIELD, J.M., and MARSHALL, H.G., Cascading disturbances in Florida Bay, USA: Cyanobacteria blooms, sponge mortality, and implications for juvenile spiny lobsters Panulirus argus. Marine Ecology Progress Series, 129, CERP (COMPREHENSIVE EVERGLADES RESTORATION PLAN), Central and Southern Florida Project. Comprehensive Everglades Restoration Plan Report to Congress. U.S. Department of the Interior and U.S. Army Corps of Engineers. l14p. DAVIS, S.M. and OGDEN, J.C., Toward ecosystem restoration. In: DAVIS, S.M. and OGDEN, J.C. (eds.), Everglades. The Ecosystem and Its Restoration. Delray Beach, Florida: St. Lucie Press, pp DIAZ, G.A., Population Dynamics and Assessment of Pink Shrimp (Farfantepenaeus duorarum) in Subtropical Nursery Grounds. Miami, Florida: University of Miami, Ph.D. dissertation, 175p. DURAKO, M.J., Seagrass die-offin Florida Bay (USA): Changes in shoot demographic characteristics and population dynamics in Thalassia testudinum. Marine Ecology Progress Series, 110, DURAKO, M.J.; HALL, M.O., and MERELLO, M., Patterns of change in the seagrass dominated Florida Bay hydroscape. In: PORTER, J.W. and PORTER, K.G. (eds.), The Everglades, Florida Bay, and Coral Reefs of the Florida Keys. An Ecosystem Sourcebook. Boca Raton, Florida: CRC Press, pp FOURQUREAN, J.W.; BOYER, J.N.; DURAKO, M.J.; HEFTY, L.N., and PETERSON, B.J., Forecasting responses of seagrass distributions to changing water quality using monitoring data. Ecological Applications, 13, FOURQUREAN, J.W.; DURAKO, M.J.; HALL, M.O., and HEFTY, L.N., Seagrass distribution in South Florida: A multi-agency coordinated monitoring program. In: PORTER, J.W. and PORTER, K.G. (eds.), The Everglades, Florida Bay, and Coral Reefs of the Florida Keys. An Ecosystem Sourcebook. Boca Raton, Florida: CRC Press, pp FOURQUREAN, J.W.; POWELL, G.V.N.; KENWORTHY, W.J., and ZIE- MAN, J.W., The effects of long-term manipulation of nutrient supply on competition between the seagrasses Thalassia testudinum and Halodule wrightii in Florida Bay. Oikos, 72, HALL, M.O.; DURAKO, M.J.; FOURQUREAN, J.W., and ZIEMAN, J.C., Decadal changes in seagrass distribution and abundance in Florida Bay. Estuaries, 22, HARRIGAN, P.; ZIEMAN J.C., and MACKO. S.A., The base of nutritional support for the gray snapper (Lutjanus griseus): An evaluation based on a combined stomach content and stable isotope analysis. Bulletin of Marine Science, 44, KANTRUD, H.A., Wigeongrass (Ruppia maritima L.): A Literature Review. Fish and Wildlife Research Report 10, U.S. Fish and Wildlife Service, 58p. KIECKBUSCH, D.K.; KOCH, M.S.; SERAFY, J.E., and ANDERSEN, W.T., Trophic linkages of primary producers and consumers in fringing mangroves of tropical lagoons. Bulletin of Marine Science, 74, LIGHT, S.S. and DINEEN, J.W., Water control in the Everglades: A historical perspective. In: DAVIS, S.M. and OGDEN, J.C. (eds.), Everglades. The Ecosystem and Its Restoration. Delray Beach, Florida: St. Lucie Press, pp LIRMAN, D. and CROPPER JR., W.P., The influence of salinity on seagrass growth, survivorship, and distribution within Biscayne Bay, Florida: Field, experimental, and modeling studies. Estuaries, 26, LIRMAN, D.; ORLANDO, B.; MACIA, S.; MANZELLO, D.; KAUFMAN, L.; BIBER, P., and JONES, T., Coral communities of Biscayne Bay, Florida and adjacent offshore areas: Diversity, abundance, distribution, and environmental correlates. Aquatic Conservation, 13, MADER, G.L., Kinematic and rapid static (KARS) GPS positioning: Techniques and recent experiences. In: BEUTLER, G.; HEIN, G.W.; MELBOURNE, W.G., and SEEBER, G. (eds.), JAG Symposia No Berlin: Springer-Verlag, pp MCIVOR, C.C.; LEY, J.A., and BJORK, R.D., Changes in freshwater inflow from the Everglades to Florida Bay including effects on biota and biotic processes: A review. In: DAVIS, S.M. and Oc- DEN, J.C. (eds.), Everglades. The Ecosystem and Its Restoration. Delray Beach, Florida: St. Lucie Press, pp MCMAHAN, C.A., Biomass and salinity tolerance of shoalgrass and manateegrass in Lower Laguna Madre, Texas. Journal of Wildlife Management, 32, MCMILLAN, C., Salt tolerance of mangroves and submerged aquatic plants. In: REIMOLD, R.J. and QUEEN, W.H. (eds.), Ecology of Halophytes. New York: Academic Press, pp MEEDER, J.F.; ALVALORD, J.; BYRN, M.; Ross, M.S., and RENSHAW, A., Distribution of Benthic Nearshore Communities and Their Relationship to Ground Water Nutrient Loading. Final Report to Biscayne National Park from the Southeast Environmental Research Program. Miami, Florida: Florida International University. MONTAGUE, C.L. and LEY, J.A., A possible effect of salinity fluctuation on abundance of benthic vegetation and associated fauna in Northeastern Florida Bay. Estuaries, 16, NRC (NATIONAL RESEARCH COUNCIL), Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution. Washington, DC: National Academy Press. QUAMMEN, M.L. and ONUF, C.O., Laguna Madre: Seagrass changes continue decades after salinity reduction. Estuaries, 16, RIEGL, B.M.; KORRUBEL, J.L., and MARTIN, C., Mapping and monitoring of coral communities and their spatial patterns using a surface-based video method from a vessel. Bulletin of Marine Science, 69, RIEGL, B.M.; MOYER, R.P.; MORRIS, L.J.; VIRNSTEIN, R.W., and PURKIS, S.J., Distribution and seasonal biomass of drift macroalgae in the Indian River Lagoon (Florida, USA) estimated with acoustic seafloor classification (QTCView, Echoplus). Journal of Experimental Marine Biology and Ecology, 326, ROBBLEE, M.B.; BARBER, T.R.; CARLSON, P.R.; DURAKO, M.J.; FOURQUREAN, J.W.; MUEHLSTEIN, L.K.; PORTER, D.; YARBRO, L.A.; ZIEMAN, R.T., and ZIEMAN, J.C., Mass mortality of the tropical seagrass Thalassia testudinum in Florida Bay (USA). Marine Ecology Progress Series, 71, ROOKER, J.R. and DENNIS, G.D., Diel, lunar and seasonal changes in a mangrove fish assemblage of southwestern Puerto Rico. Bulletin of Marine Science, 49, SARGENT, F.J.; LEARY, T.J.; CREWZ, D.W., and KRUER, C.R., Scarring of Florida's seagrasses: Assessment and management options. FMRI technical report TR-1. St. Petersburg, Florida: Florida Department of Environmental Protection. SERAFY, J.E.; AULT, J.S.; ORTNER, P., and CURRY, R., Coupling Biscayne Bay's natural resources and fisheries to environmental quality and freshwater inflow management. Science Team Final Reports. Miami, Florida: Biscayne Bay Partnership Initiative, pp SERAFY, J.E.; FAUNCE, C.H., and LORENZ, J.J., Mangrove shoreline fishes of Biscayne Bay, Florida. Bulletin of Marine Science, 72, SERAFY, J.E.; LINDEMAN, K.C.; HOPKINS, T.E., and AULT, J.S., Effects of freshwater canal discharge on fish assemblages in a subtropical bay: Field and laboratory observations. Marine Ecology Progress Series, 160, SKLAR, F.; McVoY, C.; VANZEE, R.; GAWLIK, D.E.; TARBOTON, K.; RUDNICK, D.; MIAO, S., and ARMENTANO, T., The effects of altered hydrology on the ecology of the Everglades. In: PORTER, J.W. and PORTER, K.G. (eds.), The Everglades, Florida Bay, and Journal of Coastal Research, Vol. 24, No. 1A (Supplement), 2008