Documenting Everglades restoration impacts on Biscayne Bay’s shallowest benthic habitats

Journal of Coastal Research 24 1A 135-145 West Palm Beach, Florida I January 2008

Geospatial Video Monitoring of Nearshore Benthic Habitatsof Western Biscayne Bay (Florida) Using the Shallow-WaterPositioning System (SWaPS)

Diego Lirmanl, Gregory Deangelo", Joseph E. Serafy*•, Amit Hazra1 , Destiny Smith Hazra1 , and

Alexandra Brownft

tUniversity of MiamiRosenstiel School of Marine

and Atmospheric Science4600 Rickenbacker Causeway

Miami, FL 33149, [email protected]

*NOAA/National GeodeticSurvey

1315 East-West HighwaySilver Spring, MD 20910U.S.A.

ýNOAA/National MarineFisheries Service

75 Virginia Beach DriveMiami, FL 333149, U.S.A.

ttUniversity of MiamiDepartment of Marine

Science, Coral Gables, FL33124, U.S.A.

ABSTRACT

LIRMAN, D.; DEANGELO, G.; SERAFY, J.E.; HAZRA, A.; HAZRA, D.S.; and BROWN, A., 2008. Geospatial video

monitoring of nearshore benthic habitats of western Biscayne Bay (Florida) using the shallow-water positioning sys-

tem (SWaPS). Journal of Coastal Research, 24(1A), 135-145. West Palm Beach (Florida), ISSN 0749-0208.

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 submergedaquatic 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 abun-

dance 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.

ADDITIONAL INDEX WORDS: Geospatial video surveys, salinity patterns, seagrass distribution, Everglades resto-ration.

INTRODUCTION

Bay and estuarine systems are among the world's most pro-

ductive ecosystems, providing essential habitat for wildlifeand both commercial and recreational fisheries resources. In

the United States, a disproportionately large percentage of

the population lives along coastal areas, placing an increas-ing level of environmental pressure on these critical habitats.

Locally, rapid levels of coastal development and exploitationhave resulted in habitat losses and significant reductions inproductivity (NRC, 2000). The coastal bays of South Florida

are uniquely productive and diverse natural resources locatedjust downstream of the Everglades and an upland drainagebasin that has been highly modified by human activities.

DOI:10.2112/04-0428.1 received 17 December 2004; accepted in re-vision 12 January 2006.

Within the coastal lagoons of South Florida, benthic habitatsdominated 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 claritypatterns (see FOURQUREAN et al., 2002, and references there-

in; WALKER et al., 2001).

Because of their ecological role, seagrasses have been pro-

posed as one of the best indicators of the status and changein 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 meth-odology that is especially well suited for conducting georefer-

enced surveys of the shallowest benthic habitats of coastallagoons, which have been largely neglected primarily because

of logistic problems associated with boat access. These habi-tats support healthy seagrass and hardbottom communities

e#11lll116,

136 Lirman et al.

(LIRMAN et al., 2003; LIRMAN and CROPPER, 2003) and arecritical nursery habitats for pink shrimp, Farfantepenaeusduoraraum, and fishes such as gray snapper, Lutjanus gri-seus, and spotted seatrout, Cynoscion nebulosus (AULT et al.,1999a, 1999b; DIAZ, 2001; SERAFY et al., 1997, 2001). How-ever, because of their location at the land-sea interface, theseliving resources are especially vulnerable to changes in fresh-water flows from upland sources.

The hydrology of the Everglades drainage basin has beenseverely modified over the last 70 years by the constructionof the Central and Southern Florida Project, an extensive wa-ter management system comprised of levees, canals, controlstructures, and pumping stations that have modified the nat-ural timing, quantity, and quality of freshwater flows acrossthe landscape (BROWDER and OGDEN, 1999; McIvoR, LEY,and BJORK, 1994). In Florida Bay and Biscayne Bay, themodifications to the natural drainage system have resultedin (1) a reduction in the total amount of freshwater reachingthe bays and (2) a shift from overland and groundwatersources of freshwater to highly concentrated releases fromcanals (LIGHT and DINEEN, 1994; SKLAR et al., 2002). Thesemodifications have caused adverse ecological changes that in-clude (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 mortal-ity of marine sponges (BUTLER et al., 1995).

In response to these patterns of decline, the Comprehen-sive Everglades Restoration Plan (CERP) has been designedto restore, through a series of large-scale engineering pro-jects, some of the lost natural hydrologic characteristics of theSouth Florida regional ecosystem. One of the explicit goals ofCERP is the modification of freshwater delivery patterns intocoastal bays to recover estuarine conditions that once pre-vailed along littoral environments (BROWDER and WANLESS,2001; DAVIS and OGDEN, 1994; STEINMAN et al., 2002). In-creased freshwater inflows through canals, tidal creeks, andmarshes are expected to lower salinities at the point of dis-charge and expand areas of mesohaline conditions with large-ly unknown ecological effects on the organisms found there(BROWDER and WANLESS, 2001). Considering the potentialfuture impacts of CERP projects, it is crucial to documentpresent-day patterns in abundance, distribution, and condi-tion of benthic communities at nearshore habitats likely tobe affected by CERP activities.

Previous monitoring, modeling, and experimental studieshave documented the significant influence of salinity pat-terns on the abundance, distribution, diversity, growth, andmortality of submerged aquatic vegetation (SAV), especiallyseagrasses 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 andvariable salinity are typically dominated by mixed commu-nities of Halodule wrightii and Ruppia maritima, two sea-grass 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 Syrin-godium filiforme in deeper habitats (HALL et al., 1999; ZIE-MAN, FOURQUREAN, and IVERSON, 1989; ZIEMAN, FOURQUR-EAN, and FRANKOVICH, 1999). However, limited informationis available to evaluate whether present-day seagrass abun-dance and distribution patterns are influenced by salinitypatterns in Biscayne Bay. In this study, we describe the useof the shallow-water positioning system (SWaPS), a geospa-tial video-based survey methodology, to (1) evaluate whetherdistinct patterns in abundance and distribution of submergedaquatic vegetation can be observed along the littoral zone ofwestern Biscayne Bay in relation to patterns in salinity cre-ated by the inflow of freshwater from upland sources and (2)provide a spatially explicit baseline database on the abun-dance and distribution of benthic organisms against whichthe effects of future watershed restoration activities may bediscerned.

METHODS

Study Area

Biscayne Bay is a shallow subtropical lagoon adjacent tothe city of Miami (Figure 1A). The location of Biscayne Bayalong a highly populated, rapidly growing urban center anddirectly downstream of CERP activities on the watershedmakes this important natural resource especially vulnerableto human activities and changes in water quality and flow(SERAFY et al., 2001; LIRMAN et al., 2003). Salinity fieldswithin Biscayne Bay are influenced by precipitation; fresh-water inputs from land, canal, and groundwater sources; andtidal influx of oceanic water. Presently, freshwater from up-land sources is carried into western Biscayne Bay primarilythrough control structures managed to meet agricultural,municipal, and flood-control needs and objectives (WANG,Luo, and AULT, 2003). The spatial and temporal distribu-tions of these influences delineate regions with distinct salin-ity characteristics. Areas with low and variable salinity arefound along the western margin due to freshwater inflowsfrom canals, groundwater sources, and surface runoff, whilehigher, more stable salinities are found where oceanic influ-ences 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 sa-linity variability can be observed in the area between BlackPoint and north of Turkey Point, where four canals dischargedirectly into the bay (AULT et al., 1999b; WANG, Luo, andAULT, 2003) (Figure 1A).

The Shallow-Water Positioning System

The shallow-water positioning system, developed by sci-entists from the National Oceanographic and AtmosphericAdministration's National Geodetic Survey, uses a global po-sitioning system (GPS) receiver integrated with a video cam-era and installed on a shallow-draft vessel (14-ft Carolinaskiff) (Figure 1B). The GPS receiver is centered over a gim-balled digital video camera that is suspended over a Plexiglasenclosure that provides a looking view of the bottom. A staticGPS base station is established in the vicinity of SWaPS op-

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136 Lirman et al.

Geospatial Video Surveys of Benthic Marine Communities

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 SWaPSsurvey vessel. Both receivers record the GPS LI and L2 car-rier phases and code ranges every second during operations.After each survey period, both data files are postprocessedusing the software program KINPOS as described in MADER(1996). The position of the base station is accurately deter-

mined using OPUS, a GPS processing service created by theNational Geodetic Survey (see http://www.ngs.noaa.gov/OPUS). The code ranges are used in differential mode to lo-

cate the position of the SWaPS platform. The data collectedby the base station are relayed via radio modem to the

SWaPS survey platform where the data can be processed inreal 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 preciselocation of each frame based on the location of the vessel with

respect to the base station. Although not used in the presentstudy, water depth, pitch, roll, camera specifications, andlens specifications can be used to obtain density and size es-timates for organisms and/or for other benthic features.

The video surveys conducted with SWaPS provide a contin-

uous digital video track of the bottom. The data are archivedin video format and by grabbing frames at a rate of one frameper second and storing these as digital still images. A geo-

spatial information system (GIS) is used subsequently to linkthe geospatial (locations) and thematic (descriptive) data totheir respective images.

Spatial Precision of SWaPS

While high spatial precision can be achieved on land usinga GPS base station, field conditions (e.g., winds, currents,wave action) can often limit the ability to return to the same

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location in water. The spatial precision (i.e., the ability of thesystem to return to a given location after a position has beenestablished) of SWaPS was tested using ceramic tiles (15 cmx 15 cm). The location of each tile on the bottom was deter-mined by maneuvering the boat over the tiles and capturingeach one at the center of the video frame. The following day,the position of each tile was retrieved from the video andentered as waypoints into a real-time kinematic GPS-navi-gation software package. With the GPS unit as a guide, theboat was repositioned over each waypoint. If any portion ofthe tile could be seen on the video screen, the tile was countedas a hit. If the tile was not seen on the screen, the distancebetween the position of the boat and the position of the tilewas estimated to determine the extent of the miss.

Field tests showed that SWaPS can provide submeter pre-cision consistently in shallow coastal areas. Out of the 30 tilesdeployed, 24 (80%) were relocated within the video frame us-ing only the positions recorded during deployment. At thedepths at which the tiles were deployed (75-100 cm), the vid-eo frames covered a maximum width of approximately 60 cmat the bottom. Therefore, the minimum spatial precision forthe relocation of these tiles was 50 cm, the maximum dis-tance between the center of the frame where the tile wasoriginally positioned during deployment and the center of atile for which only a small portion is showing in the videoframe during recovery. For the tiles that were not relocatedwithin the video screen (20%), the mean distance to the cen-ter of the frame was 75 cm (SD = + 12).

Field Surveys

In February-April of 2003, field surveys were conducted inwestern Biscayne Bay following the shoreline at a distanceof <300 m from shore at depths of 50-75 cm. A subset ofstations (n = 130) was sampled at -300-m intervals from thecontinuous video data set collected along the survey path(Figure 2). For each survey location (i.e., a transect <25 malong the survey track), 10 nonoverlapping frames were cho-sen at random from the image library. This approach waschosen to be consistent with the methods used by existinglong-term SAV monitoring programs in the region, wheremultiple sites are sampled visually using 0.25-M2 quadrats(FOURQUREAN et al., 2002). For each georeferenced digitalimage (i.e., the sample unit for that site), community type,species observed, and abundance (percentage cover) were re-corded from a computer monitor. The contrast and brightnessof each image were adjusted to improve classification. Per-centage cover was determined as the fraction of each frameoccupied by each taxon and the values recorded for eachframe were averaged by site (n = 10 frames per site).

The hypothesized direct influence that freshwater dis-charge from canals may have on the abundance and distri-bution of benthic organisms was tested by conducting de-tailed SWaPS surveys in the immediate vicinity of two ca-nals, Military Canal and Mowry Canal (Figure 3A), in August2005. These canals were chosen because they are in closeproximity to each other (2.2 km) but differ significantly inthe amount of freshwater they deliver to western BiscayneBay (http://www.sfwmd.org). These areas were sampled in a

regular grid pattern following parallel tracks centered on thecanal structures (Figures 3C, 3F). Survey tracks (n = 8) wereseparated by a distance of 75 m, and sites along each trackwere surveyed at 75-m intervals using preloaded GPS tracks(n = 6 sites per track). For each survey location (i.e, a tran-sect of 15-20 m along the survey track), 10 nonoverlappingframes were chosen at random from the image library andanalyzed using the methods previously described. The ben-thic coverage data obtained were averaged for each site andused to develop percentage cover surface contours usingArcView's spatial analyst using an inverse distance weightedinterpolation procedure. The distance between each surveypoint and the discharge point of each canal (i.e., the point atwhich the canal structure enters the bay) was used to testthe hypothesis that distance to canals influences the abun-dance and distribution of seagrasses using regression analysis.

Calibration between SWaPS and Visual Surveys

Because other benthic research programs in the region usevisual surveys performed by divers to estimate abundanceand distribution of benthic organisms (FOURQUREAN et al.,2002), it is important to compare the values obtained usingSWaPS with those obtained by trained observers at the samesites. To provide this calibration, a subset of sites (n = 22)with different characteristics (e.g., seagrass-dominated, algal-dominated, and mixed communities) was surveyed using bothmethods, and benthic attributes and time and effort esti-mates were compared. Trained observers followed the meth-ods outlined by FOURQUREAN et al. (2002) to estimate thepercentage cover of benthic organisms from 10 haphazardlyplaced PVC quadrats (0.25 M 2) along a 20-25-m transect. Thesame area was surveyed using SWaPS, and the data wereanalyzed as described above. The estimates of the mean per-cent cover for each seagrass species and macroalgal groupwere compared between survey methods for each site indi-vidually as well as among all sites (i.e., pooled data) using aWilcoxon test due to nonconformity to the normality assump-tion required for parametric tests.

Salinity Patterns

Salinity patterns in the shallowest habitats of western Bis-cayne Bay were measured using miniature sensors (1.5 X 5.0cm) developed by Star-Oddi (see http://www.star-oddi.com)deployed between March and September 2005. Salinity rec-ords for these habitats are generally lacking because of prob-lems associated with boat access and because most waterquality instruments available are too large to be deployed inextremely shallow habitats. The probes were positioned at adistance of <50 m from shore in two areas influenced by ca-nal flows (Military and Mowry canals), as well as in the Tur-key Point area where no canals are found (Figures 1A, 3A).Data were collected at 30-minute intervals. In addition, theamount of freshwater discharged through Military and Mow-ry canals as well as the precipitation values in the vicinity ofthese canals were obtained from historical records from theSouth Florida Water Management District (http://www.sfwmd.org). The daily canal discharge (cubic feet/s) andprecipitation values were added to obtain monthly values,


138 Lirman et al.


Figure 2. Abundance of seagrasses (mean percentage cover) in western Biscayne Bay in February-April 2003. (A) Thalassia testudinum, (B) Halodulewrightii, (C) Syringodium filiforme, (D) Ruppia maritima.

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Figure 3. Abundance and distribution of seagrasses in the vicinity of water management canals that carry freshwater from upland sources into westernBiscayne Bay in August 2005. (A) Map of the study area with the location of Military and Mowry canals, (B) Mean daily salinity recorded in 2005 usingsalinity loggers placed at a distance of -50 m from the point of freshwater discharge. Data collected at 30-minute intervals were averaged to obtain thedaily 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.


Nicaii % Cover

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Figure 4. Percentage cover of (A) Thalassia testudinum and (B) Halodule wrightii in the region between Black Point and Turkey Point, an area withlow and variable salinity because of the inflow of freshwater from water management canals.

and monthly values were averaged for the period of 1995-2004, providing a 10-year mean. Monthly values for precipi-tation and canal flows were also obtained for the periodMarch-September 2005.

RESULTS

Field Surveys

The skiff used by SWaPS performed well in the shallowenvironment of western Biscayne Bay, and approximately 35km of littoral habitats at depths <75 cm were easily sur-veyed. Moreover, the shallow draft of the vessel allowed usto survey efficiently even the shallowest habitats (<40 cm)that are hard to access using most other platforms. Depend-ing on the depth of the habitats, survey speeds of <1 knotprovide the best image quality during continuous surveys.The average time required to collect georeferenced videoalong a 25-m transect is 2-3 minutes and, depending on thespacing of the survey locations, a large number of sites canbe easily surveyed in a short period of time. In applicationssuch as the canal surveys where sites are closely spaced, upto 100 sites can be surveyed in a day.

Three main benthic community types were documented inwestern Biscayne Bay: (1) seagrass communities composed ofone or more of four seagrass species (T testudinum, H. wrigh-tii, S. filiforme, R. maritima); (2) macroalgal communitieswith attached and/or drift components; and (3) hardbottomcommunities composed of sponges, soft corals, and hard cor-als. While these were the three main categories, mixed ben-thic communities composed of organisms from two or more ofthese broad categories were commonly observed.

The most abundant seagrass species, T testudinum, wasfound throughout the study area (82% of sites [107 of 130];mean percentage cover [SD] = 43.0 [38.9]). The lowest abun-dance of this species was recorded in the areas south of BlackPoint and directly opposite Military and Mowry canals (Fig-ures 2A, 4A). In contrast, H. wrightii (18% of sites; meanpercentage cover = 6.0 [17.9]), S. filiforme (9% of sites; meanpercentage cover = 4.8 [16.9]), and R. maritima (12% of sites;mean percentage cover = 0.6 [3.7]) had lower overall abun-dance and restricted spatial distributions (Figure 2B). Syr-ingodium filiforme was restricted to the northern section ofthe survey area, while R. maritima was restricted to thesouthern region in areas directly influenced by freshwaterinflows from canals. The highest abundance of R. maritimawas documented in the vicinity of Black Point and MilitaryCanal. Similarly, the distribution of H. wrightii was associ-ated with areas of relatively high freshwater inflow reachinghigh abundance only in the immediate vicinity of the CoralGables Canal, Snapper Creek Canal, Princeton Canal, Mili-tary Canal, and Mowry Canal (Figures 2B, 4B). Finally, onlya few sites in western Biscayne Bay were completely devoidof submerged aquatic vegetation. In fact, only 2% of sites hadno seagrass biomass present and 10% of sites had no ma-croalgal biomass present.

Attached and drift macroalgae are important componentsof the benthic communities of western Biscayne Bay and werefound throughout the area at 47% and 70% of sites respec-tively. The main components of the attached macroalgalgroup included Halimeda spp., Caulerpa spp., Penicitlus spp.,Batophora spp., and Acetabularia sp., while Laurencia spp.,Chondria spp., and Dictyota spp. were the most abundant


140 Lirman et al.


components of the drift macroalgal group. Sponge-dominatedhardbottom communities were only found at two sites in thenorthern region of the study area.

Only three seagrass species, T testudinum, H. wrightii, andR. maritima, were found in the vicinity of Military and Mow-ry canals in August 2005. The benthic habitats near bothcanals were dominated by H. wrightii and mats of the driftalga Laurencia (Figures 3D, 3G). The low abundance (<3%maximum cover) and distribution (present at only 21% ofsites) of R. maritima precluded meaningful statistical anal-yses. At Military Canal, the abundance of both H. wrightiiand T testudinum was significantly influenced by distance tothe canal discharge point (linear regression, p < 0.01). Theabundance of H. wrightii decreased with distance to the canaldischarge point, and the opposite pattern was documented forT testudinum (Figures 3D, 3E). At Mowry Canal, only theabundance T testudinum decreased significantly with dis-tance to the canal (p < 0.01), while no significant spatialpatterns with respect to canal influences were observed forH. wrightii (Figures 3G, 3H). Only six stations (12.5%) werecompletely devoid of seagrass in the vicinity of Mowry Canal,while no stations devoid of seagrass biomass were observedin the vicinity of Military Canal.

Calibration between SWaPS and Visual Surveys

Twenty-two sites were surveyed visually by trained ob-servers in situ and by SWaPS. The time it took a trainedobserver to collect percentage cover data from ten 0.25-iM2

quadrats in the field ranged from 10 to 20 minutes, depend-ing on the composition of the benthic community. UsingSWaPS, video was collected over a 2-3-minute period. Thetime required to score on a computer monitor the 10 digitalframes chosen from each site ranged from 5 to 12 minutes.When data were pooled among all sites, no significant differ-ences in the percentage cover of T testudinum, H. wrightii,S. filiforme, R. maritima, drift macroalgae, and attached ma-croalgae 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 testu-dinurn were found for any of the sites. For the other speciesor groups, no significant differences were found for H. wrigh-tii 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 influenceof precipitation and canal outflow on the salinity patterns ofwestern Biscayne Bay as well as the wide fluctuation in sa-linity that can be caused by precipitation and canal releases.During the dry season (March-May), mean salinity was high-er at all three sites compared to the values obtained duringthe wet season (June-September) (Figure 5A). Moreover, theareas in the vicinity of Military and Mowry canals had con-sistently lower mean monthly salinity compared to the Tur-key Point area, where no canal discharge is present (Figure5A). Similarly, the variability in salinity (expressed as thecoefficient of variation [CVD) increased with decreasing salin-

ity in the wet season, and canal-influenced areas had a high-er degree of variability compared to non-canal-influenced ar-eas (with the exception of September, when a higher CV wasobtained for the Turkey Point area compared to the MilitaryCanal area) (Figure 5B). These patterns were accentuated inthe wet season when both precipitation and canal dischargesincrease (Figure 5C, 5D). While both canal-influenced areashad lower salinity than the non-canal-influenced area, meansalinity in the vicinity of Mowry Canal was consistently lowerthan the mean salinity recorded in the vicinity of MilitaryCanal (with the exception of May when this pattern was re-versed). Finally, the high-resolution data collected in the im-mediate vicinity of canals show that these areas can experi-ence drastic fluctuations in salinity, especially during SouthFlorida's wet season when mean daily salinity can drop by asmuch 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 thecanal-specific and seasonal differences in freshwater flow.Canal discharge rates were higher in the wet, rainy seasonthan in the dry season. Moreover, discharge values were con-siderably higher for Mowry Canal compared to Military Ca-nal throughout the year. The only time when canal flow val-ues were higher at Mowry Canal in 2005 was in May (MowryCanal = 67 cubic feet/s, Military Canal = 32 cubic feetds),which corresponds with the period when monthly mean sa-linity was lower in the vicinity of Mowry Canal (Figure 5A).Finally, the 10-year record of precipitation correlates with theobserved salinity patterns, with a marked increase in precip-itation in June at the onset of the rainy season that is cap-tured in the drop in salinity at all three sites at this time(Figures 5A, 5D). The precipitation values recorded for 2005were within the 95% confidence interval recorded for the 10-year precipitation mean (Figure 5D).

DISCUSSION

The abundance and distribution patterns recorded withSWaPS are consistent with previous reports that showed thatthe large-scale abundance, diversity, and spatial distributionof benthic organisms are often correlated with salinity pat-terns. The high abundance of Halodule wrightii and Ruppiamaritima in areas of freshwater discharge from canals cor-responds well with their reported tolerance for low and var-iable salinity (BIRD et al., 1993; McMAHAN, 1968; McMIL-LAN, 1974). Halodule wrightii is commonly regarded as anearly successional species able to survive where other sea-grass species would be removed by disturbance and remaindominant under fluctuating conditions (FOURQUREAN et al.,1995; MONTAGUE and LEY, 1993), while R. maritima is aspecies that thrives under low-salinity conditions (KANTRUD,1991). The distribution of S. filiforme, restricted to the north-ern portion of the study area, is also consistent with studiesthat suggest that this species can only sustain high produc-tivity in well-flushed areas (FOURQUREAN et al., 2002; ZIE-

MAN, FOURUREAN, and FRANKOVICH, 1999) as in the north-ern section of the study region where oceanic influences pre-vail (WANe,, Luo, and AULT, 2003).

The detailed surveys conducted in the vicinity of two canals


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[] Military Canal E Mowry Canal E] Turkey Point

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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 salinityvalues (-- 1 SD). (B) Salinity variability (coefficient of variation, CV). Data for A and B were collected using miniature loggers deployed in 2005. Valueswere 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 SouthFlorida Water Management District. Daily discharge and precipitation data were added to obtain a monthly total, and these values were averaged forthe period between 1995 and 2004 to obtain 10-year means. The diamonds in D show precipitation values for 2005.

emphasized the localized correlation between salinity pat-terns (determined by precipitation and canal flow rates) andseagrass abundance and distribution. The decreasing abun-dance of H. wrightii with increasing distance from the mouthof Mowry Canal is consistent with experimental data thatshow optimal growth for this species at the low end of thesalinity spectrum (LIRmAN and CROPPER, 2003). The mostabundant seagrass species, T testudinum, was foundthroughout western Biscayne Bay, but an increase in abun-dance with increasing distance from the point of freshwaterdischarge was still apparent. Similarly, MEEDER et al. (1997)documented a negative relationship between freshwaterseepage from groundwater sources and abundance of T tes-tudinum in western Biscayne Bay.

While this study focused on salinity influences, there areclearly other, highly intercorrelated, factors such as hydro-dynamic regime, substrate characteristics, sedimentation,light, temperature, grazing, nutrients, diseases, and compe-

tition that can also influence recruitment, growth, and sur-vivorship patterns of SAV. These factors need to be consid-ered in future studies if we are to fully understand the dis-tribution patterns of benthic organisms. Salinity, however, isthe driving factor that is potentially under the greatest man-agement control in South Florida (FOURQUREAN et al., 2003).In fact, the ability to control salinity patterns through mod-ifications in freshwater deliveries forms the basis of the res-toration targets for Biscayne Bay. One of the hypotheses ofthe Comprehensive Everglades Restoration Plan is that therestoration of favorable flow and salinity regimes will extendthe range of Halodule seaward, reduce the region of Thalas-sia dominance, and increase both Halodule and Ruppia coverinto areas presently devoid of seagrass (USACE andSFWMD, 1999). An important finding of our video surveys isthat only a few sites were completely devoid of submergedaquatic vegetation at the present time. Moreover, sites devoidof SAV were only found at the mouths of canals with high


142 Lirman et at.


rates of discharge such as Mowry Canal, indicating that thedetrimental impacts of point discharges of freshwater arespatially restricted and species specific. This information pro-vides useful scientific input into the restoration process andsuggests that if additional freshwater is to be directed intoBiscayne Bay, significant efforts need to be undertaken tomodify the existing delivery patterns away from point dis-charges such as canals toward a more natural overland flowpattern to avoid the expansion of areas devoid of SAV. Theproposed Biscayne Bay Coastal Wetlands project, a compo-nent of CERP, was specially developed to rehydrate wetlandsand reduce point sources of freshwater discharge through theuse of a spreader system aimed at restoring sheetflow pat-terns in southern Biscayne Bay (CERP, 2005). Future sur-veys will be needed as the proposed changes are implementedto fully evaluate their impacts on the extent and condition ofSAV in western Biscayne Bay.

The SWaPS platform was shown to be well suited for mon-itoring benthic communities in shallow coastal environmentssuch as western Biscayne Bay where a large number of siteswere surveyed rapidly, providing an extensive, spatially ex-plicit dataset directly comparable to the data collected in situby trained observers. Video-based surveys with position in-formation have been used elsewhere to document the large-scale patterns of abundance and distribution of coral reef or-ganisms (RIEGi, et al., 2005) and macroalgae (RIEGL, KOR-

RUBEL, and MARTIN, 2001), but the present study representsthe first time in which such a system was used to surveyextremely shallow (<1 m in depth) marine environments.

The SWaPS methodology presented here provides a rela-tively fast, spatially precise, moderate cost (startup costs arecommonly less than $25,000) alternative to diver-based sur-veys that can be especially useful when (1) a large numberof closely spaced sites need to be surveyed rapidly; (2) fieldtime is limited, as is often the case prior to the onset of anacute disturbance (e.g., hurricane, dredging project); (3) theavailability of trained field personnel is limited; (4) resourcesneed to be precisely mapped; (5) a permanent visual archiveof the extent and condition of benthic resources is needed;and (6) the same locations require surveying repeatedly with-out establishing permanent markers. Field surveys usingSWaPS can be easily conducted by a single boat operatorwithout any scientific training, thereby removing the need forspecialized field personnel and reducing the cost of field op-erations. Moreover, the collection of the benthic informationis shifted from the field to the lab, where time and personnelconstraints are generally lower. The main limitations of thevideo-based methodology are those imposed by water clarityand sea conditions. Low visibility and rough sea conditionslimit the quality of the images collected and significantly hin-der species identification. Similarly, video-based surveys maynot be adequate when a high taxonomic resolution of benthicorganisms is needed. For taxa like macroalgae that are oftensmall and exhibit subtle morphological differences amongspecies, a trained observer may need to get close to the sub-strate to be able to detect rare species and tell similar speciesapart. However, it is expected that future developments invideo technology will provide improvements in image and

taxonomic resolution without significant increases in the costof the equipment.

Because of the high spatial precision provided by SWaPS,

surveys with this system can also be used effectively to doc-ument damage and recovery patterns in shallow habitats.

One of the main sources of disturbance for seagrass and hard-

bottom communities in shallow coastal lagoons is the physi-

cal 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 andthe rate of scar closure can be quantified directly from the

video frames taking into account pitch, roll, depth, and cam-

era/lens information. Similarly, the damage caused by roller-

frame 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 shore-

line habitats by day derive their nutrition by foraging noc-

turnally 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 be-

tween shoreline fishes, their smaller fish and invertebrate

prey base, and the nearshore seagrass communities that sup-

port 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). Re-

search activities were conducted under permit BISC-2002-SCI-0052. We are grateful for the support and guidance pro-

vided by S. Bellmund and R. Curry (Biscayne National Park),

and J. Browder and C. Rivero (NOAA/National Marine Fish-

eries Service). R. Clausing and J. Herlan provided assistance

in the field. We appreciate the helpful comments of two anon-

ymous reviewers.

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Journal of Coastal Research, Vol. 24, No. LA (Supplement), 2008

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TITLE: Geospatial Video Monitoring of Nearshore BenthicHabitats of Western Biscayne Bay (Florida) Using theShallow-Water Positioning System (SWaPS)

SOURCE: J Coast Res 24 no1A Ja 2008 supp

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Documenting Everglades restoration impacts on Biscayne Bay’s shallowest benthic habitats

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