SEAGRASS COVERAGE TRENDS IN THE INDIAN RIVER LAGOON SYSTEM

Transcription

1 Biological Sciences SEAGRASS COVERAGE TRENDS IN THE INDIAN RIVER LAGOON SYSTEM R. W. VIRNSTEIN *,J.S.STEWARD, AND L. J. MORRIS St. Johns River Water Management District, Palatka, FL ABSTRACT: We assess seagrass status and trends by two methods: (1) seagrass mapping every 2 3 years since 1986, plus 1943, and (2) monitoring of 59 fixed transects twice a year since In 2005, there were 26,101 ha of seagrass between Ponce de Leon Inlet and Vero Beach, which is at least as much as the 1943 coverage. Trends based on mapping show an increase in coverage in 12 of the 18 segments from 1994 to Similarly, average extent of seagrass beds from shore to the deep edge (transect length) increased from 1996 to 1999, then leveled off through The hurricanes of 2004 did not alter these positive trends, but may have contributed to reduction in seagrass densities in 2006 from Melbourne to Vero Beach. Key Words: Seagrass, seagrass trends, Indian River Lagoon, seagrass mapping, seagrass transects, seagrass monitoring, hurricane impacts INTRODUCTION FOR the Indian River Lagoon (IRL) system, seagrass restoration and protection is a mutual goal of both state (Steward et al., 2003) and federal (IRLNEP, 1996) programs. Seagrass trend analysis, which is the purpose of this paper, is used to assess the health of the resource and the success of restoration and protection programs. Seagrass status is a related but separate assessment comparing, for example, coverage and deep-edge depths to coverage and depth targets (Steward et al., 2005) and is planned for a future paper. Nonetheless, results from a trend analysis can provide a partial status report on the resource if there are signs of consistent improvement or decline. This paper describes the trends of seagrass over the past decades based on two monitoring methods, interpretation of aerial photos and field monitoring of fixed transects. The primary objective is to determine whether seagrass is improving or declining in the IRL. METHODS The study area is that portion of the IRL system between Ponce de Leon Inlet and the St. Lucie/Indian River County line, just north of Ft. Pierce Inlet (within the boundaries of the St. Johns River Water Management District), including Mosquito Lagoon, Banana River, and the northern two-thirds of Indian River proper (Fig. 1). Because the IRL has high spatial biological diversity (Virnstein, 1990), we chose to address seagrass trends for each of study area s 18 seagrass * Corresponding Author: RVirnstein@sjrwmd.com 397

2 398 FLORIDA SCIENTIST [VOL. 70 FIG. 1. Map of the study area with seagrass management segments and sub-lagoons indicated. The outline of catchment boundaries indicates the overall IRL basin boundary. management segments (Fig. 1) (Steward et al., 2005), with boundaries generally at natural constrictions or causeways and where spatial differences in water quality were statistically defined (Sigua et al., 1996; Steward and Green, 2006). Two complementary seagrass monitoring methods are used to assess seagrass trends in the IRL study area: (1) Lagoon-wide seagrass mapping every 2 3 years since 1986, plus 1943, and (2) field monitoring of 59 fixed transects.

3 No ] VIRNSTEIN ET AL. SEAGRASS COVERAGE TRENDS 399 FIG. 2. Overall Lagoon-wide (study area) average seagrass coverage (hectares) versus time, 1943 to Only those years for which there are complete Lagoon-wide maps are included map data are preliminary. Seagrass mapping is based on manual photo-interpretation of 1:24,000-scale aerial photos taken specifically for the mapping of seagrass. Seagrass maps are available for the years 1943, 1986, 1989, 1992, 1994, 1996, 1999, 2001, 2003, and 2005 although Mosquito Lagoon was not mapped in the 1980 s. The 2005 maps at the time of this writing are only available for the Melbourne and Vero Beach segments. [Note at time of proofs: All 2005 map data have been verified as final.] The 59 fixed field seagrass transects have been monitored twice a year (summer and winter), beginning in Generally, at each transect, seagrass is monitored within a 1-m 2 quadrat every 10 m along a measured line from shore out to the deep edge of the seagrass bed. Parameters measured include transect length, species composition, density, and canopy height, plus macroalgae and epiphyte abundance. Transect length is the distance from a fixed shore pole to the deep edge of the seagrass bed. Density is reported here as percent cover, based on visual estimates. For more details of the mapping and transect monitoring methods, see Virnstein and Morris (1996), Morris et al. (2000), Virnstein (2000), and Morris et al. (2001). RESULTS Results are presented Lagoon-wide first, i.e., over the entire study area (Fig. 1). Then two site-specific examples are presented, the Melbourne and Vero Beach segments, for which 2005 mapping data are available. Based on both the mapping and transect data, a general pattern emerges: an increase in the areal extent (i.e., acreage or coverage) since the early 1990 s, with some leveling off in the most recent few years. Trends based on mapping In the study area overall, there was loss of seagrass coverage between 1943 and the early 1990 s of 13% (Fig. 2), but up to 90% in some segments (Fig. 3). Seagrass coverage increased since the mid 1990 s and generally leveled off from 1999 to 2003, but with incremental increases in several segments. Lagoon-wide, there is at least as much seagrass mapped in 2005 (26,101 ha, 64,496 acres, 101 square miles) as in 1943 (25,358 ha) (Fig. 2). Of the 18 segments, 12 showed an increase in seagrass coverage from 1994 to 2005 (Fig. 3). Preliminary results from an on-going mapping effort indicate that this positive trend may continue through 2005 (Fig. 3). For example, in the

4 400 FLORIDA SCIENTIST [VOL. 70 FIG. 3. Seagrass coverage (hectares) versus time, 1943 to 2005 for each segment (see Fig. 1 for segment map). Scales vary map data are preliminary. * 5 segment not mapped in that year. Melbourne and Vero Beach segments (IR12 and IR16-20), seagrass coverage has consistently increased since 1992/94, following large losses from 1943 to the 1980 s (Fig. 3). This trend continued even in 2005, following the active hurricane season of 2004 (Steward et al., 2006). Trends based on monitoring of transects Data from monitoring seagrass transects indicate a trend similar to that derived from seagrass mapping. Average summer transect length increased from 109 m to 152 m from 1994 to 1999 and then generally leveled off (Fig. 4). In addition, the summer-winter

5 No ] VIRNSTEIN ET AL. SEAGRASS COVERAGE TRENDS 401 FIG. 4. Average transect length of all transects versus time, 1994 to 2006, summer and winter. variability in transect length has increased in recent years (Fig. 4). Despite the overall increase in the extent of seagrass (Fig. 4), average overall summer seagrass density along all transects decreased from 47% in 1998 and 1999 to 28% in 2005 (Fig. 5). Transect lengths within the Melbourne and Vero Beach segments also increased from 1994 to 1998, an average summer increase of 93 m for the 3 transects within IR12 and 3 m for the 7 transects within IR16-20 (Fig. 6). Again, despite the increase in seagrass extent, summer seagrass density decreased sharply from 2004 to 2005: decreases of 53% and 43% in the Melbourne and Vero Beach segments, respectively (Fig. 7). DISCUSSION Both monitoring methods show similar trends of expansion and persistence in seagrass extent throughout much of the study area, reinforcing the conclusion that these trends are real. Some of this improvement may be attributed to improved management. For example, an order of magnitude decrease in discharge from wastewater treatment plants since 1994, reduced volumes of discharges from major drainage canals (e.g., the frequency of large magnitude discharges from C-54 Canal into Sebastian River and C-1 Canal into Turkey Creek has diminished over the past 12 years; Steward et al., 2003), and new micro-irrigation and fertilizing practices for citrus gradually over recent decades may be resulting in less nutrient loading. However, some of this improvement may simply be climate and long-term natural variability. For example, the seagrass growing seasons (spring/summer) in 1998 and 1999 were unusually dry, resulting in less runoff, clearer water, and more light for seagrass growth. These seagrass trends could not have been detected without a long-term, Lagoon-wide monitoring program; short-term or site-specific data would have generated misleading information. Seagrass coverage in some north IRL segments remain essentially the same in recent years as in 1943, especially those adjacent to watersheds protected

6 402 FLORIDA SCIENTIST [VOL. 70 FIG. 5. Average summer seagrass density (percent cover) of all transects versus time, 1994 to Winter data for latter years were collected only at the middle and end of transects and are thus not comparable to average summer data. FIG. 6. Average summer and winter transect length versus time for Melbourne (IR12) and Vero Beach (IR16-20) segments (Fig. 1), 1994 to Scales vary.

7 No ] VIRNSTEIN ET AL. SEAGRASS COVERAGE TRENDS 403 FIG. 7. Average summer seagrass density (percent cover) versus time for Melbourne (IR12) and Vero Beach (IR16-20) segments (Fig. 1), 1994 to from development by reason of NASA ownership. However, there are several segments where coverage is well below that of 1943 (Fig. 3). Surprisingly, the extent of seagrass was largely unaffected by the hurricanes of We suggest that physical wave impacts, e.g., scour, on seagrass was dampened due to the storm-surge doubling of water depth over most seagrass beds in the Melbourne to Vero Beach area (Steward et al., 2006). Although extent of seagrass was not affected (Fig. 3), density of seagrass decreased within this central IRL region (Fig. 7), perhaps as a result of the lingering low salinities (,20 ppt) due to the heavy rainfall associated with the hurricanes and subsequent winter storms. Why summer-winter variability of seagrass extent has increased in recent years remains a puzzle. The statement that seagrass was largely unaffected by the hurricanes of 2004 may not apply to the south IRL. Large and long-term (.1 year) discharges from Lake Okeechobee into the St. Lucie River, not all associated with the hurricanes, resulted in complete loss of seagrass at some sites in the south IRL near the St. Lucie River (unpublished data and pers. comm. from Becky Robbins and Grant Gilmore). Seagrass coverage and density is dynamic, indicating high resiliency. The fact that seagrass can expand coverage within a year demonstrates that seagrass does respond rapidly to improvements in water quality. We believe that the potential for rapid and continued recovery can be realized if the

8 404 FLORIDA SCIENTIST [VOL. 70 pollutant loading of soils and nutrients, which impact water clarity, can be reduced. There are several watershed projects, planned under state and federal programs, which will achieve such reductions once they are constructed. Monitoring continues. Future work will examine the status of seagrass coverage relative to depth and coverage targets and whether seagrass density recovers after the active 2004 hurricane season. ACKNOWLEDGMENTS Many people generously contributed to this paper over the years. Jan Miller prepared Figures 2 and 3. Joe Beck and Samuel Rajasekhar did all the GIS analyses of seagrass maps. Special thanks to John Windsor and the Florida Academy of Sciences for organizing the 25 th Anniversary of the Indian River Lagoon Symposium. LITERATURE CITED IRLNEP, INDIAN RIVER LAGOON NATIONAL ESTUARY PROGRAM The Indian River Lagoon Comprehensive Conservation & Management Plan. Indian River Lagoon National Estuary Program, Melbourne, Florida. MORRIS, L. J., R. W. VIRNSTEIN,J.D.MILLER, AND L. M. HALL Monitoring seagrass changes in Indian River Lagoon, Florida using fixed transects. Pp In:BORTONE, S. A. (ed.), Seagrasses: Monitoring, Ecology, Physiology, and Management. CRC Press, Marine Science Series, Boca Raton, Florida., L. M. HALL, AND R. W. VIRNSTEIN Field guide for fixed seagrass transect monitoring in the Indian River Lagoon. Technical Memorandum, St. Johns River Water Management District, Palatka, Florida. 28 p. plus appendices. SIGUA, G., J. S. STEWARD, AND W. A. TWEEDALE Indian River Lagoon water quality monitoring network: Proposed modifications. Technical Memorandum No. 12, St. Johns River Water Management District, Palatka, Florida. 70 p. STEWARD, J. S., R. BROCKMEYER, R.VIRNSTEIN, P.GOSTEL, P.SIME, AND JVANARMAN Indian River Lagoon Surface Water Improvement and Management (SWIM) Plan, 2002 Update. St. Johns River Water Management District, Palatka, Florida, and South Florida Water Management District, West Palm Beach, Florida. 272 p. AND W. C. GREEN Setting pollutant loading targets for the Indian River and Banana River lagoons based on loadings vs. seagrass depth limit relationship. St. Johns River Water Management District, Division of Environmental Sciences Technical Memorandum to U.S. EPA and Florida Department of Environmental Protection, February 24, 2006, Palatka, FL., R. W. VIRNSTEIN, M. A. LASI, L. J. MORRIS, L. M. HALL, AND W. A. TWEEDALE The impacts of the 2004 hurricanes on hydrology, water quality, and seagrass in the central Indian River Lagoon, Florida. Estuaries 29: ,, L. J. MORRIS, AND E. F. LOWE Setting seagrass depth, coverage, and light targets for the Indian River Lagoon system, Florida. Estuaries 28: VIRNSTEIN, R. W The large spatial and temporal biological variability of the Indian River Lagoon. Florida Scientist 53: Seagrass management in Indian River Lagoon, Florida: dealing with issues of scale. Pacific Conservation Biology 5: AND L. J. MORRIS Seagrass preservation and restoration: A diagnostic plan for the Indian River Lagoon. St. Johns River Water Management District, Technical Memorandum No. 14, Palatka, Florida. 18 p. plus appendices. Florida Scient. 70(4): Accepted: September 9, 2006