The Impacts of the 2004 Hurricanes on Hydrology ...

Estuaries and Coasts

Vol. 29, No. 6A, p. 954–965

December 2006

The Impacts of the 2004 Hurricanes on Hydrology, Water Quality, and Seagrass in the Central Indian River Lagoon, Florida JOEL S. STEWARD1,*, ROBERT W. VIRNSTEIN1, MARGARET A. LASI1, LORI J. MORRIS1, JANICE D. MILLER1, LAUREN M. HALL2, and WENDY A. TWEEDALE1 1

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St. Johns River Water Management District, Division of Environmental Sciences, 4049 Reid Street, Palatka, Florida 32177 St. Johns River Water Management District, Division of Environmental Sciences, 525 Community College Parkway SE, Palm Bay, Florida 32909

ABSTRACT: Between August 14 and September 26, 2004, four tropical weather systems (Charley, Frances, Ivan, and Jeanne) affected the central Indian River Lagoon (IRL). The central IRL received a prodigious amount of rainfall for the 2 mo, between 72 and 83 cm, which is a once-in-50-yr rainfall event. High stream discharges were generated that, combined with wind-suspended sediments, significantly reduced salinities and water transparency. In September, salinities among central IRL segments dropped from 30 psu or more to # 15 psu, color increased from a low of 10 pcu to $ 100 pcu, and turbidity increased from # 3 NTU up to 14 NTU. Evidence of the hurricanes’ physical effects on seagrasses (burial, no scour) was limited to just one of the more than 25 sites inspected. Within 2 to 3 mo following the hurricane period, most parameters related to water transparency returned to or showed improvement over their prehurricane (February–July 2004) levels. Unseasonably low salinities (,20 psu) and moderately high color (.20 pcu) were observed through spring 2005, largely attributable to a relatively long residence time and a wetter-than-average spring season in 2005. By the end of the study period (July 2006), the central IRL generally showed a continuation of two opposite seagrass trends—an increase in depthlimit coverage but a decline in coverage density—that began before 2004. Also, within a limited reach of the central IRL, there was a temporary shift in species composition in summer 2005 (Ruppia maritima increased as Halodule wrightii decreased). It is likely that the persistently low salinities (not color) in 2004–2005 affected the species composition and coverage density. This study reveals that seagrasses are resilient to the acute effects of hurricanes and underscores the need to reduce chronic, anthropogenic effects on seagrasses.

waves? Were there any observable effects on salinity and water transparency? And, if so, was salinity or water transparency so affected as to affect seagrass depth-limit distribution, coverage density, or species composition?

Introduction The effects of tropical storms and hurricanes on coastal ecosystems can be extensive (Saloman and Naughton 1977; Michot et al. 2002; Heck and Byron 2005; Sheikh 2005). The relatively quick succession of four hurricanes passing through or near the central Indian River Lagoon (IRL) basin in 2004 provides an opportunity to assess the magnitude and duration of effects from multiple storms and the system’s resiliency with respect to water quality (salinity and water transparency factors) and seagrass coverage. We present and discuss the immediate (weeks), short-term (weeks to months), and longer-term (half year to years) status of hydrology, water quality, and seagrass in the central IRL following the passage of 4 hurricanes in 2004: Charley (August 13–14), Frances (September 5–6), Ivan (September 21), and Jeanne (September 26). Specific questions that we want to address with this study are: Did any seagrass beds suffer physical effects (erosion or burial) due to storm winds or

A DESCRIPTION OF THE IRL SYSTEM WITH EMPHASIS ON THE CENTRAL IRL The portion of the IRL system that includes the central IRL extends about 190 km from Ponce de Leon Inlet, Volusia County, to just north of Ft. Pierce Inlet, St. Lucie County (Fig. 1). The entire IRL system actually extends south another 62 km into what is known as the south IRL, from Ft. Pierce Inlet to Jupiter Inlet. Large spatial variability with respect to hydrography and biology is a major characteristic of the IRL system (Steward and VanArman 1987; Virnstein 1990; Gilmore 1995). Much of the biological diversity can be explained by the system’s geographic location and length, transitioning between two biotic provinces, the temperate Carolinean province and subtropical Caribbean province. The IRL system is home to 7 species of seagrass, the most diverse assemblage of seagrass species found in any

* Corresponding author; tele: 386/329-4363; fax: 386/329-4585; e-mail: [email protected] ß 2006 Estuarine Research Federation

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Hurricane Impacts in Central IRL

Fig. 1. Map of the Indian River Lagoon system with emphasis on the central IRL. The central IRL map shows major tributaries and canals and the locations of the water elevation gage and water quality sampling sites. It is not possible to show the locations of the 25 seagrass transects given the scale of the map, but the number of transects per segment is provided.

estuary within the United States (Indian River Lagoon National Estuary Program 1996). In order of decreasing abundance, these species are Halodule wrightii, Syringodium filiforme, Thalassia testudinum, Halophila johnsonii, Halophila decipiens, Halophila engelmannii, and Ruppia maritima. Like the IRL system overall, the central IRL is shallow (average 1.7 m depth mean water level) and microtidal. Semidiurnal tides propagate through two inlets, Sebastian and Ft. Pierce (Fig. 1). Tidal forcing is weak, exerting tidal amplitudes less than

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10 cm, except within a few kilometers of the inlets where amplitudes increase twofold to threefold (Smith 1987). Weekly wind or seasonal oceanic forcings actually cause greater water level variations: 6 10–30 cm (Smith 1993, 2001). Average water residence times vary throughout the IRL system, primarily as a function of distance from an inlet. The entire central IRL is within 28 km of either Sebastian Inlet or Ft. Pierce Inlet, much closer to inlets, and more influenced by oceanic exchange than the north IRL and Banana River. The average residence time in the central IRL can vary between a week to over 2 mo, depending on the time of year and distance from an inlet, and is typically 10 to 15 times shorter than in the north IRL and Banana River, respectively (Christian 2004). Rainfall in the IRL basin averages 127 cm annually (Steward and VanArman 1987; Knowles 1995). The dry season typically begins around mid November and ends by mid June as the wet season begins. Rainfall runoff enters the system as diffuse drainage and through creeks, ditches, and outfall structures. The IRL system’s largest concentration of tributary streams and major canal networks are in the central IRL (see Fig. 1). Most of the canals were constructed between 1920 and 1970 for land development. The potential annual runoff volume is much higher in the central IRL (5,410 m3 ha21 yr21 or a total of 665 million m3 yr21) than in the north IRL (4,400 m3 ha21 yr21 or a total of 218 million m3 yr21), based on current land use (2000) and 127 cm annual rainfall (SJRWMD unpublished data). Large reaches of the central IRL have been affected by decades of increased drainage and land-use intensification as evidenced by its generally poorer water quality and seagrass habitat relative to much of the northern IRL system (Steward et al. 2003). Watershed development in the central IRL has increased the loadings of freshwater, eroded soils, organic material, and nutrients to the estuary. Turbidity, color, and chlorophyll a (chl a), in that order, were shown to be most responsible for attenuating downwelling light (Hanisak 2001; Steward et al. 2003), and light is the primary factor limiting seagrass coverage and depth distribution in the IRL system (Kenworthy and Haunert 1991; Morris and Tomasko 1993; Virnstein et al. 2000). Seagrass abundance and diversity in the IRL can be negatively affected by salinities , 20 psu (Phillips 1960; Hanisak 2001; Steward et al. 2003). Over the past 15 yr, average annual salinity and light attenuation levels within the urbanized reaches of the central IRL, Cocoa-Palm Bay and Vero Beach, were worse than their respective targets (proposed annual-average target for salinity is . 20 psu and

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for the light attenuation coefficient, Kd, is , 1.0 m21; Steward et al. 2003, 2005). As a consequence, seagrass coverages in those two reaches have been the most variable of any IRL reach north of Ft. Pierce Inlet (Steward et al. 2005). Between 1943 and 1996, the greatest loss in seagrass coverage in the IRL system occurred in the Cocoa-Palm Bay reach (82% loss; 1,148 to 204 ha) and Vero Beach (50% loss; 717 to 359 ha). Then from 1999 through 2003, coverages increased in both Cocoa-Palm Bay (546 ha in 2003) and Vero Beach (509 ha in 2003). While coverage increased in those two reaches and throughout the central IRL, seagrass density decreased (central IRL: an average 45% cover in 1999, 43% in 2001, and 33% in 2003). These coverage and density trends will be further discussed in the context of the 2004 hurricane effect observations presented in this paper. Methods A comprehensive suite of data on hydrology, water quality, and seagrass was evaluated to assess the magnitude and duration of hurricane effects in the central IRL. We monitored rainfall, stream discharge, Lagoon water elevation, salinity, light attenuation (Kd), Secchi depth, the major parameters that affect water transparency (turbidity, color, and chl a), and seagrass parameters (coverage, density, and species composition). DATA COLLECTION Hydrologic Data Daily rainfall data were obtained from National Oceanic and Atmospheric Administration (NOAA) National Weather Service meteorological stations in Melbourne and Vero Beach. Data on daily discharge measured at major tributary streams in the central IRL were obtained from the U.S. Geological Survey (USGS). The major tributaries are Eau Gallie River, Crane Creek, and Turkey Creek in the MelbournePalm Bay segment; Sebastian River in the Sebastian segment; and the North, Main, and South canals in the Vero segments (Fig. 1). For this study, both the rainfall and discharge data were computed as either cumulative or average volumes over different time intervals: annual, 60-d, monthly, or weekly. IRL water elevation data (15 min to hourly measurements, NAVD88, July 1996–January 2006) were obtained from Florida Department of Environmental Protection Bureau of Survey and Mapping and USGS for the Cocoa-Melbourne reach (Fig. 1). The data were reduced to daily average elevations to assess water elevation trends and magnitudes before, during, and after the 2004 hurricane period.

Water Quality Data The term water quality is used in this study to denote levels of salinity and the water transparency factors, turbidity, true color, chl a, Kd, and Secchi depth, that can have significant bearing on seagrass abundance and species composition. The water quality data presented here were collected from 10 sites in the central IRL segments: Melbourne-Palm Bay (MP), Grant (G), Sebastian (S), and North and South Vero (NV and SV; see Fig. 1 for sampling site locations). A long-term, monthly data set (January 1989–May 2006) and a short-term, semiweekly to biweekly data set (September 2004–January 2005) were analyzed for this study. The long-term monthly data were collected under the Indian River Lagoon Water Quality Monitoring Network, a cooperative, multiagency program managed by SJRWMD (Steward et al. 1994, 2003; Sigua et al. 1996). The additional short-term data were collected by the SJRWMD for 4 mo during and following the hurricane period and include only salinity, true color, turbidity, and Secchi depth. The short-term data were initially taken twice per week, tapering off to a weekly then biweekly frequency. The monthly measurements of turbidity (NTU), true color (pcu), and chl a (mg l21) were determined by laboratory analyses following standard U.S. Environmental Protection Agency (EPA) methods and laboratory protocols (turbidity EPA 180.1, color EPA 110.2, chl a SM-10200H). During the hurricane sampling period, turbidity and true color were measured using a portable turbidimeter and a bench-top color comparator, rather than the laboratory methods referenced above. The reason for the change in turbidity and color methods during the hurricane period sampling was to generate results more rapidly (usually the same day as sampling) in order to assess and report on conditions frequently to resource managers. Comparisons of results between the different methods for turbidity and true color yielded no significant differences based on both a paired t-test on logtransformed data and the nonparametric MannWhitney test. Field measurements of photosynthetically active radiation (PAR) were made using a LI-COR instrument equipped with three, 4p spherical sensors (two sensors at 0.2 and 0.5 m from surface, and the third sensor at 0.3 m above bottom). Kd values (m21) were calculated as the slope of a semi-log regression of PAR with depth using the method of least squares. Secchi depth (m) was measured using a standard, 18-cm black and white Secchi disk attached to a rope marked at 0.1-m intervals. Salinity (psu) was measured in the field using Hydrolab multiparameter instruments.


Seagrass Data Seagrass beds in the IRL were monitored using two methods: field monitoring of fixed seagrass transects and Lagoon-wide mapping based on interpretation of aerial photographs (Virnstein and Morris 1996; Virnstein 2000). Fixed seagrass transects have been monitored throughout the IRL system every summer and winter since 1994. Transects are oriented perpendicular to the shore, extending from the shore to the deep edge of the seagrass bed. Every 10 m along a measured line, several seagrass parameters are nondestructively measured within a 1-m2 quadrat, divided by strings into one hundred 10 3 10 cm cells (Virnstein and Morris 1996; Morris et al. 2001). In this paper, we report on just three of the parameters, transect length, density, and species composition, from 1994 to 2006 from the 25 transects in the central IRL. In addition to the summer-winter transect data from that long-term record, we also collected data from the same 25 transects in fall (October–November) 2004 immediately after the hurricanes. Transect length is an indicator of seagrass extent from shore and was measured to the nearest meter. Density was measured as a visual estimate of the number of cells that would be 100% filled if all the seagrass shoots within the quadrat could be gathered into a dense coverage (e.g., a cell that contains 30–35 shoots of H. wrightii exclusively is considered 100% filled). Species composition as a percent cover was determined by counting the number of quadrat cells where each species was present (even if it was single shoot per cell). The Lagoon-wide seagrass maps were derived from ground-truthed photo-interpretation of 1:24,000 aerial photographs (Dobson et al. 1995) usually taken sometime in the spring-summer seagrass growing season. Seagrass change can be determined by comparing areal coverage or depthlimit distribution among or between mapping years. Seagrass maps are available for 1943, 1986, 1989, 1992, 1994, 1996, 1999, 2001, 2003, and 2005. The depth limit of seagrass coverage for each of the mapping years was determined by using a 1996 bathymetric data set for the IRL (Coastal Planning and Engineering, Inc. 1997) and is described in Steward et al. (2005). Only the mapped coverages within the MP, NV, and SV segments were assessed with respect to the 2004 hurricane effects because the availability of 2005 map data were limited to those segments. DATA ANALYSES Effects on hydrology, water quality, and seagrass were assessed over the short term, from August through December 2004, and over the long term,

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through 2005 and into 2006. The hurricane period (generally September–October 2004) was compared to the immediate pre-hurricane period in 2004 (generally June through August) and to past fall season extremes (minima or maxima as seasonal averages). The post-hurricane periods, both shortterm and long-term, were compared to the immediate pre-hurricane period in 2004 and to long-term seasonal average or median conditions. The nonparametric Kruskal-Wallis test (a 5 0.05) was applied to the seasonal water quality assessments as needed. Results HYDROLOGY Short Term The hurricanes of 2004, Charley, Frances, Ivan, and Jeanne, affected the IRL basin within a 6-wk period (August 14–September 26). Hurricane rainfall amounts in the central IRL (MP, NV, and SV) were 1.8–2.8 cm from Charley, 7.6–20.3 cm from Frances, 3.6–10.4 cm from Ivan’s south Florida landfall, and 13.0–15.2 cm from Jeanne (National Hurricane Center website www.nhc.noaa.gov; NOAA National Weather Service). Considerable amounts of rain also fell between hurricane events (Fig. 2). The 2-mo cumulative rainfall in August– September 2004 was very heavy in the central IRL (72.1 cm in Melbourne, 83.1 cm in Vero Beach; Fig. 2), contributing over 50% of the annual rainfall in 2004 and equivalent to 58% of the 30-yr average annual rainfall. For most of the central IRL basin, that 2-mo rainfall is statistically classified as a oncein-50-yr event (SJRWMD unpublished data). Because of the high rainfall in August–September 2004, large runoff volumes were generated and discharged through several streams and canals to the central IRL (Fig. 2). In 2004, 2-month cumulative discharge volumes increased substantially from July– August to September–October: a greater than fourfold increase for the MP tributaries (Eau Gallie River, Crane and Turkey creeks), ninefold for S, and nearly fourfold for the Vero canals (Fig. 2). Large discharges continued into mid October due to the high rainfall and floodwater drainage generated in late September. The magnitude of the September–October 2004 discharge volume for all central IRL streams and canals combined (536 million m3) was 252% of the 11-yr average September–October discharge. Coincident with the large discharges to the IRL, water elevation increased substantially in September. The elevation increase was primarily attributable to storm surges induced by Frances and Jeanne (Fig. 3) that were superimposed on the seasonal high level of the coastal Atlantic propagating through the inlets (Smith 2001). The magnitude

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Fig. 3. Daily average water elevation in the central IRL, Melbourne/Palm Bay (MP) segment (Melbourne causeway bridge gauge) between April 2004 and end of March 2005. A gap in the MP data record from late October to late November 2004 was filled with data from a gauge immediately to the north on the Merritt Island causeway bridge near Cocoa. A 10-year (1996–2005) average of daily average elevations is also depicted for comparison. Comparison between 1999–2000 and 2004–2005 water elevations in the Melbourne/Palm Bay (MP) segment shows offsetting, yet similar storm surge patterns for September– October between the two annual records.

WATER QUALITY Short Term

Fig. 2. Monthly rainfall and discharge totals in the Melbourne/Palm Bay (MP), Sebastian (S), and Vero segments from January 2004 through July 2005. An 11-year (1994–2004) monthly average discharge is also depicted for comparison.

of the elevation rise increased the average water depth of the central IRL by one-third, and more than doubled the water depth over large areas of seagrass during passage of Frances and Jeanne. By November 2004, the hydrology of the central IRL basin generally returned to normal seasonal levels with respect to rainfall, discharge, and water elevations (Figs. 2 and 3). Long Term The return to normal seasonal hydrologic conditions observed in November 2004 had ended by the spring dry season (March–June 2005) when higher than average rainfall was measured (Fig. 2). In Vero Beach, the March–June 2005 cumulative rainfall was 70 cm, well above the 30-yr average rainfall of 43 cm. In Melbourne, the March–June cumulative rainfall was 60 cm as compared to the 30-yr average of 37.5 cm. Stream flows responded in kind (Fig. 2) and collectively generated 244% of the 11-yr seasonal (March–June) average volume discharged to the central IRL.

Immediate water quality responses to the hurricanes (particularly Frances, Ivan, and Jeanne) were apparent throughout the central IRL (Fig. 4). There were significant and rapid decreases in salinity and Secchi depth and increases in color and turbidity in September, extending into mid October 2004. This water quality trend actually began earlier in August at the NV and SV segments due to canal discharges that responded to that month’s cumulative rainfall from convective storms and Hurricane Charley. Relative to the June–August pre-hurricane conditions, salinities during the hurricane period (September–October 2004) dropped from 30–35 psu to lows of 15 psu at NV and 8.4 psu at MP (Fig. 4). Comparing the same time periods, color increased sixfold to elevenfold among the segments: from 10– 15 pcu (immediate pre-hurricane) to 100–130 pcu (hurricane). Secondary color spikes occurred after Hurricane Jeanne, which were higher than the earlier September spikes (Hurricane Frances) at MP and NV. Similar to color, turbidity responded to the hurricanes with threefold to tenfold increases over immediate pre-hurricane levels (from 1–3 to 14 NTU). Additional turbidity spikes were observed in November and December at the MP and G segments, which were a consequence of wind-wave sediment resuspension during nontropical weather events.


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Fig. 4. Water quality time series—salinity, turbidity, color, Kd, Secchi depth, and chl a—in central IRL from January 1989 through May 2006. Central IRL segments are Melbourne-Palm Bay (MP), Grant (G), Sebastian (S), North Vero (NV), and South Vero (SV). The 2004 hurricane period is shaded on graphs.

Chl a concentrations during the hurricane period (September–October) were fivefold to tenfold above immediate pre-hurricane (June–August) concentrations in all segments except NV and SV where the trend was similar but slight (Fig. 4). The MP segment had the highest chl a concentration (15.8 mg l21) among all segments measured during the hurricane period, but the largest increase relative to the immediate pre-hurricane level (tenfold) was observed in G. As color, chl a, and turbidity increased during the hurricane period, Secchi depth declined and Kd increased appreciably in all segments (from immediate pre-hurricane levels of $ 1.5 m Secchi depth and # 1 m21 Kd m to # 0.5 m Secchi depth and . 1.8 m21 Kd). Kd values during the hurricane period were highest in the G and SV segments (3.2 and 3.6 m21, respectively). By December 2004 or January 2005, turbidity, chl a, Kd, and Secchi depth (but not color) at all sites returned to immediate prehurricane levels (Fig. 4). The magnitude of the hurricanes’ effect on water quality was further assessed by comparing the water

quality of September through November (fall) 2004 to previous fall extremes of 1989–2003 (Fig. 5). During fall 2004, the MP, G, NV, and SV segments experienced the highest 3-mo average color compared to previous fall seasons in the 15-yr record. The average turbidities at MP and G segments during fall 2004 also exceeded all previous fall seasons, but G was the only segment where the average Kd exceeded a prior fall extreme. For the other parameters, salinity (minimum), Secchi depth (minimum), and chl a (maximum), there were previous fall season levels that were more extreme than what was observed in fall 2004 (Fig. 5). Long Term Salinities remained below 20 psu for 20 mo (September 2004–April 2006) in the MP segment and nearly so in the G segment save for a few monthly measurements that exceeded 21 psu (Fig. 4). In the other segments (S, NV, and SV), salinities had generally increased to . 20 psu by late October or November 2004. However, all segments

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experienced median salinities in the 2005 spring season (March–May) that were significantly lower than their preceding 15-yr spring season medians (Kruskal-Wallis, a 5 0.05), and no segment returned to the consistently high salinity levels of the immediate pre-hurricane period ($ 30 psu, June– August 2004) until late spring 2006. Color declined during October 2004 through February 2005, but increased to levels above 20 pcu at all sites during the 2005 spring season (Fig. 4) and were slightly higher than their 15-yr, spring season medians (# 15 pcu). In contrast to the color and salinity trends, there was both a short-term recovery and long-term continuation of good conditions in most segments relative to turbidity, chl a, Secchi depth, and Kd (e.g., median Kd ranged from 0.6 to 0.7 m21 during January–June 2005). The 2005 spring season medians of turbidity, chl a, and Kd showed improvement over their preceding 15-yr, spring season medians, and the differences were significant (Kruskal-Wallis, a 5 0.05) for turbidity in S and for chl a and Kd in NV and SV. Good water transparency continued from spring 2005 through spring 2006 as annual median Kd values among segments ranged from 0.8 to 1.0 m21. Summary Water quality was significantly affected during the hurricane period (September–October 2004). Color, turbidity, and Kd during the fall 2004 exceeded all previous fall season levels, especially in MP and G. Salinity and color were affected most over the longer term (especially at the MP and G sites) and exhibited, at best, only a partial return to immediate pre-hurricane or seasonal median levels. It is likely that the recovery of salinity and color was hindered by the higher than average rainfall and stream discharges in the spring season of 2005. Other parameters related to water transparency (Kd, Secchi depth, turbidity, and chl a) improved within a few months following the last hurricane and, relative to seasonal medians, showed improved levels well into the 2006 seagrass growing season. SEAGRASS

Fig. 5. Comparison between average water quality in the fall 2004 vs. historical fall extremes in water quality (minima or maxima as seasonal averages). Values were averaged across sampling sites and months (September, October, November) within a segment. Lagoon segments are Melbourne-Palm Bay (MP), Grant (G), Sebastian (S), North Vero (NV), and South Vero (SV).

Seagrass transect length and density as measures of effect must be carefully considered given the large variability that exist among and within transects. Some transects are highly variable over time, whereas some are quite stable, although there is generally a summer to winter decline in both length and density. A hurricane effect assessment at each transect or of a segment, based on an aggregate of transects, must be judged against its long-term variability.


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Short Term Among the 25 transects and other sites inspected throughout the central IRL, only one transect site in NV revealed a physical effect, and it was the partial burial of seagrass (H. wrightii) within 30 m of shore. There were no signs of bottom scour or erosion anywhere in the central IRL where seagrass bed inspections were made. (Sediment burial of seagrass was also seen at several sites in the south IRL near Ft. Pierce and Stuart much closer to the landfalls of Frances and Jeanne, but there was no sign of bottom scour [Doering personal communication].) Except in G and possibly in the MP and NV segments, the coverage of seagrass in the central IRL did not appear to be substantially affected in the short term. In G, the indication of an effect on depth-limit coverage was observed in the winter 2005 when average transect length was not only shorter than expected for that time of year (at 55 m), but was the shortest measured to date in that segment (Fig. 6). In the MP and NV segments, transects lengths receded some in the immediate post-hurricane period (October–November) then increased in winter 2005 (Fig. 6), possibly indicative of an immediate effect on depth-limit coverage, albeit slight and brief. In the remaining segments (S and SV), the immediate post-hurricane transect lengths were intermediate between the summer 2004 and winter 2005 lengths and consistent with the typical summer to winter decline; the average winter 2005 transect lengths were within the longterm variability of winter lengths (Fig. 6). The short-term effect on density was more widespread and intense than the limited effect on depth-limit coverage. This was evident by the precipitous drop in percent cover from summer 2004 to the immediate post-hurricane period in all segments (Fig. 6). By winter 2005, there was some recovery in densities in the S and Vero segments, but densities continued to drop in MP and G. The winter 2005 densities in MP and G (c. 1.2% cover in each segment) were the lowest measured since the winters of 1995 and 1996 (Fig. 6), which were part of a notably long wet period in the mid to late 1990s that strongly affected salinities in the MP segment (Steward et al. 2003; see MP’s salinity time series in Fig. 4). By winter 2005, there was no indication of a short-term effect on seagrass species composition anywhere in the central IRL. Long Term The shallow area in NV where seagrass (H. wrightii) was buried showed same species recovery by summer 2005 and at a coverage density equivalent to summer 2004. Based on transect lengths and mapped coverages (Figs. 6 and 7), there was no

Fig. 6. Seagrass transect length and density (average percent cover) from 1994 to 2006 for summer (S) winter (W) and fall 2004 hurricane sampling period (H).

long-term hurricane effect on the depth-limit coverage of seagrass in the central IRL. The depth-limit coverage expanded to or beyond what it was in summer 2004. By summer 2005, all segments had average transect lengths that were either greater than or comparable to the average lengths in summer 2004 (Fig. 6). By summer 2006, the average lengths had increased further. The rate of transect length increase was most remarkable in the NV, SV, and MP segments where average lengths in the summer 2006 were 10 to 84 m beyond what was measured in summer 2004 (Fig. 6). Based on mapped coverages, this increase in depth-limit coverage is part of a long-term positive trend in central IRL that began as early as the mid 1990s (as shown in Fig. 7 for the MP, NV, and SV segments). We did not observe the same positive trends for densities. Save for S, summer 2005 densities in the central IRL were lower than in summer 2004, extending the general negative trend in densities that began in 2001 or 2003, depending on location

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Fig. 8. Long-term average percent cover of seagrass species at the one transect (#31, MP segment) where there was a detectable shift in species composition following the 2004 hurricanes. The dates on the x-axis indicate summer (S) and winter (W) from 1994 to 2006 and also depicts the 2004 fall-season hurricane sampling period (H).

maritima was also observed in the MP segment during the mid to late 1990s (Fig. 8) when seagrass densities overall declined (Fig. 6). It is believed that both phenomena were probably a response to low salinities (, 20%; Steward et al. 2003; see salinity time series in Fig. 4). Summary

Fig. 7. Seagrass areal coverage (ha) and depth-limit extent (m, MWL) within the Melbourne-Palm Bay (MP) and Vero segments (NV and SV) for several mapping years before and one mapping year after the 2004 hurricanes.

(Fig. 6). The MP and G segments were the only two segments to continue their density declines into summer 2006. Their average densities in summer 2006 are some of the lowest summer averages in the central IRL to date (13% cover in MP and 11% in G). In the Vero segments, the trend had reversed by summer 2006 with small increases in average densities over the summer 2005 averages. The S segment was the least affected of any segment over the long term, exhibiting a steady increase in density since the immediate post-hurricane period, with average summer densities in 2005 and 2006 that exceeded the summer 2004 average (Fig. 6). By summer 2005, only one transect site, a MP site, showed a substantial change in species composition (Fig. 8). From summer 2004 to summer 2005, H. wrightii decreased from 68% to 40% cover, as transect averages, while there was a nearshore emergence of R. maritima (0% to 18% transect average) and a deep-edge emergence of H. engelmannii (0% to 1% transect average). By summer 2006, H. wrightii had nearly regained its prehurricane coverage density while R. maritima was disappearing. This transient emergence of R.

The immediate to long-term effects on seagrass were largely limited to density. While central IRL seagrass beds progressively thinned out, as was the trend even before the 2004 hurricanes, their depthlimit coverage continued its decade-long expansion apparently undeterred by any hurricanes. It is quite likely that the persistently low salinities in 2004 through 2005 (not wholly attributable to the hurricanes) may have promoted the continuation of the negative trend in densities. The lowest salinities of the 2004–2006 period were measured in MP and G where the lowest average densities and emergence of R. maritima (in MP) were observed. Discussion The 2004 hurricane period demonstrated how important hydrologic factors, such as rainfall, stream discharge, and water elevation or depth, are to water quality and seagrass distribution. During September and October 2004, the seasonal high water elevation coupled with the hurricane storm surges (Frances and Jeanne) produced water levels in the IRL that were well above the 10-yr seasonal average. The storm surges more than doubled the water depth over large areas of seagrass. The increased depth likely dampened the effect of storm waves as we found no sign of seagrass bed scour or erosion, but waves did erode shorelines throughout the central IRL, and at one transect it was evident that freshly eroded sands buried nearshore seagrass (H. wrightii). Hurricane storm surges (Frances and Jeanne) produced a precipitous rise and fall of IRL water


levels, resulting in a large hydraulic head that rapidly flushed water out Sebastian and Ft. Pierce inlets. At the same time, storm winds resuspended sediment and detritus, and rainfall runoff conveyed huge loads of upland material to the IRL. Much of that bottom and upland material remained in suspension, affecting water quality (especially color and turbidity) over a short term (September– November 2004) as the material was being flushed out. Although it may be possible that this short-term effect on water quality and transparency accelerated or augmented the usual winter decline in seagrass coverage, there was no longer-term effect on seagrass depth distribution in the central IRL as evidenced by the increased 2005 and 2006 seagrass transect lengths and 2005 mapped coverages. The nearshore area where H. wrightii had been buried, was fully recovered by summer 2005. While the hurricanes helped generate runoff conditions that decreased water quality, they also produced conditions (wind setup and abrupt water elevation changes) that increased flushing rates. We examined the residence time output from a CH3D hydrodynamic model simulation of 1999 conditions in the central IRL (Sheng and Davis 2003; Christian 2004) when there were two hurricanes, one in mid September (Floyd) and the other in mid October (Irene), that produced a storm surge pattern very similar to Hurricanes Frances and Jeanne (Fig. 3). The model output indicates that the timing and magnitude of storm surges that were observed in 1999 as well as in 2004 (0.4–0.6 m rise and fall within a few days) can halve residence times in the central IRL. The average fall season R50 values (time it takes for 50% of a conservative tracer to be removed) ranges from 2 wk in the Vero segments to . 1 mo in the MP segment. The 1999 and, presumably, the 2004 fall season R50 values were 9 d in the Vero segments and 2 wk in the MP segment (Christian 2004). Those results also indicate that the MP segment can entrain freshwater and pollutants much longer than other areas in the central IRL. The unusually wet spring season in 2005 caused salinity to drop or remain depressed, as it did in the MP and G segments, while color remained slightly above seasonal medians. As such conditions persist, seagrass beds can be affected. In the MP segment, there was a transient shift in seagrass species composition, and throughout the central IRL, there was a continued decline in density. In the MP segment, the percent cover of R. maritima increased as H. wrightii decreased. R. maritima is the most tolerant of low salinities compared to the other seagrass species found in the IRL. The periodic emergence of R. maritima has been observed before in the MP segment as well as in segments immediately north during the 1990s

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when salinities dropped and remained below 20 psu for months (Provancha and Scheidt 2000; Steward et al. 2003; see Fig. 8). Low salinities may have also contributed to the general thinning of seagrass beds from 2004 to 2005, particularly in the MP and G segments where the lowest salinities and densities were measured. Other investigators have found a relationship between salinity and seagrass density. Hanisak (2001) for the central IRL and Morrison et al. (1989) for the southwest Florida estuary of the Matlacha Pass Aquatic Preserve both reported a strong positive correlation between seagrass abundance and salinity. Montague and Ley (1993) observed that fluctuating salinities affected seagrass abundance in Florida Bay. Thompson (1976, 1978) reported a seagrass density decline in the Vero segments when salinities dropped due to the influence of discharges from North, Main, and South canals. Phillips (1960) found that H. wrightii coverage in the southern IRL became sparse at salinities , 17 psu. Although color remained unseasonably high during the post-hurricane period, it was on average only 5 to 10 units above the 15-yr, spring season medians (# 15 pcu) for all segments. Both turbidity and chl a levels in 2005 were well below their 15-yr, spring season medians, and water transparency was good in all segments well into the seagrass growing season of 2005 (median Kd values # 1.0 m21, March 2005 to May 2006). We assume that color, at the post-hurricane levels observed, did not play a significant role in attenuating light and would not have affected seagrass coverage density or species composition. The theoretical basis for this assumption was validated through application of an optical model that was developed for the south IRL and predicts Kd based on turbidity, chl a, and color (Gallegos 1993; Gallegos and Kenworthy 1996). The model overpredicts Kd for the central IRL but can still provide a relative estimate of Kd for a range of color as turbidity and chl a are held constant. For turbidity and chl a, the highest post-hurricane averages (January–July 2005) among the segments were chosen as the constant values for each model run: 2.6 NTU and 3.0 mg l21, respectively. Color values of 10 pcu were used in one model run and 30 pcu in the other, which bracketed the range of post-hurricane segment medians. The predicted Kd results were 0.8 m21 at 10 pcu and 1.0 m21 at 30 pcu; certainly not a wide variance and still under the target Kd of 1.0 m21 currently proposed for the central IRL. Conclusion A decrease in seagrass production (including defoliation) typically occurs in the fall when IRL

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water elevation is at its annual high, when rainfall and stream discharge levels are normally at or just past their seasonal peaks, and when salinity and water transparencies can be correspondingly low. The arrival of hurricanes during that time, as was the case in 2004, should presumably have minimal effects on seagrass beds already in seasonal senescence. Despite the magnitude of the hurricanes’ short-term effects on hydrology and water quality, the resultant effects on seagrasses appeared to be minimal: the transitory burial of seagrass at only one transect site, a slight interannual shift in species composition at only one transect site in Melbourne, and contribution toward an ongoing decline in density into the 2005 seagrass growing season. The latter two effects (species change and density decline) were mostly due to a prolonged period of low salinities that cannot be wholly attributable to the 2004 hurricanes; the above average rainfall and discharges in early 2005 also contributed. One consequence of human development in a coastal basin is the loss of natural hydrologic buffers (e.g., loss of wetlands and natural drainage features and their flood storage and flow attenuation capacities) that can compromise an estuary’s resiliency or capacity to rapidly recover from hurricanes. One manifestation of a compromised capacity may be seen in water quality changes that are substantial and sustained, ultimately and negatively affecting biological resources. This study revealed that central IRL seagrass beds were resilient to acute hurricane effects, but are showing signs of chronic instability (large variability in coverage and density over the long term) in segments that have relatively long residence times and are continually affected by drainage from upland developments. This study underscores the need for water managers to remain focused on implementing the strategies for minimizing chronic effects on seagrass in the IRL. Because of increased development and drainage in the central IRL, both water quality and seagrass beds are subjected to more frequent discharges at higher rates and longer durations than when the basin was undeveloped (Steward and VanArman 1987; Woodward-Clyde et al. 1994). Periods of low salinity (such as those observed in this study) and the potential for poor water transparencies, along with their negative effects on seagrass beds, can be minimized if watershed discharges and pollutant loadings are reduced. That is the aim of several watershed projects whose purpose is the reduction of discharge magnitude (rates and volumes) and duration, as well as the reduction of annual loadings of nutrients and soils. SJRWMD and its agency partners have made considerable progress toward curtailing base-flow

and storm discharges from Sebastian River and Turkey Creek (MP segment), and more work is planned to significantly reduce freshwater (salinity) and pollutant load (water transparency) effects on seagrass (Steward et al. 2003). ACKNOWLEDGMENTS The authors thank Estuaries and Coasts for the making this special issue possible and Holly Greening for inviting us to contribute. We also thank our colleagues in the Division of Engineering, David Clapp, David Christian, and Peter Sucsy, for their data input and reviews on the hydrologic assessment of this study. Finally, we want to thank the journal reviewers for their significant technical contributions.

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