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 St. Johns River Water Management District, Division of Environmental Sciences, 4049 Reid Street,

Palatka, Florida 32177

2 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.

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

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?

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: jsteward@sjrwmd.com

Estuaries and Coasts Vol. 29, No. 6A, p. 954–965 December 2006

2006 Estuarine Research Federation 954
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

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

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.

Hurricane Impacts in Central IRL 955
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.

956 J. S. Steward et al.
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,

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

Hurricane Impacts in Central IRL 957
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.

WATER QUALITY

Short Term

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.

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.

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.

958 J. S. Steward et al.
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

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.

Hurricane Impacts in Central IRL 959
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

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.

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).

960 J. S. Steward et al.
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

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

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).

Hurricane Impacts in Central IRL 961
(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.

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

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

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).

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.

962 J. S. Steward et al.
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

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

Hurricane Impacts in Central IRL 963
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.

LITERATURE CITED

CHRISTIAN, D. 2004. Indian River Lagoon Flushing. St. Johns River

Water Management District Technical Memorandum. St. John

River Water Management District, Palatka, Florida.

COASTAL PLANNING AND ENGINEERING. 1997. Indian River Lagoon

bathymetric survey. A final report to St. Johns River Water

Management District, Contract #95W142, Palatka, Florida.

DOBSON, J. E., E. A. BRIGHT, R. L. FERGUSON, D. W. FIELD, L. L.

WOOD, K. D. HADDAD, H. IREDALE, III, R. R. JENSEN, V. V. KLEMAS,

R. J. ORTH, AND J. P. THOMAS. 1995. NOAA Coastal Change

Analysis Program (C-CAP) for Regional Implementation.

NOAA Technical Report NMFS 123. NOAA, Silver Spring,

Maryland.

GALLEGOS, C. L. 1993. Determination of optical water quality

requirements for growth of seagrasses in the Indian River near

Ft. Pierce, FL, with emphasis on the impact of colored water

discharges. Report to South Florida Water Management

District, Contract # C-3311, West Palm Beach, Florida.

GALLEGOS, C. L. AND W. J. KENWORTHY. 1996. Seagrass depth limits

in the Indian River Lagoon (Florida, U.S.A.): Application of an

optical water quality model. Estuarine Coastal and Shelf Science 42:

267–288.

GILMORE, R. G. 1995. Environmental and biogeographic factors

influencing ichthyofaunal diversity: Indian River Lagoon.

Bulletin of Marine Science 57:153–170.

HANISAK, M. D. 2001. Photosynthetically active radiation, water

quality, and submerged aquatic vegetation in Indian River

Lagoon. Final report to St. Johns River Water Management

District, Contract #93W199, Palatka, Florida.

HECK, JR., K. L. AND D. BYRON. 2005. Post-Hurricane Ivan damage

assessment of seagrass resources of coastal Alabama. Dauphin

Island Sea Lab report, Dauphin Island, Alabama.

INDIAN RIVER LAGOON NATIONAL ESTUARY PROGRAM. 1996. The

Indian River Lagoon Comprehensive Conservation and Management Plan. St. Johns River Water Management District and

South Florida Water Management District and U.S. Environmental Protection Agency. Palm Bay, Florida.

KENWORTHY, W. J. AND D. E. HAUNERT. 1991. The Light

Requirements of Seagrasses: Proceedings of a Workshop to

Examine the Capability of Water Quality Criteria, Standards,

and Monitoring Programs to Protect Seagrasses. NOAA

Technical memorandum, NMFS-SEFC-287. Washington, D.C.

KNOWLES, JR., L. 1995. Rainfall and freshwater discharge in the

Indian River basin within the St. Johns River Water Management District, east-central Florida, 1989–91. U.S. Geological

Survey Water-Resources Investigations Report 94-4193. Tallahassee, Florida.

MICHOT, T. C., J. N. BURCH, A. ARRIVILLAGA, P. S. RAFFERTY, T. W.

DOYLE, AND R. S. KEMMERER. 2002. Impacts of Hurricane Mitch

on seagrass beds and associated shallow reef communities along

the Caribbean coast of Honduras and Guatemala. U.S. Geo964 J. S. Steward et al.
logical Survey Open File Report 03-181, National Wetlands

Research Center, Lafayette, Louisiana.

MONTAGUE, C. L. AND J. A. LEY. 1993. A possible effect of salinity

fluctuation on abundance of benthic vegetation and associated

fauna in northeastern Florida Bay. Estuaries 16:703–717.

MORRIS, L. J., L. M. HALL, AND R. W. VIRNSTEIN. 2001. Field guide

for fixed seagrass transect monitoring in the Indian River

Lagoon. St. Johns River Water Management District, Palatka,

Florida.

MORRIS, L. J. and D. A. TOMASKO (eds.). 1993. Proceedings and

conclusions of workshops on: Submerged aquatic vegetation

and photosynthetically active radiation. St. Johns River Water

Management District, Special Publication SJ93-SP13. Palatka,

Florida.

MORRISON, D., J. MALSI, P. RENAULT, P. LIGHT, AND C. MARX. 1989.

Impact of freshwater discharge from finger canals on estuarine

seagrass and mangrove ecosystems in southwest Florida. Final

Report, Florida Department of Environmental Regulation

Contract CM-230. City of Cape Coral, Florida.

PHILLIPS, R. C. 1960. Observations on the ecology and distribution

of the Florida seagrasses. Florida State Board of Conservation

Marine Laboratory, Professional Paper Series #2. St. Petersburg,

Florida.

PROVANCHA, J. A. AND D. M. SCHEIDT. 2000. Long-term trends in

seagrass beds in the Mosquito lagoon and northern Banana

River, Florida, p. 211–218. In S. A. Bortone (ed.), Seagrasses:

Monitoring, Ecology, Physiology, and Management. CRC Press

Marine Science Series, Boca Raton, Florida.

SALOMAN, C. H. AND S. P. NAUGHTON. 1977. Effect of Hurricane

Eloise on the benthic fauna of Panama City beach, Florida,

USA. Marine Biology 42:357–363.

SHENG, Y. P. AND J. R. DAVIS. 2003. Indian River Lagoon Pollutant

Load Reduction (IRLPLR) model development project, Volume 1, A 3-D IRL hydrodynamics/salinity model (UF-CH3D).

University of Florida, Civil and Coastal Engineering Department, Gainesville, Florida.

SIGUA, G., W. A. TWEEDALE, J. D. MILLER, AND J. S. STEWARD. 1996.

Inter-agency implementation of a modified water quality

monitoring program for the Indian River Lagoon: Methods

and QA/QC Issues. St. Johns River Water Management District

Technical Memorandum #19. Palatka, Florida.

SMITH, N. P. 1987. An introduction to the tides of Florida’s Indian

River Lagoon. I. Water levels. Florida Scientist 50:49–61.

SMITH, N. P. 1993. Tidal and nontidal flushing of the Florida’s

Indian River Lagoon. Estuaries 16:739–746.

SMITH, N. P. 2001. Seasonal-scale transport patterns in a multiinlet coastal lagoon. Estuarine Coastal and Shelf Science 52:15–28.

STEWARD, J. S., R. BROCKMEYER, R. VIRNSTEIN, P. GOSTEL, P. SIME,

AND J. VANARMAN. 2003. 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.

STEWARD, J. S., D. HAUNERT, F. LUND, AND R. VIRNSTEIN. 1994. IRL

Surface Water Improvement and Management (SWIM) Plan

(update). St. Johns River Water Management District, Palatka,

Florida: and South Florida Water Management District, West

Palm Beach, Florida.

STEWARD, J. S. AND J. VANARMAN. 1987. The Indian River Lagoon

Joint Reconnaissance Report. A report to Florida Office of

Coastal Management of the Florida Department of Environmental Regulation, and NOAA Office of Coastal Resource

Management. St. Johns River and South Florida Water

Management Districts. Palatka and West Palm Beach, Florida.

STEWARD, J. S., R. W. VIRNSTEIN, L. J. MORRIS, AND E. F. LOWE. 2005.

Setting seagrass depth, coverage, and light targets for the

Indian River Lagoon system, Florida. Estuaries 28:923–935.

THOMPSON, M. J. 1976. Photomapping and species composition of

the seagrass beds in Florida’s Indian River estuary. Harbor

Branch Foundation, Inc. Technical Report #10. Ft. Pierce,

Florida.

THOMPSON, M. J. 1978. Species composition and distribution of

seagrass beds in the Indian River Lagoon, Florida. Florida

Scientist 41:90–96.

VIRNSTEIN, R. W. 1990. The large spatial and temporal biological

variability of the Indian River Lagoon. Florida Scientist 53:249–

256.

VIRNSTEIN, R. W. 2000. Seagrass management in Indian River

Lagoon, Florida: Dealing with issues of scale. Pacific Conservation

Biology 5:299–305.

VIRNSTEIN, R. W., E. W. CARTER IV, L. J. MORRIS, AND J. D. MILLER.

2000. Utility of seagrass restoration indices based on area,

depth, and light, p. 69–80. In Proceedings of a Symposium:

Seagrass Management: It’s Not Just Nutrients! St. Petersburg,

Florida.

VIRNSTEIN, R. W. AND L. J. MORRIS. 1996. Seagrass preservation and

restoration: A diagnostic plan for the Indian River Lagoon. St.

Johns River Water Management District, Technical Memorandum No. 14. Palatka, Florida.

WOODWARD-CLYDE CONSULTANTS, MARSHALL MCCULLY AND ASSOCIATES, AND NATURAL SYSTEMS ANALYSTS. 1994. Physical features of

the Indian River Lagoon. Final Technical Report to the Indian

River Lagoon National Estuary Program, Melbourne, Florida.

SOURCES OF UNPUBLISHED MATERIALS

DOERING, PETER. personal communication. South Florida Water

Management District, 3301 Gun Club Road, West Palm Beach,

Florida 33406.

SHEIKH, P. A. 2005. The impact of Hurricane Katrina on biological

resources. Congressional Research Service Report for Congress.

Order Code RL33117 http://www.ncseonline.org/NLE/CRS/.

ST. JOHNS RIVER WATER MANAGEMENT DISTRICT (SJRWMD). unpublished data. Hydrologic results. 4049 Reid Street, Palatka,

Florida 32177.

Received, January 3, 2006

Revised, August 4, 2006

Accepted, August 16, 2006

Hurricane Impacts in Central IRL 965