ArticlePDF Available

Technical Synthesis of Sarasota Bay

Authors:

Show all 10 authorsHide

Content uploaded by Y. Peter Sheng

Content may be subject to copyright.

Page 1 of 17

by David A. Tomasko, Ph.D.

Sarasota Bay National Estuary Program

Mark Alderson

Ernest Estevez, Ph.D.

Peter Clark

Michael

Hey1

James Culter

Susan Lowrey

Kellie Dixon

Y. Peter Sheng, Ph.D.

Randy Edwards, Ph.D.

John Stevely

Table of Contents

l Executive Summary

Impacts of Pollutants on Sarasota Bay

l Water Clarity

l Dissolved Oxygen

l Present and Future Habitat

l Literature Cited

Executive Summary

Much has been learned from the technical work conducted on Sarasota Bay (from Anna Maria Sound

to Venice Inlet) during the past few years. The extent and severity of the problems within Sarasota

Bay are more substantial than originally believed, particularly in regard to the levels of toxic

contaminants found in tributaries and the degree of habitat loss. In addition, recent evidence suggests

that the extent of eutrophic conditions is most probably underestimated throughout the Bay.

The nomination document supporting entrance to the National Estuary Program stated that the

majority of problems in Sarasota Bay were related to overuse of its rather limited resources, At that

time, it was believed that habitat loss and overuse were the most pressing issues. However, data

collected through the Program indicate larger-scale problems than originally perceived.

Metals contamination, as well as contaminants from pesticides and

PCB’s,

is believed to be a

significant issue in Hudson Bayou, Cedar Hammock Creek, Bowlees Creek, Whitaker Bayou and

Phillippi Creek. In addition, the potential exists for contamination in the unsampled upstream portions

of other tributaries.

While such areas comprise a relatively small proportion of the total area of Sarasota Bay, they make

up a large proportion of the extremely important low-salinity nursery habitat for Sarasota Bay’s

fisheries. Data also indicate that vitally important tidal wetlands, some of them located in contaminated

tributaries, have declined by approximately 39 percent during the past 40 years, with equally dramatic

declines occurring in freshwater wetlands.

8/l/97

9:28:47

Technical Synthesis of Sarasota Bay

Page2of17

Oysters from tributaries with sediment contamination showed elevated levels of copper and zinc

compared to other sites in Florida, and were greatly elevated in lead when compared to both state and

national data sets. This information indicates that metals contamination, the product of stormwater

runoff and illicit point-source discharges, is an important issue in Sarasota Bay. Stormwater pollution

is also the major source of nutrient loadings, accounting for approximately 47 percent of the nitrogen

and phosphorus entering the Bay.

Data collected for the Program suggest that nutrient pollution is an important issue, since Sarasota

Bay currently receives approximately three times as much nitrogen as would be loaded from a pristine,

undeveloped watershed

Heyl, personal communication). It is also apparent that Bay circulation

and flushing patterns, as well as sediment resuspension and transport, play an important role in

determining the magnitude of water-quality degradation associated with nutrient over-enrichment.

In some areas of Sarasota Bay, anecdotal information and preliminary studies indicate that the animal

communities found in seagrass meadows have reduced species diversity, perhaps due to recurrent

hypoxia associated with algal blooms. Algal blooms, which are not uncommon in parts of the Bay with

reduced circulation, appear to be related to nutrient over-enrichment, Persistent and noxious algal

blooms can indicate water-quality problems that might require large-scale, potentially expensive

remedies.

Preliminary data from continuous monitoring of dissolved oxygen (D.O.) and faunal-utilization studies

suggest that parts

of the

Bay are more degraded than is indicated by the State of Florida’s Trophic

State Index (TSI). The TSI classifies almost all of Sarasota Bay as “good,” with only Little Sarasota

Bay ranking as “fair.” However, the index does not contain a specific term for critical pre-dawn D.O.

sags. Given the well documented importance of recurrent low D.O. levels on species diversity and

abundance within estuarine locations, including preliminary information from Sarasota Bay, it might

prove useful to incorporate such information into a modified

TSI.

Impacts of Pollutants on Sarasota Bay

Metals

Habitats located outside the mouths of the tributaries to Sarasota Bay do not appear to be heavily

impacted by metals contamination (Dixon, 1992; Lowrey, 1992). The data from the sediment- and

shellfish-contamination studies indicate that elevated metals concentrations appear primarily in the

tributaries, with anthropogenic enrichment typically increasing as one progresses upstream.

Areas ofnotable metals enrichment include Hudson Bayou, Cedar Hammock Creek, Phillippi Creek,

Whitaker Bayou and Bowlees Creek, as well as areas near points of substantial stormwater runoff.

Levels of mercury (the only regulated metal) were below federal action limits for health and safety, but

metals concentrations in shellfish were well above Florida averages for lead, zinc and copper.

For metals, the routes of entry into Sarasota Bay vary Most of the zinc entering Sarasota Bay comes

from direct atmospheric deposition and precipitation, while most lead enters via stormwater runoff

(CDM, 1992). Metals deposited on paved surfaces by direct atmospheric deposition would then be

incorporated into stormwater runoff.

The routes of entry for metals other than lead and

zinc

have not been determined for Sarasota Bay, but

they might be expected to behave in similar manners as have been documented in other major

estuaries, specifically Chesapeake Bay. Data from Chesapeake Bay indicate that in addition to zinc,

significant amounts of lead, copper and cadmium enter the Bay via direct precipitation on the open

!2/1197

Technical Synthesis of Sarasota Bay Page 3 of 17

water (Haberman et at, 1983). Lead, along with cadmium, is incorporated into stormwater runoff via

dry deposition of automobile exhaust onto paved surfaces, as well as through the deterioration of

brakes and tires (Haberman et al., 1983).

Another source of metals contamination comes from marine activities. In Chesapeake Bay, copper

loadings related to boater’s uses of antifouling bottom paints were thought to equal loadings from

industrial and municipal sources (Haberman et at, 1983). With more than 30,000 registered boats in

Manatee and Sarasota counties, the potential role of antifouling paints on copper loading into Sarasota

Bay deserves further attention. In addition, the use of copper-containing herbicides for weed control

along roads may be associated with elevated copper levels found in stormwater-control structures

(Lowrey, personal communication).

In some tributaries, problems with metals enrichment are exacerbated by contamination from pesticide

residues and PCBs (Dixon, 1992; Lowrey, 1992). The synergism between different metals, or metals

and pesticide residues, is mostly unknown. Consequently, more detailed investigations would seem

appropriate to determine the biological effects of sediment contamination by multiple factors.

Low-salinity habitats are essential for juvenile snook, redfish, tarpon, spotted seatrout, striped mullet

and pink shrimp (Edwards, 1991). As these areas become increasingly contaminated by metals, both

lethal and sub-lethal effects would act to reduce the sizes of future populations of recreationally and

commercially important species (Haberman et at, 1983).

Role of Nutrients

The nutrients nitrogen and phosphorus play important roles in determining the trophic status of

Sarasota Bay. Under conditions of increased nutrient availability, one would expect elevated levels of

phytoplankton (with reduced water clarity), elevated levels of epiphytic algae (which would shade

seagrasses) and greater amounts of drift algae (capable of shading seagrasses and producing recurrent

low pre-dawn dissolved-oxygen levels). With lower nutrient loads, less algae can be supported.

Nitrogen, rather than phosphorus, appears to be the primary limiting nutrient for phytoplankton in

Tampa Bay (Johansson, 199

l),

and nitrogen is most probably the limiting nutrient for phytoplankton,

epiphytic algae on seagrass blades and drift macroalgae in Sarasota Bay (see review in Lapointe et al.,

1992a). However, even low levels of phosphorus enrichment might be sufftcient to stimulate algal

blooms in freshwater ponds and streams throughout the watershed (Taylor, 1967).

Baywide, approximately half of all nitrogen and phosphorus loadings come from stormwater runoff,

and roughly one-quarter of loadings come from direct atmospheric deposition (CDM, 1992). The

remaining nutrient loads are divided among baseflow (groundwater contributions to tributaries), septic

tanks and point sources.

Wastewater

Point sources of pollution can cause localized water-quality problems, but the overall status of water

quality in Sarasota Bay does not seem to be strongly impacted by point sources of pollution (CDM,

1992). In addition, many point sources of nutrient pollution have been upgraded in recent years. The

documented reduction over time of phosphorus and nitrogen levels in waters offshore of Whitaker

Bayou (Lowrey, 1992) may be associated with the upgrade to nutrient-removal technology at the City

of Sarasota’s wastewater-treatment plant.

While not prominent Baywide, septic systems play a significant role in nitrogen loading in Bay

segments whose watersheds have concentrations of septic tanks (i.e., Roberts Bay, Little Sarasota

8/l

I97

9:28:48

Technical Synthesis of Sarasota Bay

Page4of17

Bay, Blackbum Bay), While properly functioning septic systems do not pose health problems, their

primary function is that of minimizing health risks through reducing bacterial contamination, rather

than the removal ofnitrogen and phosphorus in effluent.

For the soils characteristic of the Sarasota Bay region, carbonate-binding sites generally prevent

groundwater transport of phosphorus to nearby surface waters

(IFAS,

1985).

In contrast, the

processes of absorption, biological uptake, denitrification and volatilization might remove only 20-40

percent of the nitrogen load before septic-tank effluent reaches groundwater

(IFAS,

1985).

Once in the groundwater, nitrate is relatively free to travel, as opposed to ammonium, which might

still absorb onto binding sites, The method used in the CDM study (1992) for calculating the impact of

septic tanks on nitrogen loadings is the best effort to date for the Sarasota Bay area, as it was locally

calibrated using data on nutrient concentrations in receiving waters. Given that Sarasota County

contains approximately 45,000 septic tanks -the vast majority of systems in the Bay watershed- it is

essential that their impacts on nutrient loadings be documented.

In parts

of the

Bay watershed, particularly in Sarasota County, package sewage-treatment plants are

common. The levels of treatment and means of effluent disposal for these plants vary. Plants with

direct surface discharge must meet advanced wastewater-treatment (AWT) levels for biological

oxygen demand, total suspended solids, total nitrogen and total phosphorus; respectively, 5, 5, 3,

(mg/l). In contrast, several plants treat effluent only to secondary levels, with up to seven times the

nitrogen concentration of AWT effluent (approximately 20 mg/l), and four times the phosphorus

concentration of AWT effluent (approximately 4

mgA).

If percolation ponds are used for these secondary-treatment plants, the nutrient-loading potential for

these plants might be best estimated using information on groundwater migration of nutrients within

septic-tank effluent streams. Package plants with secondary treatment and percolation ponds would be

a more condensed source of nutrient pollution compared to an equivalent number of customers using

septic tanks.

Consequently, replacing septic systems with secondary- treatment plant using percolation ponds may

exacerbate problems in some areas, and might not result in any reductions in total nutrient loadings to

nearby surface waters. Connecting septic systems to secondary plants with re-use of

efIIuent,

or to

advanced wastewater-treatment plants with or without re-use, would be the only way to ensure a

decline in nutrient loads associated with wastewater

Stormwater

Stormwater loadings of nitrogen and phosphorus would be expected to decrease if agricultural land is

replaced by residential land uses. However, if natural areas are developed for housing, stormwater

loadings of nutrients would be expected to increase (data from CDM, 1992). Estimates of

nutrient-removal efficiencies of wet detention ponds, the most efficient stormwater-treatment systems,

average only 30-percent removal for nitrogen, and only SO-percent removal for phosphorus (Heyl,

1992). Currently, stormwater-control structures are required only for new developments involving the

subdivision of land, not for development of single homes on single lots.

Approximately 40 percent of the Bay’s watershed is in residential land use (CDM, 1992). Due to

extensive use of lawn fertilizers, the nutrient concentration of runoff from these residential areas is

second only to various agricultural land uses (CDM, 1992). As such, it seems obvious that source

control of nutrient runoff(e.g., educating homeowners about the impacts of lawn fertilizers on Bay

waters) would be an essential tool for improving water quality in Sarasota Bay.

9~28~48

Land-Use

Patterns

WBhfn

the

Watershed of Hudson Bayou

Land-Use Patterns Within the

Watershed

of North Creek

Technical Synthesis of Sarasota Bay

PageSof

Management options must consider the diversity of land-use patterns that occur throughout our

watershed.

Figure

1 shows the difference in land-use patterns among Hudson Bayou (an urbanized

watershed), North Creek (a rural watershed) and

Phillippi

Creek (intermediate between urban and

rural). As such, management strategies for stormwater control must be designed on a

watershed-by-watershed basis.

Connections Between Nutrient Lords and Water Quality

The recently completed nutrient-loading evaluation provides

useful

data on the sources and quantities

of nutrient loading on a watershed-by-watershed basis. However, the model cannot predict changes in

water quality associated with increased or decreased nutrient loads. Factors such as circulation and

sediment-nutrient fluxes need to be taken into account.

Figures 2. 3. 4

For instance, when nitrogen loadings from various watersheds are plotted against the annual average

total Kjeldahl nitrogen (organic N plus ammonium;

TKN)

concentration within receiving waters, no

clear pattern appears (Figure 2). The same lack of correlation occurs when phosphorus loadings are

plotted against annual average-water-column total phosphorus (TP) levels, and when nitrogen

loadings are plotted against annual average-water- column chlorophyll a concentrations

(Figure

3).

Reasons for this lack of correlation include differences in the segment volumes to which loads are

applied as well as differences in flushing rates of various segments and potential differences in nutrient

cycling associated with dissimilar sediment dynamics. As such, areas degraded by elevated loading

8/l/97

9.3R.4R

Technical Synthesis of Sarasota Bay

Page 6 of 17

would not necessarily be found in the immediate vicinity of the loading point, but possibly would be

some distance away.

As an example of the tenuous relationship between nutrient loading and segment-wide water quality,

Little Sarasota Bay has much poorer water quality (higher TKN and TP, lower clarity, shallower

depths for its seagrass meadows) than both Roberts Bay and Blackbum Bay, even though Roberts Bay

and Blackburn Bay receive considerably greater nitrogen loads than Little Sarasota Bay. A likely

reason for this apparent discrepancy is the location of Little Sarasota Bay in the null zone for

circulation within this region, as well as the proximity of Roberts Bay and Blackburn Bay to Big Pass

and the Venice inlet, respectively, which provide better flushing (Sheng and Peene 1992).

Similarly, recently obtained current and salinity data indicate that water quality in Anna Maria Sound

and Palma Sola Bay is influenced by the Manatee River and Tampa Bay (Sheng and Peene, 1992).

This influence might result in poorer water quality than that produced by the nutrient-loading

estimates for the watersheds directly draining into these areas.

Within a given area, with hydraulic variables remaining similar from measurement to measurement,

water quality can correlate well with loadings. A plot of nitrogen loadings versus chlorophyll a levels

in Hillsborough Bay (a part of Tampa Bay) shows a clear pattern over a period of 22 years (Figure 4).

The Sarasota Bay data set, on the other hand, is from a single year, and represents the initial stages of

developing specific relationships between water quality and nutrient loads on a segment-by-segment

basis.

Water quality in areas of Sarasota Bay with reduced flushing would probably be slower to respond to

nutrient-loading reductions, due to the higher residence times and the increased importance of nutrient

release from sediments, which would be less likely to be transported to other locations. In the cases of

Little Sarasota Bay and Palma Sola Bay, water quality might not improve as quickly and/or

dramatically after reducing land-based pollution as would be expected to occur in areas such as

Roberts Bay, Blackburn Bay and the areas offshore of Bowlees Creek and Tidy Island.

Without a coupled circulation/water-quality model, only limited qualitative forecasts can be made as to

the expected benefits of implemented management options. Although we cannot at present predict the

exact response of the Bay’s waters to reductions in nutrient loading, much evidence exists as to the

expected positive benefits associated with reduced nutrient loadings.

In the Potomac River, a 95percent reduction in point-source loads of both total phosphorus and

biological oxygen demand was brought about mainly by upgrading municipal sewage-treatment plants

to AWT standards (Alderson, 1988). Aquatic grass habitat in the Potomac River has dramatically

increased during the past few years, with much of this increase attributed to reduced water-column

chlorophyll levels and increased water clarity (Carter and Rybicki, 1986).

Similarly,

80-

to go-percent reductions in point-source phosphorus loading into Lake Erie and Lake

Ontario have led to declines in water-column phosphorus levels and decreased abundances of nuisance

algae in these phosphorus-limited systems (Alderson, 1988).

Johansson (1992) has documented the improvements in water quality in Tampa Bay that have

accompanied reduced nutrient loading by the fertilizer industry and the upgrading of the City of

Tampa’s main wastewater-treatment plant to Advanced Wastewater Treatment (AWT). Increased

water quality has allowed seagrasses to return to areas where they had previously been killed off by

poor water quality (Johansson, 1992).

In the part of Sarasota Bay near the mouth of Whitaker Bayou, seagrasses were thought to be

8/l/97

9:28:48

Technical Synthesis of Sarasota Bay

Page

eliminated in the past due to the discharge of secondarily treated sewage from the City of Sarasota’s

wastewater-treatment plant (Dr. Robert Orth, personal communication). Perhaps associated with the

implementation of nutrient-removal technology at this plant, declines in water- column nitrogen and

phosphorus have been detected in this region (Lowrey, 1992).

As a test case using seagrass transplants as bio-indicators of improved water quality, shoal grass

(Halodule wrightii) was transplanted into an area just south of Whitaker Bayou in October 1991. The

majority of these transplants had survived at least until March 1992, indicating water quality sufficient

to maintain shoal grass in that area. At that time, an older established seagrass bed merged with the

transplants. The loss of plot markers made continued monitoring impossible (Tomasko, unpublished

data).

Water Clarity

Water Clarity and Seagrasses

Water clarity varies from region to region throughout Sarasota Bay. Nearshore areas are more heavily

influenced by terrestrial runoff and bottom resuspension due to currents and wave action (Sheng and

Peene,

1992),

with concomitant increases in suspended and dissolved substances. Suspended

substances increase both the scattering and absorption of photosynthetically active radiation (PAR),

while dissolved substances increase the absorption of PAR mostly in the region of blue light

(McPherson and Miller, 1987). Areas closer to passes are exposed to water more characteristic of the

Gulf of Mexico, with greater water clarity.

The availability of light, as modified by various light attenuators, is the primary abiotic factor

controlling the area1 extent and productivity of seagrass communities (see review in Dennison, 1987).

Seagrasses, which cover nearly 26 percent of Sarasota Bay’s bottom (83 19 acres; Culter,

1992),

are

indispensable for the roles they play in nutrient cycling, primary production, sediment stabilization and

fisheries utilization (see reviews in Zieman, 1982; Thayer et al., 1984). Accordingly, it is crucial that

we understand the relationships between various light attenuators, water clarity and the health of

seagrass systems within the Bay.

Many studies have documented the decline of seagrass associated with degraded water clarity (es.,

Cambridge and McCoomb, 1984;

Orth

and Moore, 1984; Giesen et al., 1990). In addition, a limited

amount of information exists on the resurgence of seagrasses associated with improvements in water

clarity in Australia (Shepard et al, 1989) and in Tampa Bay (Johansson, 1992).

The shallow slope of the bottom of Sarasota Bay would allow for dramatic increases in seagrass

coverage with minimal increases in water clarity. According to bathymetric data for Sarasota Bay

(Sheng et al., in preparation), roughly 46 percent of Little Sarasota Bay is less than two feet deep at

Mean Lower Low Water (approximately three feet at Mean Sea Level). This depth is equal to the

deep edge of grassbeds in the central portion of Little Sarasota Bay.

If water clarity in Little Sarasota Bay were to improve to values typically found in Roberts Bay,

seagrasses could grow to one more foot of water depth. In Little Sarasota Bay, that would result in an

increase in potential acreage from 986 acres of Bay bottom to 1,434 acres, a possible increase in

seagrass habitat of 448 acres (equal to 45 percent of existing habitat).

Light availability not only delimits most seagrass meadows at their deep edges, it can also regulate the

biomass and productivity of seagrasses within meadows. Short (1990) has shown a linear response

between light levels and the biomass of seagrasses grown under controlled conditions. Hall et al.

8/l/97

9:28:48

Technical Synthesis of Sarasota Bay Page 9 of 17

The state of the science in seagrass biology has not progressed much beyond the point where it can be

stated that increased water clarity is good and decreased water clarity is bad. This information does

little to aid resource managers in determining how improvements in water clarity can be achieved. If

the relative importance of various light attenuators (i.e., color, turbidity, chlorophyll a) is not known,

it is difficult to devise appropriate courses of action to increase water clarity.

Knowledge of which factors are most responsible for light attenuation can be used to draft specific

resource-management options. For instance, previous work on the east coast of Florida has shown

that boat wakes can create sufficient sediment resuspension to increase turbidity values, thus

decreasing water clarity (U.S.F.W.S., 1979).

Information from the water-quality monitoring program has been used to determine which light

attenuators are most closely associated with variation in water clarity in Sarasota Bay. Based on

information from multiple-regression analysis, turbidity does not seem to be an overly important

contributor to light attenuation in those segments of the Bay where this association was examined.

The data suggest that variation in turbidity accounts for only two to five percent of the variation in

light penetration for waters in various parts of the Bay. However, these data were not collected during

weekends, when boating activity increases. It might be that only at such times would a relationship

emerge between turbidity and light attenuation.

A linkage between water-column chlorophyll values and light attenuation does exist for various parts

of the Bay, where variation in chlorophyll a accounts for 23-47 percent of the variation in water

clarity. As phytoplankton populations (the source of water-column chlorophyll a) are most probably

limited by nitrogen loading in these areas (Johannson,

1992),

locating the dominant sources of

nitrogen loading could result in management activities designed to reduce loading. A predicted

consequence would be less chlorophyll a, greater water clarity and increased acreage of seagrass

habitat.

Variation in the amount of dissolved substances (color) in any particular segment of the Bay is related

to variation in circulation patterns and the relative importance of freshwater flows to a segment. In

areas where dissolved organic substances are dominant light attenuators, few options other than

increased circulation/flushing would be sufftcient to improve water clarity.

The data on circulation and transport in Sarasota Bay showed poor tidal flushing in Palma Sola Bay

and Little Sarasota Bay (Sheng and Peene, 1992); these same two areas are in the lower 25 percent of

all Bay segments for a variety of light-related water- quality variables (Lowrey et al., 1992). The

improvement in water quality achieved by increased circulation, however, must be evaluated in

context with possible increases in salinity, and any potential reductions in low-salinity habitats.

Dissolved Oxygen

Critical Levels of Dissolved Oxygen

Dissolved oxygen (D.O.) plays a critical role in regulating the health of estuarine systems.

Unfortunately, low dissolved oxygen has become increasingly common in a variety of estuarine and

marine areas, from Denmark and Sweden to Chesapeake Bay and the Gulf of Mexico (e.g., Turner

and Allen, 1972; Rosenberg, 1990; Rossignol-Strick, 1985; Stachowitsch, 1984). Typically, low

dissolved-oxygen levels are the result of human-induced nutrient enrichment of nearshore waters,

often referred to as cultural eutrophication (Ryther and Dunstan, 1971; Officer et al., 1984;

Rosenberg, 1985).

8/l/97

9128149

Technical Synthesis of Sarasota Bay

Page 10 of 17

Physiological effects of hypoxia (less than or equal to 2 ppm D.O.) on fish and shellfish are

well-known (Butler et al., 1978; Kapper and Stickle, 1987; DeFur et al., 1990). Behavioral changes in

marine organisms can also be induced by hypoxia (Hagerman and Szaniawska, 1986; Kramer, 1987).

If marine organisms cannot evade hypoxic waters, as in blue-crab migrations (Bailey and Jones,

1989),

they must be able to adapt to conditions or perish. The eggs and larvae of bay anchovies, Anchoa

mitchilli, are extremely susceptible to hypoxic conditions (Chesney,

1989),

and their survival and

geographic distribution within estuarine systems might be somewhat controlled by hypoxia.

Dissolved Oxygen Levels in Sarasota Bay

Little information currently exists to suggest that low D.O. plays an important role in reducing the

vitality of habitats in Sarasota Bay. Excepting some of the tributary stations, few areas have reported

problems with hypoxia and/or anoxia (0 ppm D.O.) during the daylight hours typically sampled.

As related to the problems of hypoxia and anoxia in Sarasota Bay, the maxim “an absence of evidence

does not constitute evidence for absence” should be kept in mind. When water-quality sampling efforts

are undertaken later in the day (accurate measurements of water clarity require the sun to be nearly

overhead), D.O. levels can be much higher than their daily minimum in areas subject to eutrophication.

Figure 7

&~;;‘;

--__--_-_----___.

IFigure8

Daily variation in D.O. is illustrated (Figure 7) using data obtained by continuous recording devices

placed in Little Sarasota Bay by the University of Florida. Figure 8, which is a rearrangement of the

same data as in Figure 7, demonstrates the typical relationship between hours of sunlight and D.O.

levels.

For most of Sarasota Bay, the evaluation of the extent of hypoxic conditions will require monitoring

efforts at pre-dawn times, or the use of continuously recording instrumentation.

Pre-dawn D.O. sags are probably the most important water- quality variable affecting species diversity

and abundance in estuarine locations. Reliance on D.O. sampling during daylight hours biases

water-quality classification schemes to the point where optimistic evaluations of water quality are

oflen unwarranted.

Preliminary data from an ongoing study on fauna1 utilization of various grassbeds indicate that

8/l/97

9:28:49

Technical Synthesis of Sarasota Bay

Page 11 of 17

“pristine” and “impacted” seagrass beds (subjectively classified as such) have quite different

fauna1

assemblages and levels of species diversity, despite occurring in areas of equal status according to the

Trophic State Index (Leverone and Marshall, 1992). Indeed, impacted meadows of shoal grass

contained only a small fraction of the numbers of caridean shrimp as were found in healthier meadows

of shoal grass.

Results from Culter (1992) indicate that approximately 4,800 acres of Bay bottom (15 percent of the

total area) are “disturbed,” with many of these locations being anoxic sinks for fine sediments.

However, it is not known if hypoxic conditions in Sarasota Bay are persistent enough to cause

substantial differences in animal communities.

Present and Future Habitat

Status and Trends of Various Habitats

Much has been written about the value of freshwater and tidal wetlands in terms of shoreline

stabilization, wildlife utilization and filtering of runoff. From 1950 to 1990, Sarasota Bay lost an

estimated 1,609 acres of tidal wetlands, a 39-percent decline (Estevez, 1992); freshwater wetlands

show a similarly dramatic decline during that time (Beaman, 1992). Also, more than 75 percent of

freshwater wetlands within the Bay’s watershed are altered to some degree by dredge-and/or-fill

activities (Beaman, 1992).

The spatial variation in patterns of wetlands loss can be summarized as follows: Manatee County has

lost proportionally more of its original freshwater wetlands than Sarasota County, and Sarasota

County has lost proportionally more of its original tidal wetlands than Manatee County. This

configuration of wetlands loss reflects dissimilar demographic trends and agricultural practices within

the watershed (Estevez 1992).

Although seagrasses have declined approximately 30 percent Baywide compared to historical

coverage (Mangrove Systems, Inc., 1988) areas such as Longboat Pass and New Pass show positive

trends for coverage. In the Longboat Pass area, it appears that seagrass increases may be due to

growth on flood-tidal shoals created by pass dredging (Darryl Hatheway, personal communication), In

the New Pass area, better water quality (Lowrey, 1992) appears to be allowing seagrasses to grow

into deeper areas that were previously unvegetated (Culter, 1992).

In Little Sarasota Bay, data indicate significant shifts in the species composition of seagrass meadows

(Culter, 1992). Areas previously vegetated with turtle grass (Thalassia testudinum) are now mainly

vegetated with shoal grass (Halodule wrightii) and widgeon grass (Ruppia maritima). As shoal grass

often replaces turtle grass in areas of degraded water-quality (Reyes and Merino, 1991): Tomasko and

Lapointe, 1991; Lapointe et al.,

1992b),

this species shift would indicate significant changes in water

quality in portions of Little Sarasota Bay.

Functions of Wetland Habitats

Mangrove ecosystems have been shown to play an important role in shoreline stabilization (see

reviews in Odum et al., 1985). Although salt-marsh grasses are important shoreline stabilizers in

higher latitudes, they have not been extensively studied in west-central Florida (Estevez and Mosura,

1985). Regardless, it can be stated with confidence that the extensive decline in tidal wetlands, both in

area and edge, produced concurrent declines in shoreline stability. Unstable shorelines erode more

easily, with resultant increased sediment resuspension, increased turbidity and decreased water clarity.

Freshwater wetlands perform similar functions in terms of shoreline stabilization along creeks and

8/l/97

9:28:49 AM