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Technical Synthesis of Sarasota Bay

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Technical Synthesis of Sarasota Bay
Technical Synthesis of Sarasota Bay
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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
l
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.
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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
M.
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
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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
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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,
1
(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.
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Land-Use
Patterns
WBhfn
the
Watershed of Hudson Bayou
Land-Use Patterns Within the
Watershed
of North Creek
30
Technical Synthesis of Sarasota Bay
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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
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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
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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.
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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).
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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
1
&~;;‘;
--__--_-_----___.
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
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“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
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“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
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In addition to stabilizing shorelines and functioning as wildlife habitat (see reviews in Odum et al.,
1985; Lewis and Estevez, 1988) wetlands filter runoff before it enters creeks, ponds and the Bay
itself Due to differences in funding sources and research directions, tidal wetlands are better
understood than freshwater wetlands in terms of shoreline stabilization, but freshwater wetlands are
better understood than tidal wetlands as relates to filtering of stormwater runoff.
Figure9
Dense vegetation along creek banks slows the velocity of runoff, thus increasing the infiltration of
water into surface soils and groundwater. As a result, the “first flush” of runoff is dampened, and
metals and nutrients are more likely to be absorbed onto soil particles and/or incorporated into plant
biomass. In the absence of filtering vegetation, stream velocities are initially elevated compared to
natural systems; in addition, after the “first flush” stream velocities drop off more rapidly in the absence
of filtering vegetation figure 9 9.
Streams and creeks without vegetative cover exhibit a pattern of “feast or famine.” When rains occur,
velocities and pollutant loads are magnified; when dry weather dominates, creeks have reduced flow
and volume. In that estuarine areas exhibit decreased productivity with both too much and too little
freshwater inflow (see review in Browder, 1991) wetlands habitats should be protected and restored
to the fullest extent if only for their function as filters of stormwater runoff. The critical importance of
reestablishing natural patterns of freshwater input into estuarine areas is evidenced by the priority
consideration granted it by the Task Force on Resource- Based Water Quality in Tampa Bay (Agency
on Bay Management, 1990).
Even if all remaining wetlands could be completely protected from loss due to development (an
unlikely scenario), Sarasota Bay would still be left with but a fraction of its original wetlands habitat.
Those few remaining wetlands exhibit various levels of disturbance, due to ditching, invasive species,
pruning, insect damage and freeze damage (Estevez, 1992). Accordingly, increased wetlands, brought
about by restoration and/or creation activities, would seem to be an appropriate course of action. With
limited
funds
for such activities, a prioritization of properties for wetlands restoration/creation could
be appropriate. Such a ranking of areas could be based on a holistic approach to estuarine functioning.
In addition to preserving remaining wetlands and restoring/creating wetlands to ameliorate the effects
of past losses, strategies must be developed to deal with anticipated future losses. One can easily
foresee additional declines in wetlands due to continued development throughout the watershed.
Although the rate of wetlands destruction due to development may be slowed by current and future
legislation, it seems reasonable to assume that both freshwater and tidal wetlands will continue to be
lost. In addition, an accelerated rate of sea-level rise, associated with global climate change, might
produce additional losses of wetlands.
Wetlands issues connected with accelerated sea-level rise include the following: 1) hardened shorelines
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Technical Synthesis of Sarasota Bay Page 13 of 17
and development of upland areas might eliminate the possibility of landward migration of wetlands; 2)
encroachment of invasive species might hinder landward migration of wetlands; 3)
sediment-accumulation rates in wetlands might be insufficient to accommodate elevated rates of
sea-level rise. Although uncertainties abound in predicting global climate change and sea-level rise, a
prudent course of action might include a variety of activities. Purchasing acreage upland from existing
wetlands might alleviate the problems associated with wetlands migration in areas with appropriate
slopes and land-use patterns. Wetlands delimited at their upland edges by seawalls, causeways and/or
extensive reaches of invasive species might be very expensive to maintain with an elevated rate of
sea-level rise.
Relationships Between Recreation and Habitat
Recreational activities are varied in their dependence on habitat quality. Some activities, such as
boating or cruising, can take place just as easily, and be just as enjoyable, regardless of the location in
Sarasota Bay. The level of enjoyment of other activities, such as fishing, snorkeling and bird-watching,
are dependent upon the health of the Bay at that particular location.
Recreational anglers engage in various types of fishing. Individuals who use cast nets to capture mullet
can do quite well in locations where adults of other species are more difficult to catch. Consequently,
recreational fishing can be a quite diverse activity, with various people requiring various habitats; one
person’s fishing hole may be viewed as a lifeless void by other anglers.
A problem that arises with characterizing the various habitats in Sarasota Bay is the emphasis placed
on determining the “value” of such habitats. While the area around the former Midnight Pass seems to
be functioning as a nursery for various juvenile fish (Edwards,
1992),
seasonal aggregations of
sought-after species
-
as are typical of open pass areas
-
no longer occur. As a result, this area is no
longer a focal point for recreational fishing. A question arises, both in this example and in many
others, as to the type of habitats we are aspiring to preserve, enhance or create.
While one may argue the merits of maintaining a mosaic of estuarine habitats, others might argue for
maximizing the area of those habitats in shortest supply. In turn, identifying habitats in shortest supply
depends on what species are being considered. Pass-type communities are obviously very different
from quiescent, lagoonal environments. Both these areas are important, but which is most vital
depends on what species are being considered, which might also vary with the age of the targeted
species.
As shown in the chapter on Fisheries, the primary issue affecting recreational fishing in Sarasota Bay is
that of more people fighting for their slice of a diminishing pie. A tenfold increase in population during
the last 40 years has greatly increased fishing pressure. During the same period, dramatic declines in
fisheries habitat have occurred (an approximate
39-
percent decline in mangrove area, and a
30-percent decline in seagrass area). A relationship appears to exist among declines in habitat,
increased fishing pressure and the finding that the average angling experience is less productive than it
used to be. Based on this scenario, it seems that protecting remaining fisheries habitats, although
essential, is not sufficient. To truly increase the level of enjoyment of recreational angling, new
fisheries habitat must be created on a continuing basis.
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... In Sarasota Bay, trends in seagrass coverage differ substantially between the northern portion of the bay, which is wider, with more pass influences and with a smaller watershed to open water ratio (Tomasko et al., 1992) than in the southern portion of the bay, which is narrower, with lower flushing rates (Sheng and Peene, 1992) and a higher watershed to open water ratio (Tomasko et al., 1992). In the northern part of the bay, seagrass coverage has increased by more than 60% over the past 20 years, while in the southern part of the bay, coverage is lower now than it was in the late 1980s (Fig. 7). ...
... In Sarasota Bay, trends in seagrass coverage differ substantially between the northern portion of the bay, which is wider, with more pass influences and with a smaller watershed to open water ratio (Tomasko et al., 1992) than in the southern portion of the bay, which is narrower, with lower flushing rates (Sheng and Peene, 1992) and a higher watershed to open water ratio (Tomasko et al., 1992). In the northern part of the bay, seagrass coverage has increased by more than 60% over the past 20 years, while in the southern part of the bay, coverage is lower now than it was in the late 1980s (Fig. 7). ...
Article
In six contiguous estuaries in Southwest Florida (USA) focused management actions over the past several decades have reduced watershed nutrient loads, resulting in an additional 11,672 ha of seagrass meadows between 1999 and 2016, an improvement of 32%. However, in September of 2017, Hurricane Irma made landfall in the state of Florida, affecting the open water and watersheds of each of these six estuaries. In response, seagrass coverage declined by 1203 ha between 2016 and 2018, a system-wide decrease of 3%. The range of decreases associated with Hurricane Irma varied from less than a 1% loss of seagrass coverage in St. Joseph Sound to declines of 7 and 11% in Clearwater Harbor and Lemon Bay, respectively. Areas with the largest losses between 2016 and 2018 were those systems where seagrass coverage had declined in prior years, indicating the effects of Hurricane Irma might have been intensified by prior impacts.
... In Sarasota Bay, trends in seagrass coverage differ substantially between the northern portion of the bay, which is wider, with more pass influences and with a smaller watershed to open water ratio (Tomasko et al., 1992) than in the southern portion of the bay, which is narrower, with lower flushing rates (Sheng and Peene, 1992) and a higher watershed to open water ratio (Tomasko et al., 1992). In the northern part of the bay, seagrass coverage has increased by more than 60% over the past 20 years, while in the southern part of the bay, coverage is lower now than it was in the late 1980s (Fig. 7). ...
... In Sarasota Bay, trends in seagrass coverage differ substantially between the northern portion of the bay, which is wider, with more pass influences and with a smaller watershed to open water ratio (Tomasko et al., 1992) than in the southern portion of the bay, which is narrower, with lower flushing rates (Sheng and Peene, 1992) and a higher watershed to open water ratio (Tomasko et al., 1992). In the northern part of the bay, seagrass coverage has increased by more than 60% over the past 20 years, while in the southern part of the bay, coverage is lower now than it was in the late 1980s (Fig. 7). ...
Article
In Southwest Florida, a variety of human impacts had caused widespread losses of seagrass coverage from historical conditions. St. Joseph Sound and Clearwater Harbor lost approximately 24 and 51%, respectively, of their seagrass coverage between 1950 and 1999, while Tampa Bay and Sarasota Bay had lost 46% and 15%, respectively, of their seagrass coverage between 1950 and the 1980s. However, over the period of 1999 to 2016, the largest of the six estuaries, Tampa Bay, added 408 ha of seagrass per year, while the remaining five estuaries examined in this paper added approximately 269 ha per year. In total, seagrass coverage in these six estuaries increased 12,171 ha between the 1980s and 2016. Focused resource management plans have held the line on nitrogen loads from non-point sources, allowing seagrass resources to expand in response to reductions in point source loads that have been implemented over the past few decades.
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Four meadows of turtle grass (Thalassia testudinum Banks ex Konig) in Sarasota Bay, Florida were sampled on a bimonthly basis from June 1992 to July 1993 to determine spatial and temporal variation in short shoot density, biomass, productivity, and epiphyte loads. Concurrent with the seagrass sampling, quarterly water-quality monitoring was undertaken at ≥3 sites in the vicinity of each studied seagrass meadow. Three months after termination of the seagrass sampling effort, a biweekly water-quality monitoring program was instituted at two of the seagrass sampling sites. In addition, a nitrogen loading model was calibrated for the various watersheds influencing the seagrass meadows. Substantial spatial and temporal differences in turtle grass parameters but smaller spatial variation in water quality parameters are indicated by data from both the concurrent quarterly monitoring program and the biweekly monitoring program instituted after termination of the seagrass study. Turtle grass biomass and productivity were negatively correlated with watershed nitrogen loads, while water quality parameters did not clearly reflect differences in watershed nutrient inputs. We suggest that traditional water-quality monitoring programs can fail to detect the onset or continuance of nutrient-induced declines in seagrass health. Consequently, seagrass meadows should be monitored directly as a part of any effort to determine status and/or trends in the health of estuarine environments. *** DIRECT SUPPORT *** A01BY074 00029
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There is a significant positive correlation between mean Secchi depth readings and the lower limits of Thalassia. Despite this significant correlation, grazing pressure by herbivores such as the urchin Diadema antillarum can reduce the depth limit of Thalassia to shallow depths where light is not a limiting factor.-from Authors
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Chronic hypoxia tolerance of Thais haemastoma was inversely related to acclimation temperature but not to salinity. Oxyregulatory ability varied inversely with temperature and did not exhibit acclimation after 28 days' exposure to 53 mm Hg O₂. The oxygen consumption rate was lower after 28 days' exposure to hypoxia than under normoxia at 20 and 30 C but was low at all PO₂'S at 10 C. Pyruvate oxidoreductase enzymes were not induced in foot tissue after 28 days' exposure to 10%-67% of normoxia. Activity ratios of alanopine dehydrogenase:lactate dehydrogenase:strombine dehydrogenase:octopine dehydrogenase were 3.8:1.2:1.0:0. Alanopine dehydrogenase activity increased on exposure to anoxia but had returned to control activity by day 2; no change occurred in the other pyruvate oxidoreductase activities over 6 days' exposure. Adenylate energy charge (AEC) was reduced at low PO₂'S. Arginine phosphate concentrations were lower in snails exposed to 10%-33% saturation than in those at 67%100%. AEC declined from 0.79 to 0.58 after 1 day of anoxia, and arginine phosphate concentration declined by 95%; no further decrease occurred in either variable. AEC cycled at the expense of arginine phosphate with cyclic exposure of snails to anoxia.
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The release by the Arzal Dam in W France of a large amount of fresh water into the Bay of Vilaine during 4 days of July 1982 led to bottom water oxygen depletion and demersal fish mortality, by stratifying the water column and preventing ventilation of the bottom waters. -from Author
Article
Turtlegrass Thalassia testudinum meadows from 0.5 m and 2.0 m depths were studied in the Florida Keys and W Caribbean. Two meadows, one offshore of a populated island with over 2000 septic tanks, and one offshore of a large bird rookery, were similar in having elevated levels of water column nutrients, greater epiphyte levels, low shoot densities, low leaf area indices, and low biomass. Increased blade turnover time was partially responsible for increased epiphyte levels offshore of the populated island, but epiphyte communities developed faster on seagrass blades there than at a paired site offshore of an uninhabitated island. Reduced irradiance moderated the effect of nutrient enrichment on epiphyte levels. Elevated levels of water column nutrients, by stimulating epiphyte growth, reduced rhizome growth rates. -from Authors
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Increased inputs of nutrients to marine coastal areas over the last decades have created a basis for eutrophication of the waters surrounding Sweden. In combination with relatively low water exchange in these vertically stratified and almost non-tidal waters, local and regional effects of increased macro-algal biomass, and decreased oxygen concentrations in bottom water leading to mortalities of benthic animals and decreased fish catches have at times been observed. The effects were first noted in the Baltic, but are now obvious also in Swedish and Danish coastal areas in the Kattegat and the Belt Sea. Similar symptoms have recently also been recorded off the Danish North Sea coast. Other shallow coastal and shelf areas, where stratification occurs, can be regarded as potentially eutrophic risk areas.
Article
Blue crabs (Callinectes sapidus) were held in hypoxic (SO-55 mm Hg) water for 7-25 days. Post- branchial blood PO2 fell by about 80% within 24 h and then remained unchanged. Postbranchial blood total CO* increased within 24 h and remained elevated for the duration of the experiment. There was no change in post- branchial blood pH, osmolality, or Cl. Lactate, urate, and Caf2 all raise the O2 affinity of blue crab hemocya- nin; by 25 days, blood lactate and urate had risen slightly, but Ca+* had increased dramatically. Hemocyanin con- centration had also increased by 25 days. At both 7 and 25 days there was an intrinsic increase in hemocyanin-0, affinity and a change in subunit composition. The highly adaptive homotropic change is believed to be due to an attendant shift in the proportions of two of the three vari- able monomeric hemocyanin subunits. Thus, both het- erotropic and homotropic adaptations enhance blood oxygenation at the gill during long-term hypoxia.