Salinity & Nutrients

1996 Abstracts

Analyzing the Isotopic Composition of Coral and Mollusk Skeletons to Relate Past Salinity and Nutrient Levels in Florida Bay

Peter K. Swart and Genevieve Healy, Marine Geology and Geophysics, Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, Miami, FL; Robert B. Halley, U. S. Geological Survey, St. Petersburg FL.

Under normal conditions the oxygen isotopic composition of waters changes in response to the amount of evaporation experienced by the water and therefore the oxygen isotopic composition usually shows a positive correlation with salinity. This signal of salinity, modified by temperature, is in turn recorded in the skeletons of various calcium carbonate secreting organisms as variations in the 18O/16O (d18O) ratio. Hence analyses of the d18O of skeletal material which can be well dated, is able to provide a retrospective view of salinity changes in an estuarine situation. These principles can also be applied to Florida Bay. A major exception however is that the region is influenced by evaporated freshwater derived from the Everglades which under normal or drought conditions has an oxygen isotopic signature similar to evaporated seawater. Consequently relationships between salinity and oxygen needed to be understood in the Florida Bay system. In addition to d18O, the ratio of 13C/ 12C (d13C) provides an indicator of the source of waters and the degree to which they are influenced by the oxidation of organic material. Waters with negative d13C values are found associated with the Everglades while more positive d13C waters are found in more marine areas. At RSMAS, a program has been initiated both to investigate the relationship between the d18O and d13C of the water and salinity in Florida Bay and to use the d18O and d13C of the calcareous material to interpret salinity variations over the past several hundred years.

An important prerequisite in using d18O and d13C of calcareous material is the ability to date the material accurately. Coral skeletons fulfill this criteria because they contain annual growth bands composed of dense and less dense material. By counting these bands from the surface of the coral it is possible to obtain an annual chronology. Assuming a constant growth rate within a year allows intra-annual comparisons to be made. We have been able to calibrate the relationship between salinity, temperature and the d18O of the species Solenastrea bournoni by performing high resolution sampling of the coral skeleton over the last six year time period for which good salinity and temperature data exist. We have then been able to utilize specimens of the species of up to 160 years in age from Lignumvitae basin to obtain a history of the water quality in this region. These results suggest that the concentration of the railway from Miami to Key West was a major factor in the decline of the health of Florida Bay. Synchronous with railway construction, the range of inter-annual salinity variability decreased and the overall salinity increased. In addition there was a large decrease in the carbon isotopic composition suggesting the increased retention of the products of the oxidation of organic material within Florida Bay.

Coral of this age do not grow throughout Florida Bay and are restricted to regions of the bay which experience more marine and equatable conditions. In order to ascertain whether we could see similar patterns in salinity in other areas of the bay, we utilized specimens of the species Siderastrea radians. This species is very resistant, but only grows to about 20 to 30 years in age and therefore is not suitable for reconstructions over longer periods of time. Nevertheless, our study indicates that variations in salinity over the 1985 to 1995 time period could be detected in all the basins from which these corals were retrieved. These variations were similar in timing, but considerably larger in magnitude (Swart et al, poster session). Future work will concentrate on the analysis of two longer cores collected from Manatee Bay and near Crane Key.

A further method whereby the isotopic composition of calcareous organisms can be used for paleoenvironmental reconstruction is to make use of fossils in well dated sediment cores. This approach has been employed by the USGS group in cores collected from three localities in northeastern Florida Bay. In particular this group is examining the d18O and d13C composition of mollusks from cores dated by using both radioisotope and anthropogenic tracers. These cores do not have the dating resolution provided by coral cores, but they provide information in the northeastern bay where old corals do not live.

In order to interpret isotopic signatures from cores, a survey of mollusks was made from the tops of cores taken at several localities across the bay. Among these mollusks a positive covarying trend is taken to indicate a marine signature, whereas a inverse trend is suggested to be more representative of Everglades conditions. The covariant distribution of mollusk d18O and d13C values from the western bay reflect "normal marine" conditions. In contrast, the distribution of mollusks from the northeastern bay, which has a highly variable salinity regime, display a wide range in both d18O and d13C. Other basins throughout the Florida Bay show isotopic distributions that are intermediate between these two end-member environments (see Roulier and Halley, poster session).

Stable isotope analyses of mollusks collected down core vary widely, but show significant shifts in mean isotopic values during the past 150 years. A negative shift in d13C beginning near the turn of the century is similar to that from coral records, but appears to have taken several decades to stabilize at current levels of variance. Excursions toward heavier d18O, indicating more evaporitic conditions, began several decades later.

Isotopic analyses from both corals and mollusks, together with a rigorous examination of the oxygen and carbon isotopic systematics of the water column, should allow us to quantify past salinity and water quality changes across Florida Bay.

 

Benthic Flux of Nutrients From Florida Bay Sediments.

Paul Carlson and Tim Barber, Florida Department of Environmental Protection, St. Petersburg, FL; Alina Szmant and Larry Brand, Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, Miami, FL.

This presentation will combine four datasets from our two research programs to compare sediment nutrient inventories and potential fluxes from sediments to the overlying water column in Florida Bay and along transects from the Keys to the reef tract. The datasets to be used are 1. porewater nutrient profiles and benthic chamber flux measurements made in Johnson Key Basin in 1990, 2. porewater nutrient profiles and benthic chamber flux measurements made along transects from the Keys to the reef tract in 1992, 3. porewater nutrient profiles measured at several sites within Florida Bay from 1994 to 1996, and 4. measurements of potential fluxes from sediment to the water column by resuspension.

In January, April, July, and September 1990, we measured sediment-water exchange of ammonium, filterable reactive phosphorus (FRP), and silica in surviving Thalassia beds and die-off patches in Johnson Key Basin. Porewater equilibrators (peepers) were sampled at the same time to compare potential fluxes calculated from vertical profiles of porewater nutrients with actual fluxes measured in benthic chambers.

Peeper nutrient flux estimates of Si, NH4, and FRP in surviving Thalassia beds were generally higher than in die-off patches. In September 1990, for example, Si, NH4, and FRP flux estimates for surviving beds were 29, 14, and 2.6 umol/m2/d, respectively, while estimates for die-off patches were 6.9, 7.0, and 2.4 umol/m2/d.

In benthic chambers, we measured silica and ammonia fluxes of 21 and 4.0 mmol/m2/d, respectively, in surviving Thalassia beds. Fluxes of silica and ammonia from unvegetated sediments in die-off patches were negligible. Phosphorus flux was not detected in surviving grass beds or die-off patches. Silica and ammonium fluxes measured in benthic chambers exceeded by far fluxes calculated from porewater profiles, possibly as the result of rapid nutrient regeneration in the water column, at the sediment-water interface (Gardner et al. 1995) and on the surfaces of seagrass leaves.

Silica and ammonium fluxes estimated from porewater profiles at Rankin Lake, Sandy Key, and Twin Key in 1994-1996 were generally higher than 1990 estimates from Johnson Key Basin. Silica flux estimates ranged from 112 umol/m2/d for Rankin Lake to 83 umol/m2/d for Sandy Key, to 67 umol/m2/d for Twin Key. Ammonia flux estimates were highest (57 umol/m2/d) at Rankin Lake, lower (25 umol/m2/d) at Twin Key, and lowest (15 umol/m2/d) at Sandy Key. Phosphorus flux estimates were highest at Sandy Key (5.3 umol/m2/d) lower at Rankin Lake (1.0 umol/m2/d), and lowest at Twin Key Basin (0.35 umol/m2/d).

These feeding estimates indicate protozoan grazers are the most important component of total grazing community.

Direct measurements of whole community zooplankton grazing (i.e. including protozoa) and whole phytoplankton community growth have been made at a single station during the past three sampling intervals. These twenty-four hour in situ experiments have shown that grazing by the whole zooplankton community markedly exceeds estimates described above. This indicates the organisms <20 um, the protozoans, are the most important grazers in this system. Total community grazing is sufficient to nearly balance the daily growth of the whole phytoplankton community. Enumeration of the phytoplankton species composition at the outset and termination of these experiments samples will identify which components of the phytoplankton community are increasing and which are being controlled by grazers.

Although protozoans are more significant grazers than metazoans, the latter are more significant food sources for planktivorous fish. Close coordination of our studies with studies of phytoplankton growth, larval and juvenile fish abundance and food requirements, and physical circulation, as well as proper scaling through integrated modeling studies, is required to quantitatively assess the factors controlling bloom dynamics in Florida Bay.

 

The Florida Bay Water Quality Monitoring Program:Assessing Status and Trends (1989-1995)

Joseph N. Boyer and Ronald D. Jones, Florida International University, Southeast Environmental Research Program, Miami, FL.

Environmental monitoring programs are essential for our understanding and management of ecosystems. Before one can recognize environmental changes, some idea of baseline variability must be established against which to evaluate gross deviations. In addition to temporal changes, it is vitally important to understand spatial patterns of water quality in these systems in an effort to direct management efforts. One of the purposes of any monitoring program should be to use the data gained by routine sampling to extend our understanding of the system by developing new hypotheses as to the governing processes.

Florida Bay is on the marine receiving-end of the Everglades, one of the largest wetland ecosystems in the world. Recent ecological changes in Florida Bay, i.e. periods of prolonged hypersalinity, a poorly understood seagrass die-off, sponge mortality events, and elevated phytoplankton abundances have focussed attention on this ecosystem. In response to these warning signs, a network of 28 fixed monitoring stations was established in July 1989 to address trends in water quality

The shallow mud banks which divide Florida Bay into relatively discrete basins serve to restrict water movement between basins, attenuating both tidal range and current speed. Sampling sites were distributed throughout the bay near the centers of these basins. Monthly sampled parameters included salinity (ppt), temperature (øC), dissolved oxygen (DO; mg l-1), DO saturation (%), NO3- (æM), NO2- (æM), NH4+ (æM), total nitrogen (TN; æM), total inorganic nitrogen (TIN; æM), total organic nitrogen (TON; æM), total phosphorus (TP; æM), soluble reactive phosphorus (SRP; æM), total organic carbon (TOC; æM), SiO4 (æM), alkaline phosphatase activity (APA; æM hr-1), chlorophyll a (Chla; æg l-1), turbidity (NTU), TN:TP ratio (molar), and TIN:SRP ratio (molar).

Stations were grouped into distinct spatial zones of similar influence (ZSI) by a multivariate analysis outlined in Boyer et al. (in review). Briefly, principal component analysis (PCA) was used to extract composite variables (principal components) which were then rotated (using VARIMAX) and the factor scores saved for each data record. Mean and SD of factor scores were used in a cluster analysis to aggregate stations into distinct ZSI (Fig. 1). The result was 3 statistically different ZSI: Eastern Bay (19 sta.) - the most freshwater dominated, acts most like a "conventional" estuary; Western Bay (6 sta.) - influenced mostly by SW Florida Shelf waters; and Core Bay (4 sta.) - located in the N-central area, physically isolated, acts as an evaporative basin.

Three different analysis types were performed on the 6 year dataset in an effort to both visualize and test for temporal trends: a seasonal approach of graphing the monthly median and range of a parameter for all years using box-and-whisker plots; a 12 month moving average for the period of record; and a seasonal Kendall-tao test. The box-and-whisker plot depicts the distribution around the median (in quartiles) as well as the 95% confidence interval of the median, allowing it to be used as a graphical, nonparametric ANOVA. Pooling all data by month showed the presence of seasonal effects in the data. The significance of these seasonal effects were also tested using the Kruskal-Wallis test. The 12 month moving average over the period of record was used to filter out annual fluctuations and thereby disclose any interannual oscillations of longer periodicity. The seasonal Kendall-tao test is a nonparametric statistic which tests for monotonic trends (whether increasing or decreasing) by determining the significance of the trend and generates a trend slope estimate (TSE; units yr-1) for the period of record. This test cannot detect reversals of direction, such as might be seen in the case of interannual oscillations, nor is it applicable with discontinuous data.

In Eastern Bay, the seasonal Kendall-tao analysis showed that salinity, DO saturation, TP, and Chla declined significantly, whereas NH4+, NO2-, TON, turbidity, TN:TP and TIN:SRP increased (Table 1). Salinity, temperature, and DO saturation declined in the Core Bay while NO2-, TOC, APA, Chla, and turbidity increased. For the Western Bay there were significant declines in both salinity and temperature with increases in NH4+, NO2-, TOC, Chla, turbidity, and TN:TP.

These short term trends must be put in perspective with more long term climate changes. The 6 year period of record corresponds with a shift to wetter conditions from the dry period of the 1980's. Our next step is to determine the relative importance of precipitation, freshwater inflow, and water management activities on these water quality trends in Florida Bay.

 

A Model of Organic Matter Production and Fate in Florida Bay: Estimates of Nutrient Cycling

Peter K. Swart, Michael Lutz , and Geoffrey Ellis, Marine Geology and Geophysics, Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, Miami, FL.

The flux and cycling of organic material (OM) is one of the least well understood aspects of the Florida Bay ecosystem, yet remineralization of OM is critical to the understanding of the cycling of nutrients, consumption of oxygen, production of hydrogen sulfide, and consequently the health of the entire bay. This presentation will outline some of the current knowledge regarding the sources of organic material and discuss the implications of these estimates to questions regarding the recycling of nutrients.

Based on the current geometry of Florida Bay mudbanks it is estimated that there are approximately 4 x 1012 moles of carbon present as organic carbon in Florida Bay sediments. This amount of carbon has accumulated over the past 4,000 years as sea level has risen and flooded the Pleistocene aged bedrock and therefore the amount of OM present in the sediments is a product of a dynamic series of interactions including the oxidation, burial, and export of OM to the adjacent marine environment. As a first approximation, the sediments in Florida Bay can be roughly separated into the sediments present as a thin veneer above the limestone floor in the basins and the sediments contained in the mudbanks. Based on the thickness and areal extent of the mudbanks, we estimate that there are approximately 3 x 1012 moles of OM stored in this reservoir. In contrast, the sediments in the basins contain an order of magnitude less carbon (6 x 1012 gms). The carbon in the basins does not accumulate here, but instead is rapidly recycled and exported either to the adjacent marine environment or to form new mudbanks within Florida Bay. The mudbanks are considered to be a temporary/permanent sink of organic material. The nature of organic material in mudbanks relative to the basins has not been well characterized, but the mudbank material is likely to have a higher concentration of the more refractory and therefore less labile organic material. Although, these mudbanks are gradually formed and destroyed at rates related to the occurrence of storms, there is a net increase in the area of mudbanks with time. Large storms resulting in the significant destruction of mudbanks will result in substantial input of organic carbon into the basin reservoir. Its diagenetic potential however is likely to be less than new OM and therefore resuspension and consequent oxidation will have less of an influence on the release of P and N than an input of new OM.

Based on our analysis of data published by numerous workers in Florida Bay and in addition data collected by our group, we have calculated that there is an input of organic carbon into Florida Bay of between approximately 1010 and 1011 M/yr-1. This input is derived from three principle sources, the Everglades (1010 to 1011 M), in situ production from sea grasses and algae (1010 M), and phytoplankton (1010 to 1011 M). In addition minor amounts of production results from coastal mangrove production and production from islands within Florida Bay. If one assumes that (I) the mudbank organic carbon is relatively immobile, (ii) that approximately 99% of all incoming organic carbon is oxidized, and (iii) there is only minimal export of organic carbon from the bay, then the residence time of organic carbon is between 300 and 2000 years. This range is extremely large and reflects the uncertainty in the estimates of organic carbon derived both from the phytoplankton and the Everglades. Regardless of the uncertainties involving amounts of organic carbon in Florida Bay, its oxidation releases large amounts of nutrients. For example, based on our input estimates of an annual input of 1010 M/yr of OM from sources such as the Everglades, and coastal mangrove production and a preservation rate of 1%, then between 106 and 107 moles of phosphate and 108 and 109 moles of nitrogen are released annually depending upon the C:N:P ratio of the organic material. This flux is large compared to an inorganic flux from the Everglades and the nutrient input associated with precipitation.

Although there are uncertainties regarding the input of organic material in Florida Bay, there are several methods whereby better fluxes can be calculated. First, the fluxes can be measured in a temporal and spatial manner. This method has been utilized by our group and data will be presented indicating fluxes over an annual cycle. Second, the OM in the sediments can be measured and certain tracers analyzed to determined how much material is derived from the relevant sources. One set of such tracers are the stable carbon, nitrogen, and sulfur isotopic compositions of the OM. An initial approach using the carbon isotopic composition will be presented. This indicates the significant influence of the Everglades on the organic content of western Florida Bay sediments, decreasing towards the central bay region.

The work presented here is ongoing and will be designed to provide an integrated model of organic matter flux and consequent nutrient release in Florida Bay. We are currently conducting surveys of C, N, and S isotopes to further constrain end members and fluxes. Future work will examine specific biomarkers characteristic of source environments and resolve source and diagenetic influences.

 

Nutrient Exchange Between Florida Bay and the Everglades' Salinity Transition Zone

David T. Rudnick, Fred H. Sklar, and Stephen P. Kelly, South Florida Water Management District, Everglades Systems Research Division, West Palm Beach, FL; Enrique Reyes, John W. Day, Jr., Brian Perez, and Martha Sutula, Coastal Ecology Institute, Louisiana State Univ., Baton Rouge, LA; Daniel L. Childers, Steven Davis, and Nick Oehm, Southeastern Environmental Research Program, Florida International University, Miami, FL.

Efforts to restore the Everglades and Florida Bay largely entail changing the supply of fresh water to these ecosystems. Changing fresh water inflow to the Bay may affect its ecological structure and function via several mechanisms. Our research is focused on quantifying how changing fresh water inflow affects the cycling of nutrients within the mangrove dominated ecotone between Florida Bay and the Everglades and affects the net transport of nutrients through this ecotone. Understanding nutrient dynamics in this ecotone is important because this region contains a large pool of nutrients and its importance as a source or sink of nutrients may change with changing fresh water flow. Furthermore, salinity in this ecotone has a wide range and high variability; the effects of changing salinity on nutrient biogeochemical cycles should be most evident in such a region.

Our research program includes both the measurement of nutrient fluxes and experiments and simulation modeling that will help us understand the mechanisms that influence these fluxes. Field sites have been chosen along three north-south transects: through the Joe Bay - Trout Cove area, the Taylor River - Little Madeira Bay area, and the Seven Palm Lake - Terrapin Bay area. Each of these areas are important sites of freshwater inflow to Florida Bay and each spans a wide range of salinity. The distribution of transects also spans a wide range of nutrient availability, with increasing nutrient availability from east to west.

Quarterly measurements of nutrient fluxes in the Taylor River area started in January 1996, and will continue for 3 years. Fluxes in the two other areas will measured for at least one year. Net nutrient exchange between the Bay and the wetland is being calculated from measurements of water flow at the mouth of Taylor River (or other creeks) concurrent with frequent (every 3 hours) sampling for all major nutrient species in the flowing water for 10 day periods. In addition to these intensive quarterly sampling periods, daily samples are being taken year-round, with flow measurements, to calculate total N and P exchange.

Along with these measurements of net exchange, nutrient fluxes within the wetland, the mangrove creeks, and in the coastal ponds and embayments are being measured during the quarterly sampling periods. Benthic fluxes in ponds and bays are being measured using dark and light in situ chambers with continuous mixing. Mangrove prop root community - water column fluxes are being measured using duplicate 15 m long flumes along the creek banks and using corrals around small mangrove islands in the scrub mangrove zone. Fringe, basin, and scrub mangrove tree net productivity and litter fall is being measured in plots with marked individual trees and litter traps. Litter and soil decomposition rates are being measured with litter bags and the measurement of sulfate reduction and methane production. Net ecosystem production is being estimated from net exchange calculations and the long-term measurement of sediment accretion or subsidence using sediment elevation tables along the transects.

In addition to these flux measurements, we are measuring the spatial distribution of water quality parameters (pH, D.O, salinity, temperature, chlorophyll, nutrient concentrations) in the coastal region and the distribution of submersed aquatic vegetation in this same region (see abstracts by Montague, Chipouras, and Morrison, and Durako and Hefty). These spatial measurements will help us to understand the relationship of nutrient dynamics to landscape features and plant community structure and also provide data necessary for future landscape model calibration.

Experiments on mechanisms that influence nutrient dynamics will include studies on the effects of salinity change on sediment nutrient fluxes and prop root community fluxes, as well as experiments on the nutrient limitation of soil decomposition processes. To date, initial experiments have been done on the influence of different water sources on the prop root community. Comparing fresh water from Taylor Slough water and higher salinity ambient water, we found that prop roots with a dense epibiont assemblage exposed to Slough water had significantly higher water uptake rates than roots with few epibionts or any roots exposed to ambient water.

To integrate information generated from our study and related studies of Everglades - Florida Bay interactions, we are concurrently developing a simulation model of material fluxes between these ecosystems. The unit model includes nutrient exchange and transformation processes in the water column and sediments. This simple model is being implementing in a spatial manner, with upstream and downstream cells. This will enable us to asses the fate of conservative and non-conservative elements as they are transported through each of the sampled creeks and enable us to test the influence of changing water management regimes on nutrient dynamics.

 

Response of Submerged Macrophytes to Freshwater Inflow to Florida Bay

Clay L. Montague and Evan Chipouras, University of Florida, Department of Environmental Engineering Sciences, Gainesville, FL; Doug Morrison, National Audubon Society, Miami, FL.

Surface water enters Florida Bay at the northern land margin and eventually evaporates in the open parts of Florida Bay to the south and west. Submerged macrophytes are potentially influenced by the flow of surface waters in ways that range from subtle to direct. Presumably the influence of surface water discharge should be greatest near the northern land margin of the bay. Work begun 10 years ago on the effect of freshwater discharge on submerged vegetation in the northern land margin resulted in a salinity fluctuation hypothesis. This is now being tested.

During 1986-87, benthos, water quality, and other environmental variables were checked at twelve sites in northeastern Florida Bay 11 times in 18 months. The work was part of a study funded by the Everglades National Park and the South Florida Water Management District to determine the major factors that control ecological development in the region influenced by the C-111 canal. The purpose was to identify controlling variables that could be influenced by canal management and to quantify the relationship of such variables to the degree of ecological development among sites. For the benthic study, density of plants and animals was used as the measure of ecological development.

Bearing in mind that only periodic spot checks were being made, most of the measured environmental variables did not correlate with density of animals or plants. Light at the bottom, for example, did not correlate to plant density and was greater than 25% of incident light, which seems enough to support healthy beds of submerged vegetation of the type found sparsely in the area (e.g., Ruppia maritima, Chara hornemanni, Halodule wrightii, Thalassia testudinum). Some variables correlated, but were considered effects of biotic densities, not causes. Water-column nutrients, for example, were lower at sites with greater plant density. Only mean salinity, mean temperature, and the standard deviation of salinity experienced at each station correlated to plant and animal density. The best correlate was standard deviation of salinity, which indicated that salinity fluctuation might be the most important effect.

A working hypothesis was developed that salinity fluctuation rather than light, nutrients, temperature, or average salinity is the single most important variable determining the distribution and abundance of submerged vegetation at the northern land margin of Florida Bay. Several predictions can be made based on this hypotheses. Among these are: 1) survival and growth of the common species of submerged vegetation will be drastically reduced by rapid, sudden, and extreme changes in salinity; 2) density of submerged vegetation will be lower at sites with less salinity fluctuation; and 3) hydrodynamic models that can make good predictions of salinity fluctuation can also predict the distribution and abundance of submerged vegetation. Tests of these predictions are now underway. Since the 1986-87 study, a network of continuous salinity recorders have been installed along the northern land margin of Florida Bay, and convenient portable salinity loggers have become more readily available. In addition, conditions have changed somewhat in Florida Bay. The northern land margin has been fresher with the return of wet years, and the outer bay water has been more turbid with the onset of a massive algal bloom. A new opportunity has been afforded to re-measure the relative effects of nutrients, light, and salinity fluctuation and to judge their relative importance.

Monitoring stations have been established along the northern land margin of Florida Bay in proximity to salinity recording stations, and at stations along two salinity gradients. The western salinity gradient (Terapin Bay to Seven Palm Lake) includes large volume estuarine lakes, while the eastern gradient (Taylor River) contains very small ponds. The monitoring stations in the large lakes are expected to span the same salinity gradient as the stations in small ponds. The large lake sites, however, should experience less salinity fluctuation, so more submerged vegetation is expected on average at these stations. Stations in the middle of the salinity gradient are likely to be most variable in salinity because they receive alternating surface drainage and bay water with every shift in wind speed and direction, and every rain event over the watershed. These stations are expected to have the least submerged vegetation. Light penetration, nutrient concentration, temperature, and the absolute value of salinity may also influence or confound these field data, so are being monitored.

Controlled experiments with the effect of salinity fluctuation on submerged vegetation can help pinpoint the role of salinity fluctuation without confounding from other field variables. To this end, an experimental mesocosm facility has been designed, approved, and will soon be under construction on Key Largo. The facility will consist of 12 experimental 1000 l tanks fed by mixtures of saltwater and freshwater from large head tanks. Salinity fluctuations of specified frequencies, amplitudes, and waveforms will be produced by manipulating flows from each head tank. As many as four patterns can be produced simultaneously (with each pattern distributed to three randomly chosen experimental tanks). Other combinations are possible.

Model predictions of abundance and distribution depend in part on the validity of the salinity fluctuation predictions, and the ecophysiological linkage of these to species of submerged vegetation. The mesocosm experiments together with studies of the osmolality of the tissues of the plants involved, will be used to calibrate and refine an existing model of the relationship between salinity and plants. This model can be expanded to include nutrient, light, and temperature effects as quantitative evidence of their influence develops.

General survey of submersed macrophytes (D. Morrison). This project is a descriptive survey of the seasonal distribution, abundance, and community structure of submerged macrophytes in the Everglades - Florida Bay mangrove ecotone. The study will correlate macrophyte seasonality with seasonal patterns in physicochemical parameters, especially salinity. The project will provide baseline data to develop manipulative field and laboratory experiments; to identify and evaluate potential biological indicators of freshwater inflow; and to assess the ecological effectiveness of management actions to restore more natural freshwater inflow patterns. Project duration is October 1995 to December 1996. The geographic focus of the study is the lakes and embayments along the north shore of Florida Bay from Seven Palms Lake west to East Cape Sable. Two sets of sites are oriented along freshwater flow paths from inland to the Bay. These are the system comprising Seven Palm Lake, Middle Lake, Monroe Lake, and Terrapin Bay; and the system West Lake, Long Lake, The Lungs, and Garfield Bight. Additional waterbodies sampled to the west of these systems are Coot Bay,and East Cape Lake. Two sampling regimes are used for submerged macrophytes. All study waterbodies are sampled at the end of the wet (October) and dry (May) seasons to assess species distributions and abundances on a waterbody-wide scale. Six to ten sites are sampled for macrophyte percent cover (10 to 15 random quadrat samples at each site) in each study waterbody. Biomass sampling (15 random 1/8 m2 quadrats per site) is conducted every two months in 1996 at one site in each of the following waterbodies: Seven Palms Lake, inner Terrapin Bay, outer Terrapin Bay, West Lake, The Lungs, and Garfield Bight. The following physicochemical parameters are sampled at least monthly in each study waterbody: salinity, nutrients, temperature, secchi depth, water depth, and turbidity.

In West Lake, an "upper" lake, macrophyte abundance, Chara and Ruppia, was greater at the end of the 1996 dry season than the 1995 wet season. Salinity did not change greatly between seasons (1995 wet 3-4 ppt; 1996 dry 7-9 ppt). However, the secchi:water depth ratio, an indicator of light penetration, in the 1996 dry season was twice that in the 1995 wet season. Preliminary 1996 wet season sampling indicates little difference in plant cover and secchi:water depth ratio from the 1996 dry season. Water depths are lower and water clarity higher in the 1996 wet season than the 1995 wet season. Irradiance appears to be a more important factor than salinity affecting macrophyte distribution and seasonality in West Lake. In the Lungs, the "middle" lake in the West Lake system, macrophyte abundance (primarily Chara) was greater in the 1995 and 1996 wet seasons than the 1995 dry season. Salinity seasonal variation was greatest in the Lungs (<5ppt wet vs. 40 ppt end of dry). The macrophyte abundance declined considerably in the Lungs following high salinity in April and May. Macrophyte seasonality in the Lungs is correlated with salinity seasonality. Seven Palms is an upper lake like West Lake; however, water clarity is greater and less seasonal and salinity seasonality is greater in Seven Palms than West Lake. Chara abundance was greater at the end of the 1995 wet season than the 1996 dry season; whereas Batophora exhibited the opposite pattern. Macrophyte seasonality and distribution was correlated with salinity. The lakes in the mangrove ecotone zone exhibit different submerged macrophyte seasonal patterns. The same lake may exhibit different seasonal patterns from year to year. These patterns are influenced by freshwater inflow quantity and timing which affects salinity and light penetration. This study emphasizes the need for manipulative field and laboratory experiments to elucidate and model the factors that determine macrophyte dynamics in the ecotone zone.

 

Last updated: 04/08/98
by: Monika Gurnée
gurnee@aoml.noaa.gov