Spatial and temporal variability of phytoplankton and environmental factors in a temperate estuary of South America (Atlantic coast,Argentina)

A spatial and temporal study on data collected along the longitudinal gradient of the Principal Channel of Bahía Blanca estuary, Argentina, was carried out during 1992–1993. At nine stations, phytoplankton abundance, chlorophyll a (Chl-a) concentration, inorganic nutrient levels, Secchi disk depth, euphotic depth:mixing depth ratio (Z_eu:Z_m), salinity and temperature were recorded. Phytoplankton abundance, Chl-a concentration and nutrient levels decreased towards the outer zone of the estuary. The inner zone (stations 1 and 2), which was characterized by high turbidity, high nutrient concentrations and high Z_eu:Z_m (>0.16, [critical mixing ratio]), registered the highest phytoplankton abundance and Chl-a concentrations. Temporal variability of data was also noteworthy in this zone. The highest biomass values thus corresponded to June, July, August and the beginning of spring (18 μg Chl-a L⁻¹ and 9×10⁶ cells L⁻¹) concomitantly with a diatom bloom. In the middle zone (stations 3–6), a strong phytoplankton biomass decrease was observed and it coincided with both deep-mixed depths and low Z_eu:Z_m (<0.16). The outer zone (stations 7–9), which was characterized by low phytoplankton biomass values and low nutrient levels all along the year, was the area mostly influenced by waters from the adjacent continental shelf. In view of the above, it can be concluded that the most important primary production in the Bahía Blanca would be produced in the shallow inner zone during winter, being the spatial reach of the phytoplankton biomass principally limited to estuarine waters. Presumably, less than 5% of such biomass may reach the coastal area of the estuary.     

Microbiological composition of native and exotic clams from Tagus estuary: Effect of season and environmental parameters

The influence of seasonal and environmental parameters on the occurrence of bacteria was investigated in two clam species (Venerupis pullastra and Ruditapes philippinarum), water and sediment from the Tagus estuary. Total viable counts (TVC), Escherichia coli, Salmonella spp. and Vibrio spp. were evaluated during one-year. Overall, significant seasonal variations were found in both sampling sites, especially for E. coli and Vibrio spp. levels. In summer, significantly higher Vibrio spp. levels were found in R. philippinarum and sediment samples, but not in V. pullastra clams and water samples. In contrast, significantly higher TVC and E. coli levels were observed in winter months in water and sediment samples. Salmonella spp. was generally isolated when higher levels of E. coli were detected, particularly in R. philippinarum. This study is useful for authorities to develop monitoring strategies for coastal contamination and to estimate human health risks associated with the consumption of bivalves.     

An overview of physical and ecological processes in the Rio de la Plata Estuary

The Rio de la Plata is a large-scale estuary located at 35°S on the Atlantic coast of South America. This system is one of the most important estuarine environments in the continent, being a highly productive area that sustains valuable artisanal and coastal fisheries in Uruguay and Argentina. The main goals of this paper are to summarize recent knowledge on this estuary, integrating physical, chemical and biological studies, and to explore the sources and ecological meaning of estuarine variability associated to the stratification/mixing alternateness in the estuary. We summarized unpublished data and information from several bibliographic sources. From study cases representing different stratification conditions, we draw a holistic view of physical patterns and ecological processes of the stratification/mixing alternateness. This estuary is characterized by strong vertical salinity stratification most of the time (the salt-wedge condition). The head of the estuary is characterized by a well-developed turbidity front. High turbidity constrains their photosynthesis. Immediately offshore the turbidity front, water becomes less turbid and phytoplankton peaks. As a consequence, trophic web in the estuary could be based on two sources of organic matter: phytoplankton and plant detritus. Dense plankton aggregations occur below the halocline and at the tip of the salt wedge. The mysid Neomysis americana, a key prey for juvenile fishes, occurs all along the turbidity front. A similar spatial pattern is shown by one of the most abundant benthic species, the clam Mactra isabelleana. These species could be taken advantage of the particulate organic matter and/or phytoplankton concentrated near the front. Nekton is represented by a rich fish community, with several fishes breeding inside the estuary. The most important species in terms of biomass is Micropogonias furnieri, the main target for the coastal fisheries of Argentina and Uruguay. Two processes have been identified as producing partially stratified conditions: persistent moderate winds (synoptic scale), or low freshwater runoff (interannual scale). Less frequently, total mixing of the salt wedge occurs after several hours of strong winds. The co-dominance of diatoms (which proliferate in highly turbulent environments) and red tides dinoflagellates and other bloom taxa (better adapted to stratified conditions), would indicate great variability in the turbulence strength, probably manifested as pulses. Microplankton and ichthyoplankton assemblages defined for the stratified condition are still recognized during the partially mixed condition, but in this case they occupy the entire water column: vertical structure of the plankton featuring the stratified condition become lost. Bottom fish assemblages, on the contrary, shows persistence under the different stratification conditions, though the dominant species of the groups show some variations. Summarizing, the Río de la Plata Estuary is a highly variable environment, strongly stratified most of the time but that can be mixed in some few hours by strong wind events that occur in an unpredictable manner, generating stratification/partially mixed (less frequently totally mixed) pulses all along the year. At larger temporal scales, the system is under the effects of river discharge variations associated to the ENSO cycle, but their ecological consequences are not fully studied.     

Observed oceanic response to tropical cyclone Jal from a moored buoy in the south-western Bay of Bengal

Upper oceanographic and surface meteorological time-series observations from a moored buoy located at 9.98°N, 88°E in the south-western Bay of Bengal (BoB) were used to quantify variability in upper ocean, forced by a tropical cyclone (TC) Jal during November 2010. Before the passage of TC Jal, salinity and temperature profiles showed a typical BoB post-monsoon structure with relatively warm (30 °C) and low-saline (32.8 psu) waters in the upper 30- to 40-m layer, and relatively cooler and higher salinity (35 psu) waters below. After the passage of cyclone, an abrupt increase of 1 psu (decrease of 1 °C) in salinity (temperature) in the near-surface layers (up to 40-m depth) was observed from buoy measurements, which persisted up to 10–12 days during the relaxation stage of cyclone. Mixed layer heat budget analysis showed that vertical processes are the dominant contributors towards the observed cooling. The net surface heat flux and horizontal advection together contributed approximately 33 % of observed cooling, during TC Jal forced stage. Analysis showed the existence of strong inertial oscillation in the thermocline region and currents with periodicity of ～2.8 days. During the relaxation stage of the cyclone, upward movement of thermocline in near-inertial frequencies played significant role in mixed layer temperature and salinity variability, by much freer turbulent exchange between the mixed layer and thermocline.     

Feedback between residual circulations and sediment distribution in highly turbid estuaries: An analytical model

Motivated by field studies of the Ems estuary which show longitudinal gradients in bottom sediment concentration as high as O(0.01 kg/m⁴), we develop an analytical model for estuarine residual circulation based on currents from salinity gradients, turbidity gradients, and freshwater discharge. Salinity is assumed to be vertically well mixed, while the vertical concentration profile is assumed to result from a balance between a constant settling velocity and turbulent diffusive flux. Width and depth of the model estuary are held constant. Model results show that turbidity gradients enhance tidally averaged circulation upstream of the estuarine turbidity maximum (ETM), but significantly reduce residual circulation downstream, where salinity and turbidity gradients oppose each other. We apply the condition of morphodynamic equilibrium (vanishing sediment transport) and develop an analytical solution for the position of the turbidity maximum and the distribution of suspended sediment concentration (SSC) along a longitudinal axis. A sensitivity study shows great variability in the longitudinal distribution of suspended sediment with the applied salinity gradient and six model parameters: settling velocity, vertical mixing, horizontal dispersion, total sediment supply, fresh water flow, and water depth. Increasing depth and settling velocity move the ETM upstream, while increasing freshwater discharge and vertical mixing move the ETM downstream. Moreover, the longitudinal distribution of SSC is inherently asymmetric around the ETM, and depends on spatial variations in the residual current structure and the vertical profile of SSC.     

Nitrogen over enrichment in subtropical Pearl River estuarine coastal waters: Possible causes and consequences

Hong Kong is surrounded by estuarine, coastal and oceanic waters. In this study, monthly averages over a 10 year time series of salinity, temperature, chlorophyll a (chl a), dissolved oxygen (DO), dissolved inorganic nitrogen (DIN), silicate (SiO₄) and orthophosphate (PO₄) at three representative stations around Hong Kong were used to examine if excess nitrogen in estuarine influenced waters is due to P limitation. The monthly distribution clearly shows the dominant influence of the seasonal change in river discharge in the Pearl River estuary and adjacent coastal waters. In winter, the river discharge is small and more oceanic waters are dominant and as a result, salinity is high, and chlorophyll and nutrients are low. In summer, when the river discharge is high, salinity decreases and nutrients increase. DIN is very high, reaching 100 μM in the estuary. This indicates over enrichment of nitrogen relative to P and consequently there is an excess of N in coastal waters of Hong Kong. P remains low (∼1 μM) and can potentially limit both phytoplankton biomass and N utilization which was demonstrated in field incubation experiments. P limitation would result in excess N being left in the estuarine influenced waters south of Hong Kong. Phosphate concentration is lower in the Pearl River estuary than in many other eutrophied estuaries. Therefore, this relatively low PO₄ concentration should be a significant factor limiting a further increase in the magnitude of algal biomass and in the degree of eutrophication in the Pearl River estuary. The export of the excess N offshore into the northern South China Sea may result in an increase in the size of the region that is P limited in summer.     

Physical dynamics controlling variability in nearshore fecal pollution: Fecal indicator bacteria as passive particles

We present results from a 5-h field program (HB06) that took place at California’s Huntington State Beach. We assessed the importance of physical dynamics in controlling fecal indicator bacteria (FIB) concentrations during HB06 using an individual based model including alongshore advection and cross-shore variable horizontal diffusion. The model was parameterized with physical (waves and currents) and bacterial (Escherichia coli and Enterococcus) observations made during HB06. The model captured surfzone FIB dynamics well (average surfzone model skill: 0.84 {E. coli} and 0.52 {Enterococcus}), but fell short of capturing offshore FIB dynamics. Our analyses support the hypothesis that surfzone FIB variability during HB06 was a consequence of southward advection and diffusion of a patch of FIB originating north of the study area. Offshore FIB may have originated from a different, southern, source. Mortality may account for some of the offshore variability not explained by the physical model.     

Estimation of ecologically significant circulation features of the Columbia River estuary and plume using a reduced-dimension Kalman filter

A data assimilation method was used to estimate the variability of three ecologically significant features of the Columbia River estuary and plume: the size of the plume, the orientation of the plume, and the length of the salinity intrusion in the estuary. Our data assimilation method was based on a reduced-dimension Kalman filter that enables fast data assimilation of nonlinear dynamics in the estuary and plume. The assimilated data included measurements of salinity, temperature, and water levels at 13 stations in the estuary and at five moorings in the plume.     

Stormflow dynamics and loads of Escherichia coli in a large mixed land use catchment

Storm events are major transporters of faecal microbial contaminants, but few studies have reported storm loads or concentration dynamics in relation to discharge or other pollutants, notably fine sediment. Episodically, high loads of faecal contamination during storm flows impact downstream uses of water bodies, particularly contact recreation and shellfish harvesting. We examined the storm dynamics of Escherichia coli, turbidity and discharge in the mixed land use Motueka catchment (2047 km²; 60% forest and 19% pasture) to gain insights into E. coli sources and transport. We also explored different approaches for calculating E. coli loads. Discharge and field turbidity were recorded continuously, and E. coli concentrations were sampled during events, over a 13‐month period near the mouth of the Motueka River. E. coli loads were estimated by interpolation, averaging estimators and by using linear regression with smearing correction of the log‐transformed variables: discharge, turbidity, and both turbidity and discharge. The annual E. coli load was dominated (～98%) by export during events. Comparison of monthly monitoring with the intensive storm monitoring campaign suggests that simple stratification of the sampling into storm and baseflow would greatly improve export estimates. E. coli peak concentrations always preceded discharge and turbidity peaks (which had similar timing). Turbidity can be a useful surrogate for faecal microbes in smaller catchments, but in the Motueka turbidity was no better for predicting E. coli concentration than discharge. Runoff from grazed pasture and direct deposition from livestock are probably the ultimate E. coli sources in the Motueka catchment. However, in‐channel stores seem to dominate E. coli dynamics during events and account for the typical feature of bacterial concentrations peaking ahead of discharge and turbidity. This study demonstrates the importance of storm events to faecal microbial loads and shows that E. coli concentration dynamics may contrast with those of turbidity. Copyright © 2009 John Wiley & Sons, Ltd.     

Indicators of microbial beach water quality: Preliminary findings from Teluk Kemang beach,Port Dickson (Malaysia)

This study aims to determine the concentrations of total coliforms and Escherichia coli (E. coli) in beach water, Teluk Kemang beach. This study was also aimed to determine relationship between total coliforms, E. coli and physicochemical parameters. As perceived health symptoms among beach visitors are rarely incorporated in beach water studies, this element was also assessed in this study. A total of eight water sampling points were selected randomly along Teluk Kemang beach. Total coliforms concentrations were found between 20 and 1940 cfu/100 ml. E. coli concentrations were between 0 and 90 cfu/100 ml. Significant correlations were found between total coliforms and E. coli with pH, temperature and oxidation reduction potential. Skin and eyes symptoms were the highest reported though in small numbers. Microbiological water quality in Teluk Kemang public beach was generally safe for recreational activities except sampling location near with sewage outfall.     

Mixing,hypersalinity and gradients in Hervey Bay,Australia

Hervey Bay, a large coastal embayment situated off the central eastern coast of Australia, is a shallow tidal area (average depth = 15 m), close to the continental shelf. It shows features of an inverse estuary, due to the high evaporation rate (approx. 2 m/year), low precipitation (less than 1 m/year) and on average almost no freshwater input from rivers that drain into the bay. The hydro- and thermodynamical structures of Hervey Bay and their variability are presented here for the first time, using a combination of four-dimensional modelling and observations from field studies. The numerical studies are performed with the Coupled Hydrodynamical Ecological Model for Regional Shelf Seas (COHERENS). Due to the high tidal range (>3.5 m), the bay is considered as a vertically well-mixed system, and therefore, only horizontal fronts are likely. Recent field measurements, but also the numerical simulations, indicate characteristic features of an inverse/hypersaline estuary with low salinity (35.5 psu) in the open ocean and peak values (>39.0 psu) in the head water of the bay. The model further predicts a nearly persistent mean salinity gradient of 0.5 psu across the bay (with higher salinities close to the shore). The investigation further shows that air temperature, wind direction and tidal regime are mainly responsible for the stability of the inverse circulation and the strength of the salinity gradient across the bay. Due to an ongoing drying trend, the occurrence of severe droughts at the central east coast of Australia and, therefore, a reduction in freshwater supply, the salinity flux out of the bay has increased, and the inverse circulation has also strengthened.     

Metal exposure and biological responses in resident and transplanted blue mussels (Mytilus edulis) from the Scheldt estuary

The Western Scheldt river and estuary is known to be highly polluted as it receives industrial, agricultural and domestic effluents from one of the most populated and industrialised zones in Europe. Aquatic organisms are exposed to pollutants, specifically metals that are present in different environmental phases, e.g. dissolved, suspended material or sediment phases. The objective of this study was to study the relationship that exists between environmental metal levels, the degree of metal uptake by aquatic organisms with the concomitant biological responses. For this purpose the bivalve mollusk, Mytilus edulis, was selected as bioaccumulation indicator organism. Environmental (water and sediment) and mussel samples were collected during the late winter (March 2000) from sampling sites in the Scheldt estuary. Sites were selected to represent a salinity gradient from the mouth of the estuary to the furthest distribution area of mussels upstream in the system. Together with standard water quality parameters (e.g. salinity, dissolved oxygen, dissolved organic carbon, etc.) concentrations of twelve metals were analysed in the water (dissolved and suspended matter) and sediments. Levels of these metals were also measured in the soft tissue of M. edulis, together with concomitant biomarker responses in resident mussel populations at each site. The biomarkers that were included in this study were condition index, scope for growth, survival in air, cell membrane stability, DNA damage, and metallothioneins. Data were subjected to multivariate statistical analysis. The physico-chemical parameters and metals in the environmental samples clustered the sites to reflect the distribution based on the salinity gradient. Bioaccumulation results revealed increased metal uptake along a pollution gradient with the highest metal bioaccumulation occurring at the upstream most sites and therefore closest to the in the industrial activities. However, the biomarker responses clustered the sites in a manner that reflected the influence of combination of internal exposure (bioaccumulation) and external exposure (physico-chemical conditions). These differences in biomarker responses clearly demonstrated were attributed to abiotic factors other than metal pollution alone e.g. localized short-term increases in increased suspended sediment concentrations and decreased dissolved oxygen concentrations.     

Effects of freshwater inputs on the lower trophic levels of a temperate estuary: physical, physiological or trophic forcing?

The aim of this 6?year study was to assess whether freshwater inputs (rainfall and dam discharges) were acting as physical, physiological or trophic forcing factors on phytoplankton pigment concentrations and the dominant mysids of a temperate estuary (Guadalquivir estuary; SW, Spain). The effects of natural and human-controlled freshwater inputs modified the physico-chemical conditions and consequently biological production (bottom up control). Nutrient (nitrogen hypernutrification), suspended particulate matter and allochthonous photosynthetic pigment imports linked to freshwater inputs from adjacent habitats were observed, as well as light-limited autochthonous primary production. Seasonal and/or spatial patterns were shown by all study variables, including mysids. Freshwater management effects on dominant mysids differed depending on the species?? salinity tolerance (physiological forcing) and preferred prey availability (trophic forcing). Moreover, high inorganic matter content had a negative effect on the density of Mesopodopsis slabberi (physical forcing), which led to an increased detritivory/herbivory ratio (Neomysis integer/M. slabberi ratio). In conclusion, freshwater inputs appeared to effect estuarine lower trophic levels via a combination of different forcing mechanisms. Although several general patterns can be derived, the response of the system to freshwater inputs was not always univocal.     

Establishing a spatial map of health risk assessment for recreational fishing in a highly urbanized watershed

Recreational fishing is a popular activity in many urbanized watersheds. When river water is incidentally ingested during fishing sessions, substantial waterborne fecal contamination can cause adverse health effects. This study aims to spatially map health risks for recreational fishers caused by waterborne Escherichia coli (E. coli) in the highly urbanized Tamsui River watershed. First, indicator kriging was used to probabilistically estimate the distributions of waterborne E. coli and determine the conditional cumulative distribution function (CCDF). Subsequently, to propagate the parameter variability, Monte Carlo simulation was adopted to characterize the ingestion rate and exposure duration for recreational fishers and E. coli realizations were generated using random fields on the basis of the estimated CCDF. Finally, after the three parameters were combined, the approximate beta-Poisson dose–response function was employed to quantitatively determine potential risks to recreational fishers in the Tamsui River and its tributaries. The analysis results revealed that the risks of recreational fishing exceed an acceptable level of 8 infections per 1000 fishers per day at several urban river courses. Therefore, recreational fishing activities in urban riverbanks pose a substantial health threat. Recreational fishing in urban riverbanks should be limited before the construction of complete sanitary sewer systems. The river mouth and certain upstream river sections are suitable for the development of recreational fishing.     

Phytoplankton dynamics in and near the highly eutrophic Pearl River Estuary,South China Sea

The dynamics of size-fractionated phytoplankton along the salinity gradient in the Pearl River Estuary and the adjacent near-shore oceanic water was investigated using microscopic, flow cytometric, and chlorophyll analyses in the early spring (March) and early autumn (September) of 2005. In the inner part of the estuary where salinity was less than 30, the phytoplankton community was dominated by micro- and nano-sized (3–200 μm) cells, particularly the diatom Skeletonema costatum, both in early spring and early autumn. In areas where salinity >30, including the mixing zone and nearshore oceanic water, micro- and nano-sized cell populations dominated the phytoplankton assemblage during early spring when influence of river discharge was minimal, whereas pico-sized (≤3 μm) cell populations were dominant during early autumn as a result of strong river discharge in the summer, with Synechococcus and pico-eukaryotes being predominant. Picophytoplankton were two orders of magnitude more abundant in early autumn (10⁶ cells mL⁻¹) than in early spring in the nearshore oceanic water. Nutrients delivered by freshwater input to the estuary were pushed toward high salinity (>30) areas as a result of short residence time, exerting a strong influence on phytoplankton abundance, especially picophytoplankton in the nearshore, otherwise oligotrophic, water. Influenced by high abundance of DIN and limitation in phosphorus, picophytoplankton in the adjacent nearshore oceanic water rose to prominence seasonally. Our results indicate that eutrophication in the Pearl River Estuary not only stimulates the growth of S. costatum in the nutrient-rich areas of the estuary but also appears to promote the growth of Synechococcus and pico-eukaryotes in the adjacent usually oligotrophic oceanic water at least during our autumn cruise.     

Factors controlling variability in nearshore fecal pollution: The effects of mortality

A suite of physical–biological models was used to explore the importance of mortality and fluid dynamics in controlling concentrations of fecal indicator bacteria (FIB) at Huntington Beach, CA. An advection–diffusion (AD) model provided a baseline to assess improvements in model skill with the inclusion of mortality. Six forms of mortality were modeled. All mortality models performed better than the AD model, especially at offshore sampling stations, where model skill increased from <0.18 to >0.50 (Escherichia coli) or <?0.14 to >0.30 (Enterococcus). Models including cross-shore variable mortality rates reproduced FIB decay accurately (p < 0.05) at more stations than models without. This finding is consistent with analyses that revealed cross-shore variability in Enterococcus species composition and solar dose response. No best model was identified for Enterococcus, as all models including cross-shore variable mortality performed similarly. The best model for E. coli included solar-dependent and cross-shore variable mortality.     

Harmful algal bloom species and phosphate-processing effluent: field and laboratory studies

In 2002, the Florida Department of Environmental Protection began discharging phosphate-processing effluent into Bishop Harbor, an estuary within Tampa Bay. Because of concerns that the effluent would serve as a nutrient source for blooms of the toxic dinoflagellate Karenia brevis, a field monitoring program was established and laboratory bioassays were conducted. Several harmful algal bloom (HAB) species, including Prorocentrum minimum and Heterosigma akashiwo, were observed in bloom concentrations adjacent to the effluent discharge site. Blooms of diatoms were widespread throughout Bishop Harbor. K. brevis was observed with cell concentrations decreasing with increasing proximity to the effluent discharge site. Bioassays using effluent as a nutrient source for K. brevis resulted in decreased cell yields, increased growth rates, and increased time to log-phase growth. The responses of HAB species within Bishop Harbor and of K. brevis to effluent in bioassays suggested that HAB species differ in their response to phosphate-processing effluent.     

The effects of sediment transport on contaminant dispersal: An example from Milford Haven

Milford Haven is an estuary that has been subjected to industrial pollution from tanker operations and refinery and power station effluents since about 1962. Consequently build-up of various anthropogenic heavy metals and hydrocarbons has been observed in the bottom sediments. This paper describes 1. a new technique whereby the patterns of net sediment transport are determined from the relative changes of grain-size distributions in 125 grab samples, and 2. how net sediment transport correlates with the quantity of contaminants contained in the bottom sediments.The analysis demonstrates that sediment movement in the flood direction dominates the central channel and the northern half of the estuary. Transport in the ebb direction is confined to the southern coastal zone. A comparison of the transport paths with contaminant concentrations shows clearly that the flood-dominant movement leads to concentration of heavy metals and hydrocarbons. As a result, an area of fine-grained deposition near the head of the estuary receives exceptionally high concentrations of contaminants. The ebbdominant transport, on the other hand, causes dilution in the amount of pollutants contained in the sediments.The derived transport paths agree well with known current patterns, and this technique is shown to be a rapid and inexpensive method for 1. environmental impact assessment, 2. establishing effective monitoring programmes, 3. predicting areas vulnerable to contamination, and 4. rational contingency planning.     

The use of monitoring data for identifying factors influencing phytoplankton bloom dynamics in the eutrophic Taw Estuary, SW England

Using the Taw Estuary as an example, data routinely collected by the Environment Agency for England and Wales over the period 1990-2004 were interrogated to identify the drivers of excessive algal growth. The estuary was highly productive with chlorophyll concentrations regularly exceeding 100 μg L⁻¹, mostly during periods of low freshwater input from the River Taw when estuarine water residence times were longest. However, algal growth in mid estuary was often inhibited by ammonia inputs from the adjacent sewage treatment works. The reported approach demonstrates the value of applying conventional statistical analyses in a structured way to existing monitoring data and is recommended as a useful tool for the rapid assessment of eutrophication. However, future estuarine monitoring should include the collection of dissolved organic nutrient data and targeted high temporal resolution data because the drivers of eutrophication are complex and often very specific to a particular estuary.     

A natural tracer investigation of the hydrological regime of Spring Creek Springs, the largest submarine spring system in Florida

This work presents results from a nearly two-year monitoring of the hydrologic dynamics of the largest submarine spring system in Florida, Spring Creek Springs. During the summer of 2007 this spring system was observed to have significantly reduced flow due to persistent drought conditions. Our examination of the springs revealed that the salinity of the springs' waters had increased significantly, from 4 in 2004 to 33 in July 2007 with anomalous high radon (²²²Rn, t_1/2=3.8 days) in surface water concentrations indicating substantial saltwater intrusion into the local aquifer. During our investigation from August 2007 to May 2009 we deployed on an almost monthly basis a continuous radon-in-water measurement system and monitored the salinity fluctuations in the discharge area. To evaluate the springs' freshwater flux we developed three different models: two of them are based on water velocity measurements and either salinity or ²²²Rn in the associated surface waters as groundwater tracers. The third approach used only salinity changes within the spring area. The three models showed good agreement and the results confirmed that the hydrologic regime of the system is strongly correlated to local precipitation and water table fluctuations with higher discharges after major rain events and very low, even reverse flow during prolong droughts. High flow spring conditions were observed twice during our study, in the early spring and mid-late summer of 2008. However the freshwater spring flux during our observation period never reached that reported from a 1970s value of 4.9×10⁶ m³/day. The maximum spring flow was estimated at about 3.0×10⁶ m³/day after heavy precipitation in February-March 2008. As a result of this storm (total of 173 mm) the salinity in the spring area dropped from about 27 to 2 in only two days. The radon-in-water concentrations dramatically increased in parallel, from about 330 Bq/m³ to about 6600 Bq/m³. Such a rapid response suggests a direct connection between the deep and the surficial aquifers.