A SEDIMENT RESUSPENSION AND WATER QUALITY MODEL OF LAKE OKEECHOBEE1

ABSTRACT: The influence of sediment resuspension on the water quality of shallow lakes is well documented. However, a search of the literature reveals no deterministic mass-balance eutrophication models that explicitly include resuspension. We modified the Lake Okeechobee water quality model - which uses the Water Analysis Simulation Package (WASP) to simulate algal dynamics and phosphorus, nitrogen, and oxygen cycles - to include inorganic suspend. ed solids and algorithms that: (1) define changes in depth with changes in volume; (2) compute sediment resuspension based on bottom shear stress; (3) compute partition coefficients for ammonia and ortho-phosphorus to solids; and (4) relate light attenuation to solids concentrations. The model calibration and validation were successful with the exception of dissolved inorganic nitrogen species which did not correspond well to observed data in the validation phase. This could be attributed to an inaccurate formulation of algal nitrogen preference and/or the absence of nitrogen fixation in the model. The model correctly predicted that the lake is light-limited from resuspended solids, and algae are primarily nitrogen limited. The model simulation suggested that biological fluxes greatly exceed external loads of dissolved nutrients; and sediment-water interactions of organic nitrogen and phosphorus far exceed external loads. A sensitivity analysis demonstrated that parameters affecting resuspension, settling, sediment nutrient and solids concentrations, mineralization, algal productivity, and algal stoichiometry are factors requiring further study to improve our understanding of the Lake Okeechobee ecosystem.     

FIELD MEASUREMENT OF FLOW VELOCITIES,SUSPENDED SOLIDS CONCENTRATIONS,AND TEMPERATURES IN LAKE OKEECHOBEE1

ABSTRACT: This paper presents a field investigation of collecting hydrodynamic and sediment data in Lake Okeechobee with analyses examining mechanisms affecting sediment resuspension in the lake. Lake Okeechobee is a large subtropical lake located in south central Florida. Three‐dimensional flow velocities, suspended solids concentrations (SSC), and temperatures at four locations were measured from January 18 to March 5, 2000. Analyses of these data indicate that wind is the dominant factor in driving flow velocities and therefore transporting suspended solids. Wind direction also affects the SSC, especially in the north central and west littoral areas of the lake. The surface and bottom velocity components frequently flow in opposite directions, forming a stratification of the water column and preventing suspended solids from settling out. This retention of SSC in the water column may have a strong impact on the water quality of Lake Okeechobee. This study provides valuable storm event data and mechanism analyses, which will improve our understanding of the transport of suspended solids, thermal exchanges, and flow patterns within Lake Okeechobee.     

TEMPORAL VARIABILITY OF WATER QUALITY IN THE ST. LUCIE ESTUARY,SOUTH FLORIDA1

ABSTRACT: Four years of monthly freshwater discharge and constituent concentration data from three tributaries were related to a concurrent series of data for three segments of the St. Lucie Estuary in South Florida using multiple regression and time-series analysis techniques. Water quality parameters examined were dissolved inorganic and total nitrogen and phosphorus, chlorophyll a, total suspended solids, turbidity, and color. On monthly time scales, a multiple regression, which included freshwater discharge, freshwater constituent concentration, and dilution with ocean water (salinity) as independent variables, explained 50 percent or less of the variability in estuarine constituents. No single independent variable explained more variation than another. By contrast, on seasonal (wet, dry) time scales, freshwater discharge explained the bulk of variation in estuarine water quality (up to 93 percent). On monthly time scales, variability in concentrations of nutrients and other constituents may be largely controlled by processes internal to the system. At seasonal time scales, freshwater discharge appears to drive variability in most estuarine water quality parameters examined. Results indicate that management of tributary input on a seasonal basis, with the expectation of achieving seasonal concentration goals in the estuary, would have a higher probability of success than managing on a monthly basis.     

ENVIRONMENTAL FACTORS INFLUENCING SUSPENDED SOLIDS IN THE LOXAIIATCHEE ESTUARY,FLORIDA1

ABSTRACT: A study was initiated to examine the effects of wind speed, wind direction, freshwater inflow, and tide height on suspended solid concentration and distribution in the Loxahatchee estuary, Florida. Recent efforts to increase freshwater flows in this system raised concerns that suspended solid concentrations would increase as well, which might result in negative impacts for the estuary. The data indicated that total suspended solids (TSS) in the estuary are derived primarily from the inlet and not from freshwater tributaries. In addition, total suspended solids and volatile suspended solids were correlated strongly with salinity, suggesting that suspended sediments act conservatively throughout this system. No one environmental factor had an overwhelming influence on suspended solid concentration throughout the estuary; different regions of the estuary were influenced by different factors. Freshwater inflow was negatively related to TSS in the upper reaches of the estuary but was positively related to TSS in the central embayment region of the estuary. We attribute this latter finding to the fact that extremely high inflows both prevented the normal transport upstream of tidal borne suspended sediments and promoted mixing when the freshwater front moving downstream confronted the tidal front moving upstream. Wind speed, wind direction, and tide height had relatively small effects on TSS concentration but were most influential in reaches upstream of the central embayment, where tidal velocity begins to diminish.     

Magnesium-rich minerals in sediment and suspended particulates of South Florida water bodies: implications for turbidity

Fine sediments in shallow water bodies such as Lake Okeechobee are prone to resuspension. Predominantly inorganic "mud" sediment that covers approximately 670 km2 of the lake has been recognized as a persistent source of turbidity. The objective of this study was to determine if mineral components of sediments in Lake Okeechobee and water conveyances of the northern Everglades also occur as suspended sediment and hence constitute a potential abiotic contributor to turbidity. Sediment samples were collected from nine stations within the lake and eight locations north of Water Conservation Area 2A in the Everglades. Water samples were also collected at selected locations. The silt and clay mineralogy of sediment and suspended particles was determined using X-ray diffraction, thermogravimetry, scanning-electron microscopy, energy-dispersive X-ray elemental microanalysis, and high-resolution transmission-electron microscopy. Clay fractions of the lake sediment contained the Mg silicate minerals sepiolite and palygorskite, along with smectite, dolomite, calcite, and kaolinite. Sediment silt fractions were dominated by carbonates and/or quartz, with smaller amounts of Ca phosphates and sepiolite. Mineralogy of the mud sediment was similar to that reported for geologic phosphate deposits. This suggests that the mud sediment might have accumulated by stream transport of minerals from these deposits. Suspended solids and mud-sediment mineralogy were similar, except that smectite was more abundant in suspended solids. Everglade samples also contained Mg-rich minerals. The small size, low density, and fibrous or platy nature of the prevalent mud sediment minerals make them an abiotic, hydrodynamically sensitive source of persistent turbidity in a shallow lake. Mitigation efforts focused exclusively on P-induced biogeochemical processes do not address the origin or effects of these minerals. Ecological management issues such as turbidity control, P retention, geologic P input, and suitability of dredging are related to mud-sediment properties and provenance.     

ASSESSMENT OF NUTRIENT EFFECTS AND NUTRIENT LIMITATION IN LAKE OKEECHOBEE1

ABSTRACT: Lake Okeechobee, the third largest lake in the United States, is a shallow, mixing basin with annual total phosphorus concentrations ranging from 50–100 μg P/L. Data, mainly from unpublished agency reports, are analyzed to determine if nutrients limit phytoplankton, to describe spatial and temporal variability in trophic state parameters, and to evaluate conclusions obtained from empirical trophic state models. Algal bioassay experiments that have been used to assess nutrient limitation have produced equivocal results. However, seasonal minima in orthophosphorus and inorganic nitrogen indicate that both nutrients may be limiting seasonally. Strong, but reverse north-south gradients and large seasonal changes in phosphorus and nitrogen concentrations, show that empirical models based on annual phosphorus loadings or concentrations are not adequate to predict chlorophyll concentrations or other trophic state variables. Spatially-segmented, multi-class phytoplankton-nutrient models of seasonal phytoplankton responses that are coupled with hydrodynamic models may provide predictability in assessing effects of changing nutrient loads on phytoplankton composition and standing crop. Successful modeling efforts of responses to nutrients also must deal with resuspended and benthic algae, periphyton, and emergent and submergent aquatic plants that must play important trophic roles in some of the lake basin.     

Three-dimensional numerical simulation of water quality and sediment-associated processes with application to a Mississippi Delta lake

A three-dimensional water quality model was developed for simulating temporal and spatial variations of phytoplankton, nutrients, and dissolved oxygen in freshwater bodies. Effects of suspended and bed sediment on the water quality processes were simulated. A formula was generated from field measurements to calculate the light attenuation coefficient by considering the effects of suspended sediment and chlorophyll. The processes of adsorption–desorption of nutrients by sediment were described using the Langmuir Equation. The release rates of nutrients from the bed were calculated based on the concentration gradient across the water–sediment interface and other variables including pH, temperature and dissolved oxygen concentration.The model was calibrated and validated by applying it to simulate the concentrations of chlorophyll and nutrients in a natural oxbow lake in Mississippi Delta. The simulated time series of phytoplankton (as chlorophyll) and nutrient concentrations were generally in agreement with field observations. Sensitivity analyses were conducted to demonstrate the impacts of varying suspended sediment concentration on lake chlorophyll levels.     

REVIEW OF A SEVENTEEN-YEAR CHLOROPHYLL RECORD AS IT PERTAINS TO THE TROPIIIC STATUS OF LAKE OKEECHOBEE,FLORIDA, USA1

ABSTRACT: A 17-year record of chlorophyll a at eight limnetic sampling stations was used to evaluate putative changes in the trophic status of Lake Okeechobee, a shallow polymictic lake located in the subtropical environment of South Florida. Significant spatial differences were observed in the temporal patterns and variability of chlorophyll a concentrations. The highest chlorophyll a values were found in the northern and northwestern regions of the lake. The center of the lake, subject to high levels of non-algal suspended solids, exhibited relatively low chlorophyll a values and coefficient of variation. The lowest chlorophyll a values were observed at the southernmost sampling station in the lake. This was also the station that showed a significant upward trend in annual mean chlorophyll a values over the 17-year period of record. Examination of the relationship between chlorophyll a and three key environmental variables (i.e., total phosphorus concentration, phosphorus loading, and lake stage) revealed significant correlations at two out of the eight stations. The overall results of this study indicate that spatial and temporal disparities in the distribution and dynamics of chlorophyll a in Lake Okeechobee mandate more temporally and spatially intense approaches to the evaluation of trophic state than used in previous studies.     

Nutrient Uptake in a Large Urban River1

Abstract: Small streams have been shown to be efficient in retaining nutrients and regulating downstream nutrient fluxes, but less is known about nutrient retention in larger rivers. We quantified nutrient uptake length and uptake velocity in a regulated urban river to determine the river’s ability to retain nutrients associated with wastewater treatment plant (WWTP) effluent. We measured net uptake of soluble reactive phosphorus (SRP), dissolved organic phosphorus, ammonium (NH₄), nitrate, and dissolved organic nitrogen in the Chattahoochee River, Atlanta, GA by following the downstream decline of nutrients and fluoride from WWTP effluent on 10 dates under low flow conditions. Uptake of all nutrients was sporadic. On many dates, there was no evidence of measurable nutrient uptake lengths within the reach; indeed, on several dates release of inorganic N and P within the sample reach led to increased nutrient export downstream. When uptake occurred, SRP uptake length was negatively correlated with total suspended solids and temperature. Uptake velocities of SRP and NH₄ in the Chattahoochee River were lower than velocities in less‐modified systems, but they were similar to those measured in other WWTP impacted systems. Lower uptake velocities indicate a diminished capacity for nutrient uptake.     

POTENTIAL PHOSPHORUS LOAD REDUCTIONS UNDER THE LAKE OKEECHOBEE REGULATORY PROGRAM1

ABSTRACT: Nutrient loading from beef pastures located within the northern Lake Okeechobee watershed in Florida, has been identified as a source of phosphorus contributing to the accelerated eutrophication of the lake. Since 1989 within the watershed, 557 agricultural drainage sites, mainly beef pasture, have been monitored for compliance under a regulatory program. Of those sites, 154 were actively monitored for phosphorus concentrations from October 1, 1998, to September 30, 1999. Of these 154 sites, 77 were considered to be out of compliance (OOC). An OOC site is defined as having runoff with a 12‐month average phosphorus concentration exceeding the permitted discharge limit. The average annual phosphorous load from the 77 OOC sites for an eight‐year study period from October 1, 1991, to September 30, 1999, was estimated using measured concentration values and simulated runoff obtained from an agricultural nonpoint source pollution model, CREAMS‐WT. The 77 OOC sites produced an estimated average annual 46 metric tonnes of phosphorus load, of which an estimated 22 tonnes of phosphorus reached Lake Okeechobee on an average annual basis. The remaining estimated average annual 24 tonnes of phosphorus load was retained by streams and wetlands in the discharge transport system between the sites and the lake. The estimated average annual load reaching Lake Okeechobee from the OOC sites represented 11 percent of the phosphorus load above a five‐year average annual target load for the lake. However, the OOC site drainage areas represented only 3 percent of the northern watershed that drains into the lake. Of the 77 OOC sites, 12 sites had an average annual phosphorus loading rate equal to or greater than 3.0 kg/ha and were placed on the priority list for the Critical Restoration Project in the Lake Okeechobee watershed. To estimate the possible phosphorus load reductions from the 77 sites, two scenarios were modeled. The first scenario reduced phosphorus concentrations in runoff to the permitted discharge limits under the Lake Okeechobee regulatory program. The second scenario changed current land uses to native rangeland with an estimated annual offsite total phosphorus areal loading rate of 0.114 kg/ha. These two scenarios are hypothetical with assumed concentration values and loading rate. Model results showed that the first management scenario reduced the average annual phosphorus load to the lake by an estimated 15 tonnes. The second scenario reduced the average annual phosphorus load to the lake by an estimated 21 tonnes.     

TRANSPORT AND DISTRIBUTION OF NUTRIENTS IN THE LOXAHATCHEE RIVER ESTUARY,SOUTHEASTERN FLORIDA, 1979–811

ABSTRACT: Concentrations of total nitrogen, total phosphorus, and total organic carbon in the Loxahatchee River estuary decreased with increasing salinity in a manner indicating that mixing and dilution of freshwater by seawater was the primary process controlling the down-stream concentrations of nutrients. Most of the nutrients in the surface freshwater inflows entered the estuary from five major tributaries; however, about 10 percent of the total nitrogen and 32 percent of the total phosphorus were from urban stormwater runoff. The input of nutrients was highly seasonal and storm related. During a 61-day period of above average rainfall that included Tropical Storm Dennis, the major tributaries discharged 2.7 metric tons of total phosphorus, 75 metric tons of total nitrogen, and 1,000 metric tons of organic carbon to the estuary. This period accounted for more than half of the total nutrient load from the major tributaries during the 1981 water year (October 1, 1980, through September 30, 1981). Inorganic phosphorus and nitrogen increased relative to total phosphorus and nitrogen during storm runoff. Nutrient yield from the basin, expressed as grams per square meter of basin area, was relatively low. However, because the basin area (544 square kilometers) is large compared with the volume of the estuary, the basin might be expected to contribute significantly to estuarine enrichment were it not for tidal flushing. Approximately 60 percent of the total volume of the estuary is flushed on each tide. Because the estuary is well flushed, it probably has a large tolerance for nutrient loading.     

EVALUATION OF WATER QUALITY AND MONITORING IN THE ST. LUCIE ESTUARY,FLORIDA1

ABSTRACT: The St. Lucie River and its tributaries form a major estuarine system on the southeast coast of Florida. This system is strongly affected by anthropogenic influences, including controlled releases of freshwater from Lake Okeechobee through the St. Lucie Canal and an extensive artificial drainage and irrigation system in the watershed. In the present study, three years of biweekly water quality monitoring data from the estuary were examined. The major stresses to the system stem from high variability of the salinity, frequent low dissolved oxygen (DO) events, and light limitation due to high levels of humic substances brought into the system with the fresh water. Nutrient levels also are high. Management goals for the system, including improvement of DO and water clarity, will require reduction in quantity and variability of freshwater releases.     

Assessing Water Quality at Large Geographic Scales: Relations Among Land Use,Water Physicochemistry,Riparian Condition,and Fish Community Structure

Data collected from 172 sites in 20 major river basins between 1993 and 1995 as part of the US Geological Survey's National Water-Quality Assessment Program were analyzed to assess relations among basinwide land use (agriculture, forest, urban, range), water physicochemistry, riparian condition, and fish community structure. A multimetric approach was used to develop regionally referenced indices of fish community and riparian condition. Across large geographic areas, decreased riparian condition was associated with water-quality constituents indicative of nonpoint source inputs—total nitrogen and suspended sediment and basinwide urban land use. Decreased fish community condition was associated with increases in total dissolved solids and rangeland use and decreases in riparian condition and agricultural land use. Fish community condition was relatively high even in areas where agricultural land use was relatively high (>50% of the basin). Although agricultural land use can have deleterious effects on fish communities, the results of this study suggest that other factors also may be important, including practices that regulate the delivery of nutrients, suspended sediments, and total dissolved solids into streams. Across large geographic scales, measures of water physicochemistry may be better indicators of fish community condition than basinwide land use. Whereas numerous studies have indicated that riparian restorations are successful in specific cases, this analysis suggests the universal importance of riparian zones to the maintenance and restoration of diverse fish communities in streams.     

ESTABLISHING POLLUTANT LOAD REDUCTION TARGETS FOR THE INIIAN RWER LAGOON,FLORIDA1

ABSTRACT. Establishment of seagrass‐based pollutant load reduction targets is a major aim of water quality management in the Indian River Lagoon (IRL) estuary. It is believed that light and water clarity are the limiting factors affecting seagrass abundance and distribution in the Lagoon. Thus, targeted reductions of nutrients, dissolved organic matter and for suspended sediments should achieve desired seagrass coverage goals. On an annual average, the Lagoon is receiving external loadings of 5,346, 771, and 54,408 metric tons per year of nitrogen (N), phosphorus (P), and total suspended solids (TSS), respectively. Most of the loadings are stormwater generated; thus, implementation of reduction targets will key in on major urban and agricultural stormwater systems. The process of developing targets requires the establishment of light requirements for seagrass. Two methods are being employed to develop such targets. One method is the application of a predictive, 3‐D model that accounts for the essential interactive processes of the Lagoon: hydrology, hydrodynamics, water quality, nutrient uptake/release, seagrass growth rates, and light attenuation. The other method is based simply on the comparison of loading rates between a developed sub‐basin with associated seagrass impacts and an undeveloped sub‐basin with adjacent healthy seagrass coverages. The latter method will provide the initial and preliminary targets that can be tested, confirmed, or modified by application of the 3‐D model.     

MATERIALS INPUT OF LAKE CHAMPLAIN: A SYNOPTIC APPRAISAL1

ABSTRACT: The complex morphometry of Lake Champlain requires that detailed, regional studies be made, and the results integrated, to yield total lake conditions. Using specific conductance measurements, and values of total dissolved solids calculated from them, we present an approach to assessing the materials budget of the lake. The sampling program involved inventorying all 319 tributaries, determining the watershed area for each, and dividing the Champlain basin into appropriate hydrographic regions. Data were obtained from samples collected from 41 selected streams (representing 97.5% of the annual water input), sampling occurring in all seasons of the year since 1970. Results indicate that over one million metric tonnes of total dissolved solids enter Lake Champlain annually, about two-thirds (63%) from the eastern (Vermont) portion and almost one-fourth (22%) from the western (New York) part of the drainage basin, the remainder (15%) entering from the south end. Of the total quantity added annually, 17.4% is retained in the lake, indicating that a solids build-up is occurring, at a significant rate. We suggest that specific conductance, and therefore total dissolved solids, be utilized as a convenient indicator of water quality conditions, and results applied to permit more efficient watershed management.     

Nitrogen Sources and Sinks Within the Middle Rio Grande,New Mexico1

Abstract: Relationships between discharge, land use, and nitrogen sources and sinks were developed using 5 years of synoptic sampling along a 300 km reach of the Rio Grande in central New Mexico. Average river discharge was higher during 2001 and 2005 “wet years” (15 m³/s) than during the drought years of 2002‐04 “dry years” (8.9 m³/s), but there were no differences in nitrogen loading from wastewater treatment plants (WWTPs) which were the largest and most consistent source of nitrogen to the river (1,330 kg/day). Average total dissolved nitrogen (TDN) concentrations remained elevated for 180 km downstream of the Albuquerque WWTP averaging 1.2 mg/l in wet years and 0.52 mg/l in dry years. Possible explanations for the constant elevated TDN concentrations downstream of the major point source include reduced nitrogen retention capacity, minimal contact with riparian or channel vegetation, large suspended sediment loads, and low algal biomass. Somewhat surprisingly, agricultural return flows had lower average nitrogen concentrations than river water originally diverted to agriculture in both wet (0.81 mg/l) and dry years (0.19 mg/l), indicating that the agricultural system is a sink for nitrogen. Lower average nitrogen concentrations in the river during the dry years can be explained by the input of agricultural returns which comprise the majority of river flow in dry years.     

Nitrogen and phosphorus attenuation within the stream network of a coastal, agricultural watershed

Streams alter the concentration of nutrients they transport and thereby influence nutrient loading to estuaries downstream; however, the relationship between in-stream uptake, discharge variability, and subsequent nutrient export is poorly understood. In this study, in-stream N and P uptake were examined in the stream network draining a row-crop agricultural operation in coastal North Carolina. The effect of in-stream nutrient uptake on estuarine loading was examined using continuous measurements of watershed nutrient export. From August to December 2003, 52 and 83% of the NH4+ and PO4(3-) loads were exported during storms while concurrent storm flow volume was 34% of the total. Whole-ecosystem mass transfer velocities (Vf) of NH4+ and PO4(3-), measured using short-term additions of inorganic nutrients, ranged from 0.1 to 25 mm min(-1). Using a mass balance approach, this in-stream uptake was found to attenuate 65 to 98% of the NH4+ flux and 78 to 98% of the PO4(3-) flux in small, first-order drainage ditches. For the larger channel downstream, an empirical model based on Vf and discharge was developed to estimate the percentage of the nutrient load retained in-stream. The model predicted that all of the upstream NH4+ and PO4(3-) load was retained during base flow, while 65 and 37% of the NH4+ and PO4(3-) load was retained during storms. Remineralization from the streambed (vs. terrestrial sources) was the apparent source of NH4+ and PO4(3-) to the estuary during base flow. In-stream uptake reduced the dissolved inorganic N to dissolved inorganic P ratio of water exported to the N-limited estuary, thus limiting the potential for estuarine phytoplankton growth.     

DECISION FRAMEWORK FOR SEDIMENT CONTROL IN MUDDY CREEK WATERSHED1

ABSTRACT: The tailwater of Bridgewater Dam, below Lake James, North Carolina, is a designated trout stream. It has environmental attributes for a good cold water fishery with the exception of high suspended sediments. Muddy Creek, a tributary about 1.5 km downstream of the dam, is a major source of sediments. The Muddy Creek Watershed Restoration Initiative was established to develop and implement a sediment control plan. The Watershed Analysis Risk Management Framework was applied to simulate soil erosion and sedimentation and to help determine appropriate action. The simulated sediment concentrations of the river were comparable to observed data from November 1994 to November 2001. For the base condition, the sediment load was 135,000 kg/d from surface erosion and 1,300,000 kg/d from bank erosion. Increasing the buffer strip from existing 50 to 80 percent to 100 percent of stream segments would only reduce surface erosion to 70,400 kg/d with little change in sediment concentrations. Eliminating riverbank erosion would reduce the sediment load from 920,000 to 87,700 kg/d. The bank stabilization project would not only lower suspended sediment concentrations for Muddy Creek, but also reduce the lake sediment accumulation in the downstream Lake Rhodhiss by approximately 13 percent.     

Sedimentation basin performance at highway construction sites

Sedimentation basins (SBs) are commonly used during highway construction for erosion and sedimentation pollution control as well as for attenuation of overland storm waters. In order to evaluate the sediment removal capacity of these SBs, four basins were selected for monitoring from a new highway construction that extends I-99 to I-80, in Pennsylvania. Between September 2004 and August 2005, ten sampling trips were conducted during which basin inlet and outlet water samples were obtained. The SB samples were analyzed for pH, color, turbidity, total suspended solids (TSS), volatile suspended solids (VSS), total and dissolved iron, magnesium, manganese, aluminum, calcium, sulfate and phosphate. The data showed peaks in concentrations of TSS, total aluminum, total manganese, total iron and total phosphate that closely correlated to localized rainfall peaks. For certain samples, the concentration of TSS in the outlet was higher than the TSS concentration at the basin inlet, suggesting sediment re-suspension. In general SBs managed high flows during wet weather events, but were not effective in capturing particulates. This paper discusses the need for Best Management Practices (BMPs) for the design of SBs that reflect contemporary concerns for management of particle removal and to control the release of particulate-bound metals. This paper also evaluates the water quality impacts of naturally occurring acidic drainages into SBs, as several acidic seeps with pH in the range of 5-6 and having high dissolved concentrations of metals (Fe, Mn, Mg and Ca), sulfate and phosphate were observed draining into the SBs.     

Long term changes in the eutrophication process in a shallow Mediterranean lake ecosystem of W. Greece: response after the reduction of external load

Lake Pamvotis is a shallow Mediterranean lake located in Western Greece near the city of Ioannina. The lake has been recognized as an internationally important conservation site under European Community legislation due to its rich biodiversity. However, during the last three decades the trophic status of the lake has changed as a result of anthropogenic activity (among others irrigation and domestic sewage discharge), resulting in serious problems. Here we present data about the long-term development in eutrophication of Lake Pamvotis. Water samples were collected and analyzed (water temperature, pH, dissolved oxygen, nutrients, chlorophyll-a) during three monitoring periods: 1985-1989, 1998-1999, 2004-2005. The high nutrient concentrations in the lake water during the three monitoring periods, as well as its eutrophic to hypertrophic status reflect the degree of impact anthropogenic activity has had on the lake. Commencement of a restoration plan in 1995-1996, involving sewage diversion, led to a reduction in external nutrient load and consequently to lower in-lake nutrients and Chlorophyll-a concentrations. Orthophosphate concentration decreased by about 87%, nitrates fell below 1.20mg/l, whilst the total reduction of inorganic N compounds showed a weaker downward trend, fluctuating between 0.39 and 1.24mg N/l with an average value of 0.76mg N/l. However, after a short-term recovery the eutrophic status of the lake remains eight years later (2004-2005), suggesting the importance of the internal loading process and the absence of the top-down effect of fish. This study provides evidence for the need of greater restoration efforts utilized in Mediterranean shallow lakes.