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.     

Nitrogen and Phosphorus Removal in a Rain Garden Flooded with Wastewater and Simulated Stormwater

Rain gardens, also known as bioretention cells, are low‐impact developments designed to mitigate several problems associated with urban stormwater. This four‐month field study involved a rain garden at a wastewater treatment plant in north‐central Texas in the United States of America. Partially treated wastewater from an anaerobic digester spilled into the rain garden at the beginning of the study. Subsequently, inflow and outflow concentrations of nitrogen and phosphorus were measured for nine simulated floods, preceded by dry‐spell intervals of 5, 8, or 12 days. Despite large inputs from the wastewater spill, the rain garden gradually processed and flushed the nitrogen. Longer dry spells tended to produce relatively higher outflow nitrate concentrations. A large pool of phosphorus in the soil restricted the rain garden's ability to reduce outflow orthophosphate concentrations, which were stable and lower than inflow concentrations throughout the study; however, adsorptive processes attenuated a relatively high inflow concentration by the end of the study.     

PHOSPHORUS RELEASE AND ASSIMILATORY CAPACITY OF TWO LOWER MISSISSIPPI VALLEY FRESHWATER WETLAND SOILS1

ABSTRACT: Phosphorus fluxes and water quality functions of a bottomland hardwood and freshwater marsh wetland soil were compared. The effect of soil physicochemical conditions, phosphorus loading rate, and diffusive exchange between soils and the overlying food water column on phosphorus release and retention were studied. The predominantly mineral swamp forest soil displayed greater phosphorus sorption potential than the organic freshwater marsh soil. Moreover, due to its low bulk density (0.11 g cm^?3), the freshwater marsh soil surface area required for phosphorus retention is very large compared to the bottomland hardwood wetland soil. For both wetlands, soil redox status affected P release and assimilatory capacity. The more reducing the soils, the smaller their phosphorus retention capacity (greater their release). Phosphorus removal from the overlying water column into the wetland soils followed a first-order kinetic model. Under similar hydrological conditions, phosphorus was found to diffuse 1.2 times faster to the bottom. land hardwood soil than in the freshwater marsh soil. Results indicate that while the bottomland hardwood wetland soil will serve as a sink for phosphorus entering such wetland, phosphorus will be released and exported from the freshwater marsh soil into adjacent ecosystems.     

Wastewater effects on a waterhyacinth marsh and adjacent impoundment

A waterhyacinth (Eichhornia crassipes) marsh occupying two-thirds of the basin of a small Florida impoundment has received sewage effluent for nearly 20 years. Water from the marsh flows into an area that is maintained free of waterhyacinths, and is discharged through wells at the far end of the impoundment. A water budget for the basin was estimated, and phosphorus concentrations were measured monthly at three stations in the marsh and at the discharge wells in the lake. Productivity levels were measured monthly where the marsh joins the lake and at the discharge wells. Only 16% of the phosphorus that enters the basin is stored. Gross primary productivity levels in the open-water areas are very high (22 gO₂m²day), but high respiration rates appear to keep the lake in steady-state.     

Nutrient fluxes in a eutrophic coastal Louisiana freshwater lake

Nitrogen and phosphorus cycling in a eutrophic Louisiana freshwater lake system (Lac des Allemands) was studied. Nutrients from runoff entering the lake, as well as sediment-interstitial and lake water nitrogen and phosphorus fractions, were measured seasonally. Sedimentation rates in the lake were determined using¹³⁷Cs dating.Phosphorus levels in the lake were found to be largely dependent on concentrations in the incoming bayou water from upland drainage. Lake water concentrations appear to respond to fluctuations in incoming waters. Laboratory equilibrium studies showed bottom sediments in the lake are a major sink for the incoming dissolved orthophosphate phosphorus. Total nitrogen concentrations in the lake water generally exceeded incoming runoff concentrations, suggesting fixation by the large blue-green algae population in the lake as being the major source of nitrogen to the system.Sedimentation ranged from 0.44 cm/year to 0.81 cm/year, depending on the proximity to the inlet bayous. Even though the lake is eutrophic the sediment served as a buffer by removing large amounts of carbon, nitrogen, and phosphorus through sedimentation processes. Carbon, nitrogen, and phosphorus were accumulating in the sediment at rates of 60, 7.1, and 1.1 g/m²/year, respectively.The water quality of the lake is likely to continue to decline unless measures are taken to reduce municipal, industrial, and agricultural inputs of phosphorus into the lake.     

SUBALPINE,COLD CLIMATE,STORMWATER TREATMENT WITH A CONSTRUCTED SURFACE FLOW WETLAND1

The Tahoe City Wetland Treatment System (TCWTS) was constructed in 1997 to treat stormwater runoff from 23 ha of commercial, highway, and residential land use in the Lake Tahoe Basin. This subalpine, constructed, surface flow wetland treatment system consists of two cells in series, with a design water surface area of about 0.6 ha. Water quality monitoring from October 2002 through September 2003 was conducted with autosamplers at the inflow and outflow sites during 24 sampling events, with a median duration of 53 hours, representing 42 percent of total inflow to this wetland during the year. Monitoring data indicate an improvement of 49 percent or greater in effluent concentrations of dissolved phosphorus, nitrate, orthophosphorus, and total suspended solids. On average, event mean concentrations of total phosphorus were reduced from a median 279 μg/l at the inflow to 94 μg/l at the outflow. Event mean concentrations of total nitrogen were reduced from a median 1,599 μg/l at the inflow to 810 μg/l at the outflow. Net nutrient retention for the sampling period was estimated at 3 g phosphorus (P)/m²/y and 13 g nitrogen (N)/m²/y. Almost 4,000 kg of suspended sediment was captured by this wetland system during the year.     

A Daily Time Series Analysis of StreamWater Phosphorus Concentrations Along an Urban to Forest Gradient

During a 1-year period, we sampled stream water total phosphorus (TP) concentrations daily and soluble reactive phosphorus (SRP) concentrations weekly in four Seattle area streams spanning a gradient of forested to urban-dominated land cover. The objective of this study was to develop time series models describing stream water phosphorus concentration dependence on seasonal variation in stream base flows, short-term flow fluctuations, antecedent flow conditions, and rainfall. Stream water SRP concentrations varied on average by ±18% or ±5.7 μg/L from one week to another, whereas TP varied ±48% or ±32.5 μg/L from one week to another. On average, SRP constituted about 47% of TP. Stream water SRP concentrations followed a simple sine-wave annual cycle with high concentrations during the low-flow summer period and low concentrations during the high-flow winter period in three of the four study sites. These trends are probably due to seasonal variation in the relative contributions of groundwater and subsurface flows to stream flow. In forested Issaquah Creek, SRP concentrations were relatively constant throughout the year except during the fall, when a major salmon spawning run occurred in the stream and SRP concentrations increased markedly. Stream water SRP concentrations were statistically unrelated to short-term flow fluctuations, antecedent flow conditions, or rainfall in each of the study streams. Stream water TP concentrations are highly variable and strongly influenced by short-term flow fluctuations. Each of the processes assessed had statistically significant correlations with TP concentrations, with seasonal base flow being the strongest, followed by antecedent flow conditions, short-term flow fluctuations, and rainfall. Times series models for each individual stream were able to predict ∼70% of the variability in the SRP annual cycle in three of the four streams (r² = 0.57–0.81), whereas individual TP models explained ∼50% of the annual cycle in all streams (r² = 0.39–0.59). Overall, time series models for SRP and TP dynamics explained 82% and 76% of the variability for these variables, respectively. Our results indicate that SRP, the most biologically available and therefore most important phosphorus fraction, has simpler and easier-to-predict seasonal and weekly dynamics.     

Modeling Nutrient Release in the Tai Lake Basin of China: Source Identification and Policy Implications

Because nutrient enrichment has become increasingly severe in the Tai Lake Basin of China, identifying sources and loads is crucial for watershed nutrient management. This paper develops an empirical framework to estimate nutrient release from five major sectors, which requires fewer input parameters and produces acceptable accuracy. Sectors included are industrial manufacturing, livestock breeding (industrial and family scale), crop agriculture, household consumption (urban and rural), and atmospheric deposition. Results show that in the basin (only the five sectors above), total nutrient loads of nitrogen (N) and phosphorus (P) into aquatic systems in 2008 were 33043.2 tons N a^?1 and 5254.4 tons P a^?1, and annual area-specific nutrient loads were 1.94 tons N km^?2 and 0.31 tons P km^?2. Household consumption was the major sector having the greatest impact (46 % in N load, 47 % in P load), whereas atmospheric deposition (18 %) and crop agriculture (15 %) sectors represented other significant proportions of N load. The load estimates also indicate that 32 % of total P came from the livestock breeding sector, making it the second largest phosphorus contributor. According to the nutrient pollution sectors, six best management practices are selected for cost-effectiveness analysis, and feasible options are recommended. Overall, biogas digester construction on industrial-scale farms is proven the most cost-effective, whereas the building of rural decentralized facilities is the best alternative under extreme financial constraint. However, the reduction potential, average monetary cost, and other factors such as risk tolerance of policy makers should all be considered in the actual decision-making process.     

Application of SWAT with and without Variable Source Area Hydrology to a Large Watershed

In this study, two different versions of the Soil and Water Assessment Tool (SWAT) model were used to simulate the hydrology and biogeochemical response of the Cannonsville Reservoir watershed, in New York. The first version distributes overland flow in ways that are consistent with variable source area (VSA) hydrology driven by saturation excess runoff, whereas the second version is the standard version of SWAT. These two models were each calibrated for streamflow (Flow), particulate phosphorus (PP), total dissolved phosphorus (TDP), and sediment (Sed) against measured data from the 1,200 km² Cannonsville watershed. The standard version of the model yielded an r² between the measured and simulated data of 0.85, 0.73, 0.70, and 0.72 for Flow, Sed, TDP, and PP, respectively. The VSA version yielded an r² of 0.84, 0.69, 0.72, and 0.53 for Flow, Sed, TDP, and PP, respectively. The two models were then used to determine the maximum upper bound on the reduction in phosphorus loading by removing all of the corn in the watershed. The average reductions between the two models were 65 and 37% for PP and TDP, respectively. The VSA version was also used to estimate the effect of moving corn land in the watershed from the wettest, most runoff prone areas to the driest, least runoff prone areas, which cannot be done directly with the standard SWAT model.     

Observations of Atmospheric Nitrogen and Phosphorus Deposition During the Period of Algal Bloom Formation in Northern Lake Taihu,China

Cyanobacterial blooms in Lake Taihu occurred at the end of April 2007 and had crucial impacts on the livelihood of millions of people living there. Excessive nutrients may promote bloom formation. Atmospheric nitrogen (N) and phosphorus (P) deposition appears to play an important role in algal bloom formation. Bulk deposition and rain water samples were collected respectively from May 1 to November 30, 2007, the period of optimal algal growth, to measure the bulk atmospheric deposition rate, wet deposition rate, and dry deposition rate for total nitrogen (TN; i.e., all species of nitrogen), and total phosphorus (TP; i.e., all species of phosphorus), in northern Lake Taihu, China. The trends of the bulk atmospheric deposition rate for TN and the wet deposition rate for TN showed double peaks during the observation period and distinct influence with plum rains and typhoons. Meanwhile, monthly bulk atmospheric deposition rates for TP showed little influence of annual precipitation. However, excessive rain may lead to high atmospheric N and P deposition rates. In bulk deposition samples, the average percentage of total dissolved nitrogen accounting for TN was 91.2% and changed little with time. However, the average percentage of total dissolved phosphorus accounting for TP was 65.6% and changed substantially with time. Annual bulk atmospheric deposition rates of TN and TP during 2007 in Lake Taihu were estimated to be 2,976 and 84 kg km⁻² a⁻¹, respectively. The results showed decreases of 34.4% and 78.7%, respectively, compared to 2002–2003. Annual bulk deposition load of TN for Lake Taihu was estimated at 6,958 t a⁻¹ in 2007 including 4,642 t a⁻¹ of wet deposition, lower than the values obtained in 2002–2003. This may be due to measures taken to save energy and emission control regulations in the Yangtze River Delta. Nevertheless, high atmospheric N and P deposition loads helped support cyanobacterial blooms in northern Lake Taihu during summer and autumn, the period of favorable algal growth.     

Use of SWAT to Estimate Spatial Scaling of Phosphorus Export Coefficients and Load Reductions Due to Agricultural BMPS

Phosphorus export coefficients (kg/ha/yr) from selected land covers, also called phosphorus yields, tend to get smaller as contributing areas get larger because some of the phosphorus mobilized on local fields gets trapped during transport to regional watershed outlets. Phosphorus traps include floodplains, wetlands, and lakes, which can then become impaired by eutrophication. The Sunrise River watershed in east central Minnesota, United States, has numerous lakes impaired by excess phosphorus. The Sunrise is tributary to the St. Croix River, whose much larger watershed is terminated by Lake St. Croix, also impaired by excess phosphorus. To support management of these impairments at both local and regional scales, a Soil and Water Assessment Tool (SWAT) model of the Sunrise watershed was constructed to estimate load reductions due to selected best management practices (BMPs) and to determine how phosphorus export coefficients scaled with contributing area. In this study, agricultural BMPs, including vegetated filter strips, grassed waterways, and reduction of soil‐phosphorus concentrations reduced phosphorus loads by 4‐20%, with similar percentage reductions at field and watershed spatial scales. Phosphorus export coefficients from cropland in rotation with corn, soybeans, and alfalfa decreased as a negative power function of contributing area, from an average of 2.12 kg/ha/yr at the upland field scale (~0.6 km²) to 0.63 kg/ha/yr at the major river basin scale (20,000 km²). Editor's note : This paper is part of the featured series on SWAT Applications for Emerging Hydrologic and Water Quality Challenges. See the February 2017 issue for the introduction and background to the series.     

Treatment of municipal wastewater using a contact oxidation filtration separation integrated bioreactor

A new contact oxidation filtration separation integrated bioreactor (CFBR) was used to treat municipal wastewater. The CFBR was made up of a biofilm reactor (the upper part of the CFBR) and a gravitational filtration bed (the lower part of the CFBR). Polyacrylonitrile balls (50 mm diameter, 237 m²/m³ specific surface, 90% porosity, and 50.2% packing rate) were filled into the biofilm reactor as biofilm attaching materials and anthracite coal (particle size 1–2 mm, packing density 0.947 g/cm³, non-uniform coefficient (K₈₀ = d₈₀/d₁₀) < 2.0) was placed into the gravitational filtration bed as filter media. At an organic volumetric loading rate of 2.4 kg COD/(m³ d) and an initial filtration velocity of 5 m/h in the CFBR, the average removal efficiencies of COD, ammonia nitrogen, total nitrogen and turbidity were 90.6%, 81.4%, 64.6% and 96.7% respectively, but the treatment process seemed not to be effective in phosphorus removal. The average removal efficiency of total phosphorus was 60.1%. Additionally, the power consumption of the CFBR was less than 0.15 kWh/m³ of wastewater treated, and less than 1.5 kWh/kg BOD₅ removal.     

Usefulness of natural regions for lake management: Analysis of variation among lakes in northwestern Wisconsin,USA

A map of summer total phosphorus in lakes was compiled recently for a three-state area of the upper Midwest for purposes of identifying regional patterns of total phosphorus in lakes and attainable lake trophic state. Spatial patterns in total phosphorus from approximately 3000 lakes were studied in conjunction with maps of geographic characteristics that tend to affect phosphorus balance in lakes to identify regions of similarity in phosphorus concentrations in lakes or similarity in the mosaic of values as compared to adjacent areas. While degrees of relative homogeneity are apparent at many scales, the map was designed at a scale that would yield regions with sufficient homogeneity to be useful for lake management throughout the area. In this study, data from 210 lakes in a 1560-mi² area in northwestern Wisconsin, sampled by the Wisconsin Department of Natural Resources in the spring of 1988 (subsequent to the compilation of the phosphorus map), were examined to: (1) substantiate the existence of the regions depicted on the map in northwest Wisconsin, (2) determine the nature and relative precision of the regional boundaries, (3) determine the relative importance of natural and anthropogenic watershed characteristics, lake types, lake area, and lake depth in explaining within-region differences in lake phosphorus, and (4) demonstrate how the regions might be used by local lake managers.     

MODELING SEDIMENT AND PHOSPHORUS LOSSES IN AN AGRICULTURAL WATERSHED TO MEET TMDLs1

ABSTRACT: This paper studies the effectiveness of alternative farm management strategies at improving water quality to meet Total Maximum Daily Loads (TMDLs) in agricultural watersheds. A spatial process model was calibrated using monthly flow, sediment, and phosphorus (P) losses (1994 to 1996) from Sand Creek watershed in south‐central Minnesota. Statistical evaluation of predicted and observed data gave r² coefficients of 0.75, 0.69, and 0.49 for flow (average 4.1 m³/s), sediment load (average 0.44 ton/ha), and phosphorus load (average 0.97 kg/ha), respectively. The calibrated model was used to evaluate the effects of conservation tillage, conversion of crop land to pasture, and changes in phosphorus fertilizer application rate on pollutant loads. TMDLs were developed for sediment and P losses based on existing water quality standards and guidelines. Observed annual sediment and P losses exceeded these TMDLs by 59 percent and 83 percent, respectively. A combination of increased conservation tillage, reduced application rates of phosphorus fertilizer, and conversion of crop land to pasture could reduce sediment and phosphorus loads by 23 percent and 20 percent of existing loads, respectively. These reductions are much less than needed to meet TMDLs, suggesting that control of sediment using buffer strips and control of point sources of phosphorus are needed for the remaining reductions.     

Controls on Nutrients Across a Prairie Stream Watershed: Land Use and Riparian Cover Effects

Nutrient inputs generally are increased by human-induced land use changes and can lead to eutrophication and impairment of surface waters. Understanding the scale at which land use influences nutrient loading is necessary for the development of management practices and policies that improve water quality. The authors assessed the relationships between land use and stream nutrients in a prairie watershed dominated by intermittent stream flow in the first-order higher elevation reaches. Total nitrogen, nitrate, and phosphorus concentrations were greater in tributaries occupying the lower portions of the watershed, closely mirroring the increased density of row crop agriculture from headwaters to lower-elevation alluvial areas. Land cover classified at three spatial scales in each sub-basin above sampling sites (riparian in the entire catchment, catchment land cover, and riparian across the 2 km upstream) was highly correlated with variation in both total nitrogen (r² = 53%, 52%, and 49%, respectively) and nitrate (r² = 69%, 65%, and 56%, respectively) concentrations among sites. However, phosphorus concentrations were not significantly associated with riparian or catchment land cover classes at any spatial scale. Separating land use from riparian cover in the entire watershed was difficult, but riparian cover was most closely correlated with in-stream nutrient concentrations. By controlling for land cover, a significant correlation of riparian cover for the 2 km above the sampling site with in-stream nutrient concentrations could be established. Surprisingly, land use in the entire watershed, including small intermittent streams, had a large influence on average downstream water quality although the headwater streams were not flowing for a substantial portion of the year. This suggests that nutrient criteria may not be met only by managing permanently flowing streams.     

Application of nutrient loading models to the analysis of trophic conditions in Lake Okeechobee,Florida

Lake Okeechobee (surface area = 1830 km², mean depth = 3.5 m), the largest lake in Florida, is eutrophic and has nitrogen and phosphorus loading rates in excess of nearly all established criteria. The lake is not homogeneous regarding trophic conditions, and spatial and temporal variations occur regarding nutrient limitation. Nonetheless, phosphorus loading rate and trophic state data fit reasonably well to various input-output models developed for temperate lakes. Modification of the models by regression analysis to fit data for Florida lakes resulted in improved predictions for most parameters. Analysis of nutrient management alternatives for the lake indicates that a 75% reduction of phosphorus loading from the largest source (the Taylor Creek-Nubbins Slough watershed) would reduce the average chlorophyll a concentration by less than 20%. Complete elimination of inputs from the largest nitrogen source (the Everglades Agricultural Area) would decrease the average nitrogen concentration in the lake by about 20%. Limitations of nutrient inputoutput models regarding analysis of trophic conditions and management alternatives for the lake are discussed.     

Wetland Flowpaths Mediate Nitrogen and Phosphorus Concentrations across the Upper Mississippi River Basin

Eutrophication, harmful algal blooms, and human health impacts are critical environmental challenges resulting from excess nitrogen and phosphorus in surface waters. Yet we have limited information regarding how wetland characteristics mediate water quality across watershed scales. We developed a large, novel set of spatial variables characterizing hydrological flowpaths from wetlands to streams, that is, “wetland hydrological transport variables,” to explore how wetlands statistically explain the variability in total nitrogen (TN) and total phosphorus (TP) concentrations across the Upper Mississippi River Basin (UMRB) in the United States. We found that wetland flowpath variables improved landscape-to-aquatic nutrient multilinear regression models (from R² = 0.89 to 0.91 for TN; R² = 0.53 to 0.84 for TP) and provided insights into potential processes governing how wetlands influence watershed-scale TN and TP concentrations. Specifically, flowpath variables describing flow-attenuating environments, for example, subsurface transport compared to overland flowpaths, were related to lower TN and TP concentrations. Frequent hydrological connections from wetlands to streams were also linked to low TP concentrations, which likely suggests a nutrient source limitation in some areas of the UMRB. Consideration of wetland flowpaths could inform management and conservation activities designed to reduce nutrient export to downstream waters.     

The impact of recreational boat traffic on Marbled Murrelets (Brachyramphus marmoratus)

This study evaluated the impact of small boat traffic on reaction distances of Marbled Murrelets (Brachyramphus marmoratus), in the marine waters of Pacific Rim National Park Reserve, British Columbia, Canada. Observers on moving boats recorded the minimum distance the boat came to murrelets on the water, and any disturbance reaction (fly, dive, no reaction). Out of the 7500 interactions 11.7% flew, 30.8% dove and 58.1% exhibited no flushing reaction. Using a product-limit analysis, we developed curves for the proportion of Marbled Murrelets flushing (dive or flight) as a function of reaction distance. Overall, the majority of Marbled Murrelets waited until boats were within 40 m before reacting, with 25% of the population reacting at 29.2m. A stepwise Cox regression indicated that age, boat speed, and boat density (loaded in that order), significantly affected flushing response. More juveniles flushed than adults (70.1 versus 51.7%), but at closer distances. Faster boats caused a greater proportion of birds to flush, and at further distances (25% of birds flushed at 40 m at speeds > 29 kph versus 28m at speeds <12kph). A stepwise logistic regression on diving and flight responses indicated that birds tended to fly completely out of feeding areas at the approach of boats travelling >28.8 kph and later in the season (July and August). Other secondary variables included; boat density and time of day. Discussion focused on possible management actions such as the application of speed limits, set back distances, and exclusion of boat traffic to protect Marbled Murrelets.     

PATTERNS OF SUSPENDED SEDIMENT TRANSPORT IN A COASTAL ALASKA STREAM1

Suspended sediment data from a 154 ha watershed on northeast Chichagof Island, Alaska, were collected over three fall storm seasons from 1980 to 1982. Sediment rating curves for nine pooled storms explained less than 34 percent of the variation in total suspended solids (TSS). Significantly higher concentrations of suspended sediment occurred during the rising limb of storm hydrographs than for similar flows on the falling limb, accounting for hysteresis loops in TSS versus streamflow plots for individual storms. These hysteresis loops were wider during early season storms, indicating that easily transportable fine sediment may have been flushed from the upper portion of channel banks and from behind large organic debris during early season peak flows. Regression relationships (TSS versus Q) developed for the highest stormflows (> 1 m³/s) had steeper slopes than the lower stormflows (< 1 m³/s). Turbidity correlated well (r=0.94) with TSS for all storm-flow data combined. Organic matter constituted an average of 35 percent (by weight) of TSS for all water quality samples.     

CHANGES IN WATER QUALITY IN THE NEWLY IMPOUNDED SUBTROPICAL FEITSUI RESERVOIR,TAIWAN1

ABSTRACT: Water quality and trophic conditions in the Feitsui Reservoir, a subtropical reservoir, were evaluated with data from a ten-year data base to depict the impacts of river impoundment upon the chemical and biological characteristics of a reservoir, and to discuss the effects of flushing rate on in-lake phosphorus concentrations and phytoplankton growth. The results of the investigation showed that during the incipient impounding period, the water quality in the Feitsui Reservoir was significantly affected by internal loadings from submerged vegetation and soil in the flooded area. Studies of the changes in phosphorus compounds indicated that total phosphorus concentration appeared to approach equilibrium after the seventh year of impoundment and that orthophosphate stabilized after the sixth year of impoundment. Concentrations of both phosphorus forms varied seasonally after attaining stability. Nitrogen compounds (NH₃-N, NO₃-N and NO₂-N) approached equilibrium within three years after impoundment. The seasonal variation in carbon was correlated to the number of phytoplankton. The mean value of the N:P mass ratio has remained over 110 since year seven of impoundment (1990), indicating that phosphorus constitutes the potential limiting nutrient in the growth of phytoplankton. The rapid flushing rate (132.11 and 110.43 yr^-1) in Feitsui Reservoir during the first and second impounding stages was a critical factor influencing the phytoplankton growth response to available nutrients.