摘要:

Lake Mead, Colorado River, Arizona-Nevada, is one of the most heavily used reservoirs in the Western United States, providing abundant recreational opportunities as well as downstream domestic and agricultural water for over 22 million users. Based on average nutrient levels and productivity, Lake Mead is classified as mildly mesotrophic. The insertion into the water column of the Colorado River dominates the limnology of much of the 106 km-long reservoir, and may still be identified at Hoover Dam under certain conditions. The lower basin of Lake Mead ending at Hoover Dam is known as Boulder Basin and is near the Las Vegas metropolitan area. Las Vegas Bay, which comprises the northwestern portion of Boulder Basin, receives all runoff from the Las Vegas Valley via Las Vegas Wash. Inflow includes urban storm flows, secondary and tertiary treated municipal sewage effluent and groundwater seepage. The rapid increase in population of Las Vegas Valley and subsequent increased volume of point and non-point sources to Las Vegas Wash have produced noticeable physical and ecological changes in Las Vegas Bay, including eutrophication. However, the influence of the intrusion from Las Vegas Wash, which is easily identified by its relatively high specific conductance and turbidity, extends from Las Vegas Bay out into Boulder Basin, and at times, to Hoover Dam. The intrusion's depth, thickness, and the distance to which it extends into the reservoir depend on the season of the year and are related to the degree of thermal stratification within the reservoir. Limnological data on the extent of the intrusion to station LV14 suggest future studies should focus on examining the potential threat, under certain conditions, to subsurface municipal water intakes located at Saddle Island 2.5 km south of sampling station LV14. Concentrations of bacteria and organic compounds are higher in the intrusion than surrounding deeper or shallower water.

ISSN:1040-2381    

DOI:10.1080/07438149709354301

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Aquatic plant monitoring is becoming increasingly important to document the spread and introduction of nonnative plant species. This paper presents results from a telephone poll of state-agency biologists familiar with plant monitoring; natural resources agencies in all fifty United States were contacted. Twenty-seven states conduct some form of aquatic plant monitoring. Of the states without programs, fourteen believed that macrophyte monitoring was needed. Seven states conduct full-scale aquatic plant monitoring and twenty states have partial programs (i.e., a limited number of waters, selected species, or short-term). Although quantitative information is considered ideal, most states collect semi-quantitative data. The primary reason for this situation is a lack of funding — sixteen states conducting semi-quantitative monitoring believed that program expansion was necessary. Monitoring data are routinely used in formulating management strategies, most frequently for nonnative plant control. However, a number of native plants (e.g.,Myriophyllum laxum) were listed as nuisance species, becoming the focus of control and monitoring programs in eight states. Based on the knowledge shared by the poll participants, a few general recommendations on aquatic plant monitoring are presented.

ISSN:1040-2381    

DOI:10.1080/07438149709354302

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A seasonal, two-layer, variable volume, mass balance model for total phosphorus in a stratifying eutrophic lake is developed and tested for a single spring to fall interval. The model recognizes settling, sediment release, and vertical mass transport as key in-lake processes mediating phosphorus dynamics. The application presented is termed a “zero degree of freedom” model, as all inputs are determined independently through field and laboratory studies conducted on a test system. The model is considered validated for the test system because simulation results closely match field observations with no calibration (adjustment of model coefficients). Loading reductions at an adjoining wastewater treatment plant, the lake's major phosphorus source, provided an opportunity for model verification. The model framework has utility for other eutrophic stratifying lakes and the “zero degree of freedom” approach will find application where opportunities for both calibration and verification do not exist.

ISSN:1040-2381    

DOI:10.1080/07438149709354303

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Current mean annual whole-lake total phosphorus (TP) concentration is about 20 mg·m−3, and mean June through September epilimnion TP is approximately 13 mg·m−3in Lake Sammamish. Water quality is referenced to mean June through September chlorophylla(chla) and Secchi disk transparency. Currendy, these two parameters are at approximately 2.8 mg·m−3and 4 meters, respectively. A dynamic deterministic TP simulation model indicates that lake water quality will decline, and future water quality in the lake is dependent upon die extent of land development and stormwater controls. The model simulated a lake that: 1) is seasonally stratified, 2) receives seasonal internal loading, and 3) receives seasonal variations in both external TP loading and hydraulic loading. TP movement into, out of, and within the lake was estimated based upon measured values for all of the major pathways. Past, present, and future water quality were estimated based upon the simulated TP concentrations and empirical relationships developed from both literature and in-lake relationships.

ISSN:1040-2381    

DOI:10.1080/07438149709354304

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Measurements of Secchi depth in Garner Lake, TN, were made with and without a viewing tube at approximately 20-minute intervals from sunrise to sunset on 12 days throughout a year. The goal was to determine which factors affect the apparent Secchi depth as a function of time of day as well as date. The data show that on any given day the apparent (measured) Secchi depth depends on solar altitude. This relationship has been formally modeled. A key point in the model is that the magnitude of the observed Secchi depth is determined by the trajectory of abeamof sunlight through the water. The model calculations allow the Secchi depth observed at any date, time, or geographical location to be normalized to a standard, the value that would be found with the sun in the zenith. Use of such a normalization offers the opportunity to accurately quantify and study Secchi depth variations over time, or differences between lakes in different geographical locations. Based on the experience gained in this experiment, several suggestions are offered on how to obtain the most accurate measurements on any given day.

ISSN:1040-2381    

DOI:10.1080/07438149709354305

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Algal bioassays were conducted to determine whether phosphorus available for algal growth was released during sediment resuspension in Lake Okeechobee. The bioassays included evaluations of resuspension of sediments at different times of the year. Treatments consisted of floc and surface sediment layers mixed with surface lake water, then filtered to remove all but soluble phosphorus. The presence of phosphorus available for algal growth was manifested as the growth responses of unialgal cultures of phosphorus-starved cyanobacteria isolated from Lake Okeechobee. Floc and sediment treatments caused different algal growth responses depending on the sampling dates. In two summer experiments, there was enhanced growth response of the algae in the surface water previously mixed with sediments and floc. This result contrasted with the winter experiments when algal yields were lower in surface water samples mixed with sediment or floc. The results support recent hypotheses that sediment resuspension in Lake Okeechobee can act as either source or sink for orthophosphorus.

ISSN:1040-2381    

DOI:10.1080/07438149709354306

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Otsego Lake fills a depression formed by the overdeepening of the headwaters of the Susquehanna River in New York State. The lake is dimictic. During summer stratification hypolimnetic waters become oxygen depleted impacting cold-water fish habitat and resulting in changes over time in the distributions of profundal macroinvertebrates. Changes in benthic communities have occurred concurrent with changes in water quality, but not with precipitation. There is debate whether recent trends are due to meteorologically driven anomalies or to changing conditions due to human impacts on nutrient loading. Artificial changes in lake level, impacts of legislation, alterations of on-site sewage disposal and exotic introductions further complicate analysis. Eulittoral macrobenthic species richness was calculated by combining records of taxa collected from 1968–88 and comparing them to collections made from 1989–93. Lake wide average species richness has decreased 27.5%; EPT (pollution intolerant Ephemeroptera, Plecoptera and Trichoptera) species richness by 56.1%; and Mollusca by 52.9%. Invertebrate biomass from the shoreline to the deepest profundal substrates is represented by collections from 1935, 1969, and 1993. Biomass remained stable between 1935 and 1969, but increased by 1993. In 1968 mollusks dominated the eulittoral (air, water, land interface; the shoreline) environment; arthropods from 4–30 m in depth; and oligochaetes in the deepest waters. In 1993, the littoral benthic community was dominated by mollusks. The oligochaete annelids occupied substrates at middle profundal depths while the populations of fingernail clams and the Chironomidae in those regions had declined.

ISSN:1040-2381    

DOI:10.1080/07438149709354307

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The Breckenridge Sanitation District discharges tertiary-treated sewage effluent into Lake Dillon, Colorado, a major water supply reservoir for Denver. The effluent plume was mapped using rhodamine-WT dye during ice-covered and open-water conditions and compared to a criterion issued in a U.S. Environmental Protection Agency policy statement regarding mixing-zone dilution. Rhodamine-WT dye was used as a tracer and was injected directly into the sewage effluent at the outlet of the treatment plant. Dye concentrations were determined using fluorometric analysis. Dye-concentration isolines were determined by interpolation of a rectangular sampling grid. At all sampling locations, plateau dye concentrations were reached within 48 hours after injection began. Results indicated that the Breckenridge Sanitation District's effluent mixing zone extended 158.5 m (520 ft) into the lake during ice-covered conditions on March 15, 1995 (the date of maximum effluent discharge during the study period) and extended 64 m (210 ft) during open-water conditions on September 13, 1995. The EPA criterion for lakes specifies that the maximum allowable length of a mixing zone not exceed 61-m (200 ft).

ISSN:1040-2381    

DOI:10.1080/07438149709354308

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Paleolimnological techniques were used to establish baseline environmental conditions as guidelines for lake management and to evaluate water quality changes over the last 300 years in the north basin of Upper Saranac Lake (Adirondack Park, New York). Analysis of diatoms, bulk sedimentary characteristics and fossil pigments in a 30-cm core showed that the lake was once oligo- to mesotrophic but has become increasingly productive since the turn of the century, especially since the early 1950's. Relative abundances of the diatomsFragilaria crotonensis, Tabellaria flocculosa, andAsterionella formosa– common indicators of nutrient enrichment – increased in the most recent sediments, as did loss-on-ignition, cyanobacterial pigments, and inferred total phosphorus, pH, and chloride. Comparison of core profiles with historical data suggests that the productivity increased because of human activities, in eluding shoreline development and the discharge of fish hatchery effluent into a major tributary of the north basin. Short-term variations in diatom-inferred phosphorus since 1950 may be partly attributable to weather conditions.

ISSN:1040-2381    

DOI:10.1080/07438149709354309

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor