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1.
Seafloor photography from three cruises is combined with multibeam sonar imagery to characterize benthic substrates and associated fauna of Santa Monica Bay, California. The multibeam EM1000 imagery was collected in 1996. Two sampling cruises (in 1998 and 1999) provided photographs at 142 sites throughout the Bay; a final cruise (in 2000) collected still photographs and continuous video along nine transects on the mainland shelf from Pt. Dume to the Palos Verdes peninsula. Muddy substrates (typically low backscatter) were the predominant habitat throughout the Santa Monica Bay, from the 20 m isobath to the adjacent Santa Monica basin floor (780 m). Bioturbation was pervasive as evidenced by abundant open burrows, mounds, and faunal tracks and trails. Sandy substrates (typically intermediate to high backscatter) were restricted to the innermost mainland shelf and a narrow outer shelf band north of Santa Monica Canyon. Cobble and gravel substrates (high backscatter) were restricted to the innermost shelf south of El Segundo and limited parts of the shelf edge. Rocky substrates (high backscatter) with interspersed patches of sand and gravel occurred on the high-relief marginal plateau and along parts of the shelf break offshore of Malibu.  相似文献   

2.
Seafood tissue monitoring in Santa Monica Bay, CA, for regulatory compliance has provided information about contamination around major wastewater outfalls, but has not been useful in health management. It has not always focused on species caught by sport fishing, or on key sport fishing areas. Separate programs were uncoordinated, using distinct sampling patterns to collect different species. Most importantly, monitoring was not designed to feed information into a formal health management process. The Santa Monica Bay Restoration Project oversaw the development of a regionally coordinated monitoring program. It stressed that monitoring should support risk-based health management decision making by California EPA. This led to fundamental changes in the existing compliance-based monitoring design. We describe these and examine the features of the management system that both allowed for and supported an unusual degree of change in long-standing compliance monitoring programs.  相似文献   

3.
The spatial variability of Holocene (past 10,000 years) sediment accumulation in Santa Monica Bay (California) was examined to identify controls sediment trapping in a bathymetrically complex coastal embayment and to provide geologic context for the post-industrial sedimentary record and associated pollution gradients. Sediment chronologies based on downcore AMS 14C dates were used to quantify long-term (millennia) accumulation rates in an effort to elucidate particle-transport pathways and sinks. Sediment accumulation rates for the full range of bayfloor environments (50-630 m water depths) range from 22 to 102 mg/cm(2)/year (15-88 mm/100 year), have an overall mean of 51+/-21 mg/cm(2)/year (1 sigma, n=11), and are comparable to rates reported for adjacent borderland basins. Maximal accumulation rates on the Malibu shelf and within a reentrant to Redondo canyon are interpreted to reflect (1) proximity to sediment sources and (2) localized oceanographic and topographic conditions conducive to sediment trapping and deposition. The 14C-derived accumulation rates are 2-10 times lower than rates determined through (210)Pb geochronology for the same sites in a related study, revealing that Holocene sediment accumulation has been non-steady-state. Santa Monica Bay is an important sink for suspended matter; averaged over the past several millennia a mass of sediment equivalent to 10-80% of the modern annual river supply is sequestered yearly. Net influx of suspended matter derived from the adjacent Palos Verdes shelf is evinced by a concentration gradient of p,p'-DDE in bayfloor sediments, whereas the distribution of anthropogenic silver suggests transport from Santa Monica shelf to the southeastern boundary of the bay. The results of this study provide new insight to the long-term fates of particulate matter in Los Angeles coastal waters.  相似文献   

4.
Redondo Canyon, in the southern half of Santa Monica Bay, is an active submarine canyon through which sands and silts are transported by periodic submarine slides and turbid flows.  相似文献   

5.
Santa Monica Bay was mapped in 1996 using a high-resolution multibeam system, providing the first substantial update of the submarine geomorphology since the initial compilation by Shepard and Emery [(1941) Geol. Soc. Amer. Spec. Paper 31]. The multibeam mapping generated not only high-resolution bathymetry, but also coregistered, calibrated acoustic backscatter at 95 kHz. The geomorphology has been subdivided into six provinces; shelf, marginal plateau, submarine canyon, basin slope, apron, and basin. The dimensions, gradients, and backscatter characteristics of each province is described and related to a combination of tectonics, climate, sea level, and sediment supply. Fluctuations of eustatic sea level have had a profound effect on the area; by periodically eroding the surface of Santa Monica plateau, extending the mouth of the Los Angeles River to various locations along the shelf break, and by connecting submarine canyons to rivers. A wetter glacial climate undoubtedly generated more sediment to the rivers that then transported the increased sediment load to the low-stand coastline and canyon heads. The trends of Santa Monica Canyon and several bathymetric highs suggest a complex tectonic stress field that has controlled the various segments. There is no geomorphic evidence to suggest Redondo Canyon is fault controlled. The San Pedro fault can be extended more than 30 km to the northwest by the alignment of a series of bathymetric highs and abrupt changes in direction of channel thalwegs.  相似文献   

6.
This study characterized stormwater plume development and associated phytoplankton dynamics in a coastal marine ecosystem through shipboard monitoring. We focused on plumes within Santa Monica Bay, California (USA), a coastal system that is subject to rapid pulses of untreated runoff from the urbanized watershed of Los Angeles during the winter rainy season. The physical, chemical, and biological signatures of stormwater plumes were tracked over time after each of 4 precipitation events ranging in magnitude from 1.5 cm to 9 cm. Low salinity surface plumes persisted in Santa Monica Bay for at least 2 to 5 days over spatial scales of up to 15 km. This is consistent with a 6-day residence time for surface water plume parcels, which was estimated from a drifter trajectory in the bay. Shipboard sampling and salinity measurements in the surf zone showed that plumes often persisted even longer nearshore. Plume waters were generally characterized by higher concentrations of dissolved nitrogen, colored dissolved organic matter, and higher light attenuation than non-plume waters. The magnitude of the effect of stormwater runoff on phytoplankton dynamics was dependent on the size of each storm and subsequent residence time of runoff within the bay. Rain events led to increases in primary productivity, phytoplankton biomass, and specifically, increases in diatom biomass, as measured by concentrations of biogenic silica.  相似文献   

7.
Rainfall during winter storms produces extensive turbid, freshwater plumes in the coastal waters of the Southern California Bight. When the plumes result from urban runoff they contain toxic pollutants along with pathogenic bacteria and viruses, often resulting in closure of public beaches. We examined the spatial structure and evolution of stormwater plumes in Santa Monica Bay in 1996. The plumes resulted from freshwater discharge from the Ballona Creek and Malibu Creek watersheds which supply approximately 60% of the freshwater runoff to Santa Monica Bay. The spatial scales of the plumes were determined using shipboard measurements of water properties obtained from towyo transects and surface underway sampling. Salinity maps showed that the plumes typically extended 4-7 km offshore, consistent with scaling by the internal Rossby radius of deformation. Plumes extended along shore 10 km or more. Generally the plumes occupied the upper 10 m of the water column. The persistence time of a plume offshore of Ballona Creek was about three days based on a sequence of surveys in March 1996 following rainfall of about 21 mm. Limited comparison of plumes from Ballona Creek, which drains a developed watershed, and Malibu Creek, which drains a rural watershed, suggested that Malibu Creek required greater rainfall to produce an offshore plume. A stormwater plume offshore of Malibu Creek was observed on both sides of the creek mouth, possibly due to freshwater discharge from smaller surrounding watersheds or advection of freshwater discharges from the east and south. Plumes offshore of Ballona Creek mainly resulted from the creek itself and usually extended northward from the creek mouth, consistent with the wind forcing and the Coriolis acceleration.  相似文献   

8.
Examination of a time series of foraminiferal assemblage distributions on the continental shelf and slope of Santa Monica Bay from 1955 to 1997-1998 suggests that the benthic microfauna have been greatly affected by the quality and character of the municipal sludge and wastewater discharged into the bay over the last half-century by the Hyperion Treatment Plant serving the greater Los Angeles area. Five species dominate both the living and dead foraminiferal assemblages of the 1997-1998 surface samples, including Eggerella advena, Trochammina pacifica, Bulimina denudata, Buliminella elegantissima, and Epistominella bradyana. Temporal patterns of relative species abundances for both living and dead assemblages, as well as toxicity tests measuring amphipod survival and sea urchin fertilization success, show improvement since the sewage treatment program was enhanced in 1986. None of these trends are evident 10 years earlier, coincident with the onset of a Pacific Decadal Oscillation warming trend. This fact suggests that remediation, and not climate change, is responsible for the faunal changes observed. Even with remediation, however, all foraminiferal faunal trends have not returned to early-outfall levels. The organic-waste indicating species T. pacifica shows a slow decline in abundance as sewage treatment and sludge disposal activities have improved, whereas a dramatic increase in the abundance of the pioneer colonizer of impacted regions, E. advena, has occurred, often with a reciprocal response by B. denudata. Also evident is a dramatic shift in the abundance of the once-dominant species Nonionella basispinata and Nonionella stella, which were unable to recolonize Santa Monica Bay since the two major outfalls (5- and 7-mile) began discharging. Temporal variations in species abundances, as well as range expansions, contractions, and the inability to recolonize areas previously, or presently, impacted, suggests that foraminifers are a useful tool in defining areas affected by waste discharge.  相似文献   

9.
Maps are useful scientific tools for presenting environmental information, but the statistical techniques necessary to prepare scientifically rigorous maps have primarily focused on terrestrial habitats. This study compares three popular techniques (triangulation, kriging, and co-kriging) to map sediment grain size in Santa Monica Bay, California. Two grain size data sets, one collected in 1994 (79 sites) and one collected in 1997 and 1998 (149 sites) were used for model development. A bathymetric data set collected in 1997 was used as a model covariate. A third grain size data set (40 sites) collected in 1996 from independent sites was used for model evaluation. Predictions were compared to validation data by average difference, prediction mean square error (PMSE), and a goodness-of-prediction measure, G. The average difference between prediction and truth was similar for all methods, but the PMSE for triangulation was more than twice that for kriging or co-kriging, which were similar. The G measure also shows triangulation to be a far worse predictor than kriging and co-kriging. Small-scale differences were observed between kriging and co-kriging at steep depth contours, where co-kriging predicted values commensurate with the expected depth-defined grain size.  相似文献   

10.
During the summer of 1997, sediment core samples were taken at 25 stations in Santa Monica Bay. Toxicity testing was performed on 4-cm sections of the entire length of each core using purple sea urchin fertilization and amphipod survival tests. The sea urchin test identified sections as being toxic at six stations, all located near current or former Hyperion Treatment Plant (HTP) wastewater outfall locations. The amphipod test identified sections from 17 stations as having toxic sediments. The stations having toxic sediments were scattered throughout the bay and toxicity was identified at numerous core depths. Spatial and temporal patterns indicated that toxicity was most strongly associated with the historical disposal of municipal wastewater sludge. Many of the sections toxic to the amphipods did not have chemical levels expected to cause toxicity and were in locations where a source of toxicity was not apparent.  相似文献   

11.
Clay mineralogy is useful in determining the distribution, sources, and dispersal routes of fine-grained sediments. In addition, clay minerals, especially smectite, may control the degree to which contaminants are adsorbed by the sediment. We analyzed 250 shelf sediment samples, 24 river-suspended-sediment samples, and 12 river-bed samples for clay-mineral contents in the Southern California Borderland from Point Conception to the Mexico border. In addition, six samples were analyzed from the Palos Verdes Headland in order to characterize the clay minerals contributed to the offshore from that point source. The <2 microm-size fraction was isolated, Mg-saturated, and glycolated before analysis by X-ray diffraction. Semi-quantitative percentages of smectite, illite, and kaolinite plus chlorite were calculated using peak areas and standard weighting factors. Most fine-grained sediment is supplied to the shelf by rivers during major winter storms, especially during El Ni?o years. The largest sediment fluxes to the region are from the Santa Ynez and Santa Clara Rivers, which drain the Transverse Ranges. The mean clay-mineral suite for the entire shelf sediment data set (26% smectite, 50% illite, 24% kaolinite+chlorite) is closely comparable to that for the mean of all the rivers (31% smectite, 49% illite, 20% kaolinite+chlorite), indicating that the main source of shelf fine-grained sediments is the adjacent rivers. However, regional variations do exist and the shelf is divided into four provinces with characteristic clay-mineral suites. The means of the clay-mineral suites of the two southernmost provinces are within analytical error of the mineral suites of adjacent rivers. The next province to the north includes Santa Monica Bay and has a suite of clay minerals derived from mixing of fine-grained sediments from several sources, both from the north and south. The northernmost province clay-mineral suite matches moderately well that of the adjacent rivers, but does indicate some mixing from sources in adjacent provinces.  相似文献   

12.
The flux and compositions of solvent-extractable lipid fractions were measured in particulate matter collected periodically by moored sediment traps in the Santa Monica Basin from 1985–1988. The purpose was to assess the compositional changes during settling, the carbon dynamics in the basin and to estimate the impacts of energy-related by-products on the surface sediments. Sediment traps recorded consistently high lipid fluxes in the eastern slope relative to the central basin, reflecting elevated terrigenous carbon inputs possibly from land-based human activities. Generally, lipid fluxes decrease offshore but increase vertically with water depth below 500m, implying lateral transport of particles. The steep decline of flux in the top 500m of the water is related to the rapid decomposition and mineralization of the marine-derived cellular carbon compounds. Less than 5% of the marine lipid components reach the seabed. In contrast, preferential preservation of terrigenous lipid is clearly evident from the chemistry of deeper traps and surface sediments.The lateral transport of particles is reflected in the presence of higher plant-derived lignin phenols and sewage-derived coprostanol and epicoprostanol in the deep trap material as well as in surface sediment throughout the basin. Petroleum triterpanes characteristic of natural seepage also permeates through the entire basin. Based on the data collected from both the trap particulte matter and surface sediments, a carbon budget for the Santa Monica Basin has been constructed.  相似文献   

13.
Spatial distribution of selected contaminants in the surface sediments of Santa Monica Bay (SMB), California was investigated. Sediments were analyzed for DDTs (DDT and metabolites), polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), linear alkylbenzenes (LABs) and coprostanol. Effluent samples from the Hyperion Treatment Plant (HTP), which discharges treated municipal wastewater effluents into SMB, were also analyzed. The inter-correlation in the distribution trends of contaminants was examined. The concentrations of contaminants were interpolated in a geographic information system to visualize their spatial distribution in the Bay. Inventories of the contaminants were also estimated.The concentrations of coprostanol, LABs and PCBs are very high only in the vicinity of the sewage outfall whereas PAHs and DDTs occur widespread in the Bay. The poor correlation of DDTs with LABs, PAHs or coprostanol content confirms the historic origin of DDTs and their absence in the contemporary wastewaters. Moderate correlation of DDTs with PCBs implies historic deposits as a major origin of PCBs. There are hot spots of DDTs at water depths of 60 and 100 m and the inventory of DDTs in Bay sediments is insignificant compared to that estimated in the Palos Verdes Shelf which extends from the southern edge of Redondo Canyon around Palos Verdes Peninsula. The concentration of toxic contaminants was examined according to published sediment quality guidelines. About 20 stations contain p, p′-DDE and/or total DDTs above ERM and, PCBs between ERL and ERM indicating potential for adverse biological effects.  相似文献   

14.
Spatial gradients in biomass and community composition have important consequences for ecosystem structure and function. In this study, small-scale inshore-offshore (1-10 km) and vertical (1-50 m) patterns of microphytoplankton biomass and community composition are described, and the environmental controls of microphytoplankton biomass are evaluated in a coastal ecosystem of the Southern California Bight (SCB). During a two-year period, persistent inshore-offshore gradients in phytoplankton biomass and occasional inshore-offshore gradients in community composition, coincident with regional precipitation, were found, although the strength of the gradients varied between sampling periods. The chlorophyll a maximum was generally present between 15 and 45 m, the cell abundance maximum occurred in surface waters, and there was little evidence of vertical gradients in community composition. Variability in chlorophyll a concentrations was linked to variability in environmental parameters only after some rain and upwelling events. This study demonstrates that inshore-offshore patterns in phytoplankton biomass previously documented at large spatial scales (100-700 km) in the SCB can also persist at smaller scales (1-10 km), although the mechanisms for the gradients are likely to be different at the different spatial scales. The results provide a baseline data set that can be used to focus monitoring and management efforts in the SCB. In particular, this work shows that a limited number of sampling stations are sufficient for phytoplankton monitoring in Santa Monica Bay.  相似文献   

15.
Small, steep, uplifting coastal watersheds are prolific sediment producers that contribute significantly to the global marine sediment budget. This study illustrates how sedimentation evolves in one such system where the continental shelf is largely sediment-starved, with most terrestrial sediment bypassing the shelf in favor of deposition in deeper basins. The Santa Barbara–Ventura coast of southern California, USA, is considered a classic area for the study of active tectonics and of Tertiary and Quaternary climatic evolution, interpretations of which depend upon an understanding of sedimentation patterns. High-resolution seismic-reflection data over >570 km2 of this shelf show that sediment production is concentrated in a few drainage basins, with the Ventura and Santa Clara River deltas containing most of the upper Pleistocene to Holocene sediment on the shelf. Away from those deltas, the major factor controlling shelf sedimentation is the interaction of wave energy with coastline geometry. Depocenters containing sediment 5–20 m thick exist opposite broad coastal embayments, whereas relict material (bedrock below a regional unconformity) is exposed at the sea floor in areas of the shelf opposite coastal headlands. Locally, natural hydrocarbon seeps interact with sediment deposition either to produce elevated tar-and-sediment mounds or as gas plumes that hinder sediment settling. As much as 80% of fluvial sediment delivered by the Ventura and Santa Clara Rivers is transported off the shelf (some into the Santa Barbara Basin and some into the Santa Monica Basin via Hueneme Canyon), leaving a shelf with relatively little recent sediment accumulation. Understanding factors that control large-scale sediment dispersal along a rapidly uplifting coast that produces substantial quantities of sediment has implications for interpreting the ancient stratigraphic record of active and transform continental margins, and for inferring the distribution of hydrocarbon resources in relict shelf deposits.  相似文献   

16.
Spatial variation in environmental conditions can elicit predictable size and morphological responses in marine organisms through influences on physiology. Thus, spatial and temporal variation in marine organism size and shape are often used to infer paleoenvironmental conditions, such as dissolved oxygen concentrations. Benthic foraminifera commonly serve as a tool for reconstructing past ocean oxygen levels. For example, benthic foraminiferal species assemblages, within‐ and among‐species patterns of test morphology, and geochemical analyses of carbonate tests are often employed to reconstruct past marine conditions. In this study, we measured the sizes and shapes of modern foraminifera representing four species that inhabit a steep oxygen gradient in the Santa Monica Basin on the Southern California Borderland with the aim of quantifying the influence of oxygen availability on foraminiferal morphology, both within and among species. Most foraminifera rely on aerobic respiration, but the four benthic foraminifera from the Santa Monica Basin do not show the predicted size and morphological responses to variations in dissolved oxygen concentrations based on first principles of cell physiology: Bolivina spissa shows no volume or volume‐to‐surface area response, Uvigerina peregrina increases in both test volume and volume‐to‐surface area ratio with decreasing dissolved oxygen concentrations, and both Bolivina argentea and Loxostomum pseudobeyrichi decrease in test volume, but only L. pseudobeyrichi shows a decrease in test volume‐to‐surface area ratio with decreasing oxygen concentrations as expected from physiological predictions. These findings imply that the morphological responses of individual foraminiferal species are not necessarily representative of the responses of other foraminiferal species within the community. Our findings further suggest that these species use physiological strategies such as depressed metabolic rates and alternative energy metabolisms to persist in low oxygen environments and, therefore, cannot be used in any simple way as paleo‐oxygen indicators. Should Proterozoic (1,000–542 million years ago) protists have possessed metabolic strategies similar to foraminifera, the sizes and shapes of protists in the fossil record may not usefully constrain ambient oxygen conditions during the appearance and initial taxonomic radiation of heterotrophic eukaryotes.  相似文献   

17.
Although large loads of potentially toxic constituents are discharged from coastal urban watersheds, very little is known about the fates and eventual impacts of these stormwater inputs once they enter the ocean. The goal of this study was to examine the effects of stormwater discharges on the benthic marine environment of Santa Monica Bay. Sediment samples were collected across a gradient of stormwater impact following significantly sized storm events offshore Ballona Creek (a predominantly developed watershed) and Malibu Creek (a predominantly undeveloped watershed). Sediments offshore Malibu Creek had a greater proportion of fine-grained sediments, organic carbon, and naturally occurring metals (i.e., aluminum and iron), whereas sediments offshore Ballona Creek had higher concentrations of anthropogenic metals (i.e., lead) and organic pollutants (i.e., total DDT, total PCB, total PAH). The accumulation of anthropogenic sediment contaminants offshore Ballona Creek was evident up to 2 km downcoast and 4 km upcoast from the creek mouth and sediment concentrations covaried with distance from the discharge. Although changes in sediment texture, organic content, and an increase in sediment contamination were observed, there was little or no alteration to the benthic communities offshore either Ballona or Malibu Creek. Both sites were characterized as having an abundance, species richness, biodiversity and benthic response index similar to shallow water areas distant from creek mouths throughout the Southern California Bight. There was not a preponderance of pollution tolerant, nor a lack or pollution sensitive, species offshore either creek mouth.  相似文献   

18.
Water quality impacts of stormwater discharges to Santa Monica Bay   总被引：1，自引：0，他引：1  
Urban stormwater runoff is a major source of contaminants to southern California's coastal waters, yet little is known about the fate and effects of these discharges. A 3-year multidisciplinary project was conducted to investigate the dispersion of stormwater plumes in Santa Monica Bay and the resultant impacts on the water column and benthos. This paper describes the toxicity component of the study. Sea urchin fertilization toxicity tests were conducted on stormwater from the two largest discharges into the bay: Ballona Creek, which drains a highly urbanized watershed, and Malibu Creek, which receives runoff from a largely undeveloped watershed. Every sample of Ballona Creek stormwater tested was toxic (usually >5 toxic units), while Malibu Creek stormwater had a lower frequency and magnitude of toxicity (usually <4 toxic units). Surface water samples collected within the Ballona Creek stormwater discharge plume were always toxic whenever the concentration of stormwater in the plume exceeded 10%. The toxic portion of the Ballona Creek stormwater plume extended more than 4 km offshore on one occasion. Toxicity identification studies indicated that zinc was the primary cause of toxicity in both Ballona Creek stormwater and the discharge plume. No acute sediment toxicity (10-day amphipod survival) was present in the study area, although interstitial water toxicity was present at some stations located near the mouth of Ballona Creek. Differences in watershed characteristics likely were responsible for the greater toxicity of the Ballona Creek stormwater discharge plume. The Ballona Creek watershed contained a greater degree of urbanization (83% versus 12% for Malibu Creek) and the presence of a network of concrete flood control channels resulted in a stormwater plume containing elevated concentrations of toxins that received less initial dilution (compared to Malibu Creek) in the nearshore environment.  相似文献   

19.
胶州湾沉积环境演变的分析   总被引：9，自引：0，他引：9  
从剖析表征沉积环境演变的生物学、地质动力学以及地球化学指示因子入手,系统总结了近年来胶州湾沉积环境演变研究的进展,揭示了诸如有孔虫、沉积物粒度、生源要素、污染物及有机质等众多指示因子以及它们的相关组合与环境演变的关系。综合分析表明,胶州湾沉积速率较低,属于缓慢淤积的海湾,是一个典型的潮汐汊道系统。胶州湾的沉积环境演变分为4个阶段:陆相、三角洲沼泽相、海陆过渡相和海湾相。在过去的50 a中,特别是近20 a来,由于受人类活动与工业排污的影响,胶州湾的沉积环境发生了显著的变化,海域面积大大减少,陆源输入已经成为胶州湾沉积物的主要来源,沉积速率不断增加,局部海域特别是河口附近污染加重。这些变化都是通过各种生物、地质动力和地球化学因子来综合体现的。  相似文献   

20.
Santa Monica Bay (SMB) beaches are the most heavily used in the U.S.A., despite an increased number of water quality postings over the last several years. To assess whether water quality problems are concentrated at a small number of chronically affected sites or whether the problems are widely distributed, we compiled 5 years of monitoring data collected at 59 sites, 22 of which are sampled daily. Other locally available rainfall and sewage spill monitoring information data were added to this data set to assess whether sewage spills, dry-weather runoff, or wet-weather runoff contribute the most to exceedences of water quality thresholds. Approximately 13% of the shoreline mile-days along monitored beaches in SMB exceeded the State of California's beach water quality standards during the 5-year study period. Most of the water quality exceedences occurred near urban runoff drains even though areas affected by drains represent only a small portion of the total shoreline. Although storms are relatively infrequent in southern California, the extent of water quality exceedences resulting from storm water runoff was similar to the extent of water quality exceedences found during dry weather. Sewage spills, while potentially more serious because they lead to beach closures rather than to the more limited posting of warning signs, represented less than 0.1% of the shoreline mile-days that exceeded water quality thresholds. During dry weather conditions, most of the water quality problems occurred near five of the largest drains and at two beach areas that have unique physical characteristics, which limited mixing, dispersion, and dilution. During wet weather conditions, water quality problems were more widespread.  相似文献   

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