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1.
A Poisson catch rate model for striped bass (Morone saxatilis) anglers in Chesapeake Bay was developed that incorporates the effect of bottom temperature and dissolved oxygen (DO). Angler catch rates are shown to be negatively affected by low DO. Predicted angler catch rates were then used in a random utility model of striped bass fishing location choice. Where anglers choose to fish is significantly related to expected catch rate and the travel cost and time from the anglers residence to the fishing location. Results from the random utility model were then used to simulate the economic welfare changes that result from changing DO levels in the Patuxent River. Since there are many substitute sites for fishing in the Patuxent River, the welfare effects are small. Increases in DO from current levels have a small effect on angler welfare, but if levels are allowed to deteriorate so they never exceed 5 mg l−1, the welfare effects are much larger. Under this latter scenario, the net present value of angler losses exceeds 100,000, and are almost100,000, and are almost 300,000 if the fishing grounds are anoxic. Losses are considerably higher as the area impacted by low oxygen conditions increases.  相似文献   

2.
A water quality model was developed for the Patuxent estuary using the modeling framework CE-QUAL-W2 (called W2) to address the impact of current and projected land-use changes (stress) on the water quality. The W2 code, supported by the Army Corps of Engineers Waterways Experiment Station, is a two-dimensional (longitudinalvertical) model designed for hydrodynamic and water quality simulations of reservoirs and estuaries. The code was configured for the Patuxent estuary using a finite-difference grid for the water column. Also incorporated into the model calculations are flow and nutrient load results from a watershed model. The integrated model was calibrated with data of 1 yr from 1997 to 1998 by matching results with measured temperature, dissolved oxygen, chlorophylla, and nutrient levels in the water column. The calibrated model was used to predict the water quality effects of different watershed land-use scenarios. Model projection results suggest that reductions of nutrient loads would lead to improvement of anoxic conditions in the bottom waters of the lower Patuxent estuary.  相似文献   

3.
The present study focused on detecting variations in trophic relationships among blue crab (Callinectes sapidus) consumers according to water quality along two estuaries in North Carolina. Stable isotope (δ15N and δ13C) analyses of particulate organic matter and bivalve(Rangia cuneata andCorbicula fluminea) food sources were examined in combination with an Isosource mixing model. Results suggest that blue crab δ13C values increased significantly with increasing salinity from upper to lower sites along the Neuse River estuary (NRE; R2 = 0.87, p < 0.01) and Alligator River estuary (R2 = 0.92, p < 0.01). There was a positive relationship between blue crab δ15N values and nitrate concentrations for the NRE (R2 = 0.48, p = 0.12). This study found that blue crab δ13C values increased with salinity from upper to lower regions along both estuaries. Results suggest that blue crab production may have used alternative food sources that were isotopically (δ13C) depleted, especially in the upper NRE, and enriched sources in the mid to lower regions of both estuaries. Consumers sampled from the upper NRE may be influenced by higher nitrogen input from urban land use and municipal wastewater.  相似文献   

4.
The Patuxent River, Maryland, is a nutrient-overenriched tributary of the Chesapeake Bay. Nutrient inputs from sewage outfalls and nonpoint sources (NPS) have grown substantially during the last four decades, and chlorophylla levels have increased markedly with concomitant reductions in water quality and dissolved oxygen concentrations. The Patuxent has gained national attention because it was one of the first river basins in the U.S. for which basin-wide nutrient control standards were developed. These included a reduction in NPS inputs and a limit on both nitrogen (N) and phosphorus (P) loadings in sewage discharges intended to return the river to 1950s conditions. Full implementation of point source controls occurred by 1994, but population growth and land-use changes continue to increase total nutrient loadings to the river. The present paper provides the perspectives of scientists who participated in studies of the Patuxent River and its estuary over the last three decades, and who interacted with policy makers as decisions were made to develop a dual nutrient control strategy. Although nutrient control measures have not yet resulted in dramatic increases in water quality, we believe that without them, more extensive declines in water quality would have occurred. Future reductions will have to come from more effective NPS controls since future point source loading will be difficult to further reduce with present technology. Changing land use will present a challenge to policy makers faced with sprawling population growth and accelerated deforestation.  相似文献   

5.
Flushing of dense water from cavities of the upper reaches of the Swan River estuary in Western Australia was investigated using measured salinity and dissolved oxygen profiles and a two-dimensional, laterally averaged hydrodynamic model (TISAT). Seasonal flushing of dense, hypoxic bottom waters from a relatively deep site took place over ∼3 days at the onset of winter in 1994. Model simulations of the purging of this dense water did not correspond closely with changes in the densimetric Froude number. Purging, expressed as depth of the halocline as a fraction of the total cavity depth, occurred when the simulated mean horizontal velocity at 2 m depth (top of cavity) changed from negative to strongly positive, indicating arrest of upstream flow and continuous downstream flow. This corresponded to freshwater discharge of about 50 m3 s−1. Oxygen depletion of bottom waters was closely related to stratification. Oxygen dynamics at the onset of winter river flow was analysed using an exponential decay model, assuning that there was no net inflow or outflow across the halocline and thus no vertical transport of oxygen during a period of strong stratification. The rate constant for oxygen decay at Ron Courtney Island (RCI) was estimated to be 0.232 d−1 for this period. Bottom waters at RCI declined to less than 1 mg 1−1 prior to complete flushing through increased river flows. This study provided in sights to how freshwater flows may be allocated to maintain suitable oxygen levels in the bottom waters of estuarine cavities.  相似文献   

6.
This paper addresses temporal variability in bottom hypoxia in broad shallow areas of Mobile Bay, Alabama. Time-series data collected in the summer of 2004 from one station (mean depth of 4 m) exhibit bottom dissolved oxygen (DO) variations associated with various time scales of hours to days. Despite a large velocity shear, stratification was strong enough to suppress vertical mixing most of the time. Bottom DO was closely related to the vertical salinity gradient (ΔS). Hypoxia seldom occurred when ΔS (over 2.5 m) was <2 psu and occurred almost all the time when ΔS was >8 psu in the absence of extreme events like hurricanes. Oxygen balance between vertical mixing and total oxygen demand was considered for bottom water from which oxygen demand and diffusive oxygen flux were estimated. The estimated decay rates at 20°C ranging between 0.175–0.322 d−1 and the corresponding oxygen consumption as large as 7.4 g O2 m−2 d−1 fall at the upper limit of previously reported ranges. The diffusive oxygen flux and the corresponding vertical diffusivity estimated for well mixed conditions range between 8.6–9.5 g O2 m−2 d−1 and 2.6–2.9 m2 d−1, respectively. Mobile Bay hypoxia is likely to be associated with a large oxygen demand, supported by both water column and sediment oxygen demands, so that oxygen supply from surface water during destratification events would be quickly exhausted to return to hypoxic conditions within a few hours to days after destratification events are terminated.  相似文献   

7.
Increased nutrient loadings have resulted in low dissolved oxygen (DO) concentrations in bottom waters of the Patuxent River, a tributary of Chesapeake Bay. We synthesize existing and newly collected data to examine spatial and temporal variation in bottom DO, the prevalence of hypoxia-induced mortality of fishes, the tolerance of Patuxent River biota to low DO, and the influence of bottom DO on the vertical distributions and spatial overlap of larval fish and fish eggs with their gelatinous predators and zooplankton prey. We use this information, as well as output from watershed-quality and water-quality models, to configure a spatially-explicit individual-based model to predict how changing land use within the Patuxent watershed may affect survival of early life stages of summer breeding fishes through its effect on DO. Bottom waters in much of the mesohaline Patuxent River are below 50% DO saturation during summer. The system is characterized by high spatial and temporal variation in DO concentrations, and the current severity and extent of hypoxia are sufficient to alter distributions of organisms and trophic interactions in the river. Gelatinous zooplankton are among the most tolerant species of hypoxia, while several of the ecologically and economically important finfish are among the most sensitive. This variation in DO tolerances may make the Patuxent River, and similar estuaries, particularly susceptible to hypoxia-induced alterations in food web dynamics. Model simulations consistently predict high mortality of planktonic bay anchovy eggs (Anchoa mitchilli) under current DO, and increasing survival of fish eggs with increasing DO. Changes in land use that reduce nutrient loadings may either increase or decrease predation mortality of larval fish depending on the baseline DO conditions at any point in space and time. A precautionary approach towards fisheries and ecosystem management would recommend reducing nutrients to levels at which low oxygen effects on estuarine habitat are reduced and, where possible, eliminated.  相似文献   

8.
Maryland commercial landings of the blue crab (Callinectes sapidus) and catch per unit effort (CPUE) have remained fairly stable over the past 33 yr despite occasional large deviations from the long-term average. During this time, however, significant declines in the percent of legal male crabs and the mean size of legal males caught in fishery-independent surveys near Calvert Cliffs, Maryland have become apparent. Sublegal females and two of the three legal female classes (152–177 and >178 mm CW) showed no significant trends over this 33-yr period when examined by linear regression. Males showed significant trends for all size classes. Sublegal males increased from 24% of the male population during the first 5 yr of the study (1968–1972) to 71% during 1996–2000. All classes of legal males, however, exhibited downward trends. Males 127–151 mm CW decreased from 45% of the male population in the earliest period to 22% during the last 5 yr. Males 152–177 mm decreased from 27% during 1968–1972 to only 6% during 1996–2000, and males > 178 mm declined from 4% in the earliest period to 0.5% in the recent period. These size decreases for the most valuable portion of the blue crab population are further evidence of over-exploitation. The declining trends in male size indicate that growth overfishing is occurring as intense fishing pressure removes so many male crabs from the population as they reach legal size that few remain to molt to larger size. A 3-yr data set from the Patuxent River, where commercial use of crab pots is restricted and fishing pressure is lower, suggests that legal male crabs are able to attain larger size compared to an area where the pot fishery is intense. A recommendation could be made for reducing effort where the pot fishery is intense by means of time, gear, catch limits, and/or by increasing the minimum size of legal crabs to allow larger crabs to enter the fishery.  相似文献   

9.
A comparative study of the standing crop of marsh vegetation was made of the Patuxent River and Parker Creek, two tributaries of Chesapeake Bay. The biomass of marsh vegetation in the tidal freshwater and brackish regions of the Patuxent was relatively uniform with regard to salinity, seasonally high concentrations of dissolved nitrogen, and phosphorus and nutrient gradient. Maximum values of biomass occurred in the tidal freshwater and slightly brackish water region of Parker Creek, a system whose nutrient concentrations approximated 20% of those of Patuxent River. Biomass values for the Patuxent River and Parker Creek averaged about 1417 and 895 g m?2 dry weight, respectively. Estimates of total annual marsh production based on the maximum standing crop was 27×103 and 519 metric tons, respectively, for the Patuxent River and Parker Creek.  相似文献   

10.
On August 13, 2004, Hurricane Charley came ashore in the Charlotte Harbor watershed. Surface winds at the time of landfall were estimated at 130 knots. The track of the hurricane roughly followed the floodplain of the Peace River, causing massive defoliation and mortality of native vegetation and planted citrus groves, as well as substantial damage to human habitation and various infrastructure elements. Eight days after landfall, a water quality monitoring effort documented hypoxic (<2 mg I−1) to nearly anaerobic (<0.5 mg I−1) dissolved oxygen (DO) values throughout the vast majority of the Peace River's c. 6,000 km2 watershed. Low DO values appeared to be related to high values of both dissolved organic matter and suspended materials. Hypoxic conditions in Charlotte Harbor itself, occurred within 2 wk of landfall. Approximately 3 wk after the landfall of Hurricane Charley, Hurricane Frances struck the east coast of Florida, causing further wind damage and bringing substantial amounts of rain to the Charlotte Harbor watershed. Three weeks later still, Hurricane Jeanne caused similar damage to the same area. In response to the combined effects of these three hurricanes, DO values in the Peace River did not recover to pre-hurricane levels until approximately 2–3 mo later. The spatial and temporal pattern of DO fluctuations appeared to be related to the proximity of sampling locations to the path of the eyewall of the first of the three hurricanes. Within the Harbor itself, the duration of hypoxic conditions was less than that recorded within the Peace River, perhaps reflecting greater dilution of oxygen-poor waters from the watershed with less-affected water from the Gulf of Mexico.  相似文献   

11.
A diel biogeochemical study was performed to assess the influence that periods of elevated biological activity have on the biogeochemical cycling of macronutrients and redox-sensitive elements in a natural estuarine environment. High-resolution data (15 min sampling) illustrates periodic extreme variations in dissolved oxygen (DO) in the shallow waters of Azevedo Pond, Elkhom Slough, California. During periods of low tidal flushing, DO values can range from highly oxic (>560 μM O2: >250% saturation) during sunny days to suboxic conditions (<5 μM) at night. Nutrient cycling and redox-sensitive trace element biogeochemistry were evaluated in response to the extreme daily DO fluctuations. A diel sampling study was conducted over a 26-h period, where O2 concentrations ranged from 346 μM to sustained non-detectable levels in the night hours. In concert with the DO fluctuations, diel phosphate cycling was on the order of 4 μM in response to tidal flushing events and biological assimilation and regeneration. The IO3 −/I− redox couple quickly responded to suboxic conditions in the water column by a marked increase in I− concentrations and corresponding depletion of IO3 −. The extreme fluctuations of the p∈ in the water column resulted in diel dissolved Mn2+ variations of nearly 5 μM, with observed dissolved Mn removal rates on the order of 1 μM h−1. The elevated biogeochemical cycling of oxygen, nitrogen, phosphorus, iodine, manganese, and iron found in this shallow estuarine environment suggest that tidal restrictions and anthropogenic nutrient enrichments can amplify diel variations and potentially hinder the functional and ecological stability of these systems. These data suggest that accurate chemical monitoring of the health of an estuarine ecosystem must account for the diel variability inherent in these highly productive environments.  相似文献   

12.
We linked a 2-dimensional water quality model of the Patuxent River with a spatially-explicit model of fish growth to simulate how changes in land use in the Patuxent River Basin would affect the growth rate potential (GRP) of Atlantic menhaden (Brevoortia tyrannus). Simulations of three land-use patterns that reflected current nutrient loadings, increased nutrient loadings, and decreased nutrient loadings were used to drive the water quality model. Changes in nutrient loadings caused changes in the timing and intensity of phytoplankton concentrations and the region of hypoxia increased during summer with increased nutrient loading. The spatial distribution of menhaden GRP was highly correlated with phytoplankton concentrations and localized in the middle on third of the Patuxent River. Menhaden growth rate was highest in early June and late summer. During June, menhaden GRP (and phytoplankton concentration) was lowest at the lower nutrient loading simulation. During late summer, mean menhaden growth rates were inversely proportional to nutrient loading rates and menhaden grew best when nutrient loadings were the lowest. Upriver to mid-river phytoplankton patches drove overall mean calculations. Model results suggest that more research is needed on water quality model predictions of phytoplankton levels at a high level of spatial and temporal resolution, menhaden foraging, and menhaden habitat selection.  相似文献   

13.
A sulfur hexafluoride (SF6) tracer release experiment was conducted in the Stockton Deep Water Ship Channel (DWSC) to quantify mixing and transport rates. SF6 was injected in the San Joaquin River upstream of the DWSC and mapped for 8 days. From the temporal change in SF6 distributions, the longitudinal dispersion coefficient (K x ) was determined to be 32.7 ± 3.6 m2 s−1 and the net velocity was 1.75 ± 0.03 km day−1. Based on the decrease in SF6 inventory during the experiment, the pulsed residence time for waters in the DWSC was estimated at ∼17 days. Within the DWSC from Stockton downstream to Turner Cut, dissolved oxygen concentrations maintained a steady state value of 4 mg l−1. These values are below water quality objectives for the time of year. The low flow rates observed in the DWSC and the inability of oxygen-rich waters from downstream to mix into the DWSC upstream of Turner Cut contribute to the low dissolved oxygen concentration.  相似文献   

14.
We report integrated measurements of sediment oxygen consumption (SOC) and bottom water plankton community respiration rates (WR) during eight cruises from 2003 to 2007 on the Louisiana continental shelf (LCS) where hypoxia develops annually. Averaged by cruise, SOC ranged from 3.9 to 25.8 mmol O2 m−2 day−1, whereas WR ranged from 4.1 to 10.8 mmol O2 m−3 day−1. Total below-pycnocline respiration rates ranged from 46.4 to 104.5 mmol O2 m−2 day−1. In general, below-pycnocline respiration showed low variability over a large geographic and temporal range, and exhibited no clear spatial or inter-annual patterns. SOC was strongly limited by dissolved oxygen (DO) in the overlying water; whereas, WR was insensitive to low DO, a relationship that may be useful for parameterizing future models. The component measures, WR and SOC, were similar to most prior measurements, both from the LCS and from other shallow estuarine and coastal environments. The contribution of SOC to total below-pycnocline respiration averaged 20 ± 4%, a finding that differs from several prior LCS studies, but one that was well supported from the broader estuarine and oceanic literature. The data reported here add substantially to those available for the LCS, thus helping to better understand oxygen dynamics on the LCS.  相似文献   

15.
We examined high frequency fluctuations in water quality parameters in two tropical coastal plain estuaries in response to changing tidal flow conditions. The variability in total suspended sediments (TSS), volatile suspended solids (VSS), total organic carbon (TOC) concentrations, and indicators of water quality, including pH, temperature, salinity, and dissolved oxygen, over one spring tidal cycle during the early wet monsoon season was measured in two estuaries in eastern Sumatra. The relatively high rainfall experienced throughout the year, in combination with the recent extensive vegetation clearing and modification of the landscape, resulted in significant concentrations of TSS, VSS, and TOC being discharged to coastal waters. Maximum values are reached on the ebb tide (TSS > 1,013 mg l−1; VSS > 800 mg l−1; TOC >60 mg l−1). The influence of freshwater discharge and tidal flow on water properties of the lower estuaries is also marked by the variability in salinity, dissolved oxygen, and pH over the tidal cycle, with minimum values for each of these parameters following maximum current velocities and after the completion of the strong ebb tide. Estimation of seaward sediment fluxes, which are of significant interest in a region where rapid environmental change is occurring, would require further examination of sedimentary processes, such as resuspension and advection of sediment, as well as a consideration of neap-spring tidal variations and the effect of seasonality on estuarine circulation.  相似文献   

16.
Water quality in the Northern part of Mellegue-Medjerda watershed (East Algeria) has been adversely affected by important pollutants discharged into the Medjerda wadi without, in most cases, any treatment. Chemical and physical degradation are due to agricultural and industrial practices and domestic wastewaters. Over a three-month period, a study of the low-flow water quality characteristics throughout Medjerda wadi was undertaken. Longitudinal profiles of water quality were constructed using data from fourteen sites. All sewage, agricultural, and industrial inputs were included. Analyzed properties were nutrients (NO3 −, NO2 −, NH4 +, and PO4 3−), Biochemical oxygen demand after five days (BOD5), chemical oxygen demand (COD), and dissolved oxygen (DO). Along Medjerda wadi, all values change because of conditions specific to each sampling station. Nitrate was the most important form of nitrogen-element load (94%). Its concentration reached 34.3 mg L−1 at OM4 point, downstream of domestic wastewater discharges. The spatial evolution of the organic pollution index (OPI) shows that the wastewater effluent constitutes the main source of pollution. Indeed, water quality goes from a moderate pollution state at some sampling stations not or slightly affected by wastewaters discharges to a very strong pollution state (OPI of about 1.75) downstream of the domestic effluents inputs of Souk-Ahras city.  相似文献   

17.
Sources of oxygen demand in the lower San Joaquin River,California   总被引：1，自引：0，他引：1  
Dissolved oxygen concentration below 5 mg 1−1 has characterized the lower tidal portion of the San Joaquin River downstream of Stockton, California, during the summer and fall for the past four decades. Intensive field research in 2000 and 2001 indicated low dissolved oxygen concentration was restricted to the first 14 km of the river, which was deepened to 12 m for shipping, downstream of Stockton. The persistent low dissolved oxygen concentration in the shipping channel was not caused by physical stratification that prevented aeration from vertical mixing or respiration associated wigh high phytoplankton biomass. The low dissolved oxygen concentration was primarily caused bynitrification that produced up to 81% of the total oxygen demand. Stepwise multiple regression analysis isolated dissolved ammonia concentration and carbonaceous oxygen demand as the water quality variables most closely associated with the variation in oxygen demand. Between these two sources, dissolved ammonia concentration accounted for 60% of the total variation in oxygen demand compared with a maximum of 30% for carbonceous oxygen demand. The Stockton wastewater treatment plant and nonpoint sources upstream were direct sources of dissolved ammonia in the channel. A large portion of the dissolved ammonia in the channel was also produced by oxidation of the organic nitrogen load from upstream. The phytoplankton biomass load from upstream primarily produced the carbonaceous oxygen demand. Mass balance models suggested the relative contribution of the wastewater and nonpoint upstream load to the ammonia concentration in the shipping channel at various residence times was dependent on the cumulative effect of ammonification, composition of the upstream load, and net downstream transport of the daily load.  相似文献   

18.
The northern Gulf of Mexico (NGOMEX) was surveyed to examine the broad-scale spatial patterns and inter-relationships between hypoxia (<2?mg?L?1 dissolved oxygen) and zooplankton biovolume. We used an undulating towed body equipped with sensors for conductivity, temperature, depth, oxygen, fluorescence, and an optical plankton counter to sample water column structure, oxygen, and zooplankton at high spatial resolution (1?m??vertical; 0.25?C1?km??horizontal). We contrast the distribution of zooplankton during summer surveys with different freshwater input, stratification, and horizontal and vertical extent of bottom-water hypoxia. Bottom-water hypoxia did not appear to influence the total amount of zooplankton biomass present in the water column or the areal integration of zooplankton standing stock in the NGOMEX region surveyed. However, where there were hypoxic bottom waters, zooplankton shifted their vertical distribution to the upper water column during the day where they normally would reside in deeper and darker waters. When bottom waters were normoxic (>2?mg?L?1 dissolved oxygen), the daytime median depth of the water column zooplankton was on average 7?m deeper than the median depth of zooplankton in water columns with hypoxic bottom waters. A reduction in larger zooplankton when there were hypoxic bottom waters suggests that if zooplankton cannot migrate to deeper, darker water under hypoxic conditions, they may be more susceptible to size-selective predation by visual predators. Thus, habitat compression in the northern Gulf of Mexico due to hypoxic bottom water may have implications for trophic transfer by increasing the contact between predators and prey.  相似文献   

19.
Suquía River is a medium-sized hydrological system (basin area of ~7,700 km2) that supplies fresh water to Córdoba city, a town of ~1,500,000 inhabitants in central Argentina. This paper examines the present-day hydrochemistry of Suquía River urban catchment analyzing its major and minor dissolved components, and the nutrients variability by means of QUAL-2K modeling software. The Suquía River has bicarbonate-type waters upstream the city and sulfate-type waters right downstream, whereas they exhibit a mixed-to-alkali-type cationic composition. The seasonal analysis of its major dissolved constituents clearly showed a dilution process during the wet season (i.e. austral summer). In the last 20 years, the Suquía River has modified its anionic composition, now showing higher relative concentrations of SO4 2− as a consequence of urban activities. However, trace elements dissolved concentrations do not evidence a strong pollution effect. Nutrients [nitrogen species, total phosphorous (TP)] and related parameters, such as biochemical oxygen demand (BOD), and dissolved oxygen (DO), evidence a clear influence of human activities. The QUAL-2K model was used to evaluate the spatial behavior of selected nutrients and associated variables, (i.e. TP, N–NH4 +, N–NO3 −, DO, BOD). Nutrient concentrations are affected by point sources of contaminants, particularly domestic waste and sewage, as well as by diffuse agricultural pollution. A calibrated QUAL-2K modeling exercise clearly shows the impact of the Córdoba city’s municipal wastewater treatment plant on the Suquía River water quality.  相似文献   

20.
A 16-month study of estuarine habitats in poly-, meso-, and oligohaline salinity regimes near Charleston Harbor assessed the distribution and abundance of megalopae and early crab stages of the blue crab,Callinectes sapidus. Blue crab were sampled with a plankton net and a cylindrical drop sampler. Blue crab were most abundant in plankton collections at night, accounting for 68% of the megalopae and over 88% of the juveniles collected in day and night tows combined. At night, densities of megalopae were greatest in surface samples, whereas densities in daylight collections were greater on the bottom. Juvenile densities were greatest on the bottom in both day and night collections, although catch rates at night were more variable than those of the megalopae. This suggests that megalopae, and possibly juvenile stages, experience a diel vertical migration. Results indicate that ingress to estuarine nursery areas occurs at the megalopal stage. Megalopal densities were highest at the polyhaline site, while juvenile blue crab were most abundant in the oligohaline area. Habitat utilization by juvenile blue crab was estimated using a cylindrical drop sampler and Venturi suction pump on three bottom types in the intertidal zone. Densities were greatest over the sandy-mud substrate, although catch rates were much lower than those reported for other geographical areas. These results suggest that juvenile blue crab do not occur in abundance on the marsh surface but remain on the creek bottom, possibly because creek physiography and large tidal amplitudes may restrict accessibility to the marsh surface.  相似文献   

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