Tributary Estuaries Research Articles

Effects of Heavy Metal and Nutrients on Benthic Microbial Communities in Freshwater Sediment of Poyang Lake (China)

The aim of this work was to relate sediment microbial communities with geochemical processes in a freshwater lake. Surface sediment samples were collected from seven sites (i.e., main basin and tributary estuaries of the lake) at Poyang Lake, China in May of 2011. Sediment environmental parameters such as concentration of nutrients and total contents of heavy metals (copper, zinc, lead, and cadmium) were measured. Profiles of bacterial communities were generated using DGGE (denaturing gradient gel electrophoresis). Pearson correlation analysis and CCA (canonical correspondence analysis) were employed to determine effects of sediment properties on benthic microbial communities. Experimental results showed that marginal and compound pollutions, tolerable for most benthic organisms, were found in sediments of Poyang Lake according to contents of nutrients and heavy metals. However, pollutions was severe in Rao and Xin River estuaries and especially copper pollution. Also, bacterial communities in the sediments had apparently horizontal heterogeneity and genotypic richness and diversity of microbial communities were predicted by sediment nutrients. Finally, total phosphorus (TP), heavy metal copper (Cu), and cadmium (Cd) in sediment were considered to be key factors driving spatial variation of bacterial community structures in Poyang Lake. This study suggested heavy metals together with phosphates may jointly affect benthic microbial communities and metal-reduction was an important microbial activity in freshwater sediment.

Geomorphological Variables that Determine the Magnitude of Stream Floods in Wadi Al-Hasa - South Jordan

The study aims to analyze the geomorphological variables that determine the magnitude of stream floods in Wadi Al-Hasa through the analysis of the characteristics of rainfall; the shape and areal dimensions of the basin; the topography of the surface; the gradient of the Valley; the characteristics of water discharge network; and the morphology and capacity of the channel. To achieve the objectives of the study, the Analytical Approach was adopted when examining stream floods. This approach is based on the analysis of the climatic information and the hydrological data available on stream floods in Wadi Al-Hasa; and the analysis of the morphometric, hydrological and vegetation measurements that have been extracted from the topographic maps, aerial photographs and field work. The study found several results including: the increasing likelihood of frequent flooding in the sub-reaches of Wadi Al-Hasa with the increase in their stream orders, as a result of the increase in the amount of water discharge. Moreover, the results of the statistical analysis of the stepwise regression showed the importance of the following variables: (the stream order, the gradient, the width of the river channel and vegetation) in explaining the variation in the volume of the water discharge of the river tributaries in the basin, as these variables interpreted about (53%) of the total variation. The study also found that there are three areas with increased likelihood of flooding in the basin, namely: the estuaries of the eastern tributaries in Qa' Al-Jinz area; the areas where western tributaries meet with higher stream orders; and the estuary area of the mainstream of ??Valley.

Study on Characteristics of Hydrodynamic and Pollutant Transport of the Tributary Estuary in the Three Gorges Reservoir Area

To further clarify the impact of construction of the Three Gorges Reservoir on its tributaries. In this study, a three-dimensional hydrodynamic model of Changshou-Fuling section of Yangtze River was developed based on EFDC. Then an analysis of the hydrodynamics and the long term mass transport behaviors under backwater effect in the Taohuaxi River and Longxi River was presented. The results show that mainstream backwater effect plays a significant role in tributary during low-water period. And distinct circulation was shown in tributary estuary. However, in high-water period, estuarine circulation area in Longxi River decreased and disappeared, while the one in Taohuaxi River is still evident. Different input action from mainstreams to tributaries has been discovered.

Modeling the Factors Controlling Phytoplankton in the St. Louis Bay Estuary, Mississippi and Evaluating Estuarine Responses to Nutrient Load Modifications

Several biochemical and physical factors regulate phytoplankton primary productivity and algal bloom events in estuarine environments. Some of the most important factors include nitrogen, phosphorus and silica availability, light availability, and estuarine flushing potential. A better understanding of these processes is necessary to support sound management strategies that take into account the hydrological, hydraulic, and biochemical connectivity between estuaries and their watersheds. In this paper the factors controlling phytoplankton productivity in a tributary estuary of the northern Gulf of Mexico, the St. Louis Bay estuary, Mississippi, and the system responses to nutrient load alterations are studied. For this purpose a coupled hydrodynamic and water quality model based on U.S. EPA computer models was implemented. The writers present an evaluation of the model predictive capacity, and its implementation to study the processes controlling phytoplankton dynamics, nutrient cycling, and oxygen availability. The results suggest that primary productivity is limited by nitrogen availability. Under current nutrient load conditions the concentrations of nitrogen fall below 0.025 mg/L, and the rates of primary productivity vary between 0.5 and 1.8 gO/m2/day with a mean of 0.65 gO/m2/day. The simulation of four different nutrient load scenarios suggests a low sensitivity of the estuary to the evaluated conditions in the watershed. The results also indicate that the coupled model is a cost-effective strategy to investigate the impacts of different management strategies. At a regional scale, the implemented approach can contribute to the design of integrated strategies to reduce hypoxia and algal bloom problems in the Gulf of Mexico.

Study on Evolvement of High Beach and Deep Channel of Reservoir in the Sediment-Laden Rivers

According to datas of Sanmenxia Reservoir and physical model test of Xiaolangdi Reservoir during later sediment retaining period, the paper study evolvement of high beach and deep channel of reservoir in the sediment-laden rivers. The results show that: 1The evolution of high beach and deep channel is as follows, firstly, beach and channel increase synchronously , and then scour happens in channel. Channel scour and scour amount is crucial to form high beach and deep channel. 2It is found that tributaries were equivalent to the horizontal extension of mainstream riverbed. The deposition process of tributary is closely related to many factors of natural topographic condition, deposition morphology of mainstream at the confluence area of mainstream and tributary and so on. The cross-section of tributary kept uplift horizontally, and sand bar has formed in some tributary estuary. 3Rainfall erosion formed in flood period is beneficial to forming hyper-concentrated flow, to a certain extent; it can recover channel storage capacity, and play an important role on utilizing long-term comprehensive benefits. Studying evolvement of high beach and deep channel can supply some technical supports for regulating operational mode of Xiaolangdi Reservoir during later sediment retaining period and prolonging service life.

From Headwaters to Coast: Influence of Human Activities on Water Quality of the Potomac River Estuary

The natural aging process of Chesapeake Bay and its tributary estuaries has been accelerated by human activities around the shoreline and within the watershed, increasing sediment and nutrient loads delivered to the bay. Riverine nutrients cause algal growth in the bay leading to reductions in light penetration with consequent declines in sea grass growth, smothering of bottom-dwelling organisms, and decreases in bottom-water dissolved oxygen as algal blooms decay. Historically, bay waters were filtered by oysters, but declines in oyster populations from overfishing and disease have led to higher concentrations of fine-sediment particles and phytoplankton in the water column. Assessments of water and biological resource quality in Chesapeake Bay and tributaries, such as the Potomac River, show a continual degraded state. In this paper, we pay tribute to Owen Bricker’s comprehensive, holistic scientific perspective using an approach that examines the connection between watershed and estuary. We evaluated nitrogen inputs from Potomac River headwaters, nutrient-related conditions within the estuary, and considered the use of shellfish aquaculture as an in-the-water nutrient management measure. Data from headwaters, nontidal, and estuarine portions of the Potomac River watershed and estuary were analyzed to examine the contribution from different parts of the watershed to total nitrogen loads to the estuary. An eutrophication model was applied to these data to evaluate eutrophication status and changes since the early 1990s and for comparison to regional and national conditions. A farm-scale aquaculture model was applied and results scaled to the estuary to determine the potential for shellfish (oyster) aquaculture to mediate eutrophication impacts. Results showed that (1) the contribution to nitrogen loads from headwater streams is small (about 2 %) of total inputs to the Potomac River Estuary; (2) eutrophic conditions in the Potomac River Estuary have improved in the upper estuary since the early 1990s, but have worsened in the lower estuary. The overall system-wide eutrophication impact is high, despite a decrease in nitrogen loads from the upper basin and declining surface water nitrate nitrogen concentrations over that period; (3) eutrophic conditions in the Potomac River Estuary are representative of Chesapeake Bay region and other US estuaries; moderate to high levels of nutrient-related degradation occur in about 65 % of US estuaries, particularly river-dominated low-flow systems such as the Potomac River Estuary; and (4) shellfish (oyster) aquaculture could remove eutrophication impacts directly from the estuary through harvest but should be considered a complement—not a substitute—for land-based measures. The total nitrogen load could be removed if 40 % of the Potomac River Estuary bottom was in shellfish cultivation; a combination of aquaculture and restoration of oyster reefs may provide larger benefits.

Modern state of zooplankton in Vasyugan River

This paper studies the specific diversity and quantitative indicators of zooplankton of the Vasyugan River in the middle and lower reaches, as well as in the estuaries of major tributaries. The dominant zooplankton complex is noted to have changed according to 50- and 10-year-old data. The succession of the plankton community is taking place, which is possibly due to global warming and the restructuring of the entire Vasyuganye ecosystem. No oil pollution has been detected in the river by bioindication methods.

Internal versus external drivers of periodic hypoxia in a coastal plain tributary estuary: the York River, Virginia

The formation of periodic hypoxia within tributary estuaries, and its relationship to the spring-neap tidal cycle, has been well documented in several systems along the US east coast. However, the importance and scale of other physical and biological processes, which ultimately control the frequency and spatial extent of hypoxia, are less well understood. This study synthesized in situ measurements, metabolic incubations, and high-resolution water quality monitoring into a spatially explicit, temporally integrated mass balance to examine the significance of multiple organic matter sources and oxygen sinks in relation to hypoxia in the York River estuary (YRE), Virginia, USA. Results highlight episodic peaks in gross primary production (GPP) mostly unrelated to the spring-neap cycle, with phytoplankton accounting for the bulk of total GPP. Despite extensive shoals, microphytobenthos contributed under 20% and typically under 10% of total GPP. Respiration rates were sufficient to drive bottom waters to hypoxia during transitions from spring to neap tides. While GPP generally appeared to be relatively balanced with respiration, results indicated an area at the mesohaline–polyhaline boundary that was net heterotrophic. Phytoplankton production dominated the estimated inputs of organic carbon from the tributaries and surrounding watersheds, and was 1.5 times greater than inputs from the Chesapeake Bay, which roughly balanced exports. Management efforts to alleviate hypoxia should focus on reducing internal phytoplankton production, although inputs of labile organic matter from the Chesapeake Bay represent an important source that can only be controlled by more regional efforts.

Water Level and Velocity Characteristics of a Salt Marsh Channel in the Murderkill Estuary, Delaware

ABSTRACT Dzwonkowski, B.; Wong, K.-C., and Ullman, W.J., 2014. Water level and velocity characteristics of a salt marsh channel in the Murderkill estuary, Delaware. High-frequency observations of water level and velocity over one year in Murderkill estuary, a tributary estuary of Delaware Bay, are used to examine changes in tidal and subtidal flow characteristics as water propagates from the mouth of the estuary (Bowers site) into a minor channel that connects to a contiguous salt marsh (channel site), a setting where long-term continuous data sets are uncommon. These data provide insight into the flow behavior in marsh channels, the driving mechanisms of water exchange, and the potential for particle and solute exchange between tidal marshes and their adjacent estuaries. At both sites, tidal forcing is normally the dominant driving mechanism in water level and velocity signals with remote wind forcing having a limited contribution. As the tidal signal propagates from the Bowers site into the salt marsh, ...

Habitat partitioning by fish larvae among coastal, offshore and frontal zones in the southern North Sea

This study compared marine and estuarine larval fish assemblages among near- shore, offshore and frontal habitats of the southern North Sea. In late June and early July 2005, parallel cruises collected larvae along offshore transects crossing tidal mixing and river plume frontal zones in the German Bight, and at shallow nearshore stations within the Wadden Sea and associated estuaries of major tributaries. A mixture of larvae from 17 species occurred including the (then) newly re-established 'southern' clupeids (European sardine Sardina pilchardus and anchovy Engraulis encrasicolus) as well as traditional North Sea species such as horse mackerel Trachurus trachurus, solenette Buglossidium luteum and various species of gobies. Multivariate statistics on taxonomic composition indicated that shallow and deep areas were 2 distinct habitats. Wadden Sea stations had a conspicuously different species composition and low species richness compared to all deeper areas. In deeper waters, species composition was similar regardless of frontal strength, and larger larvae occurred most frequently in the cooler areas of the German Bight. Altered species composition observed at some nearshore stations indicated that frontal dynamics and tidal advection may lead to expatriation of larval fish to presumably less favorable areas. Frontal habitats in the German Bight may not necessarily promote higher survival of larval fish than other areas but appear to form important boundaries between species-rich, deeper waters of the German Bight and relatively species-poor, shallower waters of the Wadden Sea.

Distribution and accumulation of heavy metals in the sediments of Kaohsiung Harbor, Taiwan

The distribution, enrichment, and accumulation of heavy metals in the sediments, especially those at the vicinity of tributary estuaries of Kaohsiung Harbor, Taiwan were investigated. Sediment samples from six locations in the Kaohsiung Harbor were collected quarterly in the period from 2002 to 2005 and characterized for metal content (e.g., Hg, Pb, Cd, Cr, Cu, Zn and Al), water content, organic matter, total nitrogen, total phosphorous, total grease, and grain size. Results showed that metal concentrations varied from 0.58 mg kg −1 for Cd to 596 mg kg −1 for Zn. Metal concentrations at the vicinity of river mouths were higher than those at other locations. All heavy metals studied, except Cr, had relatively high enrichment factors and geo-accumulation indices in the estuaries. Moreover, metal concentrations correlated closely to the physical-chemical properties of the sediments, which strongly suggested the influence of industrial and municipal wastewaters discharged from the neighboring industrial parks and river basins. Results would help develop strategies for pollution control and sediment remediation of Kaohsiung Harbor.

Ferry-based monitoring of surface water quality in North Carolina estuaries

Several interrelated factors affect, water quality in the Albemarle-Pamlico Estuarine System (APES) including land use change in the upland and coastal watersheds, legislatively mandated basin-wide nutrient management plans, intense storms, and global and local changes in sea level. Despite its importance as an essential fish habitat, the APES has not been monitored as intensively or extensively for habitat impacts associated with decreased water quality as other estuaries have been, such as with the North Carolina tributary estuaries or Chesapeake Bay. To support the sustainable use of these estuaries, we are developing an automated water quality monitoring system aboard ferries that traverse the APES. This program, FerryMon, provides a unique, long-term, and cost-effective monitoring system to evaluate status and trends in APES water quality. Intensive temporal and spatial data obtained from all ferry routes provide an environmental baseline and are used to assess the patterns and variability in surface water hydrography, dissolved constituents, and particulate matter. The data are useful to calibrate estimates of ocean color and sea surface temperature from aircraft and satellite sensors. We are creating a searchable geographic database that is intended for scientists, managers, and the general public. Using ferries as sampling platforms to monitor estuarine water quality is a tractable approach and FerryMon represents a model for use in other large bodies of water traversed by ferries.

On the estuarine transport reversal in deep layers of the Gulf of Finland

The Gulf of Finland is a 400-km long and 48–135-km wide tributary estuary of the Baltic Sea featuring the longitudinal two-layer estuarine flow modified by transverse circulation. Longitudinal volume transport in the deep layer is investigated by decomposing it into an averaged, slowly changing estuarine component (due to large-scale density gradients, river discharge and mean wind stress) and wind-driven fluctuating component. The derived expression relates the total deep-layer transport to the projection of wind stress fluctuation to a site-specific direction. The relationship is tested and calibrated by the results from numerical experiments carried out with the three-dimensional baroclinic circulation model. For the entrance to the Gulf of Finland, winds from northeast support standard estuarine circulation and winds from southwest work against the density-driven and riverine flow. The deep estuarine transport may be reversed if the southwesterly wind component exceeds the mean value by 4–5.5 m s −1. According to the data from hydrographic observations in the western Gulf of Finland, an event of advective halocline disappearance was documented in August 1998. Comparison of the deep-water transport estimates calculated from the wind data in 1998 with the observed salinity variations showed that the events of rapid decay of estuarine stratification were coherent with the estimated reversals of deep-layer volume transport, i.e. events of salt wedge export from the gulf.

Estimation of Net Physical Transport and Hydraulic Residence Times for a Coastal Plain Estuary Using Box Models

A box model based on salinity distributions and freshwater inflow measurements was developed and used to estimate net non-tidal physical circulation and hydraulic residence times for Patuxent River estuary, Maryland, a tributary estuary of Chesapeake Bay. The box model relaxes the usual assumption that salinity is at steady-state, an important improvement over previous box model studies, yet it remains simple enough to have broad appeal. Average monthly 2-dimensional net non-tidal circulation and residence times for 1986–1995 are estimated and related to river flow and salt water inflow as estimated by the box model. An important result is that advective exchange at the estuary mouth was not correlated with Patuxent River flow, most likely due to effects of offshore salinity changes in Chesapeake Bay. The median residence time for freshwater entering at the head of the estuary was 68 d and decreased hyperbolically with increasing river flow to 30 d during high flow. Estimates of residence times for down-estuary points of origin showed that, from the head of the estuary to its mouth, control of flushing changed from primarily river flow to other factors regulating the intensity of gravitational circulation.

Corrigendum: Downstream Effects of Water Withdrawal in a Small, High-Gradient Basin: Erosion and Deposition on the Skokomish River Delta

The purpose of this paper is to analyze downstream effects of freshwater flow diversion from a small, active-continental-margin river basin. The Skokomish River delta is a tributary estuary to Hood Canal in Washington state that receives drainage from the southeastern side of the Olympic Mountains. Its drainage basin is steep, and rainfall is high. Approximately 40% of the annual average runoff of the entire system has been diverted from the North Fork Skokomish River for power production since completion of two dams in 1930; this water does not pass through the lower river or over the delta. Extensive logging has occurred in the remainder of the basin. Comparison of prediversion (1885) and postdiversion (1972) bathymetric surveys shows that deposition (about 0.013 to 0.022 m yr−1) has occurred on the inner delta and erosion (up to 0.033 m yr−1) on the outer delta. This steepening of the delta surface has apparently been caused by a loss of sediment transport capacity in the lower river and estuary combined with an increased sediment supply due to logging. Although the total area of unvegetated tidal flats thas decreased by only about 6%, there has been a more than 40% loss of highly productive low intertidal surface area. A conservative estimate of loss of eelgrass (Zostera marina) beds is 18%; a reduction in the size of mesohaline mixing zone has also occurred. These habitat losses are similar to those observed elsewhere in the world in larger river basins that have suffered water withdrawals of the same magnitude, but their impacts cannot either be evaluated or understood causally through consideration of simple measures like changes total estuarine deltaic area. Evaluation of estuarine effects of anthropogenic modification must, therefore, include consideration of both changes in habitat function and in the physical processes. These must be evaluated within the totality of the river basin-estuary system that cause these changes. In this case, sediment transport constitutes the critical link between fluvial alterations and the remote downstream, estuarine consequences thereof.

Inputs, Transformations, and Transport of Nitrogen and Phosphorus in Chesapeake Bay and Selected Tributaries

In this paper we assemble and analyze quantitative annual input-export budgets for total nitrogen (TN) and total phosphorus (TP) for Chesapeake Bay and three of its tributary estuaries (Potomac, Patuxent, and Choptank rivers). The budgets include estimates of TN and TP sources (point, diffuse, and atmospheric), internal losses (burial in sediments, fisheries yields, and denitrification), storages in the water column and sediments, internal cycling rates (zooplankton excretion and net sediment-water flux), and net downstream exchange. Annual terrestrial and atmospheric inputs (average of 1985 and 1986 data) of TN and TP ranged from 4.3 g TN m−2 yr−1 to 29.3 g TN m−2 yr−1 and 0.32 g TP m−2 yr−1 to 2.42 g TP m−2 yr−1, respectively. These rates of TN and TP input represent 6-fold to 8-fold and 13-fold to 24-fold increases in loads to these systems since the precolonial period. A recent 11-yr record for the Susquehanna River indicates that annual loads of TN and TP have varied by about 2-fold and 4-fold, respectively. TN inputs increased and TP inputs decreased during the 11-yr period. The relative importance of nutrient sources varied among these estuaries: point sources of nutrients delivered about half the annual TN and TP load to the Patuxent and nearly 60% of TP inputs to the Choptank; diffuse sources contributed 60–70% of the TN and TP inputs to the mainstream Chesapeake and Potomac River. The direct deposition of atmospheric wet-fall to the surface waters of these estuaries represented 12% or less of annual TN and TP loads except in the Choptank River (37% of TN and 20% of TP). We found direct, although damped, relationships between annual rates of nutrient input, water-column and sediment nutrient stocks, and nutrient losses via burial in sediments and denitrification. Our budgets indicate that the annual mass balance of TN and TP is maintained by a net landward exchange of TP and, with one exception (Choptank River), a net seaward transport of TN. The budgets for all systems revealed that inorganic nutrients entering these estuaries from terrestrial and atmospheric sources are rapidly converted to particulate and organic forms. Discrepancies between our budgets and others in the literature were resolved by the inclusion of sediments derived from shoreline erosion. The greatest potential for errors in our budgets can be attributed to the absence of or uncertainties in estimates of atmospheric dry-fall, contributions of nutrients via groundwater, and the sedimentation rates used to calculate nutrient burial rates.

Upland and Coastal Sediment Sources in a Chesapeake Bay Estuary

Abstract Rapid sedimentation in the upper and central portions of tributary estuaries of the Chesapeake Bay typically has been attributed to clearing of land and subsequent upland erosion since European settlement. New data from the South River, Maryland and re-examination of existing literature suggest that coastal erosion may be the dominant process driving sediment inputs along the entire length of many tributary estuaries of the Chesapeake over the past several centuries. Shoreline erosion rates derived from historical shoreline maps and aerial photographs, coupled with information on fluvial inputs obtained from sediment rating curves, indicate that coastal contributions to the estuarine sediment budget of the South River are 4 to 12 times higher than fluvial inputs. Rates of subtidal accumulation estimated from pollen and 210Pb dating of cores and changes in estuarine bathymetry suggest that most sediment from streams is trapped in river mouth marshes and subtidal storage within 1 km of river mouths. Sediment inputs from streams therefore have little direct effect on sediment accumulation in the majority of the South River estuary. These findings imply that existing runoff-based efforts to control estuarine sedimentation also need to include nonstructural measures to reduce shoreline erosion in order to alleviate sediment-related problems in tributary estuaries of the western Chesapeake.

Spatial and Temporal Variabilities of Hypoxia in the Rappahannock River, Virginia

Hypoxia/anoxia in bottom waters of the Rappahannock River, a tributary estuary of Chesapeake Bay, was observed to persist throughout the summer in the deep basin near the river mouth; periodic reoxygenation of bottom water occurred on the shallower sill at the river mouth. The reoxygenation events were closely related to spring tide mixing. The dissolved oxygen (DO) in surface waters was always near or at the saturation level, while that of bottom waters exhibited a characteristic spatial pattern. The bottom DO decreased upriver from river mouth, reaching a minimum upriver of the deepest point of the river and increasing as the water becaume shallower further upriver. A model was formulated to describe the longitudinal distribution of DO in bottom waters. The model is based on Lagrangian concept—following a water parcel as it travels upriver along the estuarine bottom. The model successfully describes the characteristic distribution of DO and also explains the shifting of the minimum DO location in response to spring-neap cycling. A diagnostic study with the model provided insight into relationships between the bottom DO and the competing factors that contribute to the DO budget of bottom waters. The study reveals that both oxygen demand, either benthic or water column demand, and vertical mixing have a promounced effect on the severity of hypoxia in bottom waters of an estary. However, it is the vertical mixing which controls the longitudinal location of the minimum DO. The strength of gravitational circulation is also shown to affect the occurrence of hypoxia. An estuary with stronger circulation tends to have less chance for hypoxia to occur. The initial DO deficit of bottom water entering an estuary has a strong effect on DO concentration near the river mouth, but its effect diminishes in the upriver direction.

Wind‐forced salt intrusion into a tributary estuary

Moored measurements and hydrographic surveys were carried out during the summers of 1986 and 1987 to examine interaction between the mainstem of the Chesapeake Bay and the Choptank River, an eastern shore tributary estuary. The data show that an important mode of interaction is through wind‐forced intrusion of saline, hypoxic water from below the pycnocline of the Bay into the lower river. Intrusions are driven by lateral tilting of the pycnocline in the Bay, when high salinity water is upwelled on the eastern side of the Bay in response to a southward pulse of wind stress. The resulting internal surges propagate up the relict Choptank entrance channel at a speed of about 20 cm/s and spill onto the broad sill inside the mouth of the river. Intrusion‐favorable pycnocline tilts in the Bay do not always result in lower layer intrusion into the Choptank, but may be blocked or choked in the entrance channel on occasion. The data suggest that wind‐forced intrusion of salt leads to increased gravitational circulation in the Choptank during the summer months, providing a mechanism through which high frequency energy may be directly translated into lower frequency motion.

Covariability of dissolved oxygen with physical processes in the summertime Chesapeake Bay

Long, rapidly sampled time series measurements of dissolved oxygen, temperature, salinity, currents, winds, tides, and insolation were colIected during the summer of 1987 across the mesohaline Chesapeake Bay. Analyses of the data show that short term variability of dissolved oxygen was both large and spatialIy heterogeneous. Time scales of variability ranged from the longest period fluctuations resolved (several days) to the sampling interval (several minutes). The largest variability was associated with large amplitude, wind and tide forced lateral internal oscilIations of the pycnocline in the mainstem of the Bay. These resulted in advection of saline, hypoxic water from below the pycnocline onto the flanks of the Bay and into the lower reaches of the Choptank River, an adjoining tributary estuary. Advective variability at higher frequencies was likely due to internal waves, internal mixing, and/or spatial patchiness. Dissolved oxygen also responded to the daily cycle of insolation, but lagged insolation by at least 90° (6 h). Advective variability of dissolved oxygen is implicated as an important characteristic of the majority of summertime benthic environments in the mesohaline Chesapeake Bay and lower reaches of adjoining tributaries.