Spatial variability of bottom types in the lower Chesapeake Bay and adjoining estuaries and inner shelf

The spatial distributions of the bed textural and morphologic properties that influence boundary-layer roughness characteristics in the lower Chesapeake Bay, the lower portions of the York, James and Elizabeth Rivers, and the adjacent inner continental shelf were systematically mapped. A high resolution, fully-corrected side-scan sonar mapping system (100 kHz) was used for remote acoustic detection of bottom roughness, supported by ‘ground-truthing’ by direct in situ observations by divers. These complementary methods proved to be especially effective in detecting a wide range of roughness-controlling bed surface properties at various scales. Fine-scale variations in sediment size and associated bottom texture are considered to be the main source of heterogeneity in Nikuradse (skin friction) roughness. A wide variety of small- and intermediate-scale morphologic elements provide meso-scale and small-scale distributed (form drag) roughness. Depending upon location, the distributed roughness may be either biogenic or hydrodynamically induced (by currents and waves), although anthropogenic roughness prevails in certain instances (e.g. port areas). In terms of particular combinations of roughness scales and types, combined sonar and diver observation data allow the beds to be systematically but qualitatively classified into 10 bottom types, each of which is associated with a particular type of subenvironment.     

Geomorphology of a sand wave in lower Chesapeake Bay,Virginia, U.S.A.

Morphologic and sedimentologic studies of a single sand wave within a sand wave field in the lower Chesapeake Bay suggest that the bedform was originally formed by ebb currents, and is presently in static equilibrium with the circulation pattern. In this report, the concept of solitary sand wave is introduced to describe the state of a sand wave when further evolution of the sedimentary structure is mostly independent of adjacent bedforms. This concept can be applied to several bedforms in the area that are isolated from others by flats. A particular sand wave that is included in this category is discussed.Contribution number 79, Instituto Argentino de Oceanografia.     

Variations in the boundary-drag coefficient in the tidal entrance to Chesapeake Bay,Virginia

Use of the quadratic shear-stress law for estimating boundary drag requires specific knowledge of the magnitude of a drag coefficient, C_D, and sectional mean velocity, $\text{u}\text{?}$. In previous attempts to adapt the relationship for use in studies of marine-sediment transport, the flow measurement has been standardized at a level 100 cm above the bed. The particularized value of the drag coefficient has been designated as C₁₀₀.In the entrance area to Chesapeake Bay, Virginia, C₁₀₀ has been found to range through unacceptably wide limits. Two-thirds of the values obtained are between 3.5 · 10^?3 and 5.4 · 10^?2. Mean C₁₀₀ for the area is 1.3 · 10^?2 as compared to 3 · 10^?3 for tidal channels within Puget Sound, Washington.Present data suggest that, given a moveable bed, a size hierarchy of mobile bed forms, time-varying flow, and a lack of equilibrium between flow and bed, C₁₀₀ changes continuously with boundary shear stress.Accurate evaluation of boundary shear stress in tidal entrances with high flow rates and mobile beds presently requires measurement of velocity profiles.     

Short-term and interannual variability of redox-sensitive chemical parameters in hypoxic/anoxic bottom waters of the Chesapeake Bay

A combination of CTD casts, discrete bottle sampling and in situ voltammetric microelectrode profiling was used to examine changing redox conditions in the water column at a single station south of the Bay Bridge in the upper Chesapeake Bay in late July/early August, 2002–2005. Short-term (2–4 h) fluctuations in the oxic/suboxic/anoxic interface were documented using in situ voltammetric solid-state electrodes. Profiles of dissolved oxygen and sulfide revealed tidally-driven vertical fluctuations of several meters in the depth and thickness of the suboxic zone. Bottom water concentrations of sulfide, Mn²⁺ and Fe²⁺ also varied over the tidal cycle by approximately an order of magnitude. These data indicate that redox species concentrations at this site varied more due to physical processes than biogeochemical processes. Based on analysis of ADCP data, tidal currents at this station were strongly polarized, with the principal axis of tidal currents aligned with the mainstem channel. Together with the chemical data, the ADCP analysis suggests tidal flushing of anoxic bottom waters with suboxic water from north of the site. The present study is thus unique because while most previous studies have focused on processes across relatively stable redox interfaces, our data clearly demonstrate the influence of rapidly changing physical mixing processes on water column redox chemistry.Also noted during the study were interannual differences in maximum bottom water concentrations of sulfide, Mn²⁺ and Fe²⁺. In 2003, for example, heavy spring rains resulted in severe hypoxia/anoxia in June and early July. While reported storm-induced mixing in late July/early August 2003 partially alleviated the low-oxygen conditions, bottom water concentrations of sulfide, Mn²⁺ and Fe²⁺ were still much higher than in the previous year. The latter implies that the response time of the microbial community inhabiting the suboxic/anoxic bottom waters to changing redox conditions is slow compared to the time scale of episodic mixing events. Bottom water concentrations of the redox-sensitive chemical species should thus be useful as a tracer to infer prior hypoxic/anoxic conditions not apparent from ambient oxygen levels at the time of sampling.     

The spectrum of subtidal variability in Chesapeake Bay circulation

The advent of long, continuous time series records of circulation in Chesapeake Bay has revealed the existence of large amplitude fluctuations within the subtidal range 0·03–0·6 cycles day⁻¹. These fluctuations represent direct and indirect response of the estuary to variations in wind stress, fresh water inflow, and coastal sea level. The fluctuations in circulation are accompanied by synchronous fluctuations in transportable properties such as salinity and temperature. A quantitative model is presented to explain this variability within the main stem of Chesapeake Bay in terms of a linear reponse to irregular, time-varying meteorological forcing. The model calculates transfer functions and energy spectra of laterally averaged transport, surface elevation, and salinity in two layers separated by a halocline, over the frequency band 0·03–0·6 cycles day⁻¹. Transfer functions between volume transport and wind stress obtained from one-month-long field experiments at three different cross sections in Chesapeake Bay are used to constrain model friction parameters. Using existing estimates for wind stress and coastal sea level energy spectra, energy spectra for volume transport and surface elevation are calculated as a function of longitudinal position. It is found that the observed volume transport spectrum at the mouth of the Bay can be explained quantitatively as the combined response to statistically indepentdent wind stress and sea level fluctuations. Variations in sea level account for 90% of the volume transport variance at the Bay mouth and dominate the volume transport spectrum below 0·375 cycles day⁻¹. In the upper Bay, longitudinal wind stress accounts for most of the variance. A maximum in the volume transport spectrum at 0·4 cycles day⁻¹, caused by a local maximum in the wind spectrum, is found at all upper Bay cross sections.     

Polarography of the bottom sediments in the Sevastopol Bay

By the method of polarographic profiling performed with the help of an Au-Hg glass microelectrode, we obtained the first high-resolution vertical profiles of the distributions of oxygen, sulfides, oxidized and reduced forms of iron, reduced manganese, and iron monosulfide in pore waters of the bottom sediments in the Sevastopol Bay. It is shown that the regional features of the vertical distributions of the main polarographically active compounds are determined by the combination of several factors: the contents of organic carbon and iron and the sizes of particles of the sediments.     

Processes controlling Fe,Mn and Zn in sediments of northern Chesapeake Bay

The dominant physical and chemical processes that control Fe, Mn and Zn are explored by comparing the compositions of sediments and their sources. The $\text{Mn}\text{Fe}$ and $\text{Zn}\text{Fe}$ ratios in sediment are found to be largely unaffected by local hydraulic sorting (unlike the actual concentrations of Fe, Mn and Zn) and thus are useful indicators of origin. The sediments in northern Chesapeake Bay have markedly lower $\text{Mn}\text{Fe}$ and $\text{Zn}\text{Fe}$ ratios than those found in the Susquehanna River (dissolved plus suspended) under ordinary flow, but not under high flow conditions. Since high flow conditions dominate sediment transport, seaward loss of a major fraction of the river-derived Mn and Zn need not be invoked to reconcile sediment and river compositions. Sediments in the seaward end of the northern bay have higher $\text{Mn}\text{Fe}$ and $\text{Zn}\text{Fe}$ ratios than their principal external source, the eroding shore deposits. The excess Zn appears to be derived from the atmosphere; the required depositional flux of Zn is consistent with measurements of the total atmospheric flux. The excess Mn can be explained by remobilization of roughly 5% of the river-borne Mn from sediments in the landward part of the northern bay. Because rare floods influence sediment composition markedly, comparing suspended particles in the river at ordinary stages with resuspended sediment in the estuary would lead to the false interpretation that Mn and Zn were being desorbed in the saltwater.     

Consolidation and erosion of deposited cohesive sediments in Northern Chesapeake Bay,USA

Deposits of dredged cohesive sediments were monitored for changes in volume, bulk characteristics, and susceptibility to resuspension and erosion at disposal sites in Chesapeake Bay. There is a 23–48% volume reduction during the first six months, with correspondingly greater changes over longer time periods. A bulk density increase from 1.15 to 1.3 g/cm³ due to dewatering and compaction accounts for the majority of the volume change. Tidal current induced resuspension is a minor process. The observed suspended sediment load can be accounted for by erosion of only a fraction of a millimeter of sediment on each tidal cycle.     

The cause of the acoustically impenetrable,or turbid,character of Chesapeake Bay sediments

Many shallow water, fine-grained sediments are almost acoustically impenetrable to the energy from high resolution, low energy continuous seismic profilers. It has been alleged that this anomalous acoustic behavior is the result of interstitial gas bubbles that produce reverberation within the sediment, but no analyses were made until recently to test this hypothesis. Determinations of the compressibility of sediments from acoustically impenetrable, or turbid, zones and from contiguous zones of good penetration in Chesapeake Bay showed that the acoustically turbid sediments are several orders of magnitude more compressible than acoustically clear sediments of very similar grain size. The increased compressibility is a result of the presence of interstitial gas bubbles. Other acoustically turbid zones are produced by buried shell beds, and do not show an increase in compressibility.Contribution No. 181, Chesapeake Bay Institute, The Johns Hopkins University, Baltimore, Md. 21218, U.S.A.     

Methylmercury production in sediments of Chesapeake Bay and the mid-Atlantic continental margin

Methylmercury (MeHg) concentration and production rates were studied in bottom sediments along the mainstem of Chesapeake Bay and on the adjoining continental shelf and slope. Our objectives were to 1) observe spatial and temporal changes in total mercury (HgT) and MeHg concentrations in the mid-Atlantic coastal region, 2) investigate biogeochemical factors that affect MeHg production, and 3) examine the potential of these sediments as sources of MeHg to coastal and open waters. Estuarine, shelf and slope sediments contained on average 0.5 to 1.5% Hg as MeHg (% MeHg), which increased significantly with salinity across our study site, with weak seasonal trends. Methylation rate constants (k_meth), estimated using enriched stable mercury isotope spikes to intact cores, showed a similar, but weaker, salinity trend, but strong seasonality, and was highly correlated with % MeHg. Together, these patterns suggest that some fraction of MeHg is preserved thru seasons, as found by others [Orihel, D.M., Paterson, M.J., Blanchfield, P.J., Bodaly, R.A., Gilmour, C.C., Hintelmann, H., 2008. Temporal changes in the distribution, methylation, and bioaccumulation of newly deposited mercury in an aquatic ecosystem. Environmental Pollution 154, 77] Similar to other ecosystems, methylation was most favored in sediment depth horizons where sulfate was available, but sulfide concentrations were low (between 0.1 and 10 μM). MeHg production was maximal at the sediment surface in the organic sediments of the upper and mid Bay where oxygen penetration was small, but was found at increasingly deeper depths, and across a wider vertical range, as salinity increased, where oxygen penetration was deeper. Vertical trends in MeHg production mirrored the deeper, vertically expanded redox boundary layers in these offshore sediments. The organic content of the sediments had a strong impact on the sediment:water partitioning of Hg, and therefore, on methylation rates. However, the HgT distribution coefficient (K_D) normalized to organic matter varied by more than an order of magnitude across the study area, suggesting an important role of organic matter quality in Hg sequestration. We hypothesize that the lower sulfur content organic matter of shelf and slope sediments has a lower binding capacity for Hg resulting in higher MeHg production, relative to sediments in the estuary. Substantially higher MeHg concentrations in pore water relative to the water column indicate all sites are sources of MeHg to the water column throughout the seasons studied. Calculated diffusional fluxes for MeHg averaged  1 pmol m^− 2 day^− 1. It is likely that the total MeHg flux in sediments of the lower Bay and continental margin are significantly higher than their estimated diffusive fluxes due to enhanced MeHg mobilization by biological and/or physical processes. Our flux estimates across the full salinity gradient of Chesapeake Bay and its adjacent slope and shelf strongly suggest that the flux from coastal sediments is of the same order as other sources and contributes substantially to the coastal MeHg budget.     

Modelling phytoplankton deposition to Chesapeake Bay sediments during winter–spring: interannual variability in relation to river flow

The often-rapid deposition of phytoplankton to sediments at the end of the spring phytoplankton bloom is an important component of benthic–pelagic coupling in temperate and high latitude estuaries and other aquatic systems. However, quantifying the flux is difficult, particularly in spatially heterogeneous environments. Surficial sediment chlorophyll-a, which can be measured quickly at many locations, has been used effectively by previous studies as an indicator of phytoplankton deposition to estuarine sediments. In this study, surficial sediment chlorophyll-a was quantified in late spring at 20–50 locations throughout Chesapeake Bay for 8 years (1993–2000). A model was developed to estimate chlorophyll-a deposition to sediments using these measurements, while accounting for chlorophyll-a degradation during the time between deposition and sampling. Carbon flux was derived from these estimates via C:chl-a = 75.Bay-wide, the accumulation of chlorophyll-a on sediments by late spring averaged 171 mg m⁻², from which the chlorophyll-a and carbon sinking fluxes, respectively, were estimated to be 353 mg m⁻² and 26.5 gC m⁻². These deposition estimates were ∼50% of estimates based on a sediment trap study in the mid-Bay. During 1993–2000, the highest average chlorophyll-a flux was in the mid-Bay (248 mg m⁻²), while the lowest was in the lower Bay (191 mg m⁻²). Winter–spring average river flow was positively correlated with phytoplankton biomass in the lower Bay water column, while phytoplankton biomass in that same region of the Bay was correlated with increased chlorophyll-a deposition to sediments. Responses in other regions of the Bay were less clear and suggested that the concept that nutrient enrichment in high flow years leads to greater phytoplankton deposition to sediments may be an oversimplification. A comparison of the carbon flux associated with the deposition of the spring bloom with annual benthic carbon budgets indicated that the spring bloom did not contribute a disproportionately large fraction of annual carbon inputs to Chesapeake Bay sediments. Regional patterns in chlorophyll-a deposition did not correspond with the strong regional patterns that have been found for plankton net community metabolism during spring.     

Seasonal patterns of growth and composition of phytoplankton in the lower Chesapeake Bay and vicinity

A twenty-three month study of lower Chesapeake Bay phytoplankton was made from February 1982 to December 1983. The major seasonal growth periods were dominated by a diatomaceous flora and a pico-nanoplankton complex composed mainly of cyanobacteria, chlorophytes and other cells <5μm in size. There was a pattern of multiple pulses throughout the year. However, the trend for maximum development occurred during the winter-spring and fall periods. The dominant diatoms were Skeletonema costatum, Leptocylindrus danicus, and Asterionella glacialis.Seasonal comparisons were made to earlier phytoplankton composition studies in the Bay. Over the past 60 years there has been a change in phytoplankton assemblages and concentrations in the Bay. Skeletonema costatum has remained a dominant species, but a more abundant and broader base of small sized, chainforming diatoms have become established. In addition, high concentrations of the pico-nanoplankton, chrysophyceans and cryptophyceans were abundant.     

Occurrence of copepod carcasses in the lower Chesapeake Bay and their decomposition by ambient microbes

We tested and refined the Neutral Red staining method for separating live and dead copepods in natural samples. Live copepods were stained red whereas dead copepods remained unstained. The staining results were not affected by method of killing, time of death or staining time. Tow duration had no significant effect on the percent dead copepods collected. The Neutral Red staining method was applied to study the occurrence of dead copepods along the York River and the Hampton River in the lower Chesapeake Bay during June–July, 2005. The zooplankton community was dominated by copepods; on average 29% of the copepod population appeared dead. Recovery of percent dead copepods did not differ between horizontal tows and vertical tows, suggesting that dead copepods were homogenously distributed in the water column. No significant relationship was found between the percent dead copepods and surface water temperature, salinity, Secchi depth or chlorophyll concentration. In laboratory experiments, dead copepods were decomposed by ambient bacteria and the rate of decomposition was temperature-dependent. Combining field and laboratory results we estimated that the non-consumptive mortality (mortality not due to predation) of copepods in the lower Chesapeake Bay was 0.12 d⁻¹ under steady-state condition, which is within the global average of copepod mortality rate.     

Reconstructing pre-colonial oyster demographics in the Chesapeake Bay, USA

Recent estimates of growth and mortality rates in extant Chesapeake Bay, USA oyster (Crassostrea virginica) populations are used to quantify changes in both population abundance (dN/dT) and shell accretion (dS/dT) associated with modern population demographics. The demographics of oyster populations that would be required to maintain reef accretion rates commensurate with sea level rise over geological time frames are examined using estimates of oyster longevity in pre-colonial (pre -1600) times combined with parallel estimates of pre-disease endemic mortality. The analysis demonstrates that modern populations, with their disease related, age-truncated demographics, are generally not capable of maintaining and building biogenic reefs through accretion. Estimates of filtration rates associated with Chesapeake Bay oyster populations prior to 1600 considerably underestimate actual benthic-pelagic coupling during that period. Pristine oyster populations would have supported water column turnover rates on the order of minutes to hours. Thus, the spatial footprint of oyster reefs was limited by available productivity in the estuary. Accretion rate calculations for pristine (pre-1600) oyster reefs describe the intimate relationship between benthic-pelagic coupling and the presence or absence of oyster reefs and the associated communities.     

Early diagenesis in recent bottom sediments of the Dvina Bay (White Sea)

Investigations in the water column and bottom sediments including entrapped water were carried out on expeditions of the P.P. Shirshov Institute of Oceanology in the Dvina Bay, the White Sea. We studied the transformation of particulate organic matter at the biogeochemical barrier between the water and bottom and in the underlying Holocene sediments. Low rates of the early diagenesis of sediments caused by low values of primary production in the conditions of high fluxes of terrigenous organic matter were established. The low temperatures of microorganisms habitat play the secondary role.     

Trace metals in Chesapeake Bay oysters: Intra-sample variability and its implications for biomonitoring

Previous studies have shown that metals from the Susquehanna River and the industrial complexes around Baltimore Harbor and Hampton Roads are bioaccumulated in the oysters of Chesapeake Bay. This paper reports two programs, one an intensive study of oysters from mid-bay sites, the other at Swan Point. Individual variability of metals in oyster tissue is reported and the implications of such variability in respect of sample size for monitoring programs discussed.The results suggest that a reasonable sample size for the mid-bay area is 15 individuals and, for Swan Point, 20, where both tissue metal concentration and sample variability apparently increased.     

The composition of phytoplankton within the Chesapeake Bay plume and adjacent waters off the Virginia coast,U.S.A.

Phytoplankton composition differences were used to identify and plot the extent of the Chesapeake Bay plume over the continental shelf. Distinct but different phytoplankton assemblages were noted within the plume for March, June and October 1980. The major constituents in the plume were Skeletonema costatum, a variety of other diatoms and an unidentified ultraplankton component assumed to consist of several cyanophycean and chlorophycean species. The shelf plankton outside of the plume was mainly characterized by the coccolithophores and other phytoplankters that formed different seasonal assemblages than what was within the plume. Patchiness and fluctuations in the shape, size and extent of the plume were noted. A total of 235 species is given, with their presence within the plume and at shelf stations indicated.     

Arsenic and heavy metals in the bottom sediments of the Balaklava Bay (Black Sea)

We present the results of investigation of the contents of metals (As, Cr, Co, Cu, Ni, Pb, Zn, V, Sr, Mn, Ti, and Fe) in the bottom sediments of the Balaklava Bay (Black Sea) carried out in July 2005. It is shown that the pollution of the bottom sediments with metals has a polyelemental character. We establish the specific features of changes in the contents of the analyzed elements and localize the sources of their appearance in the ecosystem. On the basis of the results of evaluation of the intensity of technogenic action upon the marine medium, we determine a group of toxic elements (As, Cr, Cu, Pb, and Zn) accumulated in the bottom sediments of the bay in amounts significantly exceeding the background values typical of sediments of the Black Sea shelf. The comparative analysis of the degrees of pollution of the bay and some other coastal water areas with metals is performed.     

Potential climate-change impacts on the Chesapeake Bay

We review current understanding of the potential impact of climate change on the Chesapeake Bay. Scenarios for CO₂ emissions indicate that by the end of the 21^st century the Bay region will experience significant changes in climate forcings with respect to historical conditions, including increases in CO₂ concentrations, sea level, and water temperature of 50–160%, 0.7–1.6 m, and 2–6 °C, respectively. Also likely are increases in precipitation amount (very likely in the winter and spring), precipitation intensity, intensity of tropical and extratropical cyclones (though their frequency may decrease), and sea-level variability. The greatest uncertainty is associated with changes in annual streamflow, though it is likely that winter and spring flows will increase. Climate change alone will cause the Bay to function very differently in the future. Likely changes include: (1) an increase in coastal flooding and submergence of estuarine wetlands; (2) an increase in salinity variability on many time scales; (3) an increase in harmful algae; (4) an increase in hypoxia; (5) a reduction of eelgrass, the dominant submerged aquatic vegetation in the Bay; and (6) altered interactions among trophic levels, with subtropical fish and shellfish species ultimately being favored in the Bay. The magnitude of these changes is sensitive to the CO₂ emission trajectory, so that actions taken now to reduce CO₂ emissions will reduce climate impacts on the Bay. Research needs include improved precipitation and streamflow projections for the Bay watershed and whole-system monitoring, modeling, and process studies that can capture the likely non-linear responses of the Chesapeake Bay system to climate variability, climate change, and their interaction with other anthropogenic stressors.