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Contaminated sediments, poor water quality, and lost or altered habitat in Lower Green Bay and Fox River in the 1980s led to its listing as an Area of Concern by the International Joint Commission. Previous studies on the geophysics and health of the bay demonstrated the need for estimates of transport timescales. Such estimates can contribute to improved understanding of the transport and fate of nutrients, contaminants, and biogeochemical processes in the bay. This study reviews definitions of residence time and flushing time, estimates the temporal distribution of flushing time, the spatial and temporal distributions of residence time, and horizontal diffusivities. The study used a previously developed hydrodynamic model, a drifter experiment, a lake particle transport model, and methods appropriate for the estimation of transport timescales. The estimated residence time for lower Green Bay, 56 ± 16 days, is smaller than a previous estimate of 190 days for the whole bay, and had values similar to the estimated flushing times. The closeness between those timescales demonstrates the important role of water exchange across the Chambers Island transect. Flushing time and residence time do not follow the same trends in monthly variability because the former depends directly on water exchange across Chambers Island, while the later depends, in addition, on tributary inflows and the circulation patterns in the bay. The study includes a discussion of the relations between the timescales of transport and the previous studies of biogeochemical processes, such as trophic conditions, spatial distribution of cyanobacteria, cold-water intrusions, and hypoxia.

Sediment cores collected from several stations throughout Lake Baikal in water depths from 100 m off the Selenga River delta to the deepest basin of the lake (~1640 m), have been analyzed for sedimentary organic carbon, nitrogen, and the remineralized components in pore water. The organic carbon content of surface sediments generally varied from 2.3 to 3.2% by weight, and profiles typically showed an exponential decrease in both organic carbon and nitrogen in the upper 20–30 cm of the sediment column. Steady state models of organic matter diagenesis yield first order decomposition rate constants which range from 0.0009 to 0.022 y−1. The calculated residence times for the metabolizable fraction of the organic matter in these sediments increases roughly with increasing water depth and is on the order of 50–300 years. Pore water concentration profiles were determined for dissolved inorganic carbon, dissolved organic carbon (DOC), methane, and dissolved ammonium. At depth (25–30 cm) methane concentrations ranged from 50 to 800 μmol Lpw−1 and DOC from 400 to 900 μmol Lpw−1. Estimation of carbon recycling rates based upon diffusion along pore water concentration gradients at the sediment-water interface, indicate that combined DOC and methane fluxes generally contribute

The Green Bay watershed, draining a total area of approximately 40,468 km2, comprises about a third of the Lake Michigan drainage. In the early years, fur trade was the dominant economic activity within the watershed. Later, when timber harvesting, papermaking, and agriculture came on the scene in the 19th and early 20th centuries, major environmental changes occurred in a relatively short period of time. Nutrient and sediment loadings, accompanied by organic wastes from sawmills and paper mills, resulted in a pollutant overload in the Fox River and in the eutrophication of the waters of lower Green Bay. Citizen complaints about these severely degraded conditions initiated a period of scientific investigation. Starting slowly with a few studies and surveys in the first half of the 20th century, serious investigatory work began at mid-century with support from the University of Wisconsin Sea Grant Institute. Examples of topics that have been investigated since then with support from numerous sources are: biological oxygen demand (BOD), phosphorus and total suspended solids loads, trophic status and food chain efficiencies, coastal wetland characterization, dynamics of the benthic layer, algae and abiotic solids, phosphorus cycling and mass balance, PCBs, seasonal hypoxia, and climate change impacts. These studies have provided the scientific foundation for government-led programs such as the Green Bay Remedial Action Program, the PCB clean-up program, and the TMDL program. Progress has been made—reduction in BOD is an example—but a fuller rehabilitation of this large-scale ecosystem remains an elusive goal. The saga goes on.

Six years (2009–2015) of temperature and dissolved oxygen profile data show hypoxic conditions are common in the bottom waters of southern Green Bay, Lake Michigan during the summer. Depleted oxygen concentrations (

We present water column thermal structure for two climatically different years: 2012, which experienced abnormally warm spring and summer air temperatures preceded by a relatively low ice winter and 2013, which experienced cooler than average spring and average summer air temperatures and preceded by average ice conditions. Mean bottom water temperatures for the season and during cold water intrusions were significantly warmer in 2012 than 2013 leading to a significantly reduced stratified season in 2012. Cold water intrusions were driven into southern Green Bay by southerly winds while intrusions were terminated when winds switched to persistent northerly winds. 2012 observed a significant increase in northerly winds relative to 2013, decreasing cold water intrusion presence and duration but winds did not fully explain the difference in thermal conditions for southern Green Bay. These cold bottom waters drive stratification in polymictic southern Green Bay while dimictic waters were found to have significantly warmer bottom temperatures during 2012 and a deeper mixed layer. Our observations suggest that relatively shallow (

Green Bay has sometimes been referred to as the largest freshwater “estuary” in the world. Its watershed, much of it in intensive agriculture, comprises one-third of the Lake Michigan basin and delivers one-third of the lake's total phosphorus load. At one time, the major tributary, the Fox River, was considered the most heavily industrialized river in North America, primarily from paper manufacturing. Deterioration in water quality and the loss of beneficial and ecological uses have been extensive and began well back into the last century. More recently, the bay has also become a test case for our resolve to remediate and restore ecosystems throughout the Great Lakes and elsewhere. Green Bay has stimulated a significant amount of widely relevant research on the fate and behavior of toxics, biogeochemistry, habitat, biodiversity, and ecological processes. The bay represents a true “proving ground” for adaptive restoration. Key findings of the recent summit on the Ecological and Socio-Economic Tradeoffs of Restoration in the Green Bay Ecosystem are summarized here. Foremost among recommendations of the workshop was the creation of a “Green Bay Ecosystem Simulation and Data Consortium” serving as a data clearing house, building upon the significant progress to date, and developing a modeling framework and visualization tools, furthering public outreach efforts, and ensuring a sustained growth in scientific expertise. Funding was estimated to be on the order of ~$15–20M over the next ~5 years – a modest investment relative to the value of the ecosystem and the long-term cost of inaction.

The last historic Hexagenia specimen in lower Green Bay was officially recorded in 1955. Field surveys and Hexagenia viability studies were completed to determine if lower Green Bay could support Hexagenia re-ecesis and where in the bay egg stocking could best be accomplished. The invertebrate field data were compared with historical population data based on earlier published studies in the 1950s, 1970s and 1990s to determine the bay's ecological trajectory to better understand the re-ecesis success potential of Hexagenia. No native Hexagenia were observed during this study. Deep water invertebrate diversity within the upper lower bay appears to be improving, whereas the diversity along the lower mid-bay may be deteriorating. Shallower, nearshore samples indicated a better condition with Caenis mayflies sparsely present, amphipods, isopods, gilled snails, odonates, oligochaetes, chironomids, and meiofauna present. These results suggested improved conditions shoreward versus degraded conditions deeper. Hexagenia egg viability and neonate growth indicated Hexagenia could successfully inhabit in situ Green Bay nearshore (

In nearly every instance in which the environment has been sampled on a higher resolution in time or space, fundamental processes have come to light that were previously undetected or unobserved. In this study, an autonomous dissolved phosphate sensor was deployed at the Entrance Light station in lower Green Bay, Lake Michigan in 2012 and 2013. Hourly phosphate sensor measurements were compared with other real time sensor data to gain insights into the processes occurring at this site. Results showed that the water column at this location undergoes repeated stratification and turnover during the course of the summer. Often, the stratification results from intrusions of cold hypolimnetic bottom water from the north, while turnover is associated with significant northerly and/or easterly wind events. It was observed that, during calm periods, dissolved phosphate concentrations increased at a rate that was stoichiometrically consistent with the consumption of dissolved oxygen during the remineralization of organic matter; specifically, areal oxygen consumption rates ranged from 3.2 to 43 mmol m−2 d−1 and oxygen to phosphate ratios ranged from 120 to 210. At other times, the inverse relationship between dissolved oxygen and dissolved phosphate was not stoichiometrically linked; during these times, areal oxygen depletion rates ranged from 51 to 240 mmol m−2 d−1 and oxygen to phosphate ratios ranged from 260 to 2300. Future strategic deployment of multiple in situ dissolved phosphate and other nutrient sensors will enhance the understanding of nutrient cycling in this important aquatic system.

Water, suspended-particles, and surface-sediment samples were collected from Green Bay, Lake Michigan, for the measurements of phosphorus (P) species, including dissolved/particulate-P, inorganic/organic-P, and five different forms of particulate-P, namely exchangeable- or labile-P (Ex-P), iron-bound-P (Fe-P), biogenic-apatite and/or CaCO3-associated-P (CFA-P), organic-P (Org-P) and detrital-apatite-P (Detr-P) to elucidate their reactivity and transformation pathways in the water column. Suspended particles contained mainly Ex-P (25 ± 15%), Fe-P (28 ± 12%) and Org-P (29 ± 7%). In contrast, Detr-P (34 ± 10%) and Org-P (36 ± 12%) were the predominant P species in surface sediment. Contents of Ex-P, Fe-P, Org-P and CFA-P decreased consistently from suspended-particles to surface-sediment, but an increase was observed for the Detr-P, indicating a net loss of Ex-P, Fe-P, Org-P and CFA-P from particulate into dissolved phase. Such active regeneration of P in the water column between particulate and dissolved phases may serve as an internal phosphate source in Green Bay, especially under hypoxic conditions. Degradation of organic matter in south central bay areas seemed to promote hypoxia and enhance the reductive-dissolution of Fe-P and preservation of Org-P under low-oxygen conditions in the central bay. Overall, Ex-P, Fe-P, CFA-P and Org-P species, which comprised up to 50–90% of total particulate-P, can be collectively considered as potentially-bioavailable-P (BAP). Under low-phosphate (0.022 ± 0.014 μM in Green Bay) and summer low-oxygen/hypoxic conditions, suspended-particles may release up to 71% of their BAP before deposited in sediment although the BAP regeneration decreased along the south-north transect in Green Bay.

Quantifying rates of primary production and respiration is fundamental to understanding ecosystem function. This study utilized high-frequency time series, buoy-based sensor data to estimate daily primary production and respiration rates during the summers of 2012–2015 in southern Green Bay, Lake Michigan. Highly coherent diel oscillations of dissolved oxygen concentrations in epilimnetic waters were commonly observed for much of the summer via 30-min time intervals from the GLOS buoy (NOAA 45014) sensor array. Corrections for air–sea exchange based upon wind speed-derived gas exchange coefficients and saturation state, when combined with mixing depth, allow calculation of daytime net oxygen production and nighttime respiration. Thermistor string observations at 1-m intervals over the 13 m water depth showed the onset of thermal stratification, development of the thermocline, and occasional mixing events. For the summers of 2014 and 2015, during which a nearly continuous sensor record exists, gross primary production (GPP) and respiration (R) were estimated to be 342 ± 117 and 318 ± 83 mmol O2 m− 2 day−1 for GPP and − 325 ± 120 and − 306 ± 66 mmol O2 m− 2 day−1 for R, respectively. These results indicate that during most of the summer, southern Green Bay tends towards net autotrophy with production on average exceeding respiration by 9 ± 6% (SD). Cumulative net ecosystem production from June through September was estimated to be 3.2 and 1.3 mol C m− 2 in 2014 (118 days) and 2015 (113 days), respectively, and is sufficient to drive a significant portion of benthic respiration, the principal cause of seasonal bottom water hypoxia.

Summertime bottom water hypoxia has been a recurring water quality issue in Green Bay for decades. Evidence suggests that the magnitude and duration of hypoxia is highly variable from year to year, despite the fact that nutrient loading has been relatively constant for at least 20 years. The bay's size, orientation, and morphology (~ 20 × 200 km) with high riverine inflow and restricted mixing at the southern end, combined with extensive water mass exchange with Lake Michigan at the northern end, results in a changing hydrodynamic structure that plays a key role in the set up and persistence of stratification that leads to hypoxic conditions. Using both observed and modeled data, this study represents the first attempt to examine the spatial and temporal characteristics of the interactions among the atmospheric heat flux across the water surface, the advective heat flux driven by the circulation, and the intrusion of cold Lake Michigan bottom water into the hypolimnion. With the onset of positive solar heat fluxes in April, stratification generally exists continuously between late June and early September in regions deeper than 15–20 m, and develops discontinuously at depths less than 15 m. Critical improvements in developing realistic simulations of stratification in the bay were obtained via consideration of water clarity conditions that impact heat adsorption and varying default parameters, particularly in the vertical diffusivity, in the Lake Michigan model framework.

The nature and distribution of lipid biomarkers (n-alkanes, n-alkanols, sterols and triterpenols) were investigated in five dated sediment cores from the Pichavaram mangrove–estuarine complex in order to: 1) identify the organic matter (OM) sources and its preservation and 2) trace recent changes associated with coastal processes and anthropogenic activities. The mangrove sediment extracts have higher biomarker concentration (22.6 ± 13.3 μg/g dry weight) than the estuarine extracts (6.42 ± 4.92 μg/g dry weight). Triterpenols are dominant biomarkers in both estuarine and mangrove sediments, and constitute > 50% of the total lipid extracts. The presence of n-alkanols (n-C26,28,30) and abundance of phytosterols (stigmasterol and β-sitosterol) and triterpenols (taraxerol, β-amyrin, germanicol, and lupeol) indicate that mangrove vegetation is the primary source of sedimentary OM. The high abundance and unimodal distribution of the long-chain n-alkanes (mainly n-C25,27,29), and high values of Terrestrial Aquatic Ratio (TAR 2.4 to 41) and Carbon Preference Index (CPI > 8) indicate dominance (and better preservation) of higher plant derived immature OM in mangrove sediments. In contrast, the weak dominance of high molecular weight n-alkanes, low CPI (0.75 to 0.90) and TAR (1.9 to 5.7) values, and the presence of high C27 and C28 sterols indicate that phytoplankton/algal derived OM is more pronounced in estuarine sediments. Diagenetic changes in sedimentary OM indicate that some of the lipid fractions are reactive, and as a result, they degrade more rapidly than bulk OM. These compounds follow first order decay kinetics, and concur with the downcore diagenetic changes in coastal areas. Resistance to degradation among the different lipid classes show the trend: n-alkane > sterol > triterpenol > n-alkanol. Finally, the study indicates that less freshwater discharge from the Coleroon River is causing a gradual change in mangrove vegetation. There is less input of OM derived from mangrove vegetation into recent sediments. Biomarker trends also indicate that less salt tolerant Rhizophora spp. are gradually replaced by more salt tolerant vegetation consisting of Sueda spp. and Aviccenia spp.

Water quality impairment of Great Lakes beaches is caused by fecal pollution from point and nonpoint sources. Erosion due to wind or wave action, invasive vegetation and chronically wet, flooded or standing water are conditions that can magnify water quality problems at beaches. We investigated the hydrological and geophysical characteristics of the Bradford Beach on Lake Michigan (Milwaukee, WI) and the linkage between standing water and persistent contamination by fecal indicator bacteria (FIB). Our study showed that there is a positive correlation between high concentrations of Escherichia coli ( E. coli ) in sand and high moisture content caused by standing water. The main factor associated with the formation of standing water was rainfall. There were also notable differences in standing water and/or wet sand conditions in the northern and southern parts of the beach. These differences could be accounted for by differences in ground water elevations and beach erosion and accretion patterns. Other important physical features of the beach were the presence of rain gardens and mean grain diameter (d 50 ). Rain gardens above the beach face that capture runoff contributed to transient increases in the water table, facilitating standing water formation. Standing water, stormwater runoff infiltrating through the sand and into groundwater as well as wave run up that delivered contaminated surface water to the back beach area were of health concern following heavy rainfall events. The outcomes of this study will likely be useful to beach managers investigating mechanisms/sources of fecal indicator bacteria loading and potential mitigative approaches.

This study reports a multi-parameter geochemcial investigation in water and sediments of a shallow hyper-eutrophic urban freshwater coastal lake, Zeekoevlei, in South Africa. Zeekoevlei receives it ...

The nearshore zone of western Lake Michigan is poorly characterized both in terms of its fine scale bathymetry and its lakebed characteristics. Difficulties in characterizing the lakebed of this region arise in part because of its patchiness as well as the fact that much of the lakebed is not amenable to conventional sediment sampling techniques. With this in mind, high precision bathymetry and the lakebed characteristics of ∼17.5 km2 of the nearshore of western Lake Michigan (near Oak Creek, WI) were mapped using multi-beam and single beam sonar in conjunction with lakebed characterization and surface mapping software. Lakebed features showing nearshore bluff erosion material, glacial scouring, and isolated ridges were revealed in a 1.0 × 1.5 meter resolution map of the survey area. A map of the bottom characteristics, meanwhile, showed a patchwork of four distinct bottom types (area %): mostly rock (12.8%), cobble and sand (21.0%), mostly sand (60.3%), and clay outcrops (5.9%). All predicted bottom types were shown to be accurately characterized by direct observation with an ROV.

Benthic macroinvertebrates were sampled in the summer of 1997 using a standard D frame kick net along a transect across the Peshtigo wetland, a river delta wetland on the coast of Green Bay, Lake Michigan, to describe the spatial and temporal dynamics of the invertebrate community. Various abiotic factors, including sedimentation rates determined from 210Pb and 137Cs as a proxy for delivery of riverine organic matter, were also measured to determine which factors influenced these dynamics. Significant decreasing gradients in dissolved oxygen and pH with distance from the river, coupled with trends in sedimentation rates, chloride, and sum nitrate (nitrate + nitrite), revealed that riverine water was mixing with wetland water up to 100 m from the wetland-river interface. Aboveground primary production and total invertebrate densities exhibited Weibull type distributions, with highest rates and numbers occurring 20 to 100 m from the Peshtigo River. Invertebrate densities were largely represented by Asellus sp. isopods (12–53%) and exhibited highest numbers in September. Invertebrate diversity at the genus level linearly decreased with distance from the river based on the Simpson's index of diversity (r2 = 0.60, p < 0.05) and the Shannon-Wiener function (r2 = 0.73, p < 0.01). Patterns observed suggest that there is an “optimal” zone for benthic invertebrates in the Peshtigo wetland 20 to 60 m from the Peshtigo River that is protected from high-energy events (e.g., storms, boating) in the Peshtigo wetland by a buffer zone (0 to 20 m) but is close enough to benefit from replenished levels of dissolved oxygen, nutrients, and organic matter delivered via the Peshtigo River.

Our capability to conduct precision sampling and observations at depth in aquatic environments has been greatly expanded through the use of ROVs (Remotely Operated Vehicles). We describe a simple, relatively inexpensive ROV-based sampling system designed to collect water samples from small sublacustrine hydrothermal springs. The system consists of a peristaltic pump connected to a 16 loop flow trapping high pressure valve powered and actuated electrically from the surface. Multiple thermistors serve to detect geothermally warmed regions of the lake bottom and to indicate when heated hydrothermal solutions are being sampled. The system has been used successfully to collect water samples from high temperature springs in Yellowstone Lake in Yellowstone National Park, Wyoming.

Water samples freshly collected from southern Green Bay were innoculated with radiolabelled hydrophobic organic compounds (HOC) and, after equilibration, separated into particle bound, dissolved organic matter (DOM) bound and freely dissolved phases. Ambient suspended matter (2.7–28ppm) and dissolved organic carbon (5.2–11.5 ppm) were high in concentration relative to the open Great Lakes, but HOC distribution coefficients were not significantly different between Green Bay and previously measured open Great Lakes values. HOC in the particle phase ranged from 3 % for 4-MCB to 53 % for BaP and DDT. HOC associated with DOM was generally higher than for open lake samples, and ranged from 2% for 2,2′,5,5′-TCB to 26.5% for 4-MCB.

Methane produced in anoxic organic-rich sediments of Cape Lookout Bight, North Carolina, enters the water column via two seasonally dependent mechanisms: diffusion and bubble ebullition. Diffusive transport measured in situ with benthic chambers averages 49 and 163 μmol · m −2 · hr −1 during November–May and June–October respectively. High summer sediment methane production causes saturation concentrations and formation of bubbles near the sediment-water interface. Subsequent bubble ebullition is triggered by low-tide hydrostatic pressure release. June–October sediment-water gas fluxes at the surface average 411 ml (377 ml STP: 16.8 mmol) · m−2 per low tide. Bubbling maintains open bubble tubes which apparently enhance diffusive transport. When tubes are present, apparent sediment diffusivities are 1.2–3.1-fold higher than theoretical molecular values reaching a peak value of 5.2 × 10−5 cm2 · sec−1. Dissolution of 15% of the rising bubble flux containing 86% methane supplies 170μmol · m−2 · hr−1 of methane to the bight water column during summer months; the remainder is lost to the troposphere. Bottom water methane concentration increases observed during bubbling can be predicted using a 5–15 μm stagnant boundary layer dissolution model. Advective transport to surrounding waters is the major dissolved methane sink: aerobic oxidation and diffusive atmospheric evasion losses are minor within the bight.

The rapid rates of sediment accumulation (~ 10–20 cm/yr) in the recently formed Cape Lookout Bight, North Carolina, have resulted in the deposition of approximately 157 moles of carbon, 14 moles of nitrogen and 1.3 moles of phosphorus, per square meter annually. The metabolism of the organic matter in these anoxic sediments is dominated by sulfate reduction and fermentation reactions. Sedimentary nitrogen and phosphorus budgets are estimated using 3 related approaches: 1) a kinetic model of solid phase diagenesis; 2) direct measurements of nutrient burial and regeneration; and 3) nutrient recycling rates estimated from annual rates of sulfate reduction and the SO 4 :NH 4 and SO 4 :PO 4 stoichiometry of nutrient regeneration. The mass balances derived agree reasonably well and indicate that approximately 30% of the total nitrogen and 15% of the total phosphorus deposited in these sediments are recycled. The kinetics of nutrient regeneration are rapid. The mean residence time for recycled nutrients within the sediment is 4 to 6 months for nitrogen and 1.5 to 2 years for phosphorus. Nearly 60% of the total nitrogen regeneration and 90% of the total phosphorus regeneration occur during the 4 month summer period of June through September. Nitrogen regeneration, like carbon, appears to be controlled by the microbially-mediated metabolism of labile organic matter. The greater asymmetry and lower percent turnover in phosphorus cycling is apparently due to changes in its solubility under oxidized and reduced conditions and selective regeneration prior to deposition.

Benthic environments in Lake Superior are variable and dynamic. The complex topography and morphology of eastern Lake Superior results in a variety of depositional environments, from shallow coastal bays to deep profundal basins. Direct observations of bottom types and collection of sediments across the gradient in environments reveal benthic systems that range in sediment accumulation rates from ∼1.7 mm per year to virtually zero net long-term accumulation. Diagenetic modeling of organic carbon and nitrogen under steady state conditions indicate that 15% to 40% of the organic carbon and nitrogen deposited is recycled into the overlying water. This “metabolizable” fraction has a residence time in the sediment column estimated to be 30 to 60 years for organic carbon and 15 to 30 years for nitrogen. Concentration gradients of total dissolved inorganic carbon, ammonium, and oxygen in sediment pore waters yield fluxes of remineralized carbon and nitrogen across the sediment-water interface consistent with diagenetically modeled fluxes. Spatial variability in sediment accumulation patterns, however, is great, particularly in the deep, steep relief basins in the open lake. Here, distances of a few kilometers can radically alter deposition patterns. Evidence of aperiodic bottom currents were observed in the deepest charted basin of the lake as extensive fields of sediment ripples over hard, compacted relict silt and clays. While no currents were observed in these summertime dives, it is assumed that storms during isothermal conditions generate suffficient bottom currents at depth to very effectively scour the bottom. Differences in sediment metabolism appear to be determined by the flux of labile organic matter to the bottom coupled to the physical processes of sedimentation, resuspension, and horizontal transport.

During a series of submersible dives at depths of 300 m and 400 m in southeastern Lake Superior, Hydra were observed attached to a variety of hard substrates including individual rocks, the edges of large boulders, and the undersurfaces of sandstone ledges. Although widespread, populations were generally found in distinct clumps. Based strictly on morphological features, the species resembled H. littoralis or H. carnea. The finding of Hydra at depths of 400 m in Lake Superior, the deepest sounding in the Great Lakes, would indicate that Hydra are not limited by depth.

In situ carbon flux measurements and calculated burial rates are utilized to construct an organic carbon budget for the upper meter of sediment at a single station in Cape Lookout Bight, a small marine basin located on the Outer Banks of North Carolina, U.S.A. (34°37′N, 76°33′W). Of 149 ± 20 mole · m−2 · yr−1 of total organic carbon deposited, 35.6 ± 5.2 mole · m−2 · yr−1 is recycled to overlying waters, 84 ± 18% as ∑CO2 and 16 ± 8% as CH4. Approximately 68 ± 20% of the upward carbon flux is supported by sulfate reduction while 32 ± 16% takes place as the result of underlying methanogenesis. Measured ∑CO2 and CH4 sediment-water fluxes range seasonally from 1900–6300 and 50–2500 μmole · m−2 · hr−1 respectively. The mean residence time of metabolizable organic carbon in the upper 80 cm of sediment is approximately four months with greater than 98% of the calculated total remineralization taking place within three years. In spite of large upward fluxes of methane, larger molecules derived from metabolizable sedimentary organic carbon appear to be the dominant reductants for dissolved sulfate.

The release of remineralized N and P from the organic-rich anoxic sediments of Cape Lookout Bight is controlled by processes occurring within the sediment column and at the sediment-water interface. The relatively rapid rates of temperature dependent microbial degradation of organic matter support seasonally varying nutrient fluxes ranging from 20 to 1200 μmol·m−2·hr−1 for dissolved ammonium and from − 20 to 120 μmol·m−2·hr−1 for total dissolved phosphate (measured in situ over the period October, 1976 to October, 1978). Molecular diffusion along steep vertical pore water concentration gradients measured simultaneously cannot explain the high fluxes observed during warmer months. Gradients for ammonium and phosphate ranged from 0.33 to 1.10 and from 0 to 0.29 μmol·cm−3pw·cm−1s respectively. These high summertime fluxes appear to result from increased sediment-water transport associated with bubble tubes created and maintained by low-tide methane gas bubble ebullition. When these tubes are present, apparent bulk sediment diffusivities calculated from concurrent studies of methane and radon-222 sediment-water exchange are 1.0–3.1 times greater than molecular diffusivities. Nutrient fluxes calculated via Fick's first law taking into account this enhanced transport and the differential diffusive mobilities of dissolved ammonium, phosphate and phosphate ion pairs indicate that removal by aerobic adsorption and/or biological uptake at the sediment-water interface plays an important role in controlling nutrient exchange in these sediments.