Bottom-water Concentrations Research Articles

Sedimentary oxygen consumption and microdistribution at sites across the Arabian Sea oxygen minimum zone (Pakistan margin)

Abstract Biogeochemical processes and fluxes occurring across the sediment–water interface on continental margins impacted by oxygen minimum zones (OMZs) are important to bioelement cycles, ocean inventories, and productivity. The nature and magnitude of these processes depend heavily on spatial and temporal variability in dissolved O 2 concentrations in bottom waters and porewaters. In 2003, four research cruises to the Indus margin of the Arabian Sea (Pakistan) were undertaken to survey the benthic biogeochemical processes in the resident OMZ before and after the southwest monsoon. Sediment O 2 microdistribution and consumption rates were measured at five stations along a depth transect (140–1850 m) across the OMZ, during the spring intermonsoon and the late-to-post southwest monsoon periods, using in situ benthic research platforms (landers). Lander O 2 electrode data show that the intermonsoon and late-to-postmonsoon bottom-water O 2 levels had little to no variation (300 m, no change; 940 m, 1.7–2.8 μM; 1200 m, 10.2–12.6 μM; and 1850 m, 82–80 μM). In contrast, at the shallowest station (140 m), a large fluctuation occurred between the intermonsoon (O 2 =44.5 μM) and the late-to-postmonsoon (O 2 =1 μM), due to monsoon-forced shoaling of the upper OMZ boundary. Oxygen did not penetrate into the sediments at the 300-m site during either sampling season. During the intermonsoon season at the 140-m site, O 2 penetrated to a depth of ∼3 mm, but no measurable O 2 penetration occurred after the monsoon. At the 940 and 1200-m sites, O 2 penetration into the sediments was small (ca. 1–2 mm at both sites) and did not measurably change between the two sampling seasons. In contrast, at the 1850-m site, O 2 penetration decreased after the monsoon (18–12 mm). Calculated late-to-postmonsoon O 2 consumption rates were generally similar to or lower than intermonsoon values (0 vs. 2.22 mmol m −2 d −1 at 140 m, 0.37 vs. 0.31 mmol m −2 d −1 at 1200 m, and 0.73 vs. 1.01 mmol m −2 d −1 at 1850 m). The relatively small seasonal signal suggests that organic matter delivered during the monsoon period may have already been largely remineralized by the late-to-postmonsoon sampling period. Modelling of porewater O 2 profiles indicates that subsurface O 2 consumption associated oxidation of reduced inorganic species makes a significant contribution to total O 2 consumption at some sites. Similarly, differences in O 2 consumption rates determined by porewater profile modelling and whole-core incubations at some sites indicate significant contributions associated with bioturbation and bioirrigation.

Application of Membrane Inlet Mass Spectrometry for Online and In Situ Analysis of Methane in Aquatic Environments

A method is presented for the online measurement of methane in aquatic environments by application of membrane inlet mass spectrometry (MIMS). For this purpose, the underwater mass spectrometer Inspectr200-200 was applied. A simple and reliable volumetric calibration technique, based on the mixing of two end member concentrations, was used for the analysis of CH(4) by MIMS. To minimize interferences caused by the high water vapor content, permeating through the membrane inlet system into the vacuum section of the mass spectrometer, a cool-trap was designed. With the application of the cool-trap, the detection limit was lowered from 100 to 16 nmol/L CH(4). This allows for measurements of methane concentrations in surface and bottom waters of coastal areas and lakes. Furthermore, in case of membrane rupture, the cool-trap acts as a security system, avoiding total damage of the mass spectrometer by flushing it with water. The Inspectr200-200 was applied for studies of methane and carbon dioxide concentrations in coastal areas of the Baltic Sea and Lake Constance. The low detection limit and fast response time of the MIMS allowed a detailed investigation of methane concentrations in the vicinity of gas seepages.

Major and trace element geochemistry in Zeekoevlei, South Africa: A lacustrine record of present and past processes

This study reports a multi-parameter geochemical investigation in water and sediments of a shallow hyper-eutrophic urban freshwater coastal lake, Zeekoevlei, in South Africa. Zeekoevlei receives a greater fraction of dissolved major and trace elements from natural sources (e.g., chemical weathering and sea salt). Fertilizers, agricultural wastes, raw sewage effluents and road runoff in contrast, constitute the predominant anthropogenic sources, which supply As, Cd, Cu, Pb and Zn in this lake. The overall low dissolved metal load results from negligible industrial pollution, high pH and elevated metal uptake by phytoplankton. However, the surface sediments are highly polluted with Pb, Cd and Zn. Wind-induced sediment resuspension results in increased particulate and dissolved element concentrations in bottom waters. Low C/N ratio (10) indicates primarily an algal source for the sedimentary organic matter. Variation in sedimentary organic C content with depth indicates a change in primary productivity in response to historical events (e.g., seepage from wastewater treatment plant, dredging and urbanization). Primary productivity controls the enrichment of most of the metals in sediments, and elevated productivity with higher accumulation of planktonic debris (and siltation) results in increased element concentration in surface and deeper sediments. Aluminium, Fe and/or Mn oxy-hydroxides, clay minerals and calcareous sediments also play an important role in adsorbing metals in Zeekoevlei sediments.

Iron in the Japan Sea and its implications for the physical processes in deep water

Labile dissolved Fe (DFe, <0.22 μm fraction) and total dissolvable Fe (TDFe, unfiltered) were examined throughout the water column of the northeastern (Japan Basin) and mid‐eastern (Yamato Basin) basin regions in the Japan Sea. We observed extremely high vertically integrated TDFe inventories and low nutrient concentrations at the surface water in both basins, probably resulting from high atmospheric Fe input to nutrient‐depleted surface water. DFe in both basins was characterized by mid‐depth maxima and, below that, a slight decrease with depth in deep water and uniform concentration in bottom water. In the Japan Basin, surprisingly, TDFe concentrations in bottom water were lower than those in deep water, resulting from the injection of new bottom water. On the contrary, TDFe concentrations in bottom water of the Yamato Basin increased gradually with depth and were higher than those in deep water probably due to the supply of iron during the horizontal transport of the bottom water in the Japan Basin towards the Yamato Basin. The TDFe with a high percentage of labile particulate Fe (TDFe minus DFe) may have important implications for understanding the physical processes of bottom water in the Japan Sea.

Decadal ventilation and mixing of Indian Ocean waters

Chlorofluorocarbon (CFC) and hydrographic data from the World Ocean Circulation Experiment (WOCE) Indian Ocean expedition are used to evaluate contributions to decadal ventilation of water masses. At a given density, CFC-derived ages increase and concentrations decrease from the south to north, with lowest concentrations and oldest ages in Bay of Bengal. Average ages for thermocline water are 0–40 years, and for intermediate water they are less than 10 years to more than 40 years. As compared with the marginal seas or throughflow, the most significant source of CFCs for the Indian Ocean south of 12°N is the Southern Hemisphere. A simple calculation is used to show this is the case even at intermediate levels due to differences in gas solubilities and mixing of Antarctic Intermediate Water and Red Sea Water. Bottom water in the Australia–Antarctic Basin is higher in CFC concentrations than that to the west in the Enderby Basin, due to the shorter distance of this water to the Adelie Land coast and Ross Sea sources. However, by 40°S, CFC concentrations in the bottom water of the Crozet Basin originating from the Weddell Sea are similar to those in the South Australia Basin. Independent observations, which show that bottom water undergoes elevated mixing between the Australia–Antarctic Basin and before entering the subtropics, are consistent with high CFC dilutions (3–14-fold) and a substantial concentration decrease (factor of 5) south to north of the Southeast Indian Ridge. CFC-bearing bottom waters with ages 30 years or more are transported into the subtropical South Indian Ocean by three western boundary currents, and highest concentrations are observed in the westernmost current. During WOCE, CFC-bearing bottom water reaches to about 30°S in the Perth Basin, and to 20°S in the Mascarene Basin. Comparing subtropical bottom water-CFC concentrations with those of the South Pacific and Atlantic oceans, at comparable latitudes, Indian Ocean bottom water-CFC concentrations are lower, consistent with its high dissipation rates from tidal mixing and current fluctuations as shown elsewhere. Thus, the generally high dilutions and low CFC concentrations in bottom water of the Indian Ocean are due to distance to the water mass source regions and the relative effectiveness of mixing. While it is not surprising that at thermocline, intermediate, and bottom levels, the significant ventilation sources on decadal time scales are all from the south, the CFCs show how local sources and mixing within the ocean affect the ventilation.

Bubble-induced porewater mixing: A 3-D model for deep porewater irrigation

Porewater data from vent sites of the northeastern shelf off Sakhalin Island, Sea of Okhotsk, exhibit bottom-water concentrations down to a sediment depth of up to 300 cm. Below this depth, solute concentrations rapidly change due to methanogenesis and anaerobic methane oxidation (AMO). The profile shapes suggest an irrigation-like process that mixes on a meter scale. At these sites active gas emanation into the overlying water column and near-surface gas hydrates are commonly observed. We propose that methane gas bubbles rise through the soft surface sediments and cause mixing of the porewater. Mathematically, the bubble-induced irrigation can be described by eddy diffusion enhancing the diffusive transport of solutes by several orders of magnitude. A 3-D numerical transport-reaction model was developed to investigate the parameters defining the mixing process, such as bubble rise velocity, tube size, tube distribution in the sediment, and ebullition frequency. Model consistency with the field data requires eddy diffusivities ⩾1 × 10 5 cm 2/a, tube densities of >4 tubes/m 2 (equivalent to a tube spacing of <40 cm), active gas seepage for more than a few weeks or months, and moderate to low diagenetic reaction rates of solutes. The corresponding methane gas fluxes that are predicted from the results of the model realizations range from 1 × 10 3–5 × 10 5 L/(m 2 a). Due to bubble mixing, solute fluxes in these sediments are increased by a factor of 3 and the maximum AMO rate by a factor of 7.

Water quality of Vistonis Lagoon, Northern Greece: seasonal variation and impact of bottom sediments

The aim of this study is to present the seasonal variation of nutrients in the water column and the bottom sediments of Vistonis Lagoon, a hypereutrophic Mediterranean coastal lagoon located in Northern Greece, and to estimate the impact of bottom sediments on the water quality of this lagoon. Nutrient concentrations in the water column and in bottom sediments were determined throughout seven seasonal sampling cruises from May 2003 to October 2004. Physicochemical parameters, such as transparency, temperature, salinity, pH and dissolved oxygen, were measured in situ in the water column using suitable equipment. Nutrient concentrations in bottom water were found generally higher than those in surface water. Nitrogen, mainly as nitrates, and phosphorus are released into the water column from the bottom sediments, especially during the summer period. Anoxia in the bottom water, as well as resuspension of the sediments are the main factors affecting nutrient internal loading in this lagoon. An approximate calculation showed that total phosphorus release was about 80 mg m −2 d −1 for the period March–August 2004. Vistonis Lagoon restoration will be possible only through the minimization or elimination of both external and internal nutrient loadings.

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, Mn 2+ and Fe 2+ 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, Mn 2+ and Fe 2+. 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, Mn 2+ and Fe 2+ 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.

Change of bottom water conditions at intermediate depths of the Oyashio region, NW Pacific over the past 20,000 yrs

The North Pacific Intermediate Water (NPIW), source water (Oyashio Intermediate Water; OYIW) of which originates east of the southern Kuril Islands, plays an important role in the carbon cycle of the Pacific Ocean. The spatio-temporal changes of NPIW, however, have yet to be clarified. Because of OYIW flowing in the Oyashio region, from east of the southern Kuril Islands through south of Hokkaido to east of Honshu, it is important to understand past bottom water changes at intermediate depths of this region for study of the past NPIW. Two sediment cores, GH02-1030 and MD01-2409, were collected from south of eastern and central Hokkaido. Lithology and physical properties of the cores recorded well the surface and bottom water conditions. Higher diatom productivity in the surface waters led to diatomaceous sediments with higher water contents and lower grain densities during the last deglaciation and late Holocene. These lithological changes occurred not only at the two studied cores but also at several other sites along the western margin of the NW Pacific. Benthic foraminiferal assemblages and occurrence of parallel lamination indicated such a dysoxic bottom water condition during the last deglaciation. Absence of lamination in the late Holocene horizon suggested a somewhat higher dissolved oxygen concentration in bottom water than that during the last deglaciation. On the basis of benthic foraminifer assemblages corresponding to strengthening of intermediate water circulation inferred from coarsening of sediment grain size and decreasing of sedimentation rate indicating stronger winnowing effects, it appears that the bottom water oxygen level increased during the Younger Dryas cold period.

HYPOXIA HOTSPOTS IN THE MISSISSIPPI BIGHT

Foraminiferal proxies of hypoxia indicate apparent low oxygen to hypoxic conditions in several hotspots in the Mississippi Bight. The foraminiferal hypoxia proxies, the Ammonia to Elphidium (A/E) index and the Pseudononion-Epistominella-Buliminella (PEB) index, were tabulated from three sets of core tops collected in 1951–1956. Additionally, the oxygenation history of a site near the Balize delta was evaluated over the past one hundred years based on A/E and PEB indices and size distributions of pyrite framboids in a gravity core dated by 210Pb geochronology. The results from the 1950’s core-top collections show apparent, recurrent low-oxygen to hypoxic bottom water on the inner shelf at hotspot locales seaward of the Mississippi-Alabama barrier islands and the eastern distributaries of the Balize delta. Specifically, the A/E index exceeds 90% on the inner shelf seaward of Horn and Dauphin islands, both of the Mississippi-Alabama barrier islands, and a center between Pass a Loutre and Main Pass of the Balize delta. In partial support of these results are reports of present-day low oxygen to hypoxic concentrations in bottom waters associated with seasonally high surface chlorophyll a and seasonal strengthening of a brackish-water cap at these locales. In contrast, the PEB index in core tops suggests good oxygenation at mid-shelf depths >30 m. The PEB index, size distributions of framboidal pyrite, and other indicators in a gravity core 44 km northeast of Pass a Loutre indicate no clear change in conditions over the past century, constraining the seaward extent of the hotspot near the Balize delta.

Extremely High Methane Concentration in Bottom Water and Cored Sediments from Offshore Southwestern Taiwan

Extremely High Methane Concentration in Bottom Water and Cored Sediments from Offshore Southwestern Taiwan

SEASONAL VARIATIONS IN SEDIMENT AND BOTTOM WATER CHEMISTRY OF WESTERN LONG ISLAND SOUND: IMPLICATIONS FOR LOBSTER MORTALITY

In the fall of 1999 the lobster population of western Long Island Sound (WLIS) experienced a massive die-off (LoBue et al. 2003). In fact, the lobster populations in WLIS still have not recovered as of this writing (Benway et al. 2004). Factors suspected of playing roles in this mortality event include water temperature, hypoxia and a neoparameoba. The overall objective of this study is to obtain an understanding of bottom water chemical conditions in WLIS, and their potential influence on benthic community structure and lobster health. Beginning in May 2002, water samples were collected within 5.0 cm and at 1.0 m above the bottom at 12 selected stations in WLIS and analyzed for dissolved NH4, H2S and O2. Grab samples for benthic community determination and sediment-profile images were simultaneously obtained with the water samples at 6 of the 12 stations. The data collected provide an accurate field record of the apparent levels of dissolved oxygen in the sediments of western Long Island Sound, as reflected in apparent redox depths (as recorded in sediment-profile images) and the amount of ammonia, hydrogen sulfide and oxygen present in the bottom waters, over a period critical to the LIS lobster fishery. Levels of dissolved oxygen in actual (within 5 cm) bottom waters of WLIS remain low throughout the summer and fall, despite the increase in dissolved oxygen recorded in the fall in waters 1 m above the bottom. Dissolved sulfide and ammonia concentrations in bottom waters of WLIS increased during spring and fall. These chemicals have been shown by other researchers to be physiologic stressors on lobsters and other marine life that live within benthic environments (Knezovich et al. 1996, Lianso 1991, McMahon & Wilkens 1975, Shumway & Scott 1983, Theede et al. 1969, Vargo & Sastry 1978, Vismann 1990, Wang & Widdows 1991). Given that the lobster die-off of 1999 began in late summer to early fall, it is hypothesized that ammonia and sulfide released from the sediments during that time may have played a role in weakening the lobsters. A comparison of water quality and temperature data from this study with that collected by the CT DEP at certain stations in WLIS during 1999 revealed some similarities and further strengthens the argument that environmental stressors played a role in the lobster mortality event of 1999.

APPLICATION OF WATER QUALITY MODELING TECHNOLOGY TO INVESTIGATE THE MORTALITY OF LOBSTERS (HOMARUS AMERICANUS) IN WESTERN LONG ISLAND SOUND DURING THE SUMMER OF 1999

A phased approach was taken for applying previously developed numerical models to address quantitatively whether application of 4 pesticides (i.e., methoprene, malathion, resmethrin and sumithrin) to combat mosquitoes carrying West Nile virus could alone have caused the massive die-off of lobster observed in western Long Island Sound during 1999. Model results show that even with an overly conservative model input assumption (i.e., that the entire mass of pesticides applied in the watershed reached the open waters of Long Island Sound without any attenuation or decay in either the watershed or the Sound) the calculated 24-h average ambient levels of methoprene in the Sound were less than 0.0005 μg/L and well below the lowest reported ecologic endpoint of lobster stress (i.e., 2.8 μg/L stage 2 larvae LC50). Under the assumed conservative model loadings, results for malathion were highest in the East River (maximum 24-h average = 10.3 μg/L) and were much lower in the western Sound area of the lobster die-off. These levels are below the lowest reported ecologic endpoint of lobster stress (i.e., 4.1 μg/L larvae LC50). Model calculations for resmethrin and sumithrin were compared with ecologic endpoints for lobster stress measured for resmethrin. The lowest reported ecologic endpoints reported for resmethrin were 0.01 μg/L and 0.095 μg/L for reduction in adult phagocytosis after weekly exposure and larval 96-h LC50, respectively. Calculated resmethrin levels on a 24-h average basis were as high a 0.225 μg/L, but were significantly lower in western Long Island Sound. For sumithrin, the maximum calculated 24-h average concentration, 0.151, occurred in Eastchester Bay in near surface waters. The calculated levels of neither resmethrin nor sumithrin reached the LC50 value for adult lobsters of >1 μg/L. Malathion, resmethrin and sumithrin were also modeled with a less conservative and more realistic set of assumptions that included decay of the pesticide within the receiving water. Based on a 24-h average malathion concentrations calculated by the model were <1 μg/L in near bottom waters throughout Long Island Sound and probably did not represent a stress to the lobsters. Calculated resmethrin levels throughout near bottom waters of Long Island Sound were <0.005 μg/L and, therefore, did not represent a stress to lobsters. Calculated sumithrin concentrations in near bottom waters were as high as 0.08 μg/L in portions of western Long Island Sound. Assuming that resmethrin endpoints are applicable to sumithrin, it is unlikely that sumithrin could have caused mortality in adult lobsters, however, we cannot fully rule out the possibility that sumithrin may have been a stressor at sublethal levels.

Comparative vertical distributions of iron in the Japan Sea, the Bering Sea, and the western North Pacific Ocean

The vertical distributions of dissolvable (unfiltered) Fe concentrations in semi‐closed oceanic regions, such as the Japan Sea and the Bering Sea, are characterized by a gradual increase with depth in the intermediate and deep waters. However, there is a rapid decrease in the dissolvable Fe concentration over the narrow depth range between deep and bottom waters with constantly lower concentrations observed in the bottom waters of the Japan Sea (Japan Basin), probably from the injection of newly formed bottom water. In addition, the rapid increase in dissolvable Fe concentrations in bottom waters in the western North Pacific Ocean may be due to the resuspension of sediments from the seafloor or the slope. However, there are no differences of labile dissolved (filtered) Fe concentrations and Fe(III) hydroxide solubility between deep and bottom waters, revealing that dissolved Fe concentrations in deep waters may be controlled primarily by Fe(III) complexation with natural organic ligands. Dissolved Fe concentrations, humic‐type fluorescence intensity, and Fe(III) solubility correlate well with PO4 with different slopes between intermediate and deep waters in the North Pacific Ocean. However, there is no different depth regime between them in semi‐closed oceanic regions. The most statistically significant correlation between the Fe(III) solubility and fluorescence intensity was found with a nearly linear relation in intermediate and deep waters at all stations of the semi‐closed and open oceanic regions. This result suggests that the distributions of humic‐type fluorescent organic matter may be responsible for Fe(III) solubility and dissolved Fe concentrations in deep oceanic waters.

Sediment Raking as a Tool for Enhancement of Phosphate and Productivity of Water in Tropical Pond System

Changes of orthophosphate in water, induced by raking of surface sediment in various ways, were compared. Twelve limnocorrals were installed in a shallow mesotrophic pond using four treatments with triplicate replication:(1) raking of surface sediment (R) by means of a rake for 5min per week (2) fish (F), by introducing six common carp and two silver carp per limnocorral, (3) raking plus fish (R+F) and (4) control (C), without raking and fishless. After day 122, treatments were reversed in limnocorrals by␣removing (netting) or introducing fish and stopping or initiating raking. Samples of water and sediment were collected from each limnocorral at weekly interval and monitored for orthophosphate (OP), primary productivity, phosphate solubilizing bacteria (PSB) and sediment available phosphorus. Raking in presence of fish caused 280 and 160% increase in OP concentrations in surface and bottom water, respectively, as compared with raking in absence of fish. Net effects of fish and raking were 13-81 and 26-241%, respectively in surface water and 48-100 and 46-95% in bottom water. This implied that in bottom water, net raking effect was 13-161% greater than net fish effect. Increase in PSB population, due to raking plus␣fish (R+F) was 6-82% higher than increase by fish alone (F). Responses of available-P in sediment were opposite to that of water, being highest in concentration in control (C) followed by fish (F), raking (R) and raking plus fish (R+F). The primary productivity showed variations parallel with that of orthophosphate of water. This suggests that raking accelerated the transformations of available-P from␣sediment to overlying water mediated through oxygen-dependent sediment water exchange mechanisms.

Spatial distribution and budget for submarine groundwater discharge in Eckernförde Bay (Western Baltic Sea)

Submarine groundwater discharge (SGD) from subseafloor aquifers, through muddy sediments, was studied in Eckernförde Bay (western Baltic Sea). The fluid discharge was clearly traced by 222Rn enrichment in the water column and by the chloride profiles in pore water. At several sites, a considerable decrease in chloride, to levels less than 10% of bottom‐water concentrations, was observed within the upper few centimeters of sediment. Studies at 196 sites revealed that &gt;22% of the seafloor of the bay area was affected by freshwater admixture and active fluid venting. A maximal discharge rate of .9 L m−2 d−1 was computed by modeling pore water profiles. Based on pore water data, the freshwater flow from subseafloor aquifers to Eckernförde Bay was estimated to range from 4 x 106 to 57 × 106 m3 yr−1. Therefore, 0.3–4.1% of the water volume of the bay is replaced each year. Owing to negligible surface runoff by rivers, SGD is a significant pathway within the hydrological cycle of this coastal zone. High‐resolution bathymetric data and side‐scan sonar surveys of pockmarks, depressions up to 300 m long, were obtained by using an autonomous underwater vehicle. Steep edges, with depths increasing by more than 2 m within 8–10 m in lateral directions, equivalent to slopes with an angle of as much as 11°, were observed. The formation of pockmarks within muddy sediments is suggested to be caused by the interaction between sediment fluidization and bottom currents. Fluid discharge from glacial coastal sediments covered by mud deposits is probably a widespread, but easily overlooked, pathway affecting the cycle of methane and dissolved constituents to coastal waters of the Baltic Sea.

The concentration and distribution of different mercury species in the water columns of BaiHua reservoir

Three study sites were selected at the upper, middle and down reach of the Baihua Reservoir and the water samples from different depth were collected in June 8 to 9, 2002. Dissolved gaseous mercury (DGM), reactive and total mercury concentrations in water profiles were measured by trap pre-concentration and CVAFS detection method. The results indicated, as a whole, the concentrations of all mercury species at the three sites gradually decreased along the water flow direction, and the highest concentration site presented in the upper stream (HuaQiao) of the reservoir. In water column (except bottom water), the highest average total and reactive mercury concentrations are 35.9 ng/L and 5.7 ng/L, respectively. Total mercury concentrations in bottom water reach 807-1963 ng/L. Sediments are potential mercury source to water column of the reservoir. A preliminary study indicted that Baihua Reservoir is seriously contaminated in terms of mercury due to mercury discharge from Guizhou Organic Chemical Plant (GOCP). It is essential to carry out further study on mercury cycling in Baihua Reservoir to evaluate ecological effects from mercury contamination.

The pattern and influence of low dissolved oxygen in the Patuxent River, a seasonally hypoxic estuary

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 chang- ing 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.

Seasonal Changes in Sulfate Reduction in Sediments in the Inner Part of Tokyo Bay.

In highly eutrophicated Tokyo Bay, anoxic, sulfide-containing bottom water is formed in warm stagnant seasons. It seems to develop especially in the northeast area where there are many large holes made by dredging to collect sand in addition to navigation channels. At such dredged sites, the seasonal variation in the rate of sulfate reduction in sediment was examined by a radiotracer method together with various environmental factors for two years and the results were compared with those obtained at a site on the natural sea floor. The rate of sulfate reduction was much higher at the dredged sites than on the natural sea floor. The dissolved oxygen concentration in bottom water and sulfate concentration in the sediment layer were lower, and the density of sulfate-reducing bacteria and acid-volatile sulfides were higher at the dredged sites than the natural floor site. However, there was no significant correlation between the sulfate reduction rate and any of the environmental factors examined (temperature, ignition loss, DO in bottom water and density of sulfate-reducing bacteria). The rate on an area basis seemed to fluctuate in a similar pattern to the change of chlorophyll a in the water column. The addition of seston mainly composed of diatoms to the sediment sample greatly stimulated sulfate reduction in contrast to the addition of lactate. Sulfate reduction at these sites might be controlled by the supply of organic substrates especially those of algal origin.

Controls on temporal variability of the geochemistry of the deep Cariaco Basin

Studies of the Cariaco Basin on the continental shelf of Venezuela, as a part of the Carbon Retention In A Colored Ocean (CARIACO) program, have revealed that the chemistry of the deeper waters of the system is more variable than previously believed. Small oxygen maxima have been observed on a number of occasions at depths where oxygen was previously absent, suggesting the occurrence of intrusions of oxygenated water into the region of the oxic/anoxic interface (250–300 m). Apparently because of these events, the oxic/anoxic interface deepened by about 100 m during the period of our observations. We also observed a dramatic decrease in H 2S concentrations at all depths below the oxic/anoxic interface during this same period. Bottom waters, for example, had an H 2S concentration of about 75 μM in November 1995, but since November 1997, concentrations in bottom water have not exceeded 55 μM. Water of sufficient density to sink to the bottom of the Basin has been observed on one occasion at sill depth just north of the eastern sill. However, based on a simple box model, the decrease in deep-water sulfide does not appear to be due to intrusion of oxygenated water alone, as concentrations of other measured species, and of hydrographic parameters, have remained constant with time. Instead, we postulate that an earthquake that took place in July 1997 resulted in a turbidity current that transported large quantities of coastal sediment containing oxidized iron into the deep waters of the basin. If the final products of reaction were elemental sulfur and iron sulfide, the sediment associated with the oxidized iron would have produced a turbidite layer about 10 cm thick. Previous earthquakes have produced turbidites of similar thickness.