Showing papers on "Water column published in 1983"

The Ecological Role of Water-Column Microbes in the Sea*

Abstract: Recently developed techniques for estimating bacterial biomass and productivity indicate that bacterial biomass in the sea is related to phytoplankton concentration and that bacteria utilise 10 to 50 % of carbon fixed by photosynthesis. Evidence is presented to suggest that numbers of free bacteria are controlled by nanoplankton~c heterotrophic flagellates which are ubiquitous in the marine water column. The flagellates in turn are preyed upon by microzooplankton. Heterotrophic flagellates and microzooplankton cover the same size range as the phytoplankton, thus providing the means for returning some energy from the 'microbial loop' to the conventional planktonic food chain.

Trace Metals in Sea Water

Abstract: Session I: Metalloids and the Hydride Generation.- The Determination of the Chemical Species of Some of the "Hydride Elements" (Arsenic, Antimony, Tin, and Germanium) in Sea Water: Methodology and Results.- Antimony Content and Speciation in the Water Column and Interstitial Waters of Saanich Inlet.- Ultratrace Speciation and Biogenesis of Methyltin Transport Species in Estuarine Waters.- The Relationship of the Distribution of Dissolved Selenium IV and VI in Three Oceans to Physical and Biological Processes.- Session II: Arctic Chemistry.- Trace Metals in the Arctic Ocean.- Copper in Sub-Arctic Waters of the Pacific Northwest.- Low Level Determination of Trace Metals in Arctic Sea Water and Snow by Differential Pulse Anodic Stripping Voltammetry.- The Relationship between Distributions of Dissolved Cadmium, Iron, and Aluminium and Hydrography in the Central Arctic Ocean.- Session III: Intercalibration Exercise.- Intercomparison of Seawater Sampling Devices for Trace Metals.- The Analysis of Trace Metals in Biological Reference Materials: A Discussion of the Results of the Intercomparison Studies Conducted by the International Council for the Exploration of the Sea.- An Intercomparison of Sampling Devices and Analytical Techniques Using Sea Water from a CEPEX Enclosure.- Session IV: Estuarine Processes Involving Metals.- Role of Fresh Water/Sea Water Mixing on Trace Metal Adsorption Phenomena.- Effects of Particle Size and Density on the Transport of Metals to the Oceans.- The Effect of Sewage Effluents on the Flocculation of Major and Trace Elements in a Stratified Estuary.- Impoverishment and Decrease of Metallic Elements Associated with Suspended Matter in the Gironde Estuary.- The Significance of the River Input of Chemical Elements to the Ocean.- Session V: Air/Sea Exchange and Coastal Processes Involving Metals.- Air-Sea Exchange of Mercury.- Separation of Copper and Nickel by Low Temperature Processes.- The Fate of Particles and Particle-Reactive Trace Metals in Coastal Waters: Radioisotope Studies in Microcosms.- Trace Metals in a Landlocked Intermittently Anoxic Basin.- Session VI: Ocean Distribution and Analysis.- Thorium Isotope Distributions in the Eastern Equatorial Pacific.- Aspects of the Surface Distributions of Copper, Nickel, Cadmium, and Lead in the North Atlantic and North Pacific.- Mn, Ni, Cu, Zn, and Cd in the Western North Atlantic.- Some Recent Measurements of Trace Metals in Atlantic Ocean Waters.- Determination of the Rare Earth Elements in Sea Water.- The Cycle of Living and Dead Particulate Organic Matter in the Pelagic Environment in Relation to Trace Metals.- Trace Metal Levels in Sea Water from the Skagerrak and the Kattegat.- Mercury Concentrations in the North Atlantic in Relation to Cadmium, Aluminium and Oceanographic Parameters.- Perturbations of the Natural Lead Depth Profile in the Sargasso Sea by Industrial Lead.- Copper, Nickel and Cadmium in the Surface Waters of the Mediterranean.- Accurate and Precise Analysis of Trace Levels of Cu, Cd, Pb, Zn, Fe and Ni in Sea Water by Isotope Dilution Mass Spectrometry.- Session VII: Chemical Speciation.- Studies of the Chemical Forms of Trace Elements in Sea Water Using Radiotracers.- Trace Metals (Fe, Cu, Zn, Cd) in Anoxic Environments.- The Behavior of Trace Metals in Marine Anoxic Waters: Solubilities at the Oxygen-Hydrogen Sulfide Interface.- Variations of Dissolved Organic Copper in Marine Waters.- Trace Metals Speciation in Nearshore Anoxic and Suboxic Pore Waters.- The Contrasting Geochemistry of Manganese and Chromium in the Eastern Tropical Pacific Ocean.- Potentialities and Applications of Voltammetry in Chemical Speciation of Trace Metals in the Sea.- Studies of Cadmium, Copper and Zinc Interactions with Marine Fulvic and Humic Materials in Seawater Using Anodic Stripping Voltammetry.- Chemical Periodicity and the Speciation and Cycling of the Elements.- Potentiometric and Conformational Studies of the Acid-Base Properties of Fulvic Acid from Natural Waters.- Copper Speciation in Marine Waters.- Session VIII: Metals and the Biosphere.- Plankton Compositions and Trace Element Fluxes from the Surface Ocean.- Metals in Seawater as Recorded by Mussels.- Trace Elements and Primary Production: Problems, Effects and Solutions.- Trace Metals and Plankton in the Oceans: Facts and Speculations.- Sensitivity of Natural Bacterial Communities to Additions of Copper and to Cupric Ion Activity: A Bioassay of Copper Complexation in Seawater.- List of Participants.- Index of Authors.

The Formation of Labrador Sea Water. Part I: Large-Scale Processes

Abstract: Data obtained in the western Labrador Sea during March 1976 by Hudson are analysed to show that new Labrador Sea Water was being formed at this time. On the basis of hydrographic and moored current-meter data, it is hypothesized that a 200 km scale cyclonic gyre forms in winter in the western Labrador Sea and that this gyre retains the developing deep mixed layers in this general area long enough for the transformation to Labrador Sea Water to take place. Using a model, it is demonstrated that water columns found along the western boundary of the Labrador Sea can be modified by cooling, evaporation and mixing to form deep mixed layers with the properties of Labrador Sea Water. Approximately 105 km3 of new Labrador Sea Water was formed in 1976, an estimate that is consistent with earlier estimates of mean annual production rates. This water, 2.9°C, 34.84‰, is some 0.6°C cooler and 0.06‰ fresher than that defined by Lazier (1973) from his data collected in 1966. The variation of Labrador Sea Water ...

Phytoplankton standing crop, primary productivity, and near-surface nitrogenous nutrient fields in the Ross Sea, Antarctica

Abstract: In austral summer 1978 a study was made of the phytoplankton standing crop, primary productivity, and nutrient chemistry of the waters along a north-south transect between New Zealand and the Ross Ice Shelf and along two east-west transects in the Ross Sea. The area was characterized by a high degree of spatial biological variability, with pronounced differences in phytoplankton biomass, primary productivity, and near-surface nitrate and ammonium fields between the northern, central, and southern regions of the Ross Sea. An extensive bloom of the colonial flagelate Phaeocystis pouchetiii (Prymnesiophyceae) was encountered along the barrier edge of the Ross Ice Shelf and extended to depths of 100 to 150 m at some stations. Chlorophyll a throughout the euphotic zone along the ice shelf barrier edge averaged > 1 mg m−3, and primary productivity locally reached nearly 1 g C m−2 day−1 there. In contrast, both chlorophyll a concentration and rates of 14C uptake averaged two- to four-fold less some 200 km offshore of the Ross Ice Shelf, where P. pouchetii generally was rare. Where P. pouchetii was abundant, more than 25% of the water column primary production occurred at depths below the 1% light penetration level. Near-surface nitrate concentrations were about a third lower at the stations where P. pouchetii was abundant than at those with few or no P. pouchetii. Regionally there was a strong negative correlation between NH4+ and chlorophyll a, suggesting that the biogenic uptake of NH4+ by the phytoplankton controlled the distribution of NH4+ within the euphotic zone.

Perturbations of the Natural Lead Depth Profile in the Sargasso Sea by Industrial Lead

Abstract: Concentrations of dissolved lead were determined by isotope dilution mass spectrometry in 17 seawater samples of a depth profile collected under strict contamination control at a station in the northern Sargasso Sea, 250 km northwest of Bermuda. Dissolved lead concentrations were about 170 pmol•kg −1 from the surface down to 400 m, then declined sharply to relatively constant values of 25 pmol•kg−1 below 2000 m depth. This large concentration enrichment in the upper part of the water column is concordant with the distribution observed earlier in the central northeast Pacific, and it confirms the unique character of the occurrence of lead in open ocean water columns: the depth profile for lead differs markedly from those for most other trace metals, which show concentration depletions in surface waters instead.

Primary Production and Respiration in Pelagic and Benthic Communities at Two Intertidal Sites in the Upper Bay of Fundy

Abstract: Primary production by microalgae on intertidal sediments during ebb tide at two sites in Cumberland and Minas Basin, Bay of Fundy, amounted to 47–83 g C∙m−2∙yr−1 Phytoplankton production measured during flood tide over intertidal sediments in Cumberland Basin varied from 4–10 g C∙m−2∙yr−1 with respiration in the water column between 6 and 12 g∙C∙m−2∙yr−1 depending on concentrations of suspended matter. Respiration by undisturbed sediments (47–62 g C∙m−2∙yr−1) was measured at both locations to estimate aerobic metabolic consumption of organic matter.Maximum rates of benthic primary production occurred during early and late summer at both locations but Values at the Cumberland Basin sites were two to three times greater than those observed in Minas Basin; Chlorophyll a in surface sediments was also lower at the stations in Minas Basin where coarser grained deposits reflect extensive sediment transport. Annual benthic respiration at the two stations in Cumberland Basin, however, was only slightly greater tha...

Seasonal temperature-salinity changes and thermocline development in the mid-Atlantic Bight as recorded by the isotopic composition of bivalves

Abstract: Stable isotope records across annual growth increments in specimens of the surf clam Spisula solidissima from the mid-Atlantic Bight shelf from 10 m and 45 m depths reflect the changes in temperature and nutrient concentrations on the shelf over the year. The δ 18 O and δ 13 C records from clams at the two depths record well-mixed conditions in the water column during the winter months and the development of a thermocline during the summer. Spring high productivity and a transient salinity excursion in surface waters are also recorded. Reconstructing the paleoceanography of late Cenozoic temperate continental shelves may be possible using stable isotope records from fossil Spisula solidissima and other bivalves.

Influence of the Mediterranean Outflow on the isotopic composition of neodymium in waters of the North Atlantic

Abstract: The isotopic composition of neodymium in the water column of the eastern North Atlantic near the Strait of Gibraltar has been determined for several depths. The data show that the Mediterranean outflow results in a significant shift in e_(Nd)(0) toward more radiogenic values of ^(143)Nd/^(144)Nd in the water column at a 1000-m depth. This corresponds to a depth in the neighborhood of the salinity maximum associated with the Mediterranean outflow. The core of the Mediterranean outflow gives e_(Nd)(0) = −9.8, as compared to e_(Nd)(0) ≈ −12 in overlying and underlying waters, demonstrating that the Mediterranean waters are distinct from the Atlantic. From mixing considerations we estimate that pure Mediterranean waters have e_(Nd)(0) ≈ −6. Possible sources of this relatively radiogenic Nd could be from drainage of young continental terranes or the injection of remobilized Nd from deep-sea sediments that have a young radiogenic volcanic component. New data from a depth profile in the western Atlantic is presented. Comparison of Nd data for the eastern North Atlantic with that for the western North Atlantic shows fundamental differences in the water column structures for e_(Nd)(0). While both regions show a pronounced maximum in e_(Nd)(0), the western basin maximum occurs at the near surface rather than at 1000 m. In addition, deep waters of the eastern basin are found to be more radiogenic than the western basin. These differences indicate several sources of isotopically distinct Nd in the North Atlantic. The deep waters of the North Atlantic (>1000 m) have the lowest values of e_(Nd)(0) measured in the oceans. We believe that the source of these low e_(Nd)(0) values, which we associate with North Atlantic deep water, is either from freshwater drainage off the Precambrian shields of North America and Asia into the Arctic Ocean or from the injection of ‘older,’ continentally derived REE from deep-sea sediments. Sm and Nd concentrations are found to increase with depth and e_(Nd)(0) changes with depth, indicating both vertical and lateral transport processes from different sources. This suggests a surface source of Nd and injection of REE into the water column from deep-sea sediments or large-scale bottom currents with high REE concentrations.

Nutrients and primary production in permanently well-mixed temperate coastal waters

Abstract: Seasonal changes of nutrients and primary production were studied in Morlaix Bay (western English Channel) waters in 1979–1980. This region is characterized by strong tides and consequently the water column remains permanently well-mixed. Owing to this unique hydrological regime, primary production and nutrients follow seasonal cycles different from the classic temperate water cycles. Because of the instability of the water mass, phytoplankton development in spring is slow and attains a maximum only in summer. A minor autumnal primary production peak is absent. The slow development of phytoplankton results in a slow and prolonged utilization of nitrate and phosphate which attain their lowest values only at the end of summer. Predominance of diatoms in the spring phytoplankton results in a rapid utilization of silicon which reaches its lowest concentration as early as May. Ammonium shows a unique cycle in that it accumulates in the water column in spring-summer and is utilized by phytoplankton only when nitrate concentration is low. Nitrite changes show two peaks: a major peak in autumn associated with nitrogen remineralization and a minor peak in spring due to nitrite excretion by phytoplankton. Seasonal development of primary production in these waters is successively limited by silicon, nitrate and total inorganic nitrogen depletion in spring-summer before being eventually limited by light. This is substantiated by the seasonal changes of absolute 14 C assimilation values, solar energy utilization efficiency and assimilation number. Based on these results, it is hypothesized that the ratio between the depth of the euphotic zone and the depth of the water column will indicate whether light or nutrients limit primary productivity in well-mixed temperate waters.

The bottom‐ice microalgal community from annual ice in the inshore waters of east antarctica1

Abstract: The structure, productivity and heterotrophic potential of an extensive microalgal community growing on the underside of sea ice near the Australian Antarctic Station of Casey, are described Underwater observations made near the Australian Antarctic stations of Davis and Mawson are also reported This community develops during September, is largely suspended from the bottom surface of annual sea ice and often extends into the underlying water column as conspicuous strands up to 15 cm long The algal community structure in the strands is dominated by an unidentified tube diatom belonging to the Amphipleura/Berkeleya group and chains of a species of Entomoneis cf Amphiprora paludosa var hyperborea (Grunow) Cleve Unlike previously described bottom ice environments, a brash ice layer under the hard sea ice is absent Living cells, predominantly Nitzschia frigida Grunow, also occur in microbrine channels in the bottom 3 cm of the ice Maximal primary production rates of 81 μg C · L-1· h-1 occurred during November, then began declining near the end of December Minimal rates (28 μg C · L-1· h-1) were reached in mid-January and coincided with changes in the physical structure of the sea ice and in the stability of the water column An abundant epibacterial community associated with the microalgal strands assimilated 3H-labelled amino acids suggesting significant heterotrophic recycling of dissolved organic matter Turnover times of assimilated amino acids in the bottom ice community averaged 55 h during November while negligible turnover of these substrates occurred in the water column 15 m below the ice These bottom ice communities have higher primary productivity than typical brash ice communities; they are also accessible to marine herbivores and so may be more important to the Antarctic marine food chain than previously supposed

Upper water column methane geochemistry in the eastern tropical North Pacific1

Abstract: Upper water column (~400 m) profiles of methane, suspended matter, biological indicators, and hydrographic parameters were obtained from nine stations in the eastern tropical North Pacific Ocean. Methane was consistently supersaturated with respect to atmospheric eguilibrium and displayed a subsurface maximum containing about 2-3 times near-surface conccntrations associated with the upper part of the pycnocline. Correlations between methane and the particulate and biological parameters at all nine stations taken together were low, although several stations individually exhibited significant correspondences. The methane distribution was largely controlled by physical oceanographic processes, Microbial activity associated with suspended particles, possibly recycled by repetitive zooplankton grazing, is believed most likely responsible for the excess methane.

Vertical transport and residence time of chlorinated hydrocarbons in the open ocean water column

Abstract: The vertical transport of PCBs and chlorinated hydrocarbon pesticides such as DDT compounds and HCH (BHC) isomers in the deep sea are discussed on basis of their vertical profiles and the proportion of their adsorbed and dissolved fractions in surface water surveyed in the Western Pacific, Eastern Indian and Antarctic Oceans.

Temperature, heat, and mixing in Lake Valencia, Venezuela1

Abstract: Water temperature profiles, weather data, and lake level data spanning a 2-year period for Lake Valencia, Venezuela (10°N, 67°W), are used in an analysis of thermal regime, mixing, and heat flux. The lake, which has an area of 350 km2 and mean depth of 19 m, is warm monomictic; it becomes isothermal annually near the end of November, at the onset of the dry season. The water column during stratification consists of an upper mixed layer, which averages 13 m but varies greatly in thickness, followed by alternating thick layers of positive and negligible density gradients. The number of layers changes through time. Vertical eddy diffusivities during stratification vary from 0.23 cm2·s−1 at 20 m to 0.09 at 35 m. The mean annual heat budget is 5,308 cal · cm−2. Evaporative heat loss and net radiation flux are the two largest heat budget elements; net back radiation and sensible heat loss are important secondary terms. Seasonality is principally traceable to heat budget terms influenced by minimum air temperature, whereas week-to-week variation is influenced by other terms as well. Maximum stability of the water column is about 350 g-cm · cm−2 with little variation between years. Work of the wind in distributing heat is 2,630 g-cm·cm−2 for 1977 and 3,156 in 1978. Stability responds in a significant way to weather changes, even during stratification.

Remote sensing of phytoplankton in the sea: surface-layer chlorophyll as an estimate of water-column chlorophyll and primary production

Abstract: The utility is assessed of remotely-sensed chlorophyll data in biological oceanography. Oceanographic data, highly resolved in the vertical, are used as a basis for estimating the (weighted) proportion of the water-column chlorophyll chat is accessible to the remote sensor. Examples are calculated for the continental shelf off Nova Scotia, the Canadian Arctic and the coast of Peru. The analysis is extended to the estimation of phytoplankton production. Remotely-sensed data contain only a small (5 per cent of phytoplankton biomass and 11 per cent of the turnover), but surprisingly stable, fraction of the information for the water column. A modest ground-truthing programme is required to exploit these data to the best advantage.

Tidal exchange of nitrogen and phosphorus between a mesohaline vegetated marsh and the surrounding estuary in the lower Chesapeake Bay

Abstract: A 22-month study was conducted to determine the exchange of nitrogen and phosphorus between a mesohaline vegetated marsh in the Carter's Creek area of Virginia and the surrounding estuary, focusing on the role of the vegetated marsh surface in the processing of these constituents. On an annual basis there was a removal of NH 4 + , PO 4 3− , NO 3 − , dissolved organic nitrogen, dissolved organic phosphorus, particulate nitrogen and particulate phosphorus from the tidal water as it resided on the vegetated marsh. Only nitrite was transported from the marsh to the estuary. Most of the nitrogen and phosphorus species showed distinct seasonal trends with respect to the direction of transport except nitrate and orthophosphate. The ammonium flux data indicates that this nutrient was removed from the inundating water during late spring and fall, with a slight release of this constituent into the tidal water during the late summer. The transport of nitrite was from the estuary to the marsh for most of the year except during the fall. The large release of this nutrient into the tidal water at this time is associated with the senescence of the marsh vegetation. There was a large removal of DON from the tidal water during the fall, while the flux of DOP was from the estuary to the marsh for most of the year except during the summer. The largest removal of particulate nitrogen and phosphorus from the tidal water occurred during the summer months when the turbidity of the tidal water was highest, especially when wave scouring of the mudflats brings material into the water column. A loss of particulate nitrogen from the marsh to the estuary was evident during the fall and winter.

Geochemistry of burrow waters vented by a bioturbating shrimp in Bermudian sediments

Abstract: Waters collected at various times from the burrows of Callianassa spp. shrimp are characterized chemically by concentrations of nutrients, sulfide and organic carbon, intermediate between those of pore waters and overlying waters of Coot Pond, Bermuda. Although the shrimp vigorously flush their burrows, irrigation events are insufficently frequent to maintain a burrow water composition completely like that of oxygenated overlying waters; in instances where departure is extensive, models describing the diffusion of interstitial-water solutes into the water column via burrows would have to take this into account. Measured oxidation potentials are substantially reduced with respect to those of the water column, and relatively high levels of reduced arsenic (As III) are encountered. Burrow irrigation provides a mechanism for the rapid, conservative advection of solutes across the sediment-water interface and therefore is at least partially responsible for the concentration and speciation variations typical of coastal seawater. Of the solutes tested, only total dissolved inorganic arsenic occurs at seawater concentration in the burrows, implying that some specialized mechanism intercepts the diffusion of pore water arsenic into the burrows.

Vertical Transport and Transformation of Biogenic Organic Compounds from a Sediment Trap Experiment Off the Coast of Peru

Abstract: Sediment trap experiments using the lipid biomarker approach were undertaken in the Peru coastal upwelling region to answer three main questions concerning the sources, transport and transformation of organic matter in upwelling regimes. 1. How does the temporal and spatial variability of the vertical flux and composition of particulate matter change as it sinks to the sediment surface? 2. How suitable are the various lipids incorporated into the sedimentary record as indicators of the paleoenvironment of deposition? This involves a determination of how much of these compounds survive biological degradation in the water column and surface sediments. 3. What is the relationship between the organic matter composition of sinking particulate material and biological processes in the water column? The composition and vertical fluxes of the steroid class compounds are used as an example of how these questions may be answered.

Heterotrophic activity throughout a vertical profile of seawater and sediment in halifax harbor, Canada.

Abstract: The relative heterotrophic activity of marine microorganisms was determined at two sites by the heterotrophic uptake technique throughout the water column, the sediment-water interface, and the surface layer of sediment. In the water column, uptake was greatest at the surface and steadily decreased with depth. The percentage of the substrate that was respired also decreased with depth from 69 to 56%. The activity of the sediment-water interface was several orders of magnitude greater than that of the overlying water and twice that of the sediment immediately below. Hand-collected water samples carefully taken as close as 1 cm from the sediment-water interface had the same characteristically low activity as the bottom few meters of water. Microautoradiography with H-labeled glucose, glutamic acid, or thymidine revealed a general decrease in the percentage of active cells with depth from 35 to <1%. The number of active cells in the interface and sediment averaged <10% of the total population. The data indicate that the sediment-water interface is the most active region in this system due to an increased number of active cells rather than an increased percentage of active cells or increased per-cell activity.

Population dynamics of halobacteria in the Dead Sea water column1

Abstract: A bloom of halobacteria in the Dead Sea is described, and the factors triggering its rise and decline are analyzed. The bloom developed in summer 1980, reaching population densities of up to 1.9 X lo7 cells.ml-l and imparting a red color to the water. The bloom was restricted to the upper lo-25 m of the water column. After a sharp decrease in cell numbers during the last months of 1980 the population density stabilized at values of around 5 X 10fi cells.ml-’ surface water for more than a year. Enrichment experiments performed during that period showed that the nutrients limiting bacterial development were phosphate and suitable carbon sources. The turnover of phosphate appeared to be extremely slow (estimated turnover times as long as 240-1,200 days), suggesting that the population, a significant part of which proved viable, existed in a state of very little activity.

Mesoscale response of diatom populations to a wind event in the plume of the Hudson River

Abstract: Effects of a southwest wind event on distributions of dissolved inorganic nitrogen and diatom biomass are described and evaluated in terms of interactions between circulation, static stability of the water column, and the suspension and growth of diatom populations. A diatom bloom, dominated by Skeletonema costatum , developed in response to upwelling as a consequence of the vertical transport of biomass from the aphotic zone (deduced from distributions and rate measurements), a decrease in sinking rate from 1.0 to 0.3 m d −1 (from sediment trap collections), and lower dilution rates (from hydrography). Carbon-specific growth rate of diatoms showed little variability based on measurements made before and during the bloom, i.e., variations in diatom production were primarily due to variable loss rates rather than to growth. The influence of diatom production associated with the coastal plume of the Hudson River (areas 1000 km 2 ) was observed to extend ca. 100 km seaward of the zone of most active production. Episodes of cross-shelf transport and onshore accumulation of phytoplankton biomass appear to alternate with periods of high surface production in the plume.

Production and consumption of hydrogen in a eutrophic lake.

Abstract: The vertical distribution of hydrogen was measured in the Loclat, a eutrophic and holomictic lake near Neuchâtel, Switzerland, before and during summer stratification. H2 concentrations decreased with depth in the anaerobic hypolimnion and were often below the detection limit (2.5 nl of H2 liter−1) in the water adjacent to the lake sediment. H2 was apparently not released from the lake sediment. The highest H2 concentrations (>4 μl of H2 liter−1) were observed in the aerobic water of the epilimnion and metalimnion. There, the H2 concentrations changed with time, indicating a turnover of H2. The H2 production processes could not be studied in the laboratory since incubation of water samples in light or darkness did not result in H2 production but rather always in H2 consumption. The possible role of cyanobacteria and algae for H2 production is discussed. Aerobic or anaerobic H2 consumption activities were observed at all depths of the water column, with highest activities in the hypolimnion. Aerobic H2 consumption activity was insensitive to azide inhibition, but sensitive to heat, mercuric chloride, or cyanide. It was restricted to a particle fraction of 0.2 to 3.0 μm in size, so that it must be due to single bacterial cells. Aerobic hydrogen bacteria, on the other hand, occurred in clusters of >3.0 μm. Therefore, the hydrogen bacteria could not have caused the H2 consumption in lake water. The aerobic H2 consumption activity followed Michaelis-Menten kinetics, with a Km of 67 nM H2. This is an exceptionally low value compared with Km values of hydrogenases in hydrogen bacteria and other species, but is similar to that for H2-decomposing abiontic soil hydrogenases.