Water column

About: Water column is a(n) research topic. Over the lifetime, 13706 publication(s) have been published within this topic receiving 496626 citation(s).

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.

Denitrification in freshwater and coastal marine ecosystems: Ecological and geochemical significance

Abstract: Denitrification occurs in essentially all river, lake, and coastal marine ecosystems that have been studied. In general, the range of denitrification rates measured in coastal marine sediments is greater than that measured in lake or river sediments. In various estuarine and coastal marine sediments, rates commonly range between 50 and 250 µmol N m−2 h−1, with extremes from 0 to 1,067. Rates of denitrification in lake sediments measured at near-ambient conditions range from 2 to 171 µmol N m−2 h−1. Denitrification rates in river and stream sediments range from 0 to 345 µmol N m−2 h−1. The higher rates are from systems that receive substantial amounts of anthropogenic nutrient input. In lakes, denitrification also occurs in low oxygen hypolimnetic waters, where rates generally range from 0.2 to 1.9 µmol N liter−1 d−1. In lakes where denitrification rates in both the water and sediments have been measured, denitrification is greater in the sediments. The major source of nitrate for denitrification in most river, lake, and coastal marine sediments underlying an aerobic water column is nitrate produced in the sediments, not nitrate diffusing into the sediments from the overlying water. During the mineralization of organic matter in sediments, a major portion of the mineralized nitrogen is lost from the ecosystem via denitrification. In freshwater sediments, denitrification appears to remove a larger percentage of the mineralized nitrogen. N2 fluxes accounted for 76–100% of the sediment-water nitrogen flux in rivers and lakes, but only 15–70% in estuarine and coastal marine sediments. Benthic N2O fluxes were always small compared to N, fluxes. The loss of nitrogen via denitrification exceeds the input of nitrogen via N2 fixation in almost all river, lake, and coastal marine ecosystems in which both processes have been measured. Denitrification is also important relative to other inputs of fixed N in both freshwater and coastal marine ecosystems. In the two rivers where both denitrification measurements and N input data were available, denitrification removed an amount of nitrogen equivalent to 7 and 35% of the external nitrogen loading. In six lakes and six estuaries where data are available, denitrification is estimated to remove an amount of nitrogen equivalent to between 1 and 36% of the input to the lakes and between 20 and 50% of the input to the estuaries.

Technical basis for establishing sediment quality criteria for nonionic organic chemicals using equilibrium partitioning

Abstract: The purpose of this review paper is to present the technical basis for establishing sediment quality criteria using equilibrium partitioning (EqP). Equilibrium partitioning is chosen because it addresses the two principal technical issues that must be resolved: the varying bioavailability of chemicals in sediments and the choice of the appropriate biological effects concentration. The data that are used to examine the question of varying bioavailability across sediments are from toxicity and bioaccumulation experiments utilizing the same chemical and test organism but different sediments. It has been found that if the different sediments in each experiment are compared, there is essentially no relationship between sediment chemical concentrations on a dry weight basis and biological effects. However, if the chemical concentrations in the pore water of the sediment are used (for chemicals that are not highly hydrophobic) or if the sediment chemical concentrations on an organic carbon basis are used, then the biological effects occur at similar concentrations (within a factor of two) for the different sediments. In addition, the effects concentrations are the same as, or they can be predicted from, the effects concentration determined in water- only exposures. The EqP methodology rationalizes these results by assuming that the partitioning of the chemical between sediment organic carbon and pore water is at equilibrium. In each of these phases, the fugacity or activity of the chemical is the same at equilibrium. As a consequence, it is assumed that the organism receives an equivalent exposure from a water-only exposure or from any equilibrated phase, either from pore water via respiration, from sediment carbon via ingestion; or from a mixture of the routes. Thus, the pathway of exposure is not significant. The biological effect is produced by the chemical activity of the single phase or the equilibrated system. Sediment quality criteria for nonionic organic chemicals are based on the chemical concentration in sediment organic carbon. For highly hydrophobic chemicals this is necessary because the pore water concentration is, for those chemicals, no longer a good estimate of the chemical activity. The pore water concentration is the sum of the free chemical concentration, which is bioavailable and represents the chemical activity, and the concentration of chemical complexed to dissolved organic carbon, which, as the data presented below illustrate, is not bioavailable. Using the chemical concentration in sediment organic carbon eliminates this ambiguity. Sediment quality criteria also require that a chemical concentration be chosen that is sufficiently protective of benthic organisms. The final chronic value (FCV) from the U.S. Environmental Protection Agency (EPA) water quality criteria is proposed. An analysis of the data compiled in the water quality criteria documents demonstrates that benthic species, defined as either epibenthic or infaunal species, have a similar sensitivity to water column species. This is the case if the most sensitive species are compared and if all species are compared. The results of benthic colonization experiments also support the use of the FCV. Equilibrium partitioning cannot remove all the variation in the experimentally observed sediment- effects concentration and the concentration predicted from water-only exposures. A variation of approximately a factor of two to three remains. Hence, it is recognized that a quantification of this uncertainty should accompany the sediment quality criteria. The derivation of sediment quality criteria requires the octanol/water partition coefficient of the chemical. It should be measured with modern experimental techniques, which appear to remove the large variation in reported values. The derivation of the final chronic value should also be updated to include the most recent toxicological information.

Particulate organic carbon flux in the oceans—surface productivity and oxygen utilization

Abstract: Organic detritus passing from the sea surface through the water column to the sea floor controls nutrient regeneration, fuels benthic life and affects burial of organic carbon in the sediment record1–3. Particle trap systems have enabled the first quantification of this important process. The results suggest that the dominant mechanism of vertical transport is by rapid settling of rare large particles, most likely of faecal pellets or marine snow of the order of >200 μm in diameter, whereas the more frequent small particles have an insignificant role in vertical mass flux4–6. The ultimate source of organic detritus is biological production in surface waters of the oceans. I determine here an empirical relationship that predicts organic carbon flux at any depth in the oceans below the base of the euphotic zone as a function of the mean net primary production rate at the surface and depth-dependent consumption. Such a relationship aids in estimating rates of decay of organic matter in the water column, benthic and water column respiration of oxygen in the deep sea and burial of organic carbon in the sediment record.

Anoxic Environments and Oil Source Bed Genesis

Abstract: The anoxic aquatic environment is a mass of water so depleted in oxygen that virtually all aerobic biologic activity has ceased. Anoxic conditions occur where the demand for oxygen in the water column exceeds the supply. Oxygen demand relates to surface biologic productivity, whereas oxygen supply largely depends on water circulation, which is governed by global climatic patterns and the Coriolis force. Organic matter in sediments below anoxic water is commonly more abundant and more lipid-rich than under oxygenated water mainly because of the absence of benthonic scavenging. The specific cause for preferential lipid enrichment probably relates to the biochemistry of anaerobic bacterial activity. Geochemical-sedimentologic evidence suggests that potential oil source beds are and have been deposited in the geologic past in four main anoxic settings as follows. 1. Large anoxic lakes: Permanent stratification promotes development of anoxic bottom water, particularly in large lakes which are not subject to seasonal overturn, such as Lake Tanganyika. Warm equable climatic conditions favor lacustrine anoxia and nonmarine oil source bed deposition. Conversely, lakes in temperate climates tend to be well oxygenated. 2. Anoxic silled basins: Only those landlocked silled basins with positive water balance tend to become anoxic. Typical are the Baltic and Black Seas. In arid-region seas (Red and Mediterranean Seas), evaporation exceeds river inflow, causing negative water balance and well-oxygenated bottom waters. Anoxic conditions in silled basins on oceanic shelves also depend upon overall climatic and water-circulation patterns. Silled basins should be prone to oil source bed deposition at times of worldwide transgression, both at high and low paleolatitudes. Silled-basin geometry, however, does not automatically imply the presence of oil source beds. 3. Anoxic layers caused by upwelling: These develop only when the oxygen supply in deep water cannot match demand owing to high surface biologic productivity. Examples are the Benguela Current and Peru coastal upwelling. No systematic correlation exists between upwelling and anoxic conditions because deep oxygen supply is often sufficient to match strongest demand. Oil source beds and phosphorites resulting from upwelling are present preferentially at low paleolatitudes and at times of worldwide transgression. 4. Open-ocean anoxic layers: These are present in the oxygen-minimum layers of the northeastern Pacific and northern Indian Oceans, far from deep, oxygenated polar water sources. They are analogous, on a reduced scale, to worldwide "oceanic anoxic events" which occurred at global climatic warmups and major transgressions, as in Late Jurassic and middle Cretaceous times. Known marine oil source bed systems are not randomly distributed in time but tend to coincide with periods of worldwide transgression and oceanic anoxia. Geochemistry, assisted by paleogeography, can greatly help petroleum exploration by identifying paleoanoxic events and therefore widespread oil source bed systems in the stratigraphic record. Recognition of the proposed anoxic models in ancient sedimentary basins should help in regional stratigraphic mapping of oil shale and oil source beds.