Water column

About: Water column is a research topic. Over the lifetime, 13706 publications have been published within this topic receiving 496626 citations.

Measurement of mercury methylation in lake water and sediment samples.

Abstract: Biological mercury methylation was assayed by a new radiochemical technique in the water column and sediments of a mercury-contaminated lake. In 24 weeks during 1979, there were three episodes of methylating activity in surface floc and in water, each lasting 3 to 5 weeks. Periods of methylation in the water column coincided with surface sediment methylation and appeared to be related to overall microbial activity. Mercury was actively methylated in the presence of bound sulfide.

Vertical separation of light and available nutrients as a factor causing replacement of green algae by blue-green algae in the plankton of a stratified lake

Abstract: (1) A change from the green alga, Dictyosphaerium pulchellum to the blue-green algae Microcystis aeruginosa and Anabaena spiroides occurred in the plankton of Mt Bold reservoir, South Australia, during November and December 1978. (2) The reservoir was 30-34 m deep, thermally stratified with water mixed to 6-7 m, and a euphotic zone of less than 2.7 m. Measurements of photosynthesis and light penetration showed that photosynthesis was restricted probably to the upper 2.5 m. (3) Growth experiments with eight cultured species, and the natural phytoplankton assemblage, showed that during this change there was a decline of the potential of the water for algal growth to undetectable levels at a depth of 8 m. This could be restored by the combined addition of nitrogen, phosphorus and Fe-EDTA. (4) Continuous measurement of concentration of chlorophyll a in the water column demonstrated that the blue-green algal community was able to migrate to a depth of 12 m despite substantial density barriers. (5) In situ growth experiments, using dialysis bags, showed that populations which were artificially circulated between 0.2 and 10 m, were able to grow, whereas those maintained at a single depth (0-2 m or 10 m) were not. (6) The ability of blue-green algae to overcome successfully this spatial separation between light and nutrients was proposed as the probable cause of the change from green to blue-green algae.

Nutrients, organic carbon and organic nitrogen in the upper water column of the Arctic Ocean: implications for the sources of dissolved organic carbon

Abstract: Particulate and dissolved organic carbon and nitrogen were analyzed for shelf, slope and basin samples collected during the 1994 Arctic Ocean Section. Concentrations of organic carbon and nitrogen were highest in the surface water and decreased dramatically below 100 m. Over the shelf 40–50% of the organic material was present in the particulate phase, while in the slope and basin waters >-90% of the organic material was present in the dissolved phase. The halocline layer over the Chukchi plateau contained high concentrations of inorganic nutrients but no significant elevation of either organic carbon or organic nitrogen. Highest concentrations of total organic carbon were found in the surface water of the Makarov and Amundsen Basins between 80 and 90° N. The C:N ratio of the dissolved organic material in the upper 100 m ranged from 9 to 25, with highest ratios along the Makarov flank of the Lomonosov Ridge. We suggest that a major portion of the carbon-rich organic material in surface waters is derived from Eurasian shelf riverine inputs, which tend to flow in perimeter currents along the slope and ridges. Our preliminary dissolved organic carbon (DOC) budget suggests that the three major sources of DOC in the central Arctic are in situ production (56%), river run-off (25%), and Pacific water (19%).

Simulating the global distribution of nitrogen isotopes in the ocean

Abstract: 9 isotopes, 14 N and 15 N, in the nitrate (NO3 ), phytoplankton, zooplankton, and detritus 10 variables of the marine ecosystem model. The isotope effects of algal NO3 uptake, 11 nitrogen fixation, water column denitrification, and zooplankton excretion are considered 12 as well as the removal of NO3 by sedimentary denitrification. A global database of 13 d 15 NO3 observations is compiled from previous studies and compared to the model 14 results on a regional basis where sufficient observations exist. The model is able to 15 qualitatively and quantitatively reproduce many of the observed patterns such as high 16 subsurface values in water column denitrification zones and the meridional and vertical 17 gradients in the Southern Ocean. The observed pronounced subsurface minimum in the 18 Atlantic is underestimated by the model presumably owing to too little simulated 19 nitrogen fixation there. Sensitivity experiments reveal that algal NO3 uptake, nitrogen 20 fixation, and water column denitrification have the strongest effects on the simulated 21 distribution of nitrogen isotopes, whereas the effect from zooplankton excretion is 22 weaker. Both water column and sedimentary denitrification also have important indirect 23 effects on the nitrogen isotope distribution by reducing the fixed nitrogen inventory, 24 which creates an ecological niche for nitrogen fixers and, thus, stimulates additional N2 25 fixation in the model. Important model deficiencies are identified, and strategies for 26 future improvement and possibilities for model application are outlined.

The role of mat-forming diatoms in the formation of Mediterranean sapropels

Abstract: The origins of sapropels (sedimentary layers rich in organic carbon) are unclear, yet they may be a key to understanding the influence of climate on ocean eutrophication, the mechanisms of sustaining biological production in stratified waters and the genesis of petroleum source rocks1,2,3. Recent microfossil studies of foraminifera1 and calcareous nannofossils2 have focused attention on a deep chlorophyll maximum as a locus for the high production inferred3 for sapropel formation, but have not identified the agent responsible. Here we report the results of a high-resolution, electron-microscope-based study of a late Quaternary laminated sapropel in which the annual flux cycle has been preserved. We find that much of the production was by diatoms, both mat-forming and other colonial forms, adapted to exploit a deep nutrient supply trapped below surface waters in a stratified water column. Reconstructed organic-carbon and opal fluxes to the sediments are comparable to those at high-productivity sites in today's oceans, and calculations based on diatom Si/C ratios suggest that the high organic-carbon content of sapropels may be entirely accounted for by sedimenting diatoms. We propose that this style of production may have been common in ancient Palaeogene and Cretaceous seas, environments for which conventional appeals to upwelling-driven production to account for the occurrence of diatomites, and some organic-carbon-rich sediments, have never seemed wholly appropriate.