Water column

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

Geographical variation of the water column distrubution of suspended particulate organic nitrogen and its 15N natural abundance in the Pacific and its marginal seas

Abstract: The water column distribution of suspended particulate organic nitrogen (PON) and its natural abundance ratio of 15N: 14N were investigated to a depth of ∼4000 m at 13 stations in the North Pacific, and the South China, Philippine and Bering seas. At two stations in the northern North Pacific, sediment trap experiments were also carried out. The δ15N of PON ranged from −1.5 to 23.3 per mil. The 15N natural abundance of PON increased with depth between 0 and 200 m, while the PON concentration decreased sharply in the same depth range. In the vertical profiles, the PON in the deep water was, on an average, enriched with 15N by approximately 6 per mil as compared with that in the euphotic zone. These findings imply that the vertical transport of organic matter is mediated primarily by rapidly sinking particles, and that most of the decomposition of organic matter takes place in the shallow layer beneath the bottom of the euphotic zone (<200 m) in a similar manner at all locations. The average 15N abundance of PON in the water column was higher in the eastern tropical and central gyre portions of the Pacific than in the western Pacific, the South China Sea, the Philippine Sea, and the Bering Sea. Year-round stratification, the influence of 15N enriched nitrate produced during denitrification and the lack of significant nitrogen fixation in the surface layer probably caused the 15N enrichment in the eastern tropical Pacific.

Mobilization of radiocaesium in pore water of lake sediments

Abstract: THE deposition of large amounts of radiocaesium from nuclear weapons testing and from accidents such as Chernobyl has necessitated study of the fate of these long-lived radioisotopes in the natural environment. Caesium is known to interact strongly with micaceous clay minerals in soils and sediments1–4. Radiocaesium can therefore be removed from the water column in lake systems by settling particles and surface sediments5. This process reduces its mobility and the risk of assimilation by biota. Nevertheless, there are indications that radiocaesium may be mobilized from lacustrine anoxic sediments6. Direct evidence, however, can come only from measurements on pore water from lake sediments, and here we report such measurements. Actual in situsolid/liquid distribution coefficients (Kd values), determined from our data, provide convincing evidence for post-depositional mobilization of particle-bound 137Cs. This is probably caused by ion exchange with NH4+which reaches high concentrations in anoxic pore waters. The pore-water data indicate that radiocaesium is returned to the water column and thus becomes available for uptake by aquatic organisms.

Phytoplankton, light, and nutrients in a gradient of mixing depths: field experiments

Abstract: We studied the effects of water column mixing depth and background turbidity on phytoplankton biomass, light climate, and nutrients in two field enclosure experiments designed to test predictions of a dynamical model. In 1997 and 1998, we created gradients of mixing depth by enclosing the 100-μm-filtered phytoplankton community of a phosphorus-deficient lake in cylindrical plastic bags of varying depth (1.5–15 m) which were continuously mixed. To mimic different levels of background turbidity, we surrounded the transparent enclosure walls with a layer of opaque white (1997) or black (1998) plastic. The experiments were run for 4 wk (1997) and 6 wk (1998). The results supported two key assumptions of the model: specific production and specific sedimentation losses both decreased with increasing mixing depth. At all mixing depths, fast-sinking diatoms dominated the communities. In accordance with model predictions, algal biomass concentration and standing stock (summed over the mixed layer) showed a unimodal relationship to mixing depth when background turbidity was high (1998). When background turbidity was lower (1997), only the ascending limbs of the corresponding relationships were found, which supports the prediction that the mixing depth at which biomass peaks (i.e., becomes predominantly limited by light) increases with decreasing background turbidity. Also in accordance with predictions, light intensity at the bottom of the mixed layer decreased with increasing mixing depth and with increasing background turbidity. Finally, the data supported only the ascending limbs of the predicted inverse unimodal relationships among mixing depth and dissolved inorganic and total water column phosphorus. The absence of descending limbs in these relationships at low mixing depths was probably due to deviations of the experimental systems from two model assumptions. First, the remineralization rate of sedimented phosphorus may have been too slow to equilibrate with sedimentation losses over the experimental periods. Second, biomass yield per unit nutrient (the ratio of seston carbon to phosphorus) was not constant, but decreased with increasing mixing depth. To our knowledge, these are the first field experiments in which the effects of mixing depth on phytoplankton and its resources have been investigated systematically along a large gradient.