Water column

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

Gradients in water column nutrients, sediment parameters, irradiance and coral reef development in the Whitsunday Region, central Great Barrier Reef

Abstract: Spatial variation of water column characteristics, sediment parameters and irradiance on coral reefs was examined in the Whitsunday region of the Great Barrier Reef during five sampling events between 2004 and 2006. Sampling locations were selected along a transect from outer reefs distant from terrestrial inputs, to coastal reefs near the Australian mainland. Most of the water column variables (especially chlorophyll a, total suspended solids, particulate organic carbon and particulate nutrients) and irradiance variables (Secchi and optical depth) changed significantly along the transect. For example, concentrations of chlorophyll a and total suspended solids increased two- and three-fold, respectively, from outer to coastal locations, while sediment inorganic carbon decreased and sediment colour became darker. Similarly, Secchi and optical depth, two measures of water transparency, decreased approximately three-fold towards the coast. Most of these gradients were persistent over the five sampling events, but for several parameters, the magnitude and steepness of the gradient changed. The maximum depth of zooxanthellate corals decreased five-fold along the gradient from outer to coastal locations and was related significantly to a water quality index derived for the Whitsunday Islands. Our data of the maximal depth limit for reef development at locations where suitable settlement substrata were available suggest that the absolute minimum of light required for a reef to persist is in the range of 6–8% of surface irradiance in the Whitsunday Islands.

Water column anomalies in dissolved silica over opaline Pelagic sediments and the origin of the deep silica maximum

Abstract: Recent estimates based on pore water studies and mass balance considerations put the average flux of dissolved silica from the sediments into the deep water at about 3 μmol cm−2 yr−1. This flux, if mixed uniformly in a bottom layer 100 m thick, results in an anomaly increment of 0.3 μmol kg−1 yr−1. In basins of restricted circulation containing opaline sediments the residence time of the bottom waters should be long enough that the resulting anomaly be easily resolvable using existing data. Examination of the many hundreds of detailed, precise silica profiles presently available in unpublished reports shows that this is indeed the case. The largest effects (∼50 μmol/kg) are seen in the Weddell-Enderby Basin. Intermediate anomalies (∼20 μmol/kg) are prevalent in the northern Indian Ocean. Lesser features (∼10 μmol/kg) are clearly discernable in the other antarctic basins and in the north Pacific. No anomalies are observed over the equatorial Pacific and Indian bands of opaline deposits, even in the basins east of the East Pacific Rise or in the Central Basin of the Indian Ocean. The flux of dissolved silica from the sea floor is generally unaccompanied by any resolvable oxygen consumption, indicating that relatively minor amounts of organic material reach the sediments in these regions. However, in the north Indian Ocean there are pronounced effects on all the nutrients, oxygen, and alkalinity, with regeneration approximating the Redfield prediction. In the northern Indian Ocean and the extreme northeast Pacific (northeast of 45°N, 160°W) the silica profiles increase to the bottom. This is a strong indication that the deep silica maximum observed over much of these oceans may have a large advective component, the feature itself being induced by the northward flow of the underlying low-silica bottom waters. In the Indian Ocean the data coverage is sufficient to demonstrate unequivocally the dominance of this effect.

Undocumented water column sink for cadmium in open ocean oxygen-deficient zones

Abstract: Cadmium (Cd) is a micronutrient and a tracer of biological productivity and circulation in the ocean. The correlation between dissolved Cd and the major algal nutrients in seawater has led to the use of Cd preserved in microfossils to constrain past ocean nutrient distributions. However, linking Cd to marine biological processes requires constraints on marine sources and sinks of Cd. Here, we show a decoupling between Cd and major nutrients within oxygen-deficient zones (ODZs) in both the Northeast Pacific and North Atlantic Oceans, which we attribute to Cd sulfide (CdS) precipitation in euxinic microenvironments around sinking biological particles. We find that dissolved Cd correlates well with dissolved phosphate in oxygenated waters, but is depleted compared with phosphate in ODZs. Additionally, suspended particles from the North Atlantic show high Cd content and light Cd stable isotope ratios within the ODZ, indicative of CdS precipitation. Globally, we calculate that CdS precipitation in ODZs is an important, and to our knowledge a previously undocumented marine sink of Cd. Our results suggest that water column oxygen depletion has a substantial impact on Cd biogeochemical cycling, impacting the global relationship between Cd and major nutrients and suggesting that Cd may be a previously unidentified tracer for water column oxygen deficiency on geological timescales. Similar depletions of copper and zinc in the Northeast Pacific indicate that sulfide precipitation in ODZs may also have an influence on the global distribution of other trace metals.