Water column

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

The selective advantage of buoyancy provided by gas vesicles for planktonic cyanobacteria in the Baltic Sea

Abstract: Observations were made on the vertical distribution of colonies of Aphanizomenon flos-aquae for 9 d at a drift-station east of Bornholm Island in the Baltic Sea. The buoyant colonies were dispersed in the upper layers of the water column during periods of wind-induced mixing but floated up during calm periods. From measurements of the vertical light extinction, surface irradiance and the photosynthesis versus irradiance curve, calculations were made of the changes in the daily integral of photosynthesis with respect to time and depth throughout the water column. From these calculations it is demonstrated that net photosynthesis by the population of Aphanizomenon flos-aquae increased nearly threefold by floating up after a deep mixing event. It is estimated that, averaged over alternating periods of calm and mixing, the buoyancy provided by gas vesicles in this organism will result in a nearly twofold increase in photosynthesis. A quantitative analysis has been made of the relationship of the daily integral of photosynthesis by the Aphanizomenon population with the mean depth of the population in the water column and the daily insolation. The analysis shows that the integral decreases linearly with respect to mean depth.

Planktonic foraminifera as recorders of past surface-water stratification

Abstract: The δ 18 O difference between shallow- and deep-living planktonic foraminifera is a proxy for the stratification of surface waters. In Holocene sediments from the Atlantic this difference increases from about 0‰ in subpolar regions to ∼3‰ in the tropics. The δ 18 O values of the shallow dwellers Globigerinoides sacculifer and Neogloboquadrina pachyderma mainly reflect surface-water conditions. The calcification depth of the deep-living species Globorotalia truncatulinoides (right-coiling) is affected by the stratification of the water column. In the subtropics, shell formation of G. truncatulinoides (right-coiling) begins in weakly stratified surface waters. In the tropics, the initial shell is secreted below the mixed layer. This hydrographically induced difference in calcification depth allows the use of the isotopic difference between G. truncatulinoides and the shallow species as a proxy for the stratification of surface waters. The Δδ 18 O between G. truncatulinoides and G. sacculifer from tropical gravity Core GeoB 1523-1 was significantly lower during isotope stage 2. This suggests that the glacial temperature stratification of western equatorial Atlantic surface waters was significantly reduced relative to the Holocene.

Hydrographic variability off the Rías Baixas (NW Spain) during the upwelling season

Abstract: During the Galicia X cruise, from May to October 1989 an intensive collection of hydrographic data was carried out at a single station on the shelf off the western coast of Galicia. It allows us to follow the response of the water column to the intermittent equatorward wind stress during the upwelling season. Upwelling events occur with biweekly periodicity, bringing Eastern North Atlantic Water (ENAW) to the subsurface layer at our station. A trend of the thermohaline properties of the upwelled water to increase in time was observed. This seems to be mostly due to the southwestward displacement of the origin of this water mass during the year. Although the saltier and warmer ENAW is less nutrient-rich, nutrient levels increase because of the rapid remineralization of organic matter from the Rias, which takes place in the bottom water on the shelf.

Spatial-temporal variability in surface layer deepening and lateral advection in an embayment of Lake Victoria, East Africa

Abstract: Vertical and horizontal exchanges in Pilkington Bay, a shallow (9 m) embayment of Lake Victoria, were determined from a surface energy budget, time series measurements of temperature, and quasi synoptic transects of conductivity, temperature, and depth conducted over a 2-d period. The surface energy budget is the first from a tropical lake over a diurnal timescale. Strong stratification developed during morning and early afternoon (.40 cycles h 21 ) but was eroded beginning in the afternoon by the combination of wind and heat loss. Surface heat losses contributed .70% of the energy for surface layer deepening 82% of the time from midafternoon until midmorning. Circulation times of the surface layer were ,2 min as it deepened to 1.5 m in the afternoon and were ,12 min at night even when mixing extended to the lake bottom. Spatial differences in the rates of heating and cooling and in the depth of wind mixing caused fronts to develop on spatial scales of kilometers within the bay. Convergence of these fronts led to downwelling of surface waters and upwelling of deep waters during the stratified period. Horizontal pressure gradients due to differential heating contributed to thermocline downwelling, lateral movement of deep, anoxic waters, and generation of high-frequency internal waves, all of which contribute to vertical and horizontal transports. Although wind and heat loss at one location generally determine the depth of the surface layer and thermocline, the depths of these key features may be strongly influenced by rates of heating and cooling elsewhere in a basin. The temporal and spatial scales of advection and vertical mixing determine the flux paths of particles and solutes in aquatic ecosystems and, ultimately, lacustrine productivity. Time series measurements of surface meteorology, from which surface energy budgets can be calculated, and water column temperatures can be used as an indirect approach to determine these pathways. The data also can be used to determine internal wave dynamics, the energetics of mixing, and the role of intrusions and density driven flows in advection. With this information, the rates of circulation within the upper mixed layer and vertical and horizontal transports can be assessed. The diurnal cycle of stratification and mixing is deter

Benthic flux of biogenic elements on the Southeastern US continental shelf: influence of pore water advective transport and benthic microalgae

Abstract: In situ, paired light and dark benthic flux chamber incubations were used to estimate the exchange of nutrients, oxygen and inorganic carbon across the sediment – water interface of the South Atlantic Bight (SAB) continental shelf. The results indicate that physically forced non-diffusive pore water transport and benthic primary production (BPP) by sea floor microalgae exert a major influence on benthic exchange rates on the mid- and outer-continental shelf (depths of 14–40 m). Light fluxes to the sea floor and sediment photosynthetic pigment distributions determined on two, widely spaced cross-shelf transects suggest that BPP may occur over 84% of the SAB continental shelf area. Microalgal gross BPP rates at all study sites averaged 400±260 mg C m−2 d−1 between May and September 1996 while water column primary productivity averaged 682±176 mg C m−2 d−1, implying a total primary productivity for this region of approximately 1100 mg C m−2 d−1 (1.6 times the water column productivity alone). The results are also consistent with the advective transport of pore waters. Benthic flux chambers appear to retard this exchange, affecting the accuracy of derived net fluxes. Given our inability to relate pore water gradients to fluxes in non-diffusive regimes and to mimic natural advective transport in intact core incubations, traditional techniques such as pore water gradient diffusion calculations or shipboard core incubations also may not provide accurate flux estimates. Because of these limitations, fundamental questions remain concerning the processes that control nutrient inventories in pore waters and the magnitude of the net benthic flux of nutrients on the sandy SAB shelf.