Water column

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

Nutrient and other environmental controls of harmful cyanobacterial blooms along the freshwater-marine continuum.

Abstract: Nutrient and hydrologic conditions strongly influence harmful planktonic and benthic cyanobacterial bloom (CHAB) dynamics in aquatic ecosystems ranging from streams and lakes to coastal ecosystems Urbanization, agricultural and industrial development have led to increased nitrogen (N) and phosphorus (P) discharge, which affect CHAB potentials of receiving waters The amounts, proportions and chemical composition of N and P sources can influence the composition, magnitude and duration of blooms This, in turn, has ramifications for food web dynamics (toxic or inedible CHABs), nutrient and oxygen cycling and nutrient budgets Some CHABs are capable of N2 fixation, a process that can influence N availability and budgets Certain invasive N2 fixing taxa (eg, Cylindrospermopsis, Lyngbya) also effectively compete for fixed N during spring, N–enriched runoff periods, while they use N2 fixation to supplant their N needs during N–deplete summer months Control of these taxa is strongly dependent on P supply However, additional factors, such as molar N:P supply ratios, organic matter availability, light attenuation, freshwater discharge, flushing rates (residence time) and water column stability play interactive roles in determining CHAB composition (ie N2 fixing vs non–N2 fixing taxa) and biomass Bloom potentials of nutrient–impacted waters are sensitive to water residence (or flushing) time, temperatures (preference for >15 °C), vertical mixing and turbidity These physical forcing features can control absolute growth rates of bloom taxa Human activities may affect “bottom up” physical–chemical modulators either directly, by controlling hydrologic, nutrient, sediment and toxic discharges, or indirectly, by influencing climate Control and management of cyanobacterial and other phytoplankton blooms invariably includes nutrient input constraints, most often focused on N and/or P While single nutrient input constraints may be effective in some water bodies, dual N and P input reductions are usually required for effective long–term control and management of blooms In some systems where hydrologic manipulations (ie, plentiful water supplies) are possible, reducing the water residence time by flushing and artificial mixing (along with nutrient input constraints) can be effective alternatives Blooms that are not readily consumed and transferred up the food web will form a relatively large proportion of sedimented organic matter This, in turn, will exacerbate sediment oxygen demand, and enhance the potential for oxygen depletion and release of nutrients back to the water column This scenario is particularly problematic in long–residence time (ie, months) systems, where blooms may exert a strong positive feedback on future events Implications of these scenarios and the confounding issues of climatic (hydrologic) variability, including droughts, tropical storms, hurricanes and floods, will be discussed in the context of developing effective CHAB control strategies along the freshwater–marine continuum

Sulfidity controls molybdenum isotope fractionation into euxinic sediments: Evidence from the modern Black Sea

Abstract: Molybdenum (Mo) isotope fractionation has recently been introduced as a new proxy in oceanography and biogeochemistry. It is therefore fundamental to understand the processes controlling Mo partitioning into modern marine environments. This study identifies the availability of dissolved sulfide as the dominant control on overall Mo removal from the water column in euxinic systems. Mo isotopic composition of surface sediments from different localities of the Black Sea demonstrates complete fixation of Mo only below 400 m water depth, above a critical concentration of 11 μmol l −1 aqueous hydrogen sulfide in the bottom water. The Mo isotopic composition of these sediments reflects the homogeneous seawater isotopic composition of 2.3‰. In contrast, significant Mo isotope fractionation into less euxinic sediments is evident at shallower depths in the Black Sea, as well as in temporarily euxinic deeps of the Baltic Sea, consistent with the observed lower maximum sulfide concentrations in the respective water columns. Therefore, Mo isotope signatures in the modern Black Sea constrain the processes responsible for global Mo removal from the ocean by euxinic sediments. Furthermore, models of past ocean anoxia reconstruction have to consider that the seawater Mo isotopic composition is not per se archived in euxinic sediments.

Benthic Exchange and Biogeochemical Cycling in Permeable Sediments

Abstract: The sandy sediments that blanket the inner shelf are situated in a zone where nutrient input from land and strong mixing produce maximum primary production and tight coupling between water column and sedimentary processes. The high permeability of the shelf sands renders them susceptible to pressure gradients generated by hydrodynamic and biological forces that modulate spatial and temporal patterns of water circulation through these sediments. The resulting dynamic three-dimensional patterns of particle and solute distribution generate a broad spectrum of biogeochemical reaction zones that facilitate effective decomposition of the pelagic and benthic primary production products. The intricate coupling between the water column and sediment makes it challenging to quantify the production and decomposition processes and the resultant fluxes in permeable shelf sands. Recent technical developments have led to insights into the high biogeochemical and biological activity of these permeable sediments and their role in the global cycles of matter.

Seagrass-sediment interactions, positive feedbacks and critical thresholds for occurrence: a review

Abstract: This literature review summarizes the limiting factors for seagrass occurrence, and the effect positive feedbacks in seagrass systems have on these threshold levels. Minimum water depth is mainly determined by wave orbital velocity, tide and wave energy; and maximum depth by light availability. Besides these, other limiting factors occur, such as an upper current velocity threshold, above which seagrasses are eroded, or a lower water current velocity threshold below which carbon exchange is limiting. In some locations organic matter content, sulphide concentration or nutrient availability are limiting. N-limitation is mainly reported from temperate terrigenous sediments, and P-limitation from tropical carbonate sediments. However, limiting factors sometimes change over the year, switching from light limiting to N- or P-limiting, and show at times regional variation. The effect seagrasses have on current reduction, trapping sediment and decreasing resuspension can lead to several changes in both the sediment and the water column. In the sediment, an increase in nutrient availability has been reported, and increases in organic matter, sediment height increases, and burial of the seagrasses. In the water column the effect is a reduction of the turbidity through a decrease of the sediment load, decreasing the attenuation coefficient, thereby increasing light availability. Due to the large effect light availability has on seagrass occurrence, the effect of an improvement of the light conditions by a reduction of the turbidity by seagrasses is probably the most important positive feedback in seagrass systems. The latter effect should therefore be incorporated in models that try to understand or predict seagrass changes. Generalization are difficult due a lack of studies that try to find relationships between seagrass architecture and sediment trapping (studying both turbidity reduction and nutrient increase) on a global level under a variety of different conditions. Areas for research priorities are identified.

Seasonal fluctuations of temperature, salinity, nitrate, chlorophyll and primary production at station H3/M1 over 1989-1996 in Monterey Bay, California

Abstract: Time series of temperature, salinity, nitrate, primary production and chlorophyll over 1989–1996 at station H3/M1 in central Monterey Bay, CA, USA, are described, and an `average year’ is calculated for each parameter. Surface spatial data on temperature, salinity, chlorophyll and primary production from spring and fall 1993 also are presented. Surface water (0–5 m) was coldest and saltiest in spring (∼10–11°C; S =33.4–33.8), warmed during summer (∼14°C), remained warm but freshened in fall ( S =33.3–33.4), and cooled and freshened further in winter (∼13°C; S =32.9–33.3). Nitrate time series show high concentrations (10–20 μM) present at the surface during spring and summer; low concentrations ( 100 mg C m −3 da −1 ) occurred in upper 20 m of the water column, while high chlorophyll (>3 mg m −3 ) extended to 25–30 m. Phytoplankton blooms occurred as pulses of primary production and chlorophyll during spring, summer and occasionally in fall. Springtime spatial data show a surface `plume’ of cold, salty, low chlorophyll and low primary production water extending N–S across the mouth of the Monterey Bay. The time-series station H3/M1 lies in the path of this plume. High chlorophyll and productivity values occur on the margins of the plume. Fall spatial data show temperatures and salinities warmer and fresher than spring while chlorophyll and primary production values were low. There was less spatial variability in fall. A temperature/salinity time series for Monterey Bay from 1951 to 1991 ( Kuo, 1991 ) shows similar seasonal patterns. However, the 1989–1996 time series is warmer and fresher to at least 100 m, particularly during non-upwelling seasons, and shows a later onset of upwelling. These differences are in accord with the `regime shift’ associated changes documented for the California Current by other workers. Long-term climatologies for nitrate, chlorophyll and primary production are not available for comparison with the data presented in this paper.