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Showing papers on "Water column published in 2001"


Journal ArticleDOI
TL;DR: A clearer picture is needed of the importance of suspension versus deposit feeding and the potential advantages and tradeoffs between these two feeding modes, as well as the role of resource limitation in structuring bivalve communities.
Abstract: 1. Freshwater systems are losing biodiversity at a rapid rate, yet we know little about the functional role of most of this biodiversity. The ecosystem roles of freshwater burrowing bivalves have been particularly understudied. Here we summarize what is known about the functional role of burrowing bivalves in the orders Unionoida and Veneroida in lakes and streams globally. 2. Bivalves filter phytoplankton, bacteria and particulate organic matter from the water column. Corbicula and sphaeriids also remove organic matter from the sediment by deposit feeding, as may some unionids. Filtration rate varies with bivalve species and size, temperature, particle size and concentration, and flow regime. 3. Bivalves affect nutrient dynamics in freshwater systems, through excretion as well as biodeposition of faeces and pseudofaeces. Excretion rates are both size and species dependent, are influenced by reproductive stage, and vary greatly with temperature and food availability. 4. Bioturbation of sediments through bivalve movements increases sediment water and oxygen content and releases nutrients from the sediment to the water column. The physical presence of bivalve shells creates habitat for epiphytic and epizoic organisms, and stabilizes sediment and provides refugia for benthic fauna. Biodeposition of faeces and pseudofaeces can alter the composition of benthic communities. 5. There is conflicting evidence concerning the role of resource limitation in structuring bivalve communities. Control by bivalves of primary production is most likely when their biomass is large relative to the water volume and where hydrologic residence time is long. Future studies should consider exactly what bivalves feed upon, whether feeding varies seasonally and with habitat, and whether significant overlap in diet occurs. In particular, we need a clearer picture of the importance of suspension versus deposit feeding and the potential advantages and tradeoffs between these two feeding modes. 6. In North America, native burrowing bivalves (Unionidae) are declining at a catastrophic rate. This significant loss of benthic biomass, coupled with the invasion of an exotic burrowing bivalve (Corbicula), may result in large alterations of ecosystem processes and functions.

753 citations


Journal ArticleDOI
TL;DR: In this article, a time-averaged oxygen curve of the Posidonia Shale in SW-Germany was reconstructed using the benthic macrofauna of nine fossil communities.

432 citations


Journal ArticleDOI
TL;DR: In this article, changes to the physical environment during 1997 and 1998 were placed in context of historical meteorological and oceanographic data sets, and the authors placed these changes in the context of the Bering Sea and observed that during 1998, the sea surface temperature was cooler than in 1997 but the water column had significantly higher heat content.
Abstract: During 1997 and 1998, unusual physical conditions occurred in the Bering Sea: strong May storms and calm conditions in July; record high sea surface temperature; a shallow wind mixed layer; a fresher-than-normal water column; and abnormal cross-shelf currents. Accom- panying these conditions were changes in the dominant phytoplankton, a die-off of seabirds, increased sightings of large whales and diminished returns of salmon. Changes to the physical environment during 1997 and 1998 are placed in context of historical meteorological and oceanographic data sets. Although 1997 had the warmest sea surface temperature ever observed on the south-east Bering Sea shelf, the heat content of the water column was cooler than average. In contrast, during 1998, the sea surface temperature was cooler than in 1997 but the water column had significantly higher heat content. During recent years, the water column has freshened over the middle shelf because of increased sea ice and reduction of on-shelf transport of the saline, high-nutrient water from the slope. The timing of the spring bloom is directly related to the presence of ice. When ice is advected over the south-east shelf during March/April an early, sharp phytoplankton bloom occurs. The absence of ice during this critical time is associated with a May/June bloom. 3

349 citations


Journal ArticleDOI
15 Feb 2001-Nature
TL;DR: It is suggested that organic matter might be protected from degradation by the inorganic matrix of sinking particles, and that amino-acid-like material predominates throughout the water column in both regions of the Pacific Ocean and the Arabian Sea.
Abstract: The sinking of particulate organic matter from ocean surface waters transports carbon to the ocean interior1,2, where almost all is then recycled. The unrecycled fraction of this organic matter can become buried in ocean sediments, thus sequestering carbon and so influencing atmospheric carbon dioxide concentrations3. The processes controlling the extensive biodegradation of sinking particles remain unclear, partly because of the difficulty in resolving the composition of the residual organic matter at depth with existing chromatographic techniques4. Here, using solid-state 13C NMR spectroscopy5, we characterize the chemical structure of organic carbon in both surface plankton and sinking particulate matter from the Pacific Ocean4 and the Arabian Sea6. We found that minimal changes occur in bulk organic composition, despite extensive (>98%) biodegradation, and that amino-acid-like material predominates throughout the water column in both regions. The compositional similarity between phytoplankton biomass and the small remnant of organic matter reaching the ocean interior indicates that the formation of unusual biochemicals, either by chemical recombination7 or microbial biosynthesis8, is not the main process controlling the preservation of particulate organic carbon within the water column at these two sites. We suggest instead that organic matter might be protected from degradation by the inorganic matrix of sinking particles.

345 citations


Journal ArticleDOI
TL;DR: It is argued that motile phytoplankton can form a thin layer under poorly mixed conditions and a reaction-diffusion-taxis model can explain many patterns of algal distribution found in poorly mixed aquatic ecosystems.
Abstract: Phytoplankton often face the dilemma of living in contrasting gradients of two essential resources: light that is supplied from above and nutrients that are often supplied from below. In poorly mixed water columns, algae can be heterogeneously distributed, with thin layers of biomass found on the surface, at depth, or on the sediment surface. Here, we show that these patterns can result from intraspecific competition for light and nutrients. First, we present numerical solutions of a reaction-diffusion-taxis model of phytoplankton, nutrients, and light. We argue that motile phytoplankton can form a thin layer under poorly mixed conditions. We then analyze a related game theoretical model that treats the depth of a thin layer of phytoplankton as the strategy. The evolutionarily stable strategy is the depth at which the phytoplankton are equally limited by both resources, as long as the layer is restricted to the water column. The layer becomes shallower with an increase in the nutrient supply and deeper with an increase in the light supply. For low nutrient levels, low background attenuation, and shallow water columns, a benthic layer occurs; for intermediate nutrient levels in deep water columns, a deep chlorophyll maximum occurs; and for high nutrient levels, a surface scum occurs. These general patterns are in agreement with field observations. Thus, this model can explain many patterns of algal distribution found in poorly mixed aquatic ecosystems.

288 citations


Journal ArticleDOI
TL;DR: In this article, the role of water column methanotrophy (microbial methane oxidation) as a control on methane release was quantified by measuring water column methane profiles (concentration and δ13C) and oxidation rates at eight stations in an area of active methane venting in the Eel River Basin, off the coast of northern California.

282 citations


Journal ArticleDOI
01 Apr 2001-Ecology
TL;DR: It is tested the hypothesis that reductions in light associated with increases in phytoplankton could cause compensatory decreases in benthic algal (periphyton) primary production and the increase in whole- lake primary production caused by water column fertilization was greatly overestimated.
Abstract: The perception that primary production in lakes is positively related to phos- phorus loading is based almost entirely on studies of phytoplankton. This is partly because benthic and pelagic habitats in lakes are often treated as separate ecosystems, the processes of which can be evaluated independently. However, light and nutrients often limit primary producers in both benthic and pelagic habitats. We tested the hypothesis that reductions in light associated with increases in phytoplankton could cause compensatory decreases in benthic algal (periphyton) primary production. We monitored production of periphyton on sediments (epipelon), periphyton on wood (epixylon), and phytoplankton in four lakes in upper Michigan, USA, from 1991 to 1995. During the summers of 1993-1995, we stimulated phytoplankton production in three of the lakes by fertilizing with nitrogen and phosphorus (N:P ' 25 by atoms) at rates between 0.3 and 2.0 mg P m-3-d-l. The response of periphyton to fertilization was substratum specific: epixylon increased with fertilization, but epipelon decreased. However, when area-specific production was extrapolated to the whole-lake scale, epixylon never constituted >4% of benthic primary production. Thus, the decline in epipelic production dominated the benthic response to fertilization. We also estimated whole-lake (epipelon + phytoplankton) primary production. Epipelic algae constituted 50- 80% of whole-lake primary production at ambient nutrient levels. However, only 10-40% of primary production was benthic at the highest fertilization rates. The increase in whole- lake primary production caused by water column fertilization was greatly overestimated when we did not include the compensatory decline in epipelic algae as they were shaded by increases in phytoplankton concentrations.

257 citations


Journal ArticleDOI
TL;DR: The distribution profiles of these contaminants in water and sediments suggest that there are a number of sources contributing to total contaminant burden in the bay, including soil runoffs, wastewater discharges, sewage outfalls and shipping activites.

245 citations


Journal ArticleDOI
26 Apr 2001-Nature
TL;DR: The results indicate that today's stratification between Labrador Sea Water and North Atlantic Deep Water never developed during the last interglacial period, and the present situation, with an active site of intermediate-water formation in the Labrador Sea, has no analogue throughout the last climate cycle.
Abstract: The two main constituent water masses of the deep North Atlantic Ocean—North Atlantic Deep Water at the bottom and Labrador Sea Water at an intermediate level—are currently formed in the Nordic seas and the Labrador Sea, respectively1. The rate of formation of these two water masses tightly governs the strength of the global ocean circulation and the associated heat transport across the North Atlantic Ocean2. Numerical simulations have suggested a possible shut-down of Labrador Sea Water formation as a consequence of global warming3. Here we use micropalaeontological data and stable isotope measurements in both planktonic and benthic foraminifera from deep Labrador Sea cores to investigate the density structure of the water column during the last interglacial period, which was thought to be about 2 °C warmer than present4. Our results indicate that today's stratification between Labrador Sea Water and North Atlantic Deep Water never developed during the last interglacial period. Instead, a buoyant surface layer was present above a single water mass originating from the Nordic seas. Thus the present situation, with an active site of intermediate-water formation in the Labrador Sea, which settled some 7,000 years ago, has no analogue throughout the last climate cycle.

243 citations


Journal ArticleDOI
TL;DR: In this article, a simple model of annual average response of an estuary to mean nitrogen loading rate and freshwater residence time was developed and tested for 11 North American and European estuaries having a wide range of physical characteristics, nitrogen loading rates, and geographical and climatic settings.
Abstract: A simple model of annual average response of an estuary to mean nitrogen loading rate and freshwater residence time was developed and tested. It uses nitrogen inputs from land, deposition from the atmosphere, and first-order calculations of internal loss rate and net export to perform a steady-state analysis over a yearly cycle. The model calculates the fraction of total nitrogen input from land and the atmosphere that is exported and the fraction that is denitrified or lost to other processes within the estuary. The model was tested against data from the literature for 11 North American and European estuaries having a wide range of physical characteristics, nitrogen loading rates, and geographical and climatic settings. The model shows that the fraction of nitrogen entering an estuary that is exported or denitrified can be predicted from the freshwater residence time. The first-order rate constant for nitrogen loss within an estuary, as a fraction of total nitrogen in the water column, is 0.30 mo−1. Denitrification typically accounts for 69–75% of the total annual net nitrogen removal from the water column by processes within the estuary. The model makes explicit the dependence of nitrogen concentration in the water column on the loading rate of nitrogen, water residence time, estuary volume, and the rate constant for loss within the estuary.

230 citations


Journal ArticleDOI
TL;DR: The variable nutrient dynamics that the authors observed among the three study lakes appears to be typical for shallow lake systems, which indicates that a greater reliance on lake-specific research may be required for effective management, and a lesser role of inter-lake generalization than is possible for deeper, dimictic lake systems.

Journal ArticleDOI
TL;DR: In this article, the authors used a model-driven optimization technique to estimate the bottom depth, bottom albedo, and water absorption and backscattering coefficients, which can be used to derive concentrations of chlorophyll, dissolved organic matter and suspended sediments in the water column.
Abstract: Using Airborne Visible Infrared Imaging Spectrometer (AVIRIS) data as an example, we show in this study that the properties of the water column and bottom of a large, shallow area can be adequately retrieved using a model-driven optimization technique. The simultaneously derived properties include bottom depth, bottom albedo, and water absorption and backscattering coefficients, which in turn could be used to derive concentrations of chlorophyll, dissolved organic matter, and suspended sediments in the water column. The derived bottom depths were compared with a bathymetry chart and a boat survey and were found to agree very well. Also, the derived bottom albedo image shows clear spatial patterns, with end-members consistent with sand and seagrass. The image of absorption and backscattering coefficients indicates that the water is quite horizontally mixed. Without bottom corrections, chlorophyll a retrievals were ∼50 mg m−3, while the retrievals after bottom corrections were tenfold less, approximating real values. These results suggest that the model and approach used work very well for the retrieval of subsurface properties of shallow-water environments even for rather turbid environments like Tampa Bay, Florida.

Journal ArticleDOI
TL;DR: Correlations between viral abundances and suspended solids suggest that viruses sorbed to suspended material in the water column may settle out and contribute to the benthic viral population.
Abstract: Viruses are recognized as ubiquitous components of marine ecosystems; however, there has been limited study of viral abundance and its ecological role in sediments. Viral abundance was determined in both the water column and sediments of a eutrophic (Brisbane River/Moreton Bay; 278259S, 153859E) and oligotrophic (Noosa River; 268159S, 153809E) estuary in subtropical Queensland, Australia. Viruses, bacteria, and microalgae from both water column and extracted sediment samples were enumerated using SYBR Green I staining and epifluorescence microscopy. Sediment viral abundance ranged from 10 7 to 10 9 particles cm 23 of sediment, bacterial abundance ranged from 10 7 to 10 8 cells cm 23 of sediment, and microalgal abundance ranged from 10 4 to 10 5 cells cm 23 sediment. Pelagic abundances for all microorganisms were 10‐1,000-fold lower than sediment abundances. Correlations between viral abundances and suspended solids suggest that viruses sorbed to suspended material in the water column may settle out and contribute to the benthic viral population. Virus production was measured by a time course increase of viral abundance in seawater using a dilution technique. Virus production was highest in eutrophic waters of the Brisbane River, and addition of inorganic nutrients (NO 1 NH 1 PO 1 SiO3) stimulated viral production 21 32 34 4 rates at all stations by 14‐52% above ambient, suggesting that inorganic nutrient availability may play a key role in aquatic viral abundance.

Journal ArticleDOI
01 Oct 2001-Water SA
TL;DR: In this article, the highest heavy metal concentrations in both the estuary and river were recorded at points where runoff from informal settlements and industry entered the system, which raises concern over the long-term health of the Swartkops River ecosystem.
Abstract: Elevated levels of heavy metals in the sediment can be a good indication of man-induced pollution. Concentrations of chrome, le ad, zinc, titanium, manganese, strontium, copper and tin were measured in the sediments taken along a section of the Swartkops Rive r and its estuary. These results showed that the highest heavy metal concentrations in both the estuary and river were recorded at points where runoff from informal settlements and industry entered the system. Comparison of the results for the estuary with those obtained in a similar survey made about 20 years ago revealed some remarkable increases. This raises concern over the long-term health of the Swartkops River ecosystem. The Swartkops River catchment contains almost the entire municipal area of Uitenhage and Kwanobuhle, Despatch and Ibhayi and also half of the Port Elizabeth municipal area. It is estimated that approximately one million people live and work presently in the Swartkops River catchment. The Swartkops catchment not only contains the greatest part of the metropolitan population, but it is also the area where the greatest diversity of urban users is found and where urban growth is most rapid. High- density, low-income housing is developing in the catchment with a concomitant increase in industry, and in the quality and quantity of stormwater runoff. These developments will also necessitate the building of further road and rail crossings over the river as well as developing more telecommunication links and power supply lines (Horenz, 1988). The occurrence of elevated levels of trace metals especially in the sediments can be a good indication of man-induced pollution and high levels of heavy metals can often be attributed to anthropogenic influences, rather than natural enrichment of the sediment by geological weathering (Davies et al., 1991; Lord and Thompson, 1988). There can be significant temporal and spatial variability in water column concentrations of heavy metal contaminants, which leads to problems in obtaining representative samples. Sediments, on the other hand, integrate contaminants over time and are in constant flux with the overlying water column. The analysis of heavy metals in the sediments permits detection of pollutants that may be either absent or in low concentrations in the water column (Davies et al., 1991), and their distribution in coastal sediments provides a record of the spatial and temporal history of pollution in a particular region or ecosystem. Heavy metal concentrations in the water column can be relatively low, but the concentrations in the sediment may be elevated. Low level discharges of a contaminant may meet the water quality criteria, but long-term partitioning to the sediments could result in the accumulation of high loads of pollutants. It has been estimated that about 90% of particulate matter carried by rivers settles in estuaries and coastal areas (Martin and Whitfield, 1983). Once heavy metals are discharged into estuarine and coastal

Journal ArticleDOI
01 Mar 2001
TL;DR: For example, during a cruise in the BIOTRANS area (47°N, 20°W) of the eastern North Atlantic, a large number of planktic foraminifers and pteropods were sampled from the upper 2500 m of the water column as discussed by the authors.
Abstract: During September and October 1996 planktic foraminifers and pteropods were sampled from the upper 2500 m of the water column in the BIOTRANS area (47°N, 20°W), eastern North Atlantic, as part of the JGOFS program. Hydrography, chlorophyll fluorescence, and nutrient content were recorded at high spatial and temporal resolution providing detailed information about the transition time between summer and fall. At the beginning of the cruise a shallow pycnocline was present and oligotrophic conditions prevailed. Over the course of the cruise, the mixed layer depth increased and surface water temperature decreased by 1.5°C. Both chlorophyll-a dispersed in the upper 50 m by vertical mixing and chlorophyll-a concentrations at the sea surface increased. The nitracline shoaled and nutrient enriched waters were entrained into the mixed layer. Planktic foraminifers and pteropods closely reflected the changes in the hydrography by increased growth rates and changes in species composition. Three main groups of planktic foraminiferal species were recognized: (1) a temperate and low-productivity group dominated by Neogloboquadrina incompta characterized the shallow mixed layer depths. (2) A temperate and high-productivity group dominated by Globigerina bulloides characterized the period with wind-induced dispersal of chlorophyll-a and entrainment of nutrient-enriched waters. (3) A warm water group containing Globigerinoides sacculifer, Orbulina universa, Globigerinoides ruber (white), and Globigerinella siphonifera was most common during the first days of sampling. Synchronous with the hydrographic change from summer to fall, planktic foraminiferal and pteropod growth was stimulated by redistribution of chlorophyll-a and entrainment of nutrient-enriched waters into the mixed layer. In addition, the seasonal change in the eastern North Atlantic resulted in a transition of the epipelagic faunal composition and an increased calcareous particle flux, which could be used to trace seasonality in fossil assemblages and allow for better paleoceanographic interpretation of the boreal Atlantic.

Journal ArticleDOI
TL;DR: In this article, the authors measured the redox-sensitive trace metals (Mn, V, Mo, U, Cd and Re) in sediments from ODP Leg 169S Hole 1033B in Saanich Inlet, British Columbia, to determine changes in redox conditions associated with the onset of laminated sediments at ∼12.5kyr.

Journal ArticleDOI
TL;DR: In this article, the authors measured the distribution of hydrographic parameters, currents, phytoplankton flu- orescence, suspended sediments and zoop larvae in axial transects through the Chesapeake Bay estuarine turbidity maximum (ETM) seasonally (May, July and October 1996) and over tidal cycles within seasons.
Abstract: We measured the distribution of hydrographic parameters, currents, phytoplankton flu- orescence, suspended sediments and zooplankton in axial transects through the Chesapeake Bay estuarine turbidity maximum (ETM) seasonally (May, July and October 1996) and over tidal cycles within seasons. Zooplankton abundance was estimated with a 6-frequency, Tracor Acoustical Profil- ing System (TAPS-6) at the same vertical (0.25 to 0.50 m) and horizontal (0.5 to 1.5 km) resolution as hydrographic parameters and suspended sediments. The general pattern exhibited in axial transects through the Chesapeake Bay ETM is that sediments, fluorescence and zooplankton are in higher concentrations up-Bay of the salt wedge (defined as the intersection of the 1 isohaline with the bot- tom). The salinity front appears to trap these particles in the upper portion of Chesapeake Bay. The highest acoustically determined zooplankton biomass generally occurred near the bottom, at the toe of the salt wedge. The convergence zone associated with this feature concentrates sediments and zooplankton (primarily the copepod Eurytemora affinis). Advection appeared to dominate changes in zooplankton abundance during time series studies at a fixed station in the ETM. Zooplankton bio- mass at the fixed ETM station increased/decreased with the tidal excursion of the salt wedge. Water column zooplankton concentrations and the vertical distribution of zooplankton biomass appeared to be influenced by currents. We often found that during maximum ebb and flood tidal currents, zoo- plankton biomass and sediments in the mid and upper water column increased. Thus the hydrody- namic processes that resuspend, advect and trap suspended sediments in the ETM likely have the same effects on zooplankton. The ETM of the Chesapeake Bay appears to act as an entrapment zone for zooplankton. The lack of diel vertical migration, carrying eggs until they are ready to hatch, pos- sible reduced predation by visual predators in the turbid waters, and the ability to consume phyto- plankton, protozoa and detritus all may allow Eurytemora to persist at high concentrations in the Chesapeake Bay ETM.

Journal ArticleDOI
TL;DR: The micromolar concentrations of nitrate, nitrite, silicate, phosphate and ammonium were studied during a Lagrangian study of an anti-cyclonic eddy situated in the Iceland Basin of the North Atlantic Ocean as mentioned in this paper.
Abstract: The micromolar concentrations of nitrate, nitrite, silicate, phosphate and ammonium; and the nanomolar concentrations of nitrate, nitrite and ammonium were studied during a Lagrangian study of an anti-cyclonic eddy situated in the Iceland Basin of the North Atlantic Ocean. Underway surface transects, carried out across the eddy at the beginning and end of the survey period of 2 weeks, showed a decrease in the surface-water concentrations of nitrate and phosphate, with a concomitant increase in the phytoplankton biomass and surface-water temperature. The trajectory of the eddy was tracked by a release of SF 6 tracer, and regular water column sampling was carried out at the centre of the tracer patch over a period of 11 days. A storm after the "rst 5 days of the cruise brought about a transport into the surface mixed layer of colder sub-thermocline waters, which resulted in signi"cant nutrient enrichment of nitrate, phosphate and silicate in the euphotic zone. This transport of nutrients, in particular nitrate, into the surface-mixed layer demonstrates the importance of eddies in the open ocean for sustaining &new’ production within the euphotic zone. We also report observations of ammonium at concentrations from 45 to 935 nmol l~1 for the northeast Atlantic using an automated analytical system. The presence of an ammonium maximum in the water column related to the thermocline, chlorophyll maximum, and the primary nitrite maximum. ( 2001 Elsevier Science Ltd. All rights reserved.

Journal ArticleDOI
TL;DR: In this paper, bottom-tethered sediment traps deployed at the European continental margin at 49°N and off the Canary Islands (ESTOC) were investigated for the determination of 230Th trapping efficiencies.
Abstract: Bottom-tethered sediment traps deployed in the deep eastern North Atlantic between 54°N 20°W and 33°N 20°W (L1, L2, L3), at the European continental margin at 49°N (OMEX) and off the Canary Islands (ESTOC) were investigated for the determination of 230Th trapping efficiencies. The ratios of 230Th flux measured in the traps (Fa) to the expected 230Th flux from the production rate of 230Th in the overlying water column (Fp) ranged between 0.09 and 1.26. For the traps with deployment periods >300 days the interannual variation of Fa/Fp ratios (different years but same location and water depth) were up to 10%, suggesting that the average 230Th flux to the sediment traps did not vary significantly. The influence of lateral advection on the 230Th flux was taken into account either by applying a mass balance of 230Th and 231Pa or by assuming a constant removal rate of 230Th from the water column, an assumption based on similar 230Th concentration-depth profiles observed at most locations investigated. 230Th trapping efficiencies were between 9 and 143%, showing a trend of increasing efficiencies with increasing water depth. No relation was found between current velocities and 230Th trapping efficiencies. Our investigations suggest that the observation of constant or even increasing particle flux rates with increasing water depths in several sediment trap arrays investigated may be a result of sediment trap biases. The correction for the trapping biases is important for the understanding of the regional differences in the particle flux in the eastern North Atlantic.

Journal ArticleDOI
TL;DR: In this article, two experiments were carried out to study the effects of trawling in the muddy prodeltaic deposit of the Llobregat River in the northwestern Mediterranean.
Abstract: Two experiments were carried out to study the effects of trawling in the muddy prodeltaic deposit of the Llobregat River in the northwestern Mediterranean. Trawling was conducted in two experimental lines, and bottom morphology, sediment texture, and water turbidity were analyzed before trawling and at different time intervals afterward. The tracks of the trawl gears were still observed in sonographs of the bottom 1 yr after the first experiment. The vertical grain size distribution of bottom deposits indicated that the thickness of the sediment removed by the net between the gears was about 2–3 cm on average, though the erosion produced by the gears was deeper. Resuspended aggregates with a high silt content settled during the first hour after trawling, generating a temporary increase in the silt content of the surface sediment. One day after trawling, the surface sediment was mixed and already had a similar grain size distribution to that before trawling. After the beginning of trawling, water turbidity increased first near the bottom for a few hours and later also at shallower levels of the water column within a period of 2–5 d after trawling. At the end of the experiment, about 10% of the sediment affected by trawling was diffused in the water column and the remaining 90% had settled on the bottom. Average turbidity in the water column increased by a factor of up to three for 4–5 d after trawling. This experiment shows that intense and continued trawling on continental shelves has a noticeable effect on water turbidity, which must be considered in addition to natural processes.

Journal ArticleDOI
TL;DR: The ability of larvae to behaviourally regulate their position at scales of micrometres to metres when exposed to turbulent fluid motion in the water column, as evidenced in the benthic boundary layer, is unknown.
Abstract: For marine benthic invertebrates with meroplanktonic larvae, the relative importance of hydrodynamics and swimming behaviour in determining larval dispersal in the water column, particularly at sma...

Journal ArticleDOI
TL;DR: In this paper, the effects of bioturbation by common polychaetes (Nereis spp. and Arenicola marina) in Northern European coastal waters on sediment carbon diagenesis is summarized and assessed.
Abstract: Known effects of bioturbation by common polychaetes (Nereis spp. and Arenicola marina) in Northern European coastal waters on sediment carbon diagenesis is summarized and assessed. The physical impact of irrigation and reworking activity of the involved polychaete species is evaluated and related to their basic biology. Based on past and present experimental work, it is concluded that effects of bioturbation on carbon diagenesis from manipulated laboratory experiments cannot be directly extrapolated to in situ conditions. The 45–260% flux (e.g., CO2 release) enhancement found in the laboratory is much higher than usually observed in the field (10–25%). Thus, the faunal induced enhancement of microbial carbon oxidation in natural sediments instead causes a reduction of the organic matter inventory rather than an increased release of CO2 across the sediment/water interface. The relative decrease in organic inventory (Gb/Gu) is inversely related to the relative increase in microbial capacity for organic matter decay (kb/ku). The equilibrium is controlled by the balance between organic input (deposition of organic matter at the sediment surface) and the intensity of bioturbation. Introduction of oxygen to subsurface sediment and removal of metabolites are considered the two most important underlying mechanisms for the stimulation of carbon oxidation by burrowing fauna. Introduction of oxygen to deep sediment layers of low microbial activity, either by downward irrigation transport of overlying oxic water or by upward reworking transport of sediment to the oxic water column will increase carbon oxidation of anaerobically refractory organic matter. It appears that the irrigation effect is larger than and to a higher degree dependent on animal density than the reworking effect. Enhancement of anaerobic carbon oxidation by removal of metabolites (reduced diffusion scale) may cause a significant increase in total sediment metabolism. This is caused by three possible mechanisms: (i) combined mineralization and biological uptake; (ii) combined mineralization and abiogenic precipitation; and (iii) alleviation of metabolite inhibition. Finally, some suggestions for future work on bioturbation effects are presented, including: (i) experimental verification of metabolite inhibition in bioturbated sediments; (ii) mapping and quantification of the role of metals as electron acceptors in bioturbated sediments; and (iii) identification of microbial community composition by the use of new molecular biological techniques. These three topics are not intended to cover all unresolved aspects of bioturbation, but should rather be considered a list of obvious gaps in our knowledge and present new and appealing approaches.

Journal ArticleDOI
TL;DR: In this article, a 25-h series of measurements of the rate of dissipation of turbulent kinetic energy ( e) in the Liverpool Bay region of freshwater influence (ROFI) have revealed the form of this interaction and indicate substantial differences from regions where horizontal gradients are weak.
Abstract: In regions of large horizontal density gradient, tidal straining acts to produce a periodic component of stratification that interacts with turbulent mixing to control water column structure and flow. A 25-h series of measurements of the rate of dissipation of turbulent kinetic energy ( e) in the Liverpool Bay region of freshwater influence (ROFI) have revealed the form of this interaction and indicate substantial differences from regions where horizontal gradients are weak. In the ROFI system there is a pronounced difference between flood and ebb regimes. During the ebb the water column stratifies and strong dissipation is confined to the lower half of the water column. By contrast, during the flood, stratification is eroded with complete vertical mixing occurring at high water and high values of dissipation (3 mW m23) extending throughout the water column. The cycle of dissipation is therefore predominantly semidiurnal in the upper layers whereas, near the bottom boundary, the principal variation is at the M4 frequency as observed in regions of horizontal uniformity. Toward the end of the flood phase of the cycle, tidal straining produces instabilities in the water column that release additional energy for convective mixing. Confirmation of increased vertical motions throughout the water column during the late flood and at high water is provided by measurements of vertical velocity and the error velocity from a bottom-mounted acoustic Doppler current profiler.

Journal ArticleDOI
TL;DR: In this paper, the role of macroalgae in mediating water column concentrations and sediment-water column fluxes of DON across a nutrient gradient in Hog Island Bay, a shallow macroalgal-dominated back-barrier lagoon located on the Virginia Coast was investigated.
Abstract: The activity of the benthos, including benthic plants, is important in driving the overall system dynamics in shallow lagoons, due to the high ratio of sediment surface area relative to water volume. In Hog Island Bay, benthic macroalgae appear to be a key regulator of DON dynamics, both while alive and following senescence. We investigated the role of macroalgae in mediating water column concentrations and sediment-water column fluxes of DON across a nutrient gradient in Hog Island Bay, a shallow macroalgal-dominated back-barrier lagoon located on the Virginia Coast. Sediment-water column exchanges of DON, urea and DIN were measured in sediment cores with and without macroalgae (Ulva lactuca) at three subtidal sites from the mainland to the barrier islands in the fall of 1997 and the spring and summer of 1998. The summer sampling dates bracketed a large macroalgal bloom in the mid-lagoon. Dissolved organic nitrogen was an important component (52-98%) of the total dissolved nitrogen pool in Hog Island Bay waters and made up the majority of the sediment N flux to the water column. Macroalgae impacted benthic-pelagic coupling by preventing diffusion of DIN from the water column to the sediments and by intercepting urea fluxes from the sediment to the water column. Closest to the mainland and closest to the barrier islands, at sites with low macroalgal biomass, sediment-water column fluxes of DIN and urea-free DON were negligible or directed into the sediments. Fluxes of urea from the sediment to the water column were significant at both sites, and may play an important role in satisfying macroalgal N demand, especially at the low N island site. Overall, urea was 32% of the mean DON flux from the sediments to the water column. Fluxes of urea-free DON were highest in the mid-lagoon, where macroalgal biomass was highest. The highest overall flux rates of DON (>38 mmol m−2 d−1) and DIN (>33 mmol m−2 d−1) were measured following an isolated crash of a large macroalgal mat. These release rates were not sustained for long, however, and we estimated that the majority of the N contained in the decomposing macroalgal tissues disappeared in <2 weeks. In addition to release of organic N following senescence, macroalgae ‘ leak ’ DON into the water column during active growth; release of DON increased by 250% in cores incubated with U. lactuca relative to cores with sediment only. These algae function as a conduit whereby water-column DIN and sediment urea are taken up and released to the water column as DON over relatively short (hours) time scales. This conversion of bioavailable dissolved N to PON and DON by macroalgae is likely to be important to overall system metabolism and to the retention of N within the lagoon.

Journal ArticleDOI
TL;DR: This situation was likely to favour nitrification, which led during autumn to higher nitrate concentrations within shellfish farming zones than outside, and filter feeders were able to alter the dominant biogeochemical process in the water column by stimulating nitrifica- tion.
Abstract: Thau lagoon is a Mediterranean shellfish ecosystem with large biomasses of oysters growing in waters with high residence time due to low tidal ranges. The influence of filter feeders (oysters and their epibiota) on the spatial distribution of particulate and dissolved compounds in the water column of Thau lagoon was studied through its variation with time. In 1991/1992, daily varia- tions were investigated in pens, corridors and outside shellfish farming zones for nutrients, chloro- phyll a and primary production. Salinity, dissolved oxygen, nutrients, organic matter and chloro- phyll a were also monitored in surface waters inside and outside shellfish farming zones each week from January 1993 to March 1994. The presence of shellfish farms led to a decrease by only a few per- cent of oxygen concentrations in their vicinity, but the mean (±SE) deficits of chlorophyll a and POC concentrations were 44 ± 4% and 26 ± 9% respectively in the eastern zone (8 m). The shift induced by filter feeders in phytoplankton composition favoured picophytoplankton with higher growth rates. But the summer increase in phytoplankton growth rate was stronger than the positive feedback due to filter feeder filtration. Summer was determinant for the growth of oysters owing to enhanced regenerated primary production. During this period, filter feeders were not food limited, while they tended to control phytoplankton biomasses and production the rest of the year. The nutrient excess in shellfish farming zones was highly significant, with increases of 73 ± 16, 36 ± 12 and 19 ± 8% for ammonia, phosphates and silicate respectively in the eastern zone. In the western zone, the nutrient excess was less strong by half for ammonia and phosphate, because the lower depth (4 m) allows light to reach the bottom and enables benthic macroflora to grow on nutrients of benthic origin. The decline of phytoplankton biomasses in shellfish farms induced a decrease in the nutrient demand, especially for ammonia. This situation was likely to favour nitrification, which led during autumn to higher nitrate concentrations within shellfish farming zones than outside. Therefore, filter feeders were able to alter the dominant biogeochemical process in the water column by stimulating nitrifica- tion.

Journal ArticleDOI
TL;DR: Monitoring in the Arctic Kongsfjord on Spitsbergen from 1996 to 1998 found that organisms in the eulittoral and upper sublittoral zones are affected by UV radiation throughout the polar day, since organisms in deeper water are protected against harmful UV-B radiation.
Abstract: Solar radiation as a primary abiotic factor affecting productivity of seaweeds was monitored in the Arctic Kongsfjord on Spitsbergen from 1996 to 1998. The radiation was measured in air and underwater, with special emphasis on the UV-B (ultraviolet B, 280–320 nm) radiation, which may increase under conditions of stratospheric ozone depletion. The recorded irradiances were related to ozone concentrations measured concurrently in the atmosphere above the Kongsfjord with a balloon-carried ozone probe and by TOMS satellite. For comparison, an ozone index (a spectroradiometrically determined irradiance of a wavelength dependent on ozone concentration, standardized to a non-affected wavelength) was used to indicate the total ozone concentration present in the atmosphere. Weather conditions and, hence, solar irradiance measured at ground level were seldom stable throughout the study. UV-B irradiation was clearly dependent on the actual ozone concentration in the atmosphere with a maximal fluence rate of downward irradiance of 0.27 W m−2 on the ground and a maximal daily fluence (radiation exposure) of 23.3 kJ m−2. To characterize the water body, the light transmittance, temperature and salinity were monitored at two different locations: (1) at a sheltered shallow-water bay and (2) at a wave-exposed, deep-water location within the Kongsfjord. During the clearest water conditions in spring, the vertical attenuation coefficient (Kd) for photosynthetically active radiation (PAR) was 0.12 m−1 and for UV-B 0.34 m−1. In spring, coinciding with low temperatures and clear water conditions, the harmful UV radiation penetrated deeply into the water column and the threshold irradiance negatively affecting primary plant productivity was still found at about 5–6 m depth. The water body in spring was characterized as a Jerlov coastal water type 1. With increasing temperature in summer, snow layers and glacier ice melted, resulting in a high discharge of turbid fresh water into the fjord. This caused a stratification in the optical features, the salinity and temperature of the water body. During melt-water input, a turbid freshwater layer was formed above the more dense sea water. Under these conditions, light attenuation was stronger than defined for a Jerlov coastal water type 9. Solar radiation was strongly attenuated in the first few metres of the water column. Consequently, organisms in deeper water are protected against harmful UV-B radiation. In the surface water, turbidity decreased when rising tide caused an advection of clearer oceanic water. In the course of the summer season, salinity continuously decreased and water temperature increased particularly in shallow water regions. The impact of global climate change on the radiation conditions under water and its effects on primary production of seaweeds are discussed, since organisms in the eulittoral and upper sublittoral zones are affected by UV radiation throughout the polar day. In clearer water conditions during spring, this may also apply to organisms inhabiting greater depths.

Journal ArticleDOI
01 Mar 2001-Boreas
TL;DR: Foraminiferal assemblages were studied in northern Barents Sea core ASV 880 along with oxygen and carbon isotope measurements in planktonic (N. pachyderma sin.) and benthic (E clavatum) species as mentioned in this paper.
Abstract: Foraminiferal assemblages were studied in northern Barents Sea core ASV 880 along with oxygen and carbon isotope measurements in planktonic (N. pachyderma sin.) and benthic (E clavatum) species. AMS C-14 measurements performed on molluscs Yoldiella spp. show that this core provides a detailed and undisturbed record of Holocene climatic changes over the last 10000 calendar years. Surface and deep waters were very cold (<0°C) at the beginning of the Holocene. C. reniforme dominated the highly diverse benthic foraminiferal assemblage. From 10 to 7.8 cal. ka BP, a warming trend culminated in a temperature optimum, which developed between 7.8 and 6.8 cal. ka BP. During this optimum, the input of Atlantic water to the Barents Sea reached its maximum. The Atlantic water mass invaded the whole Franz Victoria Trough and was present from subsurface to the bottom. No bottom water, which would form through rejection of brine during winter, was present at the core depth (388 m). The water stratification was therefore greatly reduced as compared to the present. An increase in percentage of I. helenae/norcrossi points to long seasonal ice-free conditions. The temperature optimum ended rather abruptly, with the return of cold polar waters into the trough within a few centuries. This was accompanied by a dramatic reduction of the abundance of C. reniforme. During the upper Holocene, the more opportunistic species E. clavatum became progressively dominant and the water column was more stratified. Deep water in Franz Victoria Trough contained a significant amount of cold Barents Sea bottom water as it does today, while subsurface water warmed progressively until about 3.7 cal. ka BP and reached temperatures similar to those of today. These long-term climatic changes were cut by several cold events of short duration, in particular one in the middle of the temperature optimum and another, which coincides most probably with the 8.2 ka BP cold event. Both long- and short-term climatic changes in the Barents Sea are associated with changes in the flow of Atlantic waters and the oceanic conveyor belt.

Journal ArticleDOI
TL;DR: In this paper, the presence or absence of stratification had a profound influence on the wind-driven, cross-shelf circulation over this shallow shelf, which was consistent with a two-dimensional upwelling/downwelling response.
Abstract: Wind-driven, cross-shelf circulation is studied using current observations spanning the 90 km wide North Carolina shelf. Most of the shelf is less than 40 m deep. Current measurements were made at five sites within 16 km of the coast from August through October or early December 1994 and at mid- and outer-shelf sites from February 1992 through February 1994. In both studies the water column was stratified in summer and often unstratified during fall and winter. The presence or absence of stratification had a profound influence on the wind-driven, cross-shelf circulation over this shallow shelf. When the water column was stratified, the wind-driven cross-shelf circulation was consistent with a two-dimensional upwelling/downwelling response. Over the mid and outer shelf, near-surface and near-bottom cross-shelf transports had similar magnitudes but opposite directions and were approximately equal to the Ekman transports associated with the alongshelf wind stress and bottom stress, respectively. Win...

Journal ArticleDOI
TL;DR: The preliminary analysis indicates that although sediment characteristics are highly variable within 100-m river reaches, the variability across the 17 rivers is even greater; this is despite the limited geographic and trophic range of the study sites.

Journal ArticleDOI
TL;DR: This article conducted a field study including a series of cruises over an isolated offshore zebra mussel reef (7-11 m deep) in Western Lake Erie to examine the effect of Zebra mussels (Dreissena spp.) on the water column.
Abstract: We conducted a field study including a series of cruises over an isolated offshore zebra mussel reef (7–11 m deep) in Western Lake Erie to examine the effect of zebra mussels (Dreissena spp.) on the water column. The horizontal currents over the reef were found to be primarily due to the hydraulic flow and surface gravitational seiches. The turbulence generated by these currents was found to be too weak to fully mix the water column. Although seasonal stratification was not observed, solar heating during the day and intrusions of cold central basin water caused stable stratification of the water column 60% of the time. Results from the seston analysis taken at five depths showed a statistically significant mussel-feeding signature in chlorophyll a and organic seston concentrations measured within 2 m above the reef. Estimates of clearance rates based on field data were consistent with rates measured in a flow chamber using water from the site, which indicated that zebra mussels could remove up to 40% of the total seston. The detection of a zebra mussel-induced concentration boundary layer is due to: (1) reduced vertical mixing as a result of semidiurnal periodic stratification, (2) refiltration of bottom water in zebra mussel populations, and (3) in situ clearance rates that are lower than those observed in the laboratory. Thus, offshore zebra mussel colonies may have less of an effect on the water column than had been previously estimated by simple stirred reactor models, and the role of zebra mussels in the clarification of Lake Erie should be investigated further.