Showing papers on "Water column published in 1986"

Decreased depth penetration of Fucus vesicolosus (L.) since the 1940's indicates eutrophication of the Baltic Sea

Abstract: Although nutrient inputs to the Baltic Sea have increased drastically since the end of the last century, there is still little hard biological evidence of a general eutrophication of the Baltic Sea outside locally polluted areas. A revisit after 40 yr to some well-documented diving stations in the outer archipelago of the h a n d Sea gave us an opportunity to register any changes in benthic vegetation that could be linked to eutrophication, By mapping the vertical distribut~on in coverage of bladder wrack Fucus vesiculosus (L.) at 11 stations we observed that the lower limit of this alga had moved upwards at 10 stations from maximally 11.5 m in 1943/44 to 8.5 m in 1984. Also the depth of maximum development had withdrawn from 5 to 6 m in 1943/44 to 3 to 4 m in 1984, while coverage a t these depths was about the same: 58 % and 51 % respectively. The deepest specimens today at 8.5 m had the same dwarfed appearance as those found at 11.5 m in the 1940's; at that time growth at 8 .5 m was luxuriant During both studies the decrease in F. vesiculosus coverage with depth towards the lower limit could be approximately fitted to an exponentially decreasing light attenuation curve. Since the structure of Baltic hardbottom communities is almost totally governed by abiotic factors, the changes in depth penetration are probably caused by decreased transparency of the water column due to eutrophication. Results indicate that the decreased water transparency arises from a 40 to 50 % increase in summer values of chlorophyll a and nutrients in the offshore surface water of the Baltic Sea since the 1940's.

Bacterial Grazing by Planktonic Lake Algae

Abstract: Six common species of lake algae were found to ingest bacteria The ingestion rates measured were of the same magnitude as those recorded for marine microflagellates totally dependent on external sources of carbon A large biomass of Dinobryon species removed more bacteria from the water column of a lake than crustaceans, rotifers, and ciliates combined

Geochemistry and deposition of 7Be in river-estuarine and coastal waters

Abstract: The atmospheric flux of cosmogenic 7Be (53.3-day half-life) and the mode of 7Be deposition in river-estuarine and coastal environments have been examined. The atmospheric flux of 7Be commonly supports inventories ranging from 1.0 to 2.0 pCi/cm2 (1 pCi = 0.037 Bq). Beryllium 7 concentrations in water phase samples, collected across salinity gradients in several estuaries along the eastern coastline of the United States, range from 0.03 to 0.53 pCi/L and primarily reflect variations in 7Be supply and sorption kinetics. The major process controlling the concentration of 7Be on estuarine suspended particles appears to be the length of time that these particles remain in the water column. Field particle-to-water distribution coefficients for 7Be have a median value of about 4 × 104 but range over an order of magnitude reflecting short-term variations in 7Be input, particle dynamics, and particulate iron content rather than equilibrium sorption-desorption responses to changes in water salinity or particle type. Residence times of 7Be in the water column range from a few days in estuarine areas of rapid fine-particle deposition, to several weeks in high-energy environments where pronounced sediment resuspension reintroduces deposited 7Be back into the water column. Inventories of 7Be in sediments range from nondetectable to 3.3 pCi/cm2, with the highest inventories in areas where fine particles are accumulating rapidly. Such sites are also major repositories for other particle-reactive substances. A 7Be budget for the James estuary indicates that less than 5% of the expected 7Be input is in the water column and that the short-term estuarine trapping efficiency for atmospherically derived 7Be is somewhere between 50 and 100%.

Relationships among vertical mixing, nitrate uptake, and phytoplankton growth during the spring bloom in the southeast Bering Sea middle shelf

Abstract: The temporal development of a spring diatom bloom in the southeast Bering Sea middle shelf for three consecutive ice-free years is analyzed. Physical and chemical criteria are used to divide the bloom period into prebloom, bloom, and postbloom stages. At this shelf depth, the shallowing of the mixed layer was most important in triggering bloom conditions by diminishing phytoplankton respirational losses. This occurred in late April to early May during a hiatus in wind mixing associated with low pressure systems. Sver drup's (1953, Journal du Conseil. Conseil International pour l'Exploration de la Mer,18, 287–295) critical depth model accurately predicted changing prebloom growth conditions probably because it described the mean daily mixed layer light conditions to which the plant communities were exposed. The usefulness of this integrative model diminished as species composition changed since compensation depth light levels for later bloom species were not known. A nitrate advection diffusion model indicated that a cross pycnocline mixing rate of 2.1 m d−1 was associated with bloom conditions. Estimated convective vertical mixing rates in the upper water column were low during the period leading to nitrate exhaustion suggesting that cells remained at favorable light levels for extended periods. Since both net respirational loss and the supply of nutrients to the mixed layer were dependent on vertical mixing, bloom development could be described as a function of upper water stability. Discrepancies arose when rapid changes in mixing conditions decreased the correspondence between measured nitrate uptake rates and the prevailing surface layer buoyancy. Signals generated by periodic factors such as the Mf (neap-spring) tide and aperiodic storm events were resolved during the spring bloom. Approximately 37% of new nitrogen productivity was due to wind mixing events that occurred after initial water column stabilization and prolonged high rates of nitrate uptake. This enhancement varied from 10 to 50% among years which suggests that large-scale meteorological factors determine May entrainment activity and influence the intensity and pattern of production on this shelf. The passage of low pressure systems temporarily pumped nutrient-rich outer shelf water ( 100m) into the middle shelf domain (

Patterns of productivity during eutrophication: a mesocosm experiment

Abstract: In a 28 mo mesocosm experiment , levels and patterns of productivity and respiration were observed for a range of nutrient additions selected to provide a gradation from conditions in lower Narragansett Bay, Rhode Island , USA. to maximum impact for an urbalJ. t\!ary receiving present day leve1s of sewage effluents. For a 3Z. fold increase in nutrients, system apparent prod.uction increased by only a factor of 3,5. Seasonal patterns of . autotrophy during the winter-spring diatom bloom and , heterolrophy during summer and early fall occurred at all treatment levels. With the exception of the 8 x treci\ment, all treatments above 2 X had a greater respiratory demand in the water column than the benthos. The highest treatment mesocosm (32 X) wentbrieny anoxic during the second summer of the experiment during a period when little productivity was occurring in the water column.

Ecosystem analysis in the southeastern Bering sea

Abstract: During a 7-year study of the food-chain dynamics of the outer and middle shelves of the southeastern Bering Sea, numerical models were developed to test ecological hypotheses posed at the event, seasonal, and annual scales of habitat variability. Interannual variations in year class strength of Alaska pollock between warm and cold years are attributed to an order of magnitude decline in prey availability during larval drift in cold years. In cold years delayed increases in seasonal abundance of copepod nauplii, the prey of these larval fish, are attributed to low temperature effects on crustacean metabolism rather than to changes in food availability (phytoplankton). Sinking diatoms, uneaten by the zooplankton in the water column during warm or cold years, are resuspended by wind events and advected seaward every 3 to 5 days during the spring. The ungrazed phytoplankton from the outer shelf are moved offshore and deposited on the adjacent slope. Because of the unusual width of the Bering shelf (∼500km) and a seasonal decline in the number of upwelling, favorable wind events, diatoms of the middle shelf are not exported. They are consumed instead by the benthos, with 10-fold larger infaunal biomass found here than on the outer shelf. An annual carbon budget for the middle shelf suggests consumption of all the primary production in this regime, compared to an apparent export of 48% of the outer shelf primary production to the continental slope.

Estuarine nitrification: A naturally occurring fluidized bed reaction?

Abstract: The rates of nitrification in the water column of the Tamar river estuary, southwest England have been measured using the incorporation of H14CO3 in samples with and without the inhibitor of nitrification, 2-chloro-6-(trichloromethyl) pyridine (N-Serve). N-Serve proved successful in totally inhibiting NH4-oxidizing bacteria but the activity of NO2-oxidizing bacteria was inhibited by only 30%; other organisms were only slightly affected. Measurements of the nitrification rate made over the entire salinity range of the estuary (0–30‰) showed that maximum nitrification always coincided with the turbidity maximum. The field data suggest that the organisms responsible for nitrification were associated with periodically resuspended particulate material and that the turbidity maximum acts in a manner similar to a fluidized bed reactor. A dispersion model has been used to demonstrate that nitrification in the water column can account for 100% of the NO2 maximum which is apparent down estuary from the turbidity maximum.

Redox state of iron in the offshore waters of peru

Abstract: The total concentration and redox state of iron were examined along a transect across the continental shelf off the Peruvian coast during. January 1984. Total and dissolved iron (0.4~pm filter) were measured by the Co-APDC coprecipitation method. Fe(II) was measured by a preconcentration step with S-hydroxyquinoline bonded to silica as the stationary phase, followed by elution and the ferrozine method. Up to 40 nmol kg-l of Fe(II) was detected in the bottom water at 5-10 km offshore and decreased markedly upward in the water column and with distance offshore. A good correlation between the distribution of Fe(II) and nitrite in the bottom water indicated a common source from the shelf sediments. Elevated Fe(II) concentrations near the sea surface and a diel change were probably due to photochemical reactions involving iron. Total iron levels were >300-500 nmol kg-’ in the surface and the bottom water at 5-6 km offshore. About 80-90% of the iron was in the particulate form, indicating a substantial input of iron from continental dust and from the sediments on the shelf. The total iron level decreased considerably within 35 km of the coastline and the iron seemed to be trapped on the shelf. Iron is found in natural waters in both Fe(II) and Fe(III) oxidation states. The distribution of these two forms of iron is governed by several factors, including redox potential, pH, and the presence of organic material. From thermodynamic considerations, the concentration of reduced forms of iron in oxic natural waters will be much lower than that of the oxidized forms of iron due to the rapid oxidation of Fe(II) by Oz. Nevertheless, it has been suggested that nonequilibrium processes may enable Fe(II) species to persist at appreciable concentrations in natural waters even in the presence of oxygen. McMahon (1969) suggested that annual and diurnal variations of acid-soluble ferrous iron in lake water were a result of photochemical reactions or of metabolic activity of microorganisms. Recently, the photochemistry of iron in natural waters has been emphasized by several studies as reviewed by Zafiriou ( 1983). Miles and Brezonik (198 1) showed that the oxygen consumption in humic-colored freshwaters involved a photochemical ferrous-ferric catalytic cycle. Waite and Morel (1984)

Elevated Levels of Microbial Activity in the Coral Surface Microlayer

Abstract: Mcrobial biomass and activity parameters were measured in the mucus-rich, coral surface microlayer (CSM) and in the waters overlying coral reefs in the Dry Tortugas and Key West Florida, USA. For all parameters measured (thymidine ~ncorporation, bacterial direct counts, primary production, chlorophyll a, dissolved DNA, and particulate DNA), the CSM values were significantly greater than the overlying water values on a per volume basis. Enrichment in the CSM ranged from nearly equal to 7.5-fold for bacterial direct counts to 18 to 280-fold for chlorophyll a. Microautotrophic processes in the CSM were due almost exclusively to the activities of free-living zooxanthellae, while activities in overlying waters were dominated by autotrophic picoplankton. Higher cellular rates of thymldine incorporation (1.23 to 50 tlmes greater) indicated that bacteria in the CSM were growing faster than those in the water column. Bacterial production estimates on a per area basis in the CSM (thickness = several mm) were equivalent to that of the whole water column over the reef (depth 1 to 4 m). Size fractionation of the CSM indicated that most of this bacterial activity was associated with particles > l pm, while overlying waters were dominated by

Stable isotopic characterization of organic carbon accumulation on the Amazon continental shelf

Abstract: The stable carbon isotopic ratio (13C:12C) of organic matter in the water column and sediments has been used to examine the distribution and accumulation of organic carbon on the Amazon continental shelf. Near the river mouth, in 0 × 10−3 salinity surface waters, particulate organic carbon (POC) is isotopically light (−27.3 per mil). Isotopic values of POC in continental shelf waters north and east of the river mouth range from −19.5 to −25.7 per mil. The isotopic variations in water column POC can be related to productivity, turbidity, and water density. The isotopic character of water column POC is controlled, therefore, by the dynamic mixing and northwestward migration of riverine and marine shelf waters. Terrestrial organic carbon dominates the isotopic signal in surface sediments (upper 10 cm) from the river mouth to areas 400 km to the northwest. Only on the outermost and northern parts of the shelf is marine organic carbon dominant in surface sediments. A sharp boundary between terrestrial and marine isotopic values is present in continental shelf sediments at the outer edge of the prograding mud delta. This boundary is associated with an abrupt decrease in sediment accumulation rate and a change in stratigraphic setting from topset and foreset regions to bottomset regions. The average TOC content of Amazon continental shelf sediments is 0.66 ± 0.20%. Based on the average TOC of Amazon shelf sediments and current estimates of sediment accumulation rates, approximately 4.5 × 1012 g-TOC y−1 is accumulating in Amazon shelf sediments. Using δ13C-POC values at salinities of 0 and 35 × 10−3 as end-members to isotopically resolve the contribution of terrestrial and marine carbon sources, approximately 3.1 × 1012 g-TOC y−1 or 69% of the organic carbon accumulating in Amazon shelf surface sediments is from terrestrial sources. This represents 6% of current estimates for Amazon riverine carbon discharge.

Antarctic aquatic ecosystems as habitats for phytoplankton

Abstract: Summary 1. The Southern Ocean is a large-scale, relatively homogeneous upwelling ecosystem whose phytoplankton apparently grows suboptimally over much of its area. By contrast there is a wide variety of freshwater habitats in the Antarctic and in some of these phytoplankton growth efficiency is very high. The two habitats share similar temperature and irradiance regimes, but differ markedly in availability of inorganic nutrients, in grazing pressure and in the time- and space-scales on which various physical processes act. 2. Concentrations of inorganic nutrients in the marine ecosystem have been represented as being in excess of phytoplankton requirements, but the ionic composition of some nutrient pools may not conform to phytoplankton preferences. 3. Nutrient-limitation determines phytoplankton production in Antarctic lakes and gives rise to gross differences between lakes. 4. Irradiance in the water column varies greatly over the year in both marine and freshwater ecosystems. Most algae are shade-adapted, with the ability to utilize low irradiance but with sub-optimal response to high irradiance. However, local phytoplankton maxima may attain very high carbon fixation and growth rates. 5. Consistently low temperatures characterize both systems. Their effects on photo-synthetic carbon uptake mirror shade-adaptation. Division rates of marine phytoplankton may however be very much higher than predicted for ambient temperatures. 6. Vertical mixing is important in both ecosystems and influences the environment experienced by phytoplankton cells. This appears to have little effect on the average performance of phytoplankton in the strongly mixed surface water column of the Southern Ocean, where the mixed depth may exceed 100 m. This can be related partly to the shade-adapted photosynthetic response. Euphotic depths range from 20 to 100 m. 7. Strong vertical mixing under ice-free conditions in lakes may maximize photosynthetic efficiency, whilst distinct vertical stratification in permanently ice-covered lakes gives rise to segregation of nutrient uptake and regeneration. 8. Physical removal of phytoplankton biomass by grazing is locally important in the Southern Ocean, in contrast to the estimated mean mesoscale impact of grazing. Vertical sedimentation losses appear important in the context of mixing depth and generation time, and may be modified by vertical circulation of water. 9. Loss of phytoplankton biomass from lakes during the ice-free period is dominated by physical removal via the lake outflow. Grazing is generally unimportant, except where larvae of otherwise nektobenthic zooplankton hatch in synchrony with a phytoplankton maximum. Sedimentation is important under ice-cover.

Distribution patterns of benthic microalgal standing stock at McMurdo Sound, Antarctica.

Abstract: During the austral summer of 1975–76 and winter of 1977 benthic and water column chlorophyll a and phaeopigments were measured at several sites along the east and west sides of McMurdo Sound, Antarctica. Estimates of in situ primary productivity were made at some McMurdo Sound locations. Additionally, water column samples were collected at 5 stations in the Ross Sea during January, 1976. Standing stock data are analyzed to identify seasonal and spatial patterns. Variability in algal standing stock was related to ambient light levels and appeared to be mediated by ice and snow cover whereby the highest algal standing stock was present under high light conditions (low ice and snow cover, shallow water, summer). Differences in published benthic invertebrate densities appear to be closely allied to differences in benthic primary production, and less so to in situ planktonic ice microalgal production.

Effects of suspended sediments on geochemical processes near the mouth of the Amazon River: examination of biological silica uptake and the fate of particle-reactive elements

Abstract: The effects of suspended-sediment concentration on the biological uptake of silica and the chemical behavior of particle-reactive elements are examined for the continental shelf waters near the mouth of the Amazon River. Dissolved silica and suspended-sediment concentrations measured during periods of high and low discharge indicate that uptake of dissolved silica begins where the suspended-sediment concentration falls below 10–20 mg l1. Near the mouth of the Amazon River the 10 mg l1 turbidity contour and the region of initial silica uptake both occur 20–40 km farther offshore during periods of high discharge (May and June) than during periods of low discharge (October and November). The location of initial silica uptake along an across-shelf transect near the river mouth varies by as much as 20 km on a time scale of days to weeks, which is comparable to the seasonal variation. Silica uptake, chlorophyll a concentration, and suspended-sediment concentration are poorly correlated along this transect (correlation coefficients range from +0.03 to −0.3). The poor correlations may be a result of grazing by zooplankton. Phaeopigment:chlorophyll a ratios increase offshore from 0 to 0.7, which supports the hypothesis that zooplankton grazing may be responsible for the low chlorophyll concentrations in the low-turbidity offshore waters exhibiting extensive silica removal. Excess210Pb inventories for box cores collected from an across-shelf transect just south of the river mouth range from 21 to 124 dpm cm2, which are 2–10 times greater than the inventory supported by atmospheric supply and in-situ production in the overlying water column. Riverine supply and lateral transport from offshore waters are the main sources of excess210Pb to the Amazon continental shelf.210Pb and234Th data indicate that the high particulate flux from the Amazon River scavenges particle-reactive species not only from river water and continental shelf water but also scavenges these elements from offshore open-ocean water which flows onto the shelf.

The Influence of River Variability on the Circulation, Chemistry, and Microbiology of the

Abstract: Gravitational circulation of the Delaware Estuary is dominated by a single river, the Delaware River. The seasonal variation in river discharge is large. Consequently, the water column varies between vertically homogenous conditions found during most of the year and strongly stratified conditions found during the high flow of the spring freshet. Both the variation in river discharge and the extent of stratification affect chemical distributions and biological processes in the estuary. With a simple advection-diffusion model, we show that the apparent nonconservative behavior of nitrate in the Delaware Estuary can result from varying endmember concentration and varying river discharge. In addition, we illustrate the relationship between water column stratifi- cation, phytoplankton production, and concurrent bacterial activity. Finally, as an indirect chemical response to phytoplankton growth during high river discharge, we show strongly nonconservative patterns for ammonium, phosphate, and silicate in the estuary.

The influence of river variability on the circulation, chemistry, and microbiology of the Delaware Estuary

Abstract: Gravitational circulation of the Delaware Estuary is dominated by a single river, the Delaware River. The seasonal variation in river discharge is large. Consequently, the water column varies between vertically homogenous conditions found during most of the year and strongly stratified conditions found during the high flow of the spring freshet. Both the variation in river discharge and the extent of stratification affect chemical distributions and biological processes in the estuary. With a simple advection-diffusion model, we show that the apparent nonconservative behavior of nitrate in the Delaware Estuary can result from varying endmember concentration and varying river discharge. In addition, we illustrate the relationship between water column stratification, phytoplankton production, and concurrent bacterial activity. Finally, as an indirect chemical response to phytoplankton growth during high river discharge, we show strongly nonconservative patterns for ammonium, phosphate, and silicate in the estuary.

Restoration of two lowland lakes by isolation from nutrient-rich water sources with and without removal of sediment.

Abstract: SUMMARY (1) The Norfolk Broadland has suffered extreme eutrophication. Restoration of the once clear water and aquatic plant communities from the present phytoplankton dominance is sought. (2) In the first two experiments, Alderfen Broad was isolated, by diversion of an inflow stream, without removal of its recently-deposited phosphorus-rich sediments. In the second, sediment was removed from Cockshoot Broad and dams were placed against the effluent-rich River Bure, but the Broad continued to be fed by a small stream draining an agricultural and fen catchment. (3) In four years (1979-82) after isolation of Alderfen Broad, the phytoplankton crop was greatly reduced and the water became clear. Net release of P04-P from the sediment ceased and the Broad became dominated by Ceratophyllum demersum L. Then, because of the reduction in turbulence of the water column by the plants, and the organic matter supplied to the sediment surface in their decay, the mechanism for release of P04-P from the sediment was reactivated and, in 1984, supported a large phytoplankton crop in the spring. The aquatic plant population declined. In 1985 there was both spring and summer phytoplankton growth, the latter probably supported by nitrogen-fixation, and the aquatic plant growth was negligible. (4) In Cockshoot Broad a reduction in phytoplankton growth followed soon after isolation and sediment-removal, and the water became very clear and has remained so. Part of the Broad has already recolonized with a diverse collection of aquatic plants. (5) The consequences of these results for the management of the rest of the Broadland waterway are discussed.

Oxygen budget of a perennially ice-covered Antarctic lake.

Abstract: A bulk O2 budget for Lake Hoare, Antarctica, is presented. Five years of seasonal data show the lake to be persistently supersaturated with O2. Oxygen is carried into the lake in glacial meltstreams and is left behind when this water is removed as ice by ablation and sublimation. A diffusive loss of O2 from the lake through the summer moat is suggested. Measured values of the total O2 in the water column indicate that the time scale of O2 turnover is much longer than a year. Based on these results, it is suggested that the amount of O2 in the water does not change significantly throughout the year and that the lake is also supersaturated with N2.

Capture Efficiency of a Beach Seine for Seven Freshwater Fishes in a North-Temperate Lake

Abstract: Daytime seining efficiencies were estimated for shoreline fish populations in Sparkling Lake, a small mesotrophic clear-water lake in northern Wisconsin. Except for rock bass (Ambloplites rupestris), efficiency was related to the typical position of each species in the water column; efficiencies were higher for midwater fishes (cyprinids and yellow perch, Perca flavescens) than for benthic fishes (darters, Etheostoma spp.). Efficiencies for many species might be improved by modifications of seining technique or use of heavier lead lines that would keep the seine closer to the bottom.

Silica, diatoms and a freshwater productivity maximum in Atlantic Coastal Plain estuaries, Chesapeake Bay☆

Abstract: Monthly data collected in 1981 from the James, York and Rappahannock rivers show that dissolved silica (= silicic acid) exhibits two distinct types of longitudinal distributions in these estuaries during the year. Conservative behavior was observed during winter or when river discharge was high. The second distribution occurs when uptake by planktonic diatoms reduces concentrations below the levels attributable to dilution by seawater. In all three systems, the non-conservative behavior is characterized by rapid uptake of silicic acid in the tidal freshwater portion, just upstream of the limit of salt intrusion, and corresponds to a chlorophyll maximum shown to contain bloom densities of diatoms. Silicic acid is exhausted below detectable concentrations in the vicinity of the diatom maximum, representing more than 98% removal of the riverine source. Non-conservative distributions of total silica (= silicic acid + biogenic silica) in this same region indicate deposition of biogenic silica to the sediments. At the same time the riverine source is depleted by the diatoms in the freshwater reaches, silicic acid is resupplied to the water column in the oligohaline portion of the estuaries. This resupply is shown to be significant, nearly equal to the amount of silicic acid entering the estuary in river input, suggesting complete mineralization of biogenic silica produced in the freshwater zone. The spatial distributions described above were observed regularly in all three systems, indicating that enhanced phytoplankton production and nutrient regeneration in the vicinity of the transition zone is a natural feature of these partially-mixed estuarine rivers. Possible mechanisms contributing to the accumulation of phytoplankton biomass and the enhanced cycling of silicon are discussed.

Biogenic silica and phosphorus accumulation in sediments as indices of eutrophication in the Laurentian Great Lakes

Abstract: Biogenic silica (BSi), total phosphorus (TP), and biologically available phosphorus (AVP) were measured in short cores from Lake Michigan, Lake Erie, and Lake Ontario. Peaks in BSi concentration and peaks in BSi:TP or BSi:AVP ratios provided stratigraphic signals of water column silica (Si) depletion as a response of increased diatom production to P enrichment and decreased diatom production resulting from silica depletion. By contrast the stratigraphic record of P accumulation provided very weak signals of the historical nutrient enrichment in the water column. These results indicate that system P recycling has a higher rate constant than Si recycling and, as a consequence, that relatively small levels of P enrichment can increase diatom production and sedimentation eventually causing Si depletion and Si-limited diatom production in the water mass.