Showing papers on "Water column published in 1997"

New measurements of phytoplankton and ice algal production in the Arctic Ocean

Abstract: During the Canada/US 1994 Arctic Ocean Section, algal biomass (Chlorophyll a) and primary production were measured in the water column, at the ice-water interface and in the bottom 24 cm of the sea ice along a transect from the Chukchi Sea to the Nansen Basin via the North Pole Algal biomass and primary production were determined for 07-5 urn and > 5 urn size fractions The algal release rate of D014C during incubation was also measured In the Chukchi Sea and in leads of the Makarov and Nansen Basins, total maximum particulate phytoplankton production rates were 2570,73 and 521 mg C m-* day-', respectively At these stations, where ice cover varied from 55 to 90%, large phytoplankton ( > 5 pm) represented 61-98% of the total algal biomass At stations with higher ice coverage (>90%), the total phytoplankton production decreased to 9- 57mgCm-*day-' At these stations, small phytoplankton (07-5 urn) accounted for 59-88% of the total biomass and more than 64% of the total production Along the transect, the percentage of the total phytoplankton production released as extracellular carbon was generally less than 20%, except in the Canadian Basin where it ranged from 31 to 65% Total particulate ice algal production ranged from 05 to 310 mg C m-* day-' and showed maximum rates in the central Arctic Ocean Large cells ( > 5 urn) generally dominated the ice algal community, representing 5&100% of the total biomass and more than 50% of the total production Ice algae released on average 34% of total carbon fixed during the 4-12 h incubation Ice algae contributed on average 57% of the entire primary production (water column + sea ice) in the central Arctic and 3% in the surrounding regions Total primary productivity in the central Arctic Ocean is estimated at 15 g C m-* year-', a value at least 10 times higher than previously reported The difference between estimates is due in part to the previously unmeasured contribution of the particulate production by ice algae and the release of DOC by both ice and pelagic algae 0 1998 Elsevier Science Ltd All rights reserved

Regulation of algal blooms in Antarctic Shelf Waters by the release of iron from melting sea ice

Abstract: During summer 1995–96, we measured iron in the water column and conducted iron-enrichment bottle-incubation experiments at a station in the central Ross Sea (76°30′S, 170°40′W), first, in the presence of melting sea ice, and 17 days later, in ice-free conditions. We observed a striking temporal change in mixed-layer dissolved iron concentrations at this station, from 0.72–2.3 nM with sea ice present, to 0.16–0.17 nM in ice-free conditions. These changes were accompanied by a significant drawdown in macronutrients and an approximate doubling of algal (diatom) biomass. Our incubation experiments suggest that conditions were iron-replete in the presence of sea ice, and iron-deficient in the absence of sea ice. We surmise that bioavailable iron was released into seawater from the melting sea ice, stimulating phytoplankton production and the biological removal of dissolved iron from the mixed layer, until iron-limited conditions developed. These observations suggest that the episodic release of bioavailable iron from melting sea ice is an important factor regulating phytoplankton production, particularly ice-edge blooms, in seasonally ice-covered Antarctic waters.

Compositions and transport of lipid biomarkers through the water column and surficial sediments of the equatorial Pacific Ocean

Abstract: A systematic investigation of fluxes and compositions of lipids through the water column and into sediments was conducted along the U.S. JGOFS EgPac transect from l2°N to l5°S at 140°W. Fluxes of lipids out of the euphotic zone varied spatially and temporally, ranging from ≈0.20 – 0.6 mmol lipid-C m−2 day−1. Lipid fluxes were greatly attenuated with increasing water column depth, dropping to 0.002-0.06 mmol lipid-C m−2 day−1 in deep-water sediment traps. Sediment accumulation rates for lipids were ≈ 0.0002 – 0.00003 mmol lipid-C m−2 day−1. Lipids comprised ≈ 11–23% of Corg in net-plankton, 10–30% in particles exiting the euphotic zone, 2–4% particles in the deep EgPac, and 0.1-1 % in sediments. Lipids were, in general, selectively lost due to their greater reactivity relative to bulk organic matter toward biogeochemical degradation in the water column and sediment. Qualitative changes in lipid compositions through the water column and into sediments are consistent with the reactive nature of lipids. Fatty acids were the most labile compounds, with polyunsaturated fatty acids (PUFAs) being quickly lost from particles. Branchedchain C15 and C17 fatty acids increased in relative abundance as particulate matter sank and was incorporated into the sediment, indicating inputs of organic matter from bacteria. Long-chain C39 alkenones of marine origin and long-chain C20-C30 fatty acids, alcohols and hydrocarbons derived from land plants were selectively preserved in sediments. Compositional changes over time and space demonstrate the dynamic range of reactivities among individual biomarker compounds, and hence of organic matter as a whole. A thorough understanding of biogeochemical reprocessing of organic matter in the oceanic water column and sediments is, thus, essential for using the sediment record for reconstructing past oceanic environments.

Potential links between ocean plateau volcanism and global ocean anoxia at the Cenomanian-Turonian boundary

Abstract: The Cenomanian-Turonian boundary is marked globally by the extinction of marine invertebrates and an increase in the accumulation and preservation of organic carbon-rich sediments (black shales). The timing of this boundary also coincides with large-scale volcanism from at least three large igneous provinces: the Caribbean and Ontong-Java oceanic plateaus and the Madagascar flood basalts. In this paper, we assess the possibility that hydrothermalism associated with large-scale submarine magmatism was responsible for the reduction of dissolved O 2 in the oceans. We investigate two potential mechanisms: the oxidation of reduced material in hydrothermal effluents and the stimulation of primary productivity in the water column due to the injection of hydrothermal Fe into surface waters. With the first mechanism, we find that a 10,000-km 3 submarine basalt eruption (three orders of magnitude larger than recent midocean ridge eruptions) could release enough reduced hydrothermal material to consume at least 6 percent of the dissolved O 2 in seawater in a well-ventilated ocean (such as the present ocean). With the second mechanism, we calculate that even a small percentage of the hydrothermal Fe released from a single large flow could have led to a significant increase in primary productivity in areas where Fe was the biolimiting nutrient (such as open-ocean areas in the Pacific). The potential impact of both of these O 2 -reducing mechanisms at the Cenomanian-Turonian boundary would have been significantly greater if Cretaceous oceans were less well ventilated than the present oceans.

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

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

Planktonic foraminifera as recorders of past surface-water stratification

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

Continuous measurements of water tensions in the xylem of trees based on the elastic properties of wood

Abstract: According to the cohesion theory for the ascent of water in vascular plants, significant tensions should develop in the water columns of transpiring trees. These tensions cause small but detectable changes in the diameter of the xylem as a consequence of adhesive forces between water molecules and the inner xylem walls. The diurnal time course of tension in the water columns in the xylem of the trunk of mature Scots pine (Pinus sylvestris L.) was measured during the summer of 1995 by means of a displacement transducer mounted on a rigid steel frame. The apparent elastic modulus of Scots pine wood in the radial direction (E ′ r ) was determined in the laboratory and then used to estimate tensions from the measured displacement. Laboratory measurements on logs indicated that only the sapwood contributed to dimensional changes of the xylem. Corrections for thermal expansion of the system were included. Water tensions fell by 0.19 MPa over the course of the day, when needle water potentials fell by 0.50 MPa. Such data are consistent with the cohesion theory, and with the view that the hydraulic resistances to flow in above- and below-ground plant parts are of similar magnitude.

Nutrients, organic carbon and organic nitrogen in the upper water column of the Arctic Ocean: implications for the sources of dissolved organic carbon

Abstract: Particulate and dissolved organic carbon and nitrogen were analyzed for shelf, slope and basin samples collected during the 1994 Arctic Ocean Section. Concentrations of organic carbon and nitrogen were highest in the surface water and decreased dramatically below 100 m. Over the shelf 40–50% of the organic material was present in the particulate phase, while in the slope and basin waters >-90% of the organic material was present in the dissolved phase. The halocline layer over the Chukchi plateau contained high concentrations of inorganic nutrients but no significant elevation of either organic carbon or organic nitrogen. Highest concentrations of total organic carbon were found in the surface water of the Makarov and Amundsen Basins between 80 and 90° N. The C:N ratio of the dissolved organic material in the upper 100 m ranged from 9 to 25, with highest ratios along the Makarov flank of the Lomonosov Ridge. We suggest that a major portion of the carbon-rich organic material in surface waters is derived from Eurasian shelf riverine inputs, which tend to flow in perimeter currents along the slope and ridges. Our preliminary dissolved organic carbon (DOC) budget suggests that the three major sources of DOC in the central Arctic are in situ production (56%), river run-off (25%), and Pacific water (19%).

Seasonal Patterns and Long-term Trends in an Estuarine Upwelling Ecosystem (Ría de Vigo, NW Spain)

Abstract: Thermohaline properties, nutrient salts, chlorophyll a and meteorological variables have been intensively monitored since February 1987 in the Ria de Vigo (NW Spain), in order to examine the temporal variability and the relationships between these variables over different time scales. In this paper, the seasonal and the long-term components of the 1987–92 time series are analysed. The seasonal changes in thermohaline properties are forced by meteorological factors, but whereas temperature shows a well-defined annual cycle, salinity presents a higher frequency variation pattern due to the influence of upwelling–downwelling events and runoff. Nutrient concentrations change in a regular way through the seasons, so that characteristic and well-defined cycles are observed, but they are different for each nutrient and, for a given nutrient, exhibit a marked contrast between surface and bottom layers. The seasonal changes of nutrients are not explainable by advection and water column processes alone; fractionation of nutrients during recycling and, presumably, sedimentary processes should also play an important role. The annual cycle of chlorophyll a shows a bimodal pattern, which corresponds with the development of the spring and autumn blooms; even though the seasonal cycle accounts for an important amount of the observed temporal variability, variation at lower time scales is also important. Long-term trends, as a change in the mean level of the analysed time series, have been observed for most of the variables. Salinity increased and temperature decreased both for surface and bottom series. The largest trend, in terms of the percentage to the observed variability it represents, was an increase in bottom salinity. In relation to nutrient salts, there was no evidence of increasing eutrophication, although surface dissolved inorganic nitrogen, and surface and bottom phosphate increased slightly. Chlorophyll a concentration showed a decreasing trend, especially at the surface. The observed long-term trends could be attributed to changes of the meteorological factors that operate through an increase in the estuarine residual circulation.

Seasonal patterns and long-term trends in an estuarine upwelling ecosystem (Ría de Vigo: NW Spain)

Abstract: Thermohaline properties, nutrient salts, chlorophyll a and meteorological variables have been intensively monitored since February 1987 in the Ria de Vigo (NW Spain), in order to examine the temporal variability and the relationships between these variables over different time scales. In this paper, the seasonal and the long-term components of the 1987–92 time series are analysed. The seasonal changes in thermohaline properties are forced by meteorological factors, but whereas temperature shows a well-defined annual cycle, salinity presents a higher frequency variation pattern due to the influence of upwelling–downwelling events and runoff. Nutrient concentrations change in a regular way through the seasons, so that characteristic and well-defined cycles are observed, but they are different for each nutrient and, for a given nutrient, exhibit a marked contrast between surface and bottom layers. The seasonal changes of nutrients are not explainable by advection and water column processes alone; fractionation of nutrients during recycling and, presumably, sedimentary processes should also play an important role. The annual cycle of chlorophyll a shows a bimodal pattern, which corresponds with the development of the spring and autumn blooms; even though the seasonal cycle accounts for an important amount of the observed temporal variability, variation at lower time scales is also important. Long-term trends, as a change in the mean level of the analysed time series, have been observed for most of the variables. Salinity increased and temperature decreased both for surface and bottom series. The largest trend, in terms of the percentage to the observed variability it represents, was an increase in bottom salinity. In relation to nutrient salts, there was no evidence of increasing eutrophication, although surface dissolved inorganic nitrogen, and surface and bottom phosphate increased slightly. Chlorophyll a concentration showed a decreasing trend, especially at the surface. The observed long-term trends could be attributed to changes of the meteorological factors that operate through an increase in the estuarine residual circulation.

“top–down” trophic interactions in lakes: effects of fish on nutrient dynamics

Abstract: We conducted enclosure experiments over two summers in Tuesday Lake, Michigan, to assess how a gradient of zooplanktivorous fish biomass affected the dynamics of nutrients (nitrogen, N, and phosphorus, P), and their partitioning among ecosystem compartments. In both years, fish (the cyprinid Phoxinus eos) reduced the abundance of large zooplankton species and increased the biomass of phytoplankton as predicted by the top–down control hypothesis. Fish had strong effects on the dynamics and fluxes of N and P. Total P concentrations in the water column declined over time in all enclosures, but fish slowed the rate of decline. Thus total water column P increased with increasing fish biomass. Total N increased less strongly with increasing fish biomass, and thus the total N:P ratio decreased with increasing fish biomass. The concentrations of particulate carbon, nitrogen, and phosphorus in the water column also increased with increased fish biomass. Particulate N:P ratio decreased with increased fish biomass, but effects were weaker compared to effects on total N:P ratios. Nutrient ratios of the zooplankton fraction (particles >63 μm) showed a response that was transient but consistent with observed trends in zooplankton species composition. In particular, when the large cladocerans Daphnia and Holopedium increased upon exclusion of fish, C:P and N:P ratios of the zooplankton fraction showed distinct declines, corresponding to the relatively high body P contents of these taxa. Phosphorus budgets revealed that fish were a net source of P to the water column, because they lost mass during the experiments, even at densities below those in the lake. However, loss of P from fish could not account for the higher total P concentration observed in enclosures with fish compared to fishless enclosures. The absolute amount of P sinking from the water column increased with increasing fish biomass but decreased when expressed as percentage of total P sinking, again suggesting that the presence of fish increases the relative retention of P the water column. The rate of decline in water column total P in the presence of fish was accurately predicted by sedimentation of P from the water column and other fluxes. Our results support the hypothesis that fish can exert major influences on the dynamics, distribution, and ratios of limiting nutrients.

Microalgae on the arctic ocean section, 1994: species abundance and biomass

Abstract: Algal species from the ice, the water directly below the ice (the sub-ice area), and the water column from 21 stations in the Arctic Ocean were examined using epifluorescence and inverted light microscopy Biomass of autotrophic dinoflagellates and other miscellaneous autotrophic flagellates was determined for the first time in the central Arctic basins Together these two groups dominated phytoplankton biomass in 74% of samples from the central Arctic, with diatom biomass predominant in the remainder Picophytoplankton at selected stations in the Canada and Makarov Basins contributed 93% to autotroph cell numbers and 36% to autotroph biomass Diatom species achieved high biomass in ice and sub-ice samples The centric diatom Melosira arctica dominated the sub-ice area, while pennate diatoms were major contributors to the ice samples Despite ample silicate concentrations in the water, diatom frustules were often lightly silicified

Wave-induced shear stresses, plant nutrients and chlorophyll in seven shallow lakes

Abstract: 1 Sediment resuspension dynamics were investigated in relation to changes in water column nutrients (TP, TN, PO4-P, NO3-N and NH4-N), chlorophyll a and phaeopigment in seven shallow (Zm < 15 m) lakes in South Island, New Zealand, ranging in area from 01 to 180 km2 2 Benthic shear stress, calculated from wind speed, effective fetch and depth, was a considerably better predictor of nutrient and pigment concentrations than wind speed 3 For TP, TN, chlorophyll a and phaeopigment, sixteen of the possible twenty-eight linear correlations with benthic shear stress were significant at P < 005, with 16–87% of the variation being explained by shear stress 4 Wind decreased the ratios of TN : TP, with ratios exponentially approaching those of the sediments as shear stress increased in four of the lakes 5 Relationships of dissolved inorganic nutrients to shear stress were considerably weaker than those for total nutrients and showed no consistent trend over the seven lakes 6 Estimated annual mean TP inclusive of resuspension was over four times higher than that derived from measured calm samples in two lakes 7 The number of nutrient and pigment parameters that were significantly correlated with shear stress and the strengths of the relationships varied widely from lake to lake We could establish no simple relationships between these effects and any single characteristic of the lake, sediment, or water 8 A function is developed to predict the rate of entrainment of TN and TP in response to an applied shear stress, where the independent variables are sediment nutrient content and particle size, and the macrophyte density in the lake

Mass budget and dynamics of polycyclic aromatic hydrocarbons in the Mediterranean Sea

Abstract: A mass budget was constructed to examine the status and dynamics of polycyclic aromatic hydrocarbons (PAHs) in the western Mediterranean Sea Using gas chromatography-mass spectrometry, 11 PAHs have been quantified in atmospheric aerosols, rivers and seawater, sediment cores and sediment trap samples Total PAH concentrations in Mediterranean aerosols range from 02 to 2 ng m−3, with 50–70% associated with the sub-micron particles Maximum PAH concentrations were observed in winter when the concentrations were double those recorded in the spring Total PAH inputs from the atmosphere were estimated to be from 35 to 70 t year−1 with a mean value of 475 t year−1 (wet/dry mean ratio of ∼2–3) Atmospherically-deposited PAH are dominated by the benzofluoranthenes The total PAH riverine inputs amount to about 53−33 t year−1 from the Rhone river and 13 t year−1 from the Ebro river The difference in these riverine fluxes is due to differences in annual water discharges and upstream land use The total PAH accumulation rate in surficial sediments in the whole basin is estimated at 182 t year−1 Nearly 50% of the total PAHs accumulate in the 0–200 m water depth area supporting the importance of the coastal zone as a trap of terrigenous material and associated contaminants Sediment trap experiments gave a mean residence time in the water column of total PAH (considering only particle settling) of 11 years, with higher residence times for high molecular weight PAHs This supports the hypothesis that lower molecular weight PAHs are more efficiently removed from the water column Water exchange resulted in a net outflow of 20 t year−1 and 2 t year−1 through the Gibraltar and Sicilian Straits, respectively Atmospheric deposition and the Rhone River are the major contributors of PAH in the western Mediterranean Sedimentation was identified as the major net output of PAH

Gaseous emissions and oxygen consumption in hydroelectric dams: A case study in French Guyana

Abstract: Methane, carbon dioxide, and hydrogen sulfide emissions from the hydroelectric dam of Petit Saut on the Sinnamary River in French Guyana have been measured over a 2 year period. Since the beginning of the reservoir filling (January 1994), 300 km 2 of tropical forest have been submerged. Emissions of CH 4 by diffusion and by bubbling into the atmosphere or by degassing of the water released into the river, as well as the stock of dissolved gases in the lake, and their temporal evolutions were determined. Maximum emissions of 800 t CH per day were reached in February 1995, corresponding to dissolved CH 4 concentrations of 14 mg 4 L -1 in the water column. The biological oxidation of methane results in a strong oxygen consumption in lake and river waters. Total emissions of CH 4 and CO 2 from January 1994 to December 1995 were calculated from the whole data set, which also allows us to calculate the total carbon loss since reservoir filling. About 10% of the carbon stored in soil and vegetation was released in gaseous form within 2 years.

Evaluation of long-chain alkenones as paleo-temperature indicators in the Mediterranean Sea

Abstract: Long-chain alkenones were analyzed in samples collected from February to July 1989 by a time-series sediment trap located at 200 m depth in the northwestern Mediterranean Sea. The alkenone temperature signal was calibrated using water column particles collected at the sediment trap mooring site. The calibration (U37K′ = 0.041T−0.21, r2 = 0.97, 13°C < T < 19°C), differed from the culture calibration of Prahl et al. (1988, Geochimica et Cosmochimica Acta, 52, 2303–2310) and the calibration for the eastern North Atlantic waters between 16°C and 25°C (Cenre and Eglinton, 1993, Deep-Sea Research 1, 40, 1935–1961). The slope of the calibration was, however, similar to the slope of the calibration established using all Pacific Ocean samples (Sikes and Volkman, 1993, Geochimica et Cosmochimica Acta, 57, 1883–1889). The correlation of U38MeK and water temperature was also significant (U38MeK =0.042T−0.25, r2=0.87). Sediment trap results revealed that the production of alkenones during high flux periods predominantly occurred in subsurface waters. This was further supported by the depth distribution of alkenones observed during the monthly cruises: C37 alkenone concentrations in the upper 100 m of the water column indicated a subsurface maximum at approximately 50 m depth in May, June and July, whereas in March and November alkenones were more abundant in the upper 50 m. These results emphasize that accurate reconstruction of SST in core-tops must take into account the seasonality and depth variations of alkenone production. An intercalibration exercise, conducted on Mediterranean and Norwegian Sea water samples, demonstrated that high resolution chromatographic methods are essential to obtain reliable values of U37K′ and avoid the overestimation of production temperatures, particularly for colder waters where alkyl alkenoates are abundant.

Benthic-Pelagic Links: Responses of Benthos to Water-Column Nutrient Enrichment

Abstract: Although the responses of pelagic algae and invertebrates to gradients of nutrient enrichment are well known, less is known about the responses of benthos to such gradients or how benthic and pelagic responses may interact. We performed a 9-wk experiment in 2000-L mesocosms in the field to test for the effect of water-column nutrient enrichment on phytoplankton, algae on sediments (epipelon) and hard surfaces (plastic strips), as well as pelagic and benthic primary consumers. The experimental design consisted of 4 nutrient enrichment rates (0, 0.5, 1.0 and 2.0 μg P L -1 d -1 , together with N to yield an N:P ratio of 20:1 by weight). Nutrient enrichment induced significant increases in chlorophyll a in phytoplankton and attached algae, but not epipelon. Zooplankton biomass was significantly higher in enriched mesocosms than in controls over the initial 4 wk of enrichment, but the effect was not sustained over the course of the experiment. Densities of sediment-dwelling, and hard-substra...

Observations of Stratified Turbulent Mixing in an Estuary: Neap-to-spring Variations During High River Flow

Abstract: Observations of turbulent mixing, stratification and currents were made with a free-falling microstructure profiler and a 600-kHz ADCP in the Hudson River off Manhattan in May 1994. The ship was anchored in a relatively straight and uniform channel about 15 m deep. Over 4 days of intense observations from 19 to 24 May 1994, the progression from neap tides to spring tides caused a severe reduction in stratification from an initial top-to-bottom salinity difference of 18 to 3–6. While low gradient Richardson numbers, Ri , occurred only in the weakly stratified bottom layer on flood during neap tides, low Ri spanned the water column on the second half of ebb during spring tides. Turbulent dissipation rates varied roughly inversely to Ri . During neap conditions, turbulent mixing was intense only in the bottom layer on flood, where eddy diffusivities, K ρ , reached more than 10 −2 m 2 s −1 , while mixing was weak in the centre of the halocline near 5 m. During spring tides, strong mixing occurred throughout the water column during the latter part of ebb with typical K ρ ≈5×10 −3 m 2 s −1 and typical vertical salt flux of 0·0015 kg m −2 s −1 . If it existed in isolation, this flux would eliminate the haline stratification on a time scale of 4 h. Ebb-time bands of low Ri and intense mixing showed increasing phase lag with increasing distance from the bottom. The spring-tide, ebb-time internal turbulent stress was relatively uniform with depth with a typical magnitude of 0·4 Pa (4 dyne cm −2 ). Turbulent overturning (Thorpe) scales ranged from below 1 cm in the neap-tide halocline to a few metres in the weakly stratified bottom layer on flood. The strong spring-tide, ebb-time mixing had Thorpe scales of typically 0·5 m throughout the water column. Ozmidov scales tracked Thorpe scales except in the weakly stratified layer above the bottom.

Cycling of iron, manganese, and phosphate in a meromictic lake

Abstract: Lake Nordbytjernet (Southeast Norway) had up to 40 mg liter- ’ dissolved iron and 67 mg liter-’ dissolved manganese in a stagnant layer. The ratio between iron and manganese loading was around 3 : 1, while the inventory of manganese was six times higher than for iron. High concentrations of dissolved iron occurred solely under strictly anoxic conditions, while dissolved manganese was unaffected by intrusion of oxygen during the autumnal partial circulation. Mixed potentials caused by oxidation of ferrous iron and reduction of manganic oxide or 0, controlled the redox potential values, while dissolved manganese appeared to have no influence. The following main processes controlled the sedimentation rates: fluvial supply of suspended material, precipitation of ferric hydroxide and manganese oxide from the water column, and plankton production, Sorption to ferric hydroxide in the oxic zone and reductive desorption in the anoxic zone controlled the deep water stratification of phosphorus, dissolved organic carbon, and some major cations. No similar effects were seen for manganese sedimentation. The iron sedimentation rate was enhanced in the anoxic zone due to redox coupling of iron oxidation and manganese reduction. Phosphorus was depleted in the anoxic zone during periods of substantial ferric hydroxide sedimentation. Sedimentation of manganese under anoxic conditions seemed to be governed by precipitation of carbonate and phosphate. Residence times in the lake were 0.2 yr for iron and phosphorous vs. 3.3 yr for manganese and 1.4 yr for water. Meromictic lakes (with permanent stagnant depths) produce extreme types of aquatic environments that are well suited for studies of physical and biogeochemical phenomena (Walker and Likens 1975). Products of biogeochemical processes can accumulate in such concentrations that they influence the color, turbidity, and odor of water samples from

Iron and manganese in surface waters of the Australian subantarctic region

Abstract: Iron and manganese concentrations were measured in seawater from the upper water column (≤350 m water depth) in the subantarctic region to the south and west of Tasmania, Australia, during January 1995. Dissolved and total-dissolvable Fe and dissolved Mn were determined in samples from four stations along 140°E ranging from polar to subtropical waters, and from one station over the continental slope off north-west Tasmania. Dissolved Fe concentration were generally low ( 2 nM) observed in shallower waters over the continental slope. Vertical concentration profiles suggest biological removal of dissolved Fe from the photic zone at the deep-ocean stations, except in the subantarctic front. The higher mixed-layer concentrations of Fe and Mn observed over the continental slope and in the deep ocean at 40°S are thought to reflect inputs from the continental shelves or atmospheric deposition. Water-column Fe and Mn profiles from 45°S, 50°S and 53°S show no evidence for significant inputs of these metals via atmospheric deposition or upwelling of metal-rich deep waters, although there was evidence of shelf-derived particulate Fe at 45°S. The low concentrations of dissolved Fe measured at 45°S, 50°S and 53°S indicate that iron deficiency could limit summertime primary production in these high-nitrate lowchlorophyll waters.