Showing papers on "Water column published in 1985"

The distribution of 13C of ΣCO2 in the world oceans

Abstract: Measurements of the δ13C of total dissolved inorganic carbon (ΣCO2) in the world oceans are presented. Most of the samples are from the GEOSECS expeditions which covered the Atlantic, Pacific, and Indian oceans between 1971 and 1978. The results from 2252 samples from 107 hydrographic stations are presented as north-south vertical (depth) sections with δ13C contoured at intervals of 0.1%. The data show that the distribution of δ13C is controlled mainly by the input of organically produced material and its subsequent oxidation as it falls through the water column. This covariance can be summarized by the regression equation: δ13C=1.5−0.0075·AOU, where AOU represents the oxygen utilization within a water sample after leaving the surface. Other factors influencing the distribution of δ13C are the dissolution of inorganic carbonate and the addition of anthropogenic CO2 to the oceans. A complex mathematical model was employed to estimate the levels of ΣCO2 and δ13C in pre-industrial oceanic surface waters. The results suggest that the ΣCO2 of surface waters has increased by 40 μmole kg−1 from a pre-industrial level of approximately 2135 μmole kg−1. The δ13C of the ΣCO2 has decreased by 0.5% from a pre-industrial value of approximately 2.5%.

Seasonality in the flux of natural radionuclides and plutonium in the deep Sargasso Sea. Technical report

Abstract: A record of radionuclide fluxes at a deep-ocean station near Bermuda (32/sup 0/ 05 min N, 64/sup 0/ 15 min W) was obtained from analysis of a 3-year collection of sediment-trap samples. The trap was placed at a depth of 3200 m, 1000 m above the sea floor, and the samples were recovered at 2-month intervals. Concentrations of U-238, -234, Th-232, -230, -228, Pa-231, Pb-210, Po-210, and Pu-239 and -240 were measured in the trapped material. Most of the radionuclide activity was found in the <37-micron sieved fraction. All radionuclide fluxes showed seasonal variations in phase with the variations in total sediment flux, which had been previously shown to be closely tied to the annual cycle of primary production in the overlying surface water. Seasonal variations are especially noteworthy for Th-230 and Pa-231, considering that most of their production occurs in the water column below the euphotic zone. Evidently the seasonal influence is transmitted downward by the varying particle flux so that radionuclide scavenging rates at depth, as well as at the surface, are affected. It is suggested that this could be brought about by seasonal variations in the flux of marine snow or in the rate of fecal-matter productionmore » in the deep-water column. Keywords: Pelagic sedimentation.« less

Seasonality in the flux of natural radionuclides and plutonium in the deep Sargasso Sea

Abstract: A record of radionuclide fluxes at a deep-ocean station near Bermuda (32°05′N, 64°15′W) was obtained from analysis of a 3-year collection of sediment-trap samples. The trap was placed at a depth of 3200 m, 1000 m above the sea floor, and the samples were recovered at 2-month intervals. Concentrations of 238U, 234U, 232Th, 230Th, 228Th, 231Pa, 210Po, and 239, 240Pu were measured in the trapped material. Most of the radionuclide activity was found in the <37-gmgm sieved fraction. All of the radionuclide fluxes showed seasonal variations that were in phase with the variations in total sediment flux, which were shown in earlier work to be closely tied to the annual cycle of primary production in the overlying surface water. The seasonal variations are especially noteworthy for 230Th and 231Pa, considering that most of their production occurs in the water column below the euphotic zone. Evidently the seasonal influence is transmitted downward by the varying particle flux so that radionuclide scavenging rates at depth, as well as at the surface, are affected. It is suggested that this could be brought about by seasonal variations in the flux of marine snow or in the rate of fecal-matter production in the deep-water column. Fluxes of 230Th and 231Pa integrated over the annual cycle yielded a trapping efficiency of 105 ± 17% for the PARFLUX sediment trap used in this investigation.

Entrainment and vertical transport of deep-ocean water by buoyant hydrothermal plumes

Abstract: Submarine hydrothermal vents produce effluent plumes in the water column which can be easily detected with tracers such as 3He, manganese and methane1–3. Comparison of the tracer concentrations and temperature anomalies in such plumes with direct measurements of the pure vent waters shows that the plumes are typically mixtures of 1 part vent water with 104 parts ambient sea water1. However, the extent to which hydrothermal plumes entrain and transport sea water and contribute to deep-ocean mixing is not known. We present here hydrographie data collected in the vicinity of an active hydrothermal field on the Endeavour segment of the Juan de Fuca Ridge where a well-defined effluent layer resides in the water column ∼200 m above the depth of the vent field. The temperature and salinity signature of this layer indicates that a small volume of hot vent water can be very efficient at entraining ambient sea water. Directly above the Endeavour vent field, the effluent layer is composed of 0.01% vent water, 30% ambient water normally found at that depth, and ∼70% entrained water which has been transported from deeper in the water column.

The effects of suspensoids on fish

Abstract: High suspensoid loads are a common feature of many Southern Hemisphere inland waters. Case studies on a natural lake (Chilwa), a man-made lake (Le Roux) and S.W. Indian Ocean estuaries reveal that the effects of turbidity on fish in these systems differ widely. In Lake Le Roux, high suspensoid loads influence fish by causing a reduced growth rate, a decrease in size at first maturity and maximum size, and a movement inshore by large fishes to feed on phytobenthos. High turbidities in Lake Chilwa sharply reduce food availability in benthic offshore zones, and restrict fishes to pelagic and inshore food resources. The resuspension of sediments by wind action may cause fish mortalities through deoxygenation of the water column. Moderate turbidity levels appear to be beneficial to fish in estuaries by affording protection from predators in shallow, food-rich areas. Turbidity gradients may also provide a navigational aid to fish entering estuaries.

Environmental setting of San Francisco Bay

Abstract: San Francisco Bay, the largest bay on the California coast, is a broad, shallow, turbid estuary comprising two geographically and hydrologically distinct subestuaries: the northern reach lying between the connection to the Pacific Ocean at the Golden Gate and the confluence of the Sacramento-San Joaquin River system, and the southern reach (herein called South Bay) between the Golden Gate and the southern terminus of the bay. The northern reach is a partially mixed estuary dominated by seasonally varying river inflow, and the South Bay is a tidally oscillating lagoon-type estuary. Freshwater inflows, highest during winter, generate strong estuarine circulation and largely determine water residence times. They also bring large volumes of dissolved and particulate materials to the estuary. Tidal currents, generated by mixed semidiurnal and diurnal tides, mix the water column and, together with river inflow and basin geometry, determine circulation patterns. Winds, which are strongest during summer and during winter storms, exert stress on the bay’s water surface, thereby creating large waves that resuspend sediment from the shallow bay bottom and, together with the tidal currents, contribute markedly to the transport of water masses throughout the shallow estuary.

Temporal dynamics of estuarine phytoplankton: A case study of San Francisco Bay

Abstract: Detailed surveys throughout San Francisco Bay over an annual cycle (1980) show that seasonal variations of phytoplankton biomass, community composition, and productivity can differ markedly among estuarine habitat types. For example, in the river-dominated northern reach (Suisun Bay) phytoplankton seasonality is characterized by a prolonged summer bloom of netplanktonic diatoms that results from the accumulation of suspended particulates at the convergence of nontidal currents (i.e. where residence time is long). Here turbidity is persistently high such that phytoplankton growth and productivity are severely limited by light availability, the phytoplankton population turns over slowly, and biological processes appear to be less important mechanisms of temporal change than physical processes associated with freshwater inflow and turbulent mixing. The South Bay, in contrast, is a lagoon-type estuary less directly coupled to the influence of river discharge. Residence time is long (months) in this estuary, turbidity is lower and estimated rates of population growth are high (up to 1–2 doublings d−1), but the rapid production of phytoplankton biomass is presumably balanced by grazing losses to benthic herbivores. Exceptions occur for brief intervals (days to weeks) during spring when the water column stratifies so that algae retained in the surface layer are uncoupled from benthic grazing, and phytoplankton blooms develop. The degree of stratification varies over the neap-spring tidal cycle, so the South Bay represents an estuary where (1) biological processes (growth, grazing) and a physical process (vertical mixing) interact to cause temporal variability of phytoplankton biomass, and (2) temporal variability is highly dynamic because of the short-term variability of tides. Other mechanisms of temporal variability in estuarine phytoplankton include: zooplankton grazing, exchanges of microalgae between the sediment and water column, and horizontal dispersion which transports phytoplankton from regions of high productivity (shallows) to regions of low productivity (deep channels).

Shallow-water benthic and pelagic metabolism: evidence of heterotrophy in the nearshore Georgia Bight

Abstract: Pelagic primary production and benthic and pelagic aerobic metabolism were measured monthly at one site in the estuarine plume region of the nearshore continental shelf in the Georgia Bight. Benthic and water-column oxygen uptake were routinely measured and supplemented with seasonal measures of total carbon dioxide flux. Average respiratory quotients were 1.18:1 and 1.02:1 for the benthos and water column, respectively. Benthic oxygen uptake ranged from 1.23 to 3.41 g O2 m-2 d-1 and totalled 756 g O2 m-2 over an annual period. Water column respiration accounted for 60% of total system metabolism. Turnover rates of organic carbon in sediment and the water column were 0.09 to 0.18 yr-1 and 6.2 yr-1, respectively. Resuspension appeared to control the relative amounts of organic carbon, as well as the sites and rates of organic matter degradation in the benthos and water column. Most of the seasonal variation in benthic and pelagic respiration could be explained primarily by temperature and secondarily by primary productivity. On an annual basis, the shelf ecosystem appeared to be heterotrophic; primary production was 73% of community metabolism, which was 749 g C m-2 yr-1. The timing of heterotrophic periods through the year appeared to be closely related to both river discharge and the periodicity of growth and death of marsh macrophytes in the adjacent estuary. The results of this study support the estuarine “outwelling” hypothesis of Odum (1968).

Dissolved aluminium in the central North Pacific

Abstract: Aluminium is the most abundant metallic element in the Earth's crust (8.23% by weight)1, yet little is known about its oceanic distribution. Published data sets concerning aluminium in sea-water2 are primarily for the North Atlantic Ocean3–7. We report here that dissolved aluminium concentrations in the central North Pacific are 8–40 times lower than those at corresponding depths in the central North Atlantic, but the vertical distribution features are similar. The vertical distribution and inter-ocean fractionation of aluminium can be explained by geographical variations in atmospheric aluminium sources, intense particle scavenging throughout the water column, and some regeneration in bottom waters. Aluminium's short oceanic residence time (estimated here as 100–200 years) leads to its marked inter-ocean fractionation, which is the reverse of that for nutrient elements such as silicon.

Increased benthic grazing: An alternative explanation for low phytoplankton biomass in northern San Francisco Bay during the 1976–1977 drought

Abstract: Among the consequences of extremely low river flow into northern San Francisco Bay during a two-year drought were (1) a gradual increase in salinity, (2) an unusual decline in chlorophyll a concentration, and (3) the upstream migration of estuarine benthic invertebrates to the normally brackish area of the bay. Total abundance in the benthos at a shallow monitoring site increased from a normal 2000 to greater than 20 000 individuals m −2 during the summer of 1977, presumably in response to the increased salinity. Estimated filtration rates derived from equations in the literature for one of the species, the suspended-feeding bivalve Mya arenaria ranged from 1 to 4 m 3 m −2 day −1 during 1977 depending on abundance and mean size on sampling dates. Because water depth at this site is less than 2 m, Mya could have filtered all of the particles (including diatoms) from the water column on the order of once per day. Several other immigrant species undoubtedly contributed to the removal of particles from the near-bottom water as well. Increased benthic grazing, therefore, could have accounted for the anomalously low phytoplankton biomass observed during the drought. These results suggest that during periods of prolonged low river flow and increased salinity benthic food webs could become more important than planktonic food webs in the upper part of the estuary.

Early chemical diagenesis, sediment-water solute exchange, and storage of reactive organic matter near the mouth of the Changjiang, East China Sea

Abstract: A substantial proportion of the material delivered to the modern oceans is supplied by a few large rivers such as the Changjiang. Early diagenetic reactions in surficial bottom sediments determine in large part both the eventual influence of these rivers on the sea and the nature of sedimentary deposits formed. The region off the mouth of the Changjiang exemplifies the interplay between physical, chemical, and biological factors which can produce particular spatial patterns of diagenesis and sediment-water exchange. To examine these patterns measurement of pore water solute profiles, sediment-water solute fluxes, and solute reaction rates in the upper few decimeters of sediment were made at 27 stations near the Changjiang in the East China Sea. Direct measurements of dissolved Si(OH)4, NH4+, and NO3−, fluxes from or into bottom sediments made during summer and autumn periods (15 to 24°C) range from 0.13 to 13.2, −2.6 to 3.4, and −1.4 to 3.2mmol m−2 day−1, respectively. Net solute flux from the sea floor is often lowest from deposits having the highest interstitial solute concentrations. In addition, bottom regions having the highest build up of reaction products or depletion of reactants in pore waters (with respect to overlying water) actually have the lowest rates of reaction. These same areas of elevated (products) or depleted (reactants) pore water solute concentrations, low reaction rates, and low net rate of solute exchange which are located near the mouth of the Changjiang are sites of high sedimentation rates and depauperate benthic communities. High water turbidity and resuspension apparently hinder water column production and input of reactive organic matter or other biogenic material which drive many diagenetic reactions. Rapid sedimentation or disturbance hinders benthic community development, lowers biogenic reworking, and allows build up or depletion of reaction products or reactants in bottom sediments. Offshore areas of lower sedimentation, higher productivity, and active bottom communities are sites of high initial reaction rates and increased sediment-water solute exchange compared with rapid sedimentation regions. A diagenetic paradox resulting from the interaction between benthic communities and the physical environment of sedimentation is that proportionally the greatest storage of diagenetic products related to organic matter decomposition can occur in sediments that are initially the least diagenetically reactive.

Arsenic, barium, germanium, tin, dimethylsulfide and nutrient biogeochemistry in Charlotte Harbor, Florida, a phosphorus-enriched estuary

Abstract: Concentrations of dissolved nutrients (NO3, PO4, Si), germanium species, arsenic species, tin, barium, dimethylsulfide and related parameters were measured along the salinity gradient in Charlotte Harbor. Phosphate enrichment from the phosphate industry on the Peace River promotes a productive diatom bloom near the river mouth where NO3 and Si are completely consumed. Inorganic germanium is completely depleted in this bloom by uptake into biogenic opal. The GeSi ratio taken up by diatoms is about 0·7 × 10−6, the same as that provided by the river flux, confirming that siliceous organisms incorporate germanium as an accidental trace replacement for silica. Monomethylgermanium and dimethylgermanium concentrations are undetectable in the Peace River, and increase linearly with increasing salinity to the seawater end of the bay, suggesting that these organogermanium species behave conservatively in estuaries, and are neither produced nor consumed during estuarine biogenic opal formation or dissolution. Inorganic arsenic displays slight removal in the bloom. Monomethylarsenic is produced both in the bloom and in mid-estuary, while dimethylarsenic is conservative in the bloom but produced in mid-estuary. The total production of methylarsenicals within the bay approximately balances the removal of inorganic arsenic, suggesting that most biological arsenic uptake in the estuary is biomethylated and released to the water column. Dimethylsulfide increases with increasing salinity in the estuary and shows evidence of removal, probably both by degassing and by microbial consumption. An input of DMS is observed in the central estuary. The behavior of total dissolvable tin shows no biological activity in the bloom or in mid-estuary, but does display a low-salinity input signal that parallels dissolved organic material, perhaps suggesting an association between tin and DOM. Barium displays dramatic input behavior at mid-salinities, probably due to slow release from clays deposited in the harbor after catastrophic phosphate slime spills into the Peace River.

Seasonal Succession of Phytoplankton in Lake Constance

Abstract: In Lake Constance, the phytoplankton growth season starts when stratification begins in spring. Initially, maximum growth rates are favored; later, as the water column stabilizes, zooplankton grazing and competition for nutrients become the dominant selective forces. When the mixing depth increases in autumn, algae that can tolerate low light are selected for. Temperature and sedimentation turn out to be less important in succession than traditionally assumed.

Benthic nutrient remineralization in a coastal lagoon ecosystem

Abstract: In situ measurements of the exchange of ammonia, nitrate plus nitrite, phosphate, and dissolved organic phosphorus between sediments and the overlying water column were made in a shallow coastal lagoon on the ocean coast of Rhode Island, U.S.A. The release of ammonia from mud sediments in the dark (20–440 μmol per m2 per h) averaged ten times higher than from a sandy tidal flat (0–60 μmol per m2 per h), and while mud sediments also released nitrate and phosphate, sandy sediments took up these nutrients. Fluxes of nutrients from mud sediments, but not from sandy areas, markedly increased with temperature. Ammonia release rates for mud sediments in the light (0–350 μmol per m2 per h) were lower than those in the dark and it is estimated that some 25% of the ammonia released to the water column on an annual basis may be intercepted by the benthic microfloral community. Estimates of the annual net exchange of nutrients across the sediment-water interface, weighted by sediment type for the lagoon as a whole, showed a release of 450 mmol per m2 of ammonia, 5 mmol per m2 of phosphate, 5 mmol per m2 of dissolved organic phosphorus, and an uptake of 80 mmol per m2 of nitrate. Although rates of ammonia and nitrate exchange were comparable to those described for the deeper heterotrophic bottom communities of nearby Narragansett Bay, rates of benthic phosphate release were significantly lower. On an annual basis the Bay benthos released approximately 20 times more inorganic phosphate per unit area than did the lagoon benthos. As a result., the N/P ratio for the flux from the sediments was 74∶1 in the lagoon, compared with 16∶1 in “average” marine plankton and 8∶1 for the benthic flux from Narragansett Bay. The lack of remineralized phosphate in the lagoon, is reflected in water, column phosphate concentrations (always <1 μm) and water column N/P ratios (annual N/P=27) and suggests that the lagoon may show phosphate limitation rather than the nitrogen limitation commonly associated with marine systems.

The distance demersal zooplankton migrate above the benthos: implications for predation

Abstract: The distance demersal zooplankton (mobile, benthic organisms which periodically emerge from the benthos and move up into the water column) swim vertically above the bottom at night was measured quantitatively on a subtidal sand flat in the Gulf of California during July, 1979. Three patterns of migration were observed: (1) small-bodied animals, including copepods, ostracods and the amphipod Metaceradocus occidentalis, remained within 30 cm of the bottom except at full moon when a significantly higher proportion of these animals swam up at least 1 m into the water column, (2) syllid polychaetes swam up at least 2 m into the water column irregardless of the phase of the moon, and (3) large-bodied forms (animals >2 mm) swam throughout the water column but in gradually decreasing abundances nearer the surface. Since nocturnally foraging planktivorous fishes feed primarily on the large-bodied, readily visible animals, we had predicted that these large forms would remain near the relative safety of the benthos. However, the movement of the larger demersal zooplankton higher into the water column than the smaller, less visible forms, suggests that factors other than predation, possibly dispersal, may be major selective pressures governing the distance demersal zooplankton swim above the benthos.

Manganese fluxes in the benthic boundary layer

Abstract: Diagenetic modeling and mass-balance calculations were applied to box-core and sediment-trap data from three stations at 300-400-m depth in the Laurentian Trough to estimate downward and upward fluxes of manganese across the sediment-water interface, fluxes across the redox boundary in the sediment, rates of dissolution and precipitation of manganese, and rates of manganese accumulation. At all stations the cycling of manganese between the oxidizing and the reducing zone of the sediment was quantitatively more important than the cycling between the sediment and the water column. The redox boundary was the site of the largest fluxes. Downward fluxes across this boundary (0.45, 1.23, and 13.9 mmol mm2 d-l) were 3-50 times the rates of sedimentation or accumulation of manganese. The production of dissolved manganese turns over the inventory of particulate manganese in the sediment surface layer in 43-207 days. A small proportion of the dissolved manganese produced (13-29%) escapes the sediment, reprecipitates in the water column and, in part, returns to the sediment. Increased rates of bioturbation increase the rate of internal manganese cycling more than they do the rate of cycling across the sediment surface. The surface sediments of coastal marine deposits are often enriched in manganese as a result of processes taking place during the early stages of diagenesis (Calvert and Price 1977). These processes comprise burial and dissolution of particulate manganese, upward migration of dissolved manganese, and reprecipitation of particulate manganese at the sediment surface. The dynamic nature of the diagenesis of manganese was demonstrated by Aller (1980). By modeling the production rate of dissolved manganese in bioturbated sediments in Long Island Sound, he found that the production rate was sufficient to turn over the manganese contained in the enriched surface layer every 60-100 days. We here extend Aller’s approach to include cycling across the sediment-water interface as well as within the sediment itself, using data from the Gulf of St. Lawrence, diagenetic modeling, and mass balance calculations. We thank C. Gobeil for technical assistance and R. C. Aller for discussions. The estimates of the biological mixing coefficients were made possible thanks to the collaboration of V. Nguyen, Y. Yokoyama, and R. Chesselet.

Effect of fecal pollution on Vibrio parahaemolyticus densities in an estuarine environment.

Abstract: Vibrio parahaemolyticus densities in the Narragansett Bay, R.I., estuary were found to significantly correlate with the level of fecal pollution in the water. Results of field investigations showed a definite association between the levels of this organism and those of Escherichia coli, Clostridium perfringens, and enterococci. Densities of V. parahaemolyticus were greatest in the near-surface waters of contaminated areas and decreased sharply with both the distance from the sources of fecal pollution and the depth of the water column. A positive association with the amount of particulate matter in the water and specifically with its zooplankton content also was found. No association was obtained with any of the physical or chemical parameters examined except dissolved oxygen. The results of laboratory studies on the growth of V. parahaemolyticus in collected estuarine waters were consistent with the field observations in that nutrients in sewage did not directly produce any increase in natural or inoculated levels of the organism. Of several particulates added to the water, only chitin and net zooplankters (live or dead) supported the growth of V. parahaemolyticus. The addition of sewage to the water had no measurable effect, whether or not the various particulates also were present. The data show that wastewater effluents have an effect on V. parahaemolyticus densities in this estuary and indicate that the effect is indirect, probably mediated by biostimulation of the food chain and manifested at the level of the microfauna.

Temporal and spatial variation in bacterial production in the water column over a coral reef

Abstract: Production and doubling times of the bacterial populations in the water around and over the reefs at Lizard Island, Great Barrier Reef were measured during summer and winter, 1982 and 1983. Bacterial productivity, determined from the rate of tritiated thymidine incorporation into DNA, was high over the reef flats and a Thalassia hemprichii sand flat (28 to 58 μg Cl-1 d-1). Bacterial growth rates increased during the day and fell at night over the reef flats and seagrass bed. Growth rates were slower over the reef front and in open water. Doubling times ranged from about 2 d in the open water to about 3 h over the reef flat in summer. As numbers did not increase, grazing was probably intense on the reef flats. Growth rates were much slower in winter. The main source of organic nutrient used by the bacteria was probably mucus released following photosynthesis in the corals. The cyanobacterium Synechococcus sp. was sometimes very numerous, especially in summer when 2×108 cells l-1 were recorded in one water mass. The number of bacteria was also very high in summer, with values ranging from 1×109 to 2.5×109l-1.

231Pa and 230Th profiles in the open ocean water column

Abstract: 231Pa and two Th isotopes (230Th and 228Th) were measured using large-volume water samples from the western North Pacific. Although 231Pa and 230Th have uniform source distributions (by decay of uranium) throughout the oceans, their vertical profiles are discernibly different from each other. The 231Pa profile shows a mid-depth maximum around 2.5 km, while 230Th increases monotonically with depth. This demonstrates that these nuclides follow different pathways for their transport and removal from seawater to depositional sinks. The activity ratio of 230Th/231Pa in seawater increases from one at the surface to four at a depth of 4 to 5 km, which is lower by a factor of 10 than the ratios published for open ocean particulate matter at the same depth horizon. The distribution coefficient, KD for Pa, and the fractionation factor, FTh/Pa, between natural marine particulate matter and seawater are constant with depth at ∼2 × 100 and 10, respectively. This implies that the adsorptive characteristics of deep open ocean particles are independent of particle composition or the content of manganese. The mean residence time of 231Pa is estimated to be 200 years, which is considerably longer than that of 230Th, but close to that of 210Pb. There is a general similarity between 231Pa and 210Pb in their deep ocean profiles, suggesting that 231Pa may follow 210Pb in the marine geochemical system.

Thermal characteristics of standing waters: an illustration of dynamic processes

Abstract: The principles of physical limnology in the Southern Hemisphere are no different to those in other parts of the world and the same mechanisms may be expected to be active in lakes of similar size and shape. However, in actuality, it is observed (Imberger, 1982; Ward, 1982; Allanson & Jackson, 1983; Powell et al., 1984) that the temperature gradients and their seasonal occurrence are more predominant in the Southern Hemisphere. The temperature range is fixed by the winter cold inflow and the summer heating and the temperature range from the bottom of a lake to the lake surface in the Southern Hemisphere is commonly up to 10 C° or even 15 C° during the summer peak period. Combined with the overall elevated mean temperature, this leads to an extremely strong buoyancy stabilization of the water column and it is this which characterizes lakes in the Southern Hemisphere.

Benthic-pelagic coupling and nutrient cycling across an experimental eutrophication gradient

Abstract: Sediment-water exchange rates of dissolved inorganic nitrogen (NH:, NO;, NO;) and phosphorus (PO:-), water column concentrations of both dissolved and particulate forms of N and P, and net primary production (14C) were measured during a summer period in large (13 m3) experimental mesocosms that had been subjected to continuous daily nutrient additions (N, P, Si) for over 1 yr. The concentrat~on of combined dissolved inorganic plus particulate nutrient forms (N, P) was linearly related to the nutrient input rate across a loading range from 596 to 34,100 mmols N m-2 yr-l. Benthic nutrient regeneration generally increased, although not uniformly, with loading. Average summer fluxes for treatments ranged from about 150 to 1200 pmols NH: m-2 h' , 188 to 989 pmols DIN m-2 h' , and 8 to 76 pmols PO:m-2 h' , with highest rates being I-ecorded at the 2 highest loading levels. Benthic nutrient regeneration d ~ d not increase in direct proportion to loading, but appeared strongly related to net primary production rates across the enrichment gradient. Comparison, across the experimental gradient, of the external nutrient supply rate with the benthic feedback of dissolved nutrients to the water column indicated that beyond a loading of about 2000 to 5000 mmols N m-? yr-' the water column nutrient dynamics became dominated by the external supply. Results suggest that the importance of benthic-pelagic interactions to biogeochemical and ecological cycles of coastal areas may decrease markedly with nutrient enrichments which exceed the needs of the autotrophic components of the ecosystem.

Sediments and water quality

Abstract: Sediments play a crucial role in water quality. On the one hand they remove pollutants from the water column whereas on the other hand pollutants accumulated in the sediments may provide the surface water (long after the source of pollution has ceased) with contaminants. An overview of the immobilization and remobilization processes affecting heavy metals and organic micro‐pollutants is given.

Sulfur constituents and cycling in waters, seston, and sediments of an oligotrophic lake

Abstract: Organic and inorganic sulfur constituents in streams, the water column, seston, and sediments of an oligotrophic Adirondack lake were measured for 2 years (1981-1983). Soluble organic S constituents (C-bonded S and ester sulfate) were l-18% of total S in streams, the water column, and lake outlet. Seston S (0.3-1.2% dry mass) in South Lake consisted of ester sulfate (44-59%), C-bonded S (32-43%), sulfate (10-l 6%), and nonsulfate inorganic S (~2%). Rates of S deposition measured in sediment traps were highest after spring turnover. The organic matter content (5281% dry mass) of traps at 5, 8, and 15.5 m showed no significant differences. Net mineralization of seston inputs was 26% based on mass balance calculations, with 43% of the ester sulfate input mineralized. Because most ofthe S input to the sediments was not mineralized, organic S accumulated and constituted the major (74% of total S) S component of the sediment. Most studies of sulfur cycling in freshwater systems have focused on the dynamics of inorganic sulfate and sulfide for which transformations are regulated by redox reactions (Stuiver 1967; Berner 197 1; Cook 198 1). Organic S in freshwater lake sediments has been less studied. Hesse (1958) found that >90% of the total S in the sediments of Lake Victoria was in organic form. Nriagu (1968) found that organic S composed most of the total S in marl deposits and a small amount of total S in sludge sed

Plankton distribution and activity in the North Sea/Skagerrak-Kattegat frontal area in April 1984

Abstract: 47 fixed stations off the Northwest Danish coast were s a m ~ l e d svstematicallv and repeatedly between 5 and 14 April 1984, for distribution of temperature, salinity, and chlorophyll a fluorescence throuahout the water column. Low chloro~hvll concentrations were recorded at stations in . . North Sea water. In the Skagerrak/Kattegat stations sampled, dense phytoplankton concentrations (chl a up to >20 mg m-3) were observed in association with a strong halocline (surface to bottom AS up to >l5 %o). This halocline weakened somewhat in the region northhortheast of Skagen and at 'upwelling' stations near the Swedish coast. Surface patches of elevated chlorophyll concentrations were frequently associated with these weakenings of the halocline. Determination of combined nitrite/ nitrate revealed elevated values in surface waters in these less stratified regions. A strong front occurred in the approaches toward North Sea water to the west of Skagen. Over the frontal region, the water column salinity difference decreased from > l 2 to <0.1 %O over the course of 39 km. The distribution of chlorophyll in relation to the hydrographic features of this front was determined 4 times during 1 wk. Chlorophyll distribution patterns were apparently related to hydrographic features and were observed to be extremely dynamic. Primary production and light measurements were taken on 1 frontal crossing. These indicate that daily production rates may have been on the order of 25 X higher in this frontal region than in surrounding waters at the time of sampling. Copepod nauplii and egg distributions were ca I O X higher in Skagerrak/Kattegat waters than in the North Sea and did not show a clear association with the front. The highest nauplii/egg concentrations were observed in association with dense chlorophyll patches located at the pycnocline. Egg production rates of the copepod Calanus finmarchicus were shown to be positively correlated with chlorophyll concentration at the depth of collection.

Water column and sediment characteristics of Lake Fryxell, Taylor Valley, Antarctica

Abstract: Lake Fryxell is a permanently icecovered 19 m deep lake in the lower Taylor Valley, Antarctica (75°35'S, 163°35'E). The lake is thermally and chemically stratified and has a euphotic (aerobic) zone above 9 m depth and an anaerobic (anoxic) zone below. Lake waters are derived from glacial melt into which upward diffusion of brines or redissolved salts from the basin has formed a diffusion cell of about 1000 years age. Lake sediment cores contain five recognised units, three of which are calcareous. The uppermost unit (E) has, at the top of the sediment column, calcite flakes precipitated as a result of biological CO2 fixation in the euphotic zone. The calcite is either deposited as stromatolites, where the lake bed is within the euphotic zone, or from suspension over deeper parts of the lake. Unit D is a varve-like aragonite deposit dated at about 10 000 years RP. and was deposited as a result of evaporative concentration of lake waters following the retreat of the Ross Sea I expanded ice sheet fr...