Showing papers on "Water column published in 2000"

Phosphate depletion in the western North Atlantic Ocean.

Abstract: Surface waters of the subtropical Sargasso Sea contain dissolved inorganic phosphate (DIP) concentrations of 0.2 to 1.0 nanomolar, which are sufficiently low to result in phosphorus control of primary production. The DIP concentrations in this area (which receives high inputs of iron-rich dust from arid regions of North Africa) are one to two orders of magnitude lower than surface levels in the North Pacific (where eolian iron inputs are much lower and water column denitrification is much more substantial). These data indicate a severe relative phosphorus depletion in the Atlantic. We hypothesize that nitrogen versus phosphorus limitation of primary production in the present-day ocean may be closely linked to iron supply through control of dinitrogen (N2) fixation, an iron-intensive metabolic process. Although the oceanic phosphorus inventory may set the upper limit for the total amount of organic matter produced in the ocean over geological time scales, at any instant in geological time, oceanic primary production may fall below this limit because of a persistent insufficient iron supply. By controlling N2 fixation, iron may control not only nitrogen versus phosphorus limitation but also carbon fixation and export stoichiometry and hence biological sequestration of atmospheric carbon dioxide.

Transport and degradation of phytoplankton in permeable sediment

Abstract: In flume and field experiments we demonstrate that interfacial water flows, generated when bottom currents interact with sea bed topography, provide a fast and efficient pathway for the transport of suspended phytoplankton into subsurface layers of permeable sandy sediments. The advective transport, associated with small mounds and ripples as commonly found on shelf sediments, increased penetration depth of unicellular algae (Dunaliella spec.) into sandy sediment (permeability k 5 4 3 10 211 m 2 ) up to a factor of 7 and flux up to a factor of 9 relative to a smooth control sediment. The pore water flow field produced a distinct distribution pattern of particulate organic matter in the sediment with subsurface concentration maxima and zones depleted of algae. Flux chamber simulations of advective transport of algae into sands of different grain sizes revealed increasing fluxes, algal penetration depths, and degradation rates with increasing permeability of the sediment. Two experiments conducted in intertidal sand flats confirmed the importance of the advective interfacial transport of phytoplankton for natural settings, showing permeability-dependent penetration of planktonic algae into embedded sand cores of different grain sizes. The significance of our results is discussed with respect to particulate organic matter flux and mineralization in shelf sands, and we suggest the concept of a decomposition layer. In contrast to muddy sea beds with low permeabilities, where transport of solutes is mainly driven by diffusion, water can flow through marine sands, providing a fast carrier for the exchange of substances between the water column and the upper sediment layers. Surface gravity waves cause pressure oscillations that increase fluid exchange at the sediment‐water interface and dispersion of solutes within the

Black shale deposition and faunal overturn in the Devonian Appalachian Basin: Clastic starvation, seasonal water-column mixing, and efficient biolimiting nutrient recycling

Abstract: Integrated geochemical data suggest that black shale deposition in the Devonian Geneseo Formation of western New York was initiated by the coincidence of siliciclastic starvation and the intensification of seasonal water column stratification and mixing. Once established, however, black shale deposition was maintained through efficient recycling of biolimiting nutrients which enhanced primary productivity. Recycling efficiency was achieved through a positive feedback loop of oscillating benthic redox conditions that enhanced N and P regeneration from sediments, sustained high primary productivity by returning nutrients to the photic zone during mixing, and ensured a downward flux of organic matter that drove or enhanced the episodic development of benthic anoxia during stratification. This feedback was ultimately disrupted by rising siliciclastic influx, which diluted organic matter and restored benthic redox stability. The abrupt overturn of diverse, long-standing Appalachian basin marine communities may have been the result of trophic resource destabilization during Geneseo deposition.

Relationships between benthic community condition, water quality, sediment quality, nutrient loads, and land use patterns in Chesapeake Bay

Abstract: Associations between macrobenthic communities, measures of water column and sediment exposure, and measures of anthropogenic activities throughout the watershed were examined for the Chesapeake Bay, U.S. The condition of the macrobenthic communities was indicated by a multimetric benthic index of biotic integrity (B-IBI) that compares deviation of community metrics from values at reference sites assumed to be minimally altered by anthropogenic sources of stress. Correlation analysis was used to examine associations between sites with poor benthic condition and measures of pollution exposure in the water column and sediment. Low dissolved oxygen events were spatially extensive and strongly correlated with benthic community condition, explaining 42% of the variation in the B-IBI. Sediment contamination was spatially limited to a few specific locations including Baltimore Harbor and the Southern Branch of the Elizabeth River and explained about 10% of the variation in the B-IBI. After removing the effects of low dissolved oxygen events, the residual variation in benthic community condition was weakly correlated with surrogates for eutrophication—water column concentrations of total nitrogen, total phosphorus, and chlorophylla. Associations between benthic conditions and anthropogenic inputs and activities in the watershed were also studied by correlation analysis. Benthic condition was negatively correlated with measures of urbanization (i.e., population density, point source loadings, and total nitrogen loadings) and positively correlated with watershed forestation. Significant correlations were observed with population density and nitrogen loading below the fall line, but not above it, suggesting that near-field activities have a greater effect on benthic condition than activities in the upper watershed. At the tributary level, the frequency of low dissolved oxygen events and levels of sediment contaminants were positively correlated with population density and percent of urban land use. Sediment contaminants were also positively correlated with point source nutrient loadings. Water column total nitrogen concentrations were positively correlated with nonpoint nutrient loadings and agricultural land use while total phosphorus concentrations were not correlated with land use or nutrient loadings. Chlorophylla concentrations were positively correlated with nitrogen and phosphorus concentrations in the water column and with agricultural land use but were not correlated with nutrient loads.

The Late Pleistocene - Holocene palaeolimnology of Lake Victoria East Africa, based upon elemental and isotopic analyses of sedimentary organic matter

Abstract: Three piston cores from Lake Victoria (East Africa) have been analysed for organic carbon (TOC) and nitrogen (TN) content, stable isotopes (δ13C and δ15N), and Hydrogen Index (HI). These data are combined with published biogenic silica and water content analyses to produce a detailed palaeolimnological history of the lake over the past ca. 17.5 ka. Late Pleistocene desiccation produced a lake-wide discontinuity marked by a vertisol. Sediments below the discontinuity are characterised by relatively low TOC and HI values, and high C/N, δ13C and δ15N, reflecting the combined influence of abundant terrestrial plant material and generally unfavourable conditions for organic matter preservation. A thin muddy interval with lower δ13C and higher HI and water content indicates that dry conditions were interrupted by a humid period of a few hundred years duration when the lake was at least 35 m deep. The climate changed to significantly more humid conditions around 15.2 ka when the dry lake floor was rapidly flooded. Abundant macrophytic plant debris and high TOC and δ13C values at the upper vertisol surface probably reflect a marginal swamp. δ13C values decrease abruptly and HI begins to increase around 15 ka BP, marking a shift to deeper-water conditions and algal-dominated lake production. C/N values are relatively low during this period, suggesting a generally adequate supply of nitrogen, but increasing δ15N values reflect intense utilisation of the lake's DIN reservoir, probably due to a dramatic rise in productivity as nutrients were released to the lake from the flooded land surface. An abrupt drop in δ13C and δ15N values around 13.8-13.6 ka reflects a period of deep mixing. Productivity increased due to more efficient nutrient recycling, and δ13C values fell as 12C-rich CO2 released by bacterial decomposition of the organic material was brought into the epilimnion. A weak drop in HI values suggests greater oxygen supply to the hypolimnion at this time. Better mixing was probably due to increased wind intensity and may mark the onset of the Younger Dryas in the region. After the period of deep mixing, the water column became more stable. TOC, C/N, δ13C and HI values were at a maximum during the period between 10 and 4 ka, when the lake probably had a stratified water column with anoxic bottom waters. A gradual decrease in values over the last 4000 yrs suggest a change to a more seasonal climate, with periodic mixing of the water column. Rising sediment accumulation rates and a trend to more uniform surface water conditions over the last 2000 yrs are probably a result of increased anthropogenic impact on the lake and its catchment. Following a maximum at the time of the rapid lake-level rise during the terminal Pleistocene, δ15N has remained relatively low and displays a gradual but consistent trend to lower values from the end of the Pleistocene to the present. TN values have risen during the same period. The lack of correlation between δ13C and δ15N, and the absence of any evidence for isotopic reservoir effects despite the rise in TN, suggests that the atmosphere, rather than the lake's dissolved nitrogen pool has been the principal source of nitrogen throughout the Holocene. The importance of atmospheric N fixation to Lake Victoria's nitrogen cycle thus predates by a very considerable margin any possible anthropogenic eutrophication of the lake.

Do diatoms in the Swiss Alps reflect the length of ice-cover?

Abstract: Diatom analyses in the water column, sediment traps, surficial sediments as well as in a short sediment core from Hagelseewli (2339 m asl, Swiss Alps) give information about the present-day seasonal cycle of diatom blooms, taphonomic processes in the lake basin and the lake's history. Analyses of surficial sediments show that water depth and thus light and nutrient availability is the most important factor influencing the production and distribution of diatom assemblages in Hagelseewli, and that periphytic diatom valves deposited in the deeper part of the basin originate from the shallow, littoral parts and are transported to the central part by processes such as lateral currents or sediment focussing. The lake is characterised by a very short period (2-3 months) of open water. Water-column and sediment-trap data revealed that planktonic diatoms bloom during and after the ice break-up, whereas mainly periphytic Fragilaria species entered the traps during the ice-covered period. These results suggest that plankton development is strongly inhibited by the ice-cover, with longer periods of ice-cover favouring Fragilaria species in Hagelseewli. The diatom analysis of a short sediment core that includes the last five centuries revealed several changes in the proportion of planktonic diatoms to Fragilaria species. The colder phases of the Little Ice-Age correspond to phases of lower concentration of planktonic diatoms. The highest, statistically significant amount of variance in the downcore diatom data is explained by winter precipitation, which directly influences the length of the ice-cover but inversely influences the light regime.

Sediment mineralization, nutrient fluxes, denitrification and dissimilatory nitrate reduction to ammonium in an estuarine fjord with sea cage trout farms

Abstract: Sediment oxygen consumption, nutrient fluxes across the sediment-water interface, denitrification, and dissimilatory nitrate reduction to ammonium (DNRA) were measured in sediment below the cages of a rainbow trout farm in the estuary of Horsens Fjord, Denmark, and compared to that in the sediment at reference stations elsewhere in the fjord Denitrification and DNRA were measured by the isotope pairing technique and a 15N technique, respectively The measurements are the first published data of in sjtu activities in sediment below marine fish farms Oxygen consumption by the sediment and effluxes of NH,' and pod3were markedly stimulated by organic matter loading from the overlying fish cages Denitrification of nitrate from the water column (D,) was only of significance during autumn and winter when runoff from land resulted in high water colun~n itrate concentrations In contrast, coupled nitrification-denitrification (D,) occurred in the sediment throughout the summer, with the rates in the sediment below the fish cages surprisingly being of the same magnitude as those at the reference stations Competition for inorganic nitrogen from benthic microalgae probably inhibited the activity of both nitrifiers and denitrifiers a t the reference stations, while a mat of Beggiatoa spp on the sediment surface below the fish cages in late summer indicated the presence of sulfide in the uppermost layers of the sediment, which almost ceased D, While DNRA was absent in sediment unaffected by the trout farms, it was of quantitative importance in the reduced sediment below the fish cages, DNRA activity being up to 7-fold greater than denitrification activity The ecological consequence of this shift in the relative importance of the processes in response to organic matter loading was a reduced nitrogen removal by denitrification and an increased efflux of NH,' to the water column, resulting in stimulation of pelagic primary production and increased nitrogen retention by the ecosystem The excess nitrogen input to the fjord from the trout farms corresponded to approximately 12 % of the total nitrogen load from land during the summer months Only 01 % of this additional nitrogen input was removed by denitrification in the underlying sediments during the fish production season In general, shallow estuarine ecosystems hosting fish farms may therefore face increasing eutrophication problems due to the direct supply of nutrients in the fish food and to shifts in the dominant microbial processes in the sediment below the fish cages

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

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

Iron and Manganese in the Ross Sea, Antarctica: Seasonal Iron Limitation in Antarctic Shelf Waters

Abstract: Dissolved iron and manganese and total dissolvable iron were measured in water column samples collected from the polynya region of the southern Ross Sea during cruises in November–December 1994 (spring 1994) and December 1995 to January 1996 (summer 1995). Iron and manganese addition bottle incubation experiments were also performed during these cruises in order to assess the nutritional sufficiency of ambient iron and manganese concentrations for growth of the phytoplankton community. Generally high dissolved iron concentrations (>0.5 nM) and relatively complex iron and manganese vertical profiles were obtained during the spring 1994 cruise, compared with the summer 1995 data. Dissolved iron concentrations in the upper water column averaged 1.0 nM during spring 1994 and 0.23 nM in summer 1995, excluding two stations where concentrations exceeding 1 nM are attributed to inputs from melting sea ice. The observed differences in the distribution of iron and manganese between spring 1994 and summer 1995 are attributed to seasonal decreases in the up welling of bottom waters and melting of sea ice, which supply these metals into the upper water column, combined with the cumulative removal of iron and manganese from the water column throughout the spring and summer, due to biological uptake, vertical export and scavenging by suspended and sinking particles. Results of the metal addition bottle incubation experiments indicate that ambient dissolved iron concentrations are adequate for phytoplankton growth requirements during the spring and early summer, when algal production is highest and Phaeocystis antarctica dominates the algal community, whereas low dissolved iron concentrations limit algal community growth later in the summer, except in the stratified, iron-enriched waters near melting sea ice, where diatoms are able to bloom. Our observations and the inferred seasonal distributions of P. antarctica and diatoms in these waters suggest that iron availability and vertical mixing (i.e., irradiance) exert the primary controls on phytoplankton growth and community structure in the southern Ross Sea during the spring and summer.

Incorporation and preservation of Mg in Globigerinoides sacculifer: implications for reconstructing the temperature and 18O/16O of seawater

Abstract: Using bathymetric transects of surface sediments underlying similar sea surface temperatures but exposed to increasing dissolution, we examined the processes which affect the relationship between foraminiferal Mg/Ca and δ18O. We found that Globigerinoides saccculifer calcifies over a relatively large range of water depth and that this is apparent in their Mg content. On the seafloor, foraminiferal Mg/Ca is substantially altered by dissolution with the degree of alteration increasing with water depth. Selective dissolution of the chamber calcite, formed in surface waters, shifts the shell's bulk Mg/Ca and δ18O toward the chemistries of the secondary crust acquired in colder thermocline waters. The magnitude of this shift depends on both the range of temperatures over which the shell calcified and the degree to which it is subsequently dissolved. In spite of this shift the initial relationship between Mg/Ca and δ18O, determined by their temperature dependence, is maintained. We conclude that paired measurements of δ18O and Mg/Ca can be used for reconstructing δ18Owater, though care must be taken to determine where in the water column the reconstruction applies.

The record of nitrate utilization and productivity limitation provided by δ15N values in lake organic matter—A study of sediment trap and core sediments from Baldeggersee, Switzerland

Abstract: The response of nitrogen isotopic composition of organic matter to changes in nitrate utilization, N-limitation and lake productivity was studied in Baldeggersee, a small eutrophic lake in central Switzerland. Nitrogen isotope ratios were measured on organic matter accumulation in sediment traps, collected daily from March 1995 through October 1996, and on individually sampled light and dark annual laminae from a 108-yr sediment core sequence (1885‐1993). Nitrogen accumulation in the sediment traps averaged 0.04 g N m 22 d 21 . d 15 N values of the sediment trap material increased from 11‰ to 13‰ (atmospheric N 2) as primary productivity decreased surface water [NO3 2 ] from 1.4 to 0.7 mg N L 21 during the seasonal stratified periods. Very small amounts of isotopically enriched organic matter (d 15 N ; 15‐20‰) of heterotrophic and/or detrital origin accumulated in the winter months. Nitrogen accumulation in the core sediments average 4 g N m 22 per year. d 15 N values of the sediment core material increased up-core (from d 15 N ; 6‰ to d 15 N ; 11‰); several abrupt positive isotope shifts ( .2‰) occur in the upper part of the record. Comparison between sediment d 15 N values and surface water [NO3 2 ] measurements for the period of 1976‐1993 reveals that the abrupt positive d 15 N shifts occurred in years where unusually large phytoplankton blooms depleted surface waters nitrate to concentrations of ,0.7 mg N L 21 .A 3‰ negative d 15 N shift, observed between 1973‐1975 at the time of maximum anoxic conditions and meromixis, can be attributed to phytoplankton incorporation of ammonia, which was present in concentrations of up to 0.7 mg L 21 in the epilimnion. Preservation of isotopic shifts in the sediment core which are clearly related to water column processes, and the similarity of core top d 15

The biology of soft shores and estuaries

Abstract: 1: Introduction: organisms, sediments, and water movements 2: The world of particles: a variety of habitats 3: The coarse extreme: life on sandy beaches 4: A fine option: life on mudflats and in seagrass beds 5: Salt marshes and mangrove swamps 6: Life at the bottom: sublittoral sediments and community structure 7: Estuarine habitats and coastal lagoons 8: The estuarine benthos and its distribution 9: Life in the estuarine water column 10: Estuarine ecosystems 11: Uses and abuses: human impact and counter-measures.

Role of bottom sediments in the secondary pollution of aquatic environments by heavy metal compounds

Abstract: The results of long-term investigations into the concentrations of some heavy metals (Fe, Mn, Cu, Zn, Pb, Cr, and Cd) in the bottom sediments of the Dnieper reservoirs and the Dnieper–Bug estuary are considered. Maximum quantities of the metals studied are characteristic of southern water bodies located within industrial zones (the Zaporozh’e and Kakhovka reservoirs as well as the Dnieper–Bug estuary). The highest concentrations of the metals studied occurred in the clay silts (Fe, 11 600–32 400; Mn, 1504–3450; Cu, 38.9–85.5; Zn, 89.8–186.5; Cr, 48.6–193.0; and Cd, 1.9–4.4 mg kg-1 dry weight). Accumulation of heavy metals in the bottom sediments is an important factor in the self-purification of aquatic environments. However, this process is reversible and therefore provides a constant threat of secondary water pollution. Secondary water pollution is observed in summer and autumn when water consumption increases. The concentrations of heavy metals increase by a factor of 1.5–3 after the drawdown of the water level. The main reason for the rise in the concentrations of metals is exchange between the bottom sediments and the water column. The rate of heavy metal migration is connected with the forms of occurrence in solid substrates and pore solutions in the bottom sediments, as well as with physico-chemical conditions arising at the sediment/water boundary. Therefore, our investigations concentrated on the study of the fractional distribution of heavy metals among solid substrates and their forms of occurrence in interstitial solutions. This distribution depends, most of all, on the chemical properties of metals as well as the chemical and mineralogical composition of the sediments and the chemical properties of pore solutions. Most of the supply of Mn, Zn, Fe, and Cd is associated with oxides and hydroxides of iron and manganese (Mn, 74–93%; Zn, 43–70%; Fe, 27–59%; and Cd, 28–41%). Most copper and chromium is bound to organic matter and to scarcely soluble minerals. In the interstitial solutions studied, metals (except manganese) are found mainly as complex compounds with dissolved organic matter of a different molecular weight. Nevertheless, the fraction of complexes with a relatively low molecular weight (500–5000 Da) prevailed (40–70%). Dissolved manganese in the pore solutions consists chiefly of free (hydrated) ions Mn2+ (80–95%). The results obtained were used for a comparative evaluation of heavy metal mobility and the exchange ability of their associated compounds in the bottom sediment–water system.

Succession in marine benthic habitats and fauna in response to oxygen deficiency: analysed by sediment profile-imaging and by grab samples

Abstract: This study shows, for the first time, in situ images of changes in marine benthic habitats in response to oxygen deficiency. Tight coupling is demonstrated between gradual degradation of benthic habitat and faunal behaviour, species richness, abundance and biomass. The critical oxygen level that forced changes in the benthic faunal successional stages was ∼10% that of air saturation (∼0.7 ml 02 l -1 ). Before this critical saturation level was reached, tube-building polychaetes their tubes extended higher into the water column, the width of the sub-oxic sediment layer decreased, and vertical animal burrows (formerly oxidised and brown in colour) became sulphidic and black. Over a 10 mo hypoxic period (June 1997 to April 1998) in the Gullmarsfjord (Swedish west coast), benthic community successional stages declined from equilibrium to virtually azoic conditions. As normoxic conditions returned, pioneering stages gradually recolonised the area.

Phytoplankton taxonomic variability in nutrient utilization and primary production in the Ross Sea

Abstract: Patterns of nutrient utilization and primary productivity (PP) in late austral spring and early summer in the southwestern Ross Sea were characterized with respect to phytoplankton taxonomic composition, polynya dynamics, and upper ocean hydrography during the 1996–1997 oceanographic program Research on Ocean-Atmosphere Variability and Ecosystem Response in the Ross Sea. Phytoplankton biomass in the upper 150 m of the water column ranged from 40 to 540 mg chlorophyll a (Chl a) m−2, exceeding 200 mg Chl a m−2 everywhere except the extreme northern and eastern boundaries of the Ross Sea polynya. Diatom biomass was greatest in the shallow mixed layers of Terra Nova Bay, while the more deeply mixed waters of the Ross Sea polynya were dominated by Phaeocystis antarctica. Daily production computed from the disappearance of NO3 (1.14 g C m−2 d−1) and total dissolved inorganic carbon (TDIC, 1.29 g C m−2 d−1) is consistent with estimates made from an algorithm forced with satellite measurements of Chl a (1.25 g C m−2 d−1) and from measurements of 14C uptake (1.33 g C m−2 d−1). Phytoplankton PP in the Ross Sea averaged 100 g C m−2 yr−1 during 1996–1997. Despite the early formation of the Terra Nova Bay polynya the diatom bloom there did not reach its peak PP until middle to late January 1997 (most likely because of more intense wind mixing in November), ∼6 weeks after the P. antarctica bloom in the Ross Sea polynya had reached the same stage of development. From 70 to 100% of the C and N deficits in the upper 150 m could be accounted for by particulate organic matter, indicating that there had been little dissolved organic matter production or export of particulate material prior to our cruise. This suggests that early in the season, PP and zooplankton grazing are decoupled in the southwestern Ross Sea. The NO3∶PO4 disappearance ratio in waters dominated by P. antarctica (19.0±0.61) was significantly greater than in waters where diatoms were most common (9.52±0.33), and both were significantly different from the Redfield N∶P ratio of 16. Vertical profiles of TDIC suggest that P. antarctica took up 110% more CO2 per mole of PO4 removed than did diatoms, an important consideration for climate models that estimate C uptake from the removal of PO4.

Benthic flux of biogenic elements on the Southeastern US continental shelf: influence of pore water advective transport and benthic microalgae

Abstract: In situ, paired light and dark benthic flux chamber incubations were used to estimate the exchange of nutrients, oxygen and inorganic carbon across the sediment – water interface of the South Atlantic Bight (SAB) continental shelf. The results indicate that physically forced non-diffusive pore water transport and benthic primary production (BPP) by sea floor microalgae exert a major influence on benthic exchange rates on the mid- and outer-continental shelf (depths of 14–40 m). Light fluxes to the sea floor and sediment photosynthetic pigment distributions determined on two, widely spaced cross-shelf transects suggest that BPP may occur over 84% of the SAB continental shelf area. Microalgal gross BPP rates at all study sites averaged 400±260 mg C m−2 d−1 between May and September 1996 while water column primary productivity averaged 682±176 mg C m−2 d−1, implying a total primary productivity for this region of approximately 1100 mg C m−2 d−1 (1.6 times the water column productivity alone). The results are also consistent with the advective transport of pore waters. Benthic flux chambers appear to retard this exchange, affecting the accuracy of derived net fluxes. Given our inability to relate pore water gradients to fluxes in non-diffusive regimes and to mimic natural advective transport in intact core incubations, traditional techniques such as pore water gradient diffusion calculations or shipboard core incubations also may not provide accurate flux estimates. Because of these limitations, fundamental questions remain concerning the processes that control nutrient inventories in pore waters and the magnitude of the net benthic flux of nutrients on the sandy SAB shelf.

Dynamics of nutrients and phytoplankton biomass in the Pearl River estuary and adjacent waters of Hong Kong during summer : preliminary evidence for phosphorus and silicon limitation

Abstract: The Pearl River in the south of China is the second largest river in China in terms of discharge volume. Two cruises were made to investigate the dynamics of nutrients and phytoplankton biomass in June and July 1998, across the Pearl River estuary to the adjacent territorial waters of Hong Kong. On-deck incubation experiments of 5 mixtures of freshwater from the surface with seawater from below the halocline were conducted to simulate time scales of phytoplankton blooms for each freshwater/seawater mixture and to examine uptake of nutrients. In July, phytoplankton growth rates increased with salinity of the mixtures, with the lowest growth rate (0.81 d -1 ) in freshwater and the highest (2.41 d -1 ) in 100% seawater (salinity = 29). PO 4 was lower in freshwater (0.3 μM) than in seawater (1.2 μM), whereas concentrations of NO 3 +NH 4 +urea (-80 pM) and SiO 4 (150 pM) were higher in freshwater than those in seawater (25 pM for nitrogen and 26 pM for SiO 4 ). During the incubation PO 4 disappeared first, indicating that P was limiting the phytoplankton biomass. All mixtures reached the maximum in phytoplankton biomass in 3 to 4 d. There was a regional maximum of phytoplankton biomass that occurred at the seaward edge of the estuarine plume during June. The region moved eastwards (away from the estuary) to the southern waters between Lantau Island and Hong Kong Island during July. Mixing diagrams of NO 3 and SiO 4 showed conservative behaviour with salinity in the estuary. These observations suggest that dilution by freshwater outflow was a controlling factor in determining the distribution of nutrients and phytoplankton biomass in the estuary due to high flow during June and July. The regional maximum of phytoplankton biomass was comparable to that resulting from the incubation and coincided with the exhaustion of PO 4 during July. On the estuarine (west) side of the regional maximum, chl a fluorescence increased during 24 h incubations, but decreased at the station with the maximum and on the east side, suggesting the possible limitation of nutrients to the phytoplankton community. In the same eastern waters, both PO 4 and SiO 4 were very low. However, NO 3 and NH 4 remained abundant, suggesting possible co-limitation by phosphorus and silicon. We hypothesize that considerable seasonal rainfall in June and early July might have contributed an additional source of nitrogen to the water column, which resulted in the exhaustion of PO 4 and SiO 4 before nitrogen.

Bacterioplankton distribution and production in deep Pacific waters: Large-scale geographic variations and possible coupling with sinking particle fluxes

Abstract: Bacterial abundance and leucine incorporation rate were measured throughout the water column (depth, 4,000– 6,000 m) at stations occupied in the equatorial, subtropical, and subarctic Pacific as well as in the Bering Sea during three cruises conducted between 1993 and 1997. In general, depth-dependent decreases of bacterial abundance and leucine incorporation in the bathypelagic layer (depth, >1,000 m) were well described by a power function with remarkably uniform exponents among distant locations: average exponents were –0.900 and –1.33 for abundance and leucine incorporation, respectively. Depth profiles of bacterial properties were complex at some subarctic stations, suggesting lateral transport of organic carbon by local eddies. Organic carbon fluxes from abyssal sediment to overlying water would explain increases in bacterial abundance and leucine incorporation in near-bottom layers. Biomass was twofold to fourfold and the production was threefold to sevenfold greater in subarctic than in subtropical regions. This latitudinal pattern was consistent with the basin-scale distribution of sinking fluxes of particulate organic carbon (POC) reported in the literature. Rates of bacterial carbon uptake accounted for 51% (range, 31–153) and 23% (14–58) of deep sinking POC fluxes in subarctic and subtropical regions, respectively. Average turnover time of deep bacterial assemblages was estimated to be 1–30 yr. These results suggest that deep bacterial biomass and production are generally coupled with sinking POC fluxes and that organic carbon is substantially transformed within bathypelagic environments via a sinking POC → dissolved organic carbon → bacteria pathway, as previously suggested in the mesopelagic zone.

In situ nutrient enrichment: Methods for marine benthic ecology.

Abstract: Nutrient enrichment of marine sediments or the water column has been used to study plant nutrient limitation and its cascading effects on community structure, Here we develop methodological recommendations for in situ enrichment. We review 18 published enrichment methods. Nutrient concentrations varied through time and among sites, with sediment depth, distance from the source, fertilizer type and load. Combining available data, we could predict an increase in sediment porewater phosphate (r 2 = 0.48) but not ammonium (r 2 = 0.07) concentrations in a multiple regression model. In three comparative field experiments we applied a coated slow-release fertilizer in the sediment and the water column and followed nutrient concentrations over time. We recommend coated fertilizer pellets, because they provide gradual nutrient release, allow for realistic nutrient gradients, and even application but we emphasize that nutrient concentrations need to be monitored through time.

Long-term phytoplankton-nutrient interactions in a shallow coastal sea: Algal community structure, nutrient budgets, and denitrification potential

Abstract: In the eutrophic Marsdiep, the westernmost tidal inlet of the Wadden Sea, phytoplankton biomass, and production almost doubled at the end of the 1970s and remained high ever since. Principal component analysis of 21-yr (1974‐ 1994) high-resolution time series of the 32 most numerous marine algal species revealed that the phytoplankton community changed drastically both between 1976 and 1978 and again between 1987 and 1988, and that it was relatively stable in-between (1974‐1976, 1978‐1987) and thereafter (1988‐1994). These major changes in phytoplankton biomass and species composition coincided with changes in absolute and relative (TN : TP) nutrient concentrations. During the summer of 1977, the Marsdiep shifted from a rich, but phosphorus-controlled system to an even more eutrophic but nitrogen-controlled environment. The system reshifted towards P-control between 1987 and 1988. The coincidence of the shifts in relative nutrient concentrations and phytoplankton species composition implies a strong causal relationship between TN : TP ratios and phytoplankton community structure. Among diatoms, the observed increase in phytoplankton biomass under eutrophic N-controlled conditions was particularly due to an increase of the abundance of larger algae. Our results indicate that the N budget of the area is correlated with the community structure, suggesting enhanced loss of nitrogen to the sediment through increased deposition of larger algal cells. In shallow coastal marine waters, the total primary production and biomass of phytoplankton is generally assumed to be hyperbolically related to nutrient loadings from land and subsequent availability of these nutrients in the water column (e.g., Borum and Sand-Jensen 1996). However, understanding the effect of nutrient enrichment on living resources requires detailed knowledge of how nutrients enter and leave these waters. Dissolved and particulate materials as well as living organisms are exchanged between the coastal waters and the open sea, and net material fluxes appear to depend on physical and biological responses within these systems to changes in nutrient loadings from land (e.g., Dame and Allen 1996).

Effects of bottom-layer hypoxia on abundances and depth distributions of organisms in Patuxent River, Chesapeake Bay

Abstract: Oxygen depletion, seasonally common in bottom waters of many stratified aquatic systems, may have strong effects on abundances, distributions, and interactions among organisms, and therefore community dynamics. To examine effects of bottom-layer hypoxia on densities and vertical distributions in a stratified subestuary, fish larvae, their gelatinous predators, and copepod prey were surveyed near-surface, within the pycnocline, and near-bottom in the Patuxent River (Chesapeake Bay) under a range of near-bottom dissolved oxygen (DO) conditions. Overall abundances of fish larvae and copepod nauplii were lower throughout the water column when bottom-layer DO was low (≤2 mg DO 1 -1 ). When bottom-layer DO was low ( 2 mg 1 -1 ) DO conditions, and overall density of copepod nauplii declined by >50%. Depth-distributions of several organisms also were affected by bottom-oxygen depletion: fish larvae, scyphomedusae, and copepods were much less common near the bottom when bottom-layer DO was low than when it was >2 mg 1 -1 . The ctenophore Mnemiopsis leidyi occurred in high densities at DO as low as 1.3 mg 1 -1 , but was nearly absent at <1 mg 1 -1 . The results indicate the potential for substantial differences in organism interactions, especially predator-prey relationships, between times of hiqh and low bottom-layer DO.

Pelagic C:N:P Stoichiometry in a Eutrophied Lake: Responses to a Whole-Lake Food-Web Manipulation

Abstract: Changes in the ecological stoichiometry of C, N, and P in the pelagic zone are reported from a whole-lake manipulation of the food web of Lake 227, an experimentally eutrophied lake at the Experimental Lakes Area, Canada. Addition of northern pike eliminated populations of planktivorous minnows by the third year (1995) after pike introduction, and in the fourth year after pike addition (1996), a massive increase in the abundance of the large-bodied cladoceran Daphnia pulicaria occurred. Accompanying this increase in Daphnia abundance, zooplankton community N:P declined, seston concentration and C:P ratio decreased, and dissolved N and P pools increased. During peak abundance, zooplankton biomass comprised a significant proportion of total epilimnetic phosphorus (greater than 30%). During the period of increased Daphnia abundance, concentrations of dissolved inorganic nitrogen (TIN) increased more strongly than dissolved phosphorus (TDP), and thus TIN:TDP ratios were elevated. Sedimentation data indicated that increased grazing led to greatly reduced residence times of C, N, and especially P in the water column during 1996. Finally, previously dominant N-fixing cyanobacteria were absent during 1996. Our results show that strong effects of food-web structure can occur in eutrophic lakes and that stoichiometric mechanisms play a potentially important role in generating these effects.

The vertical flux of biogenic and lithogenic material in the Ross Sea : moored sediment trap observations 1996-1998

Abstract: As part of the USJGOFS-AESOPS program, we deployed an array of moored sediment traps and determined the composition and vertical flux of particulate matter through the water column of the continental shelf environment of the Ross Sea. We deployed two moorings, with two traps each. One mooring was located in the southcentral Ross Sea along the line of AESOPS process studies; the other mooring was located in the northcentral Ross Sea. The flux in the southern Ross Sea was generally double that in the north, and there was a significant increase in flux into the deep traps indicating horizontal transport and possible focusing of particulate matter. The particles are primarily biogenic although lithogenic contributions can increase to over 30% of the flux in the near-bottom trap nearer to the ice shelf. The fluxes collected by the traps were generally low but within the range of previous observations. Several unique observations stand out from these data: (1) The export of organic matter to 200 m during the spring and summer was extremely low, consistent with observations that the ongoing phytoplankton bloom was being stored or recycled in the upper-water column; (2) A major pulse of biogenic silica export occurred in the fall, just as ice was reforming. The correlation of this event with lithogenous particles, excess Fe, and with the early breakout of katabatic winds suggests that this event might be related to Fe fertilization of diatom production and export; (3) The largest flux of organic matter during 1996–1997 was carried by the pteropod Limacina helicina and was observed at both moorings under the ice in the late fall, just after the diatom pulse. Although it is difficult to estimate the proportion of these organisms that were swimmers contaminating the flux, we believe that a significant fraction settled into the traps and may represent the die-out of this herbivore population after the suspended POM concentration had dropped to very low levels. Fluxes to the upper traps were much lower than estimates of export flux made by other methods during synoptic ship studies. These differences are discussed and the fluxes are compared with preliminary data from the ROAVERRS trap program, which was simultaneously deployed in the Ross Sea.