Showing papers on "Water column published in 2004"

The flux of iron from continental shelf sediments: A missing source for global budgets

Abstract: [1] The flux of dissolved iron from sediments to the water column was measured with flux chambers along the California coast over a five-year period. High fluxes were observed from sediments on the continental shelf. The measured fluxes were an average of 75 times larger than flux values derived from pore-water iron gradients. The iron flux was significantly correlated with the oxidation of organic matter, which allows an extrapolation to the global shelf. The input from shelf sediments is at least as significant as the global input of dissolved iron from aerosols, which has been presumed to be the dominant external iron source. Evidence of this input is seen 100's of kilometers offshore where it can enable the high productivity of broad coastal regions seen in satellite images.

Influence of bioturbation by three benthic infaunal species on microbial communities and biogeochemical processes in marine sediment

Abstract: Benthic invertebrates play a key role in the physical, chemical, and biological properties of the marine water-sediment interface. The influences of invertebrates on biogeochemical processes have mainly been attributed to their sediment reworking and bioirrigation activities. The aim of this study was to compare the influences of bioturbation activities by 3 dominant species of shallow water habitats (Cerastoderma edule, Corophium volutator, and Nereis diversicolor) on microbial communi- ties and biogeochemical processes in sediment cores. C. edule acted as a biodiffuser, mixing surface particles in the top 2 cm of the sediment. Despite this mixing activity, this species had little effect on O2 consumption, water exchange between the water column and the sediment, microbial character- istics, and release of nutrients from the sediment. In contrast, C. volutator and N. diversicolor produced burrows in the sediment that allowed transport of surface particles into biogenic structures. These 2 species doubled the solute exchange between the water column and the sediment. Such modifications of sediment structure and solute transport increased the O2 consumption and the release of nutrients from the sediment. Both C. volutator and N. diversicolor stimulated the microbial communities as indicated by higher percentages of active bacteria. Reduction of the numbers of sulphate reducing bacteria was observed when the 3 invertebrates were present and could be attrib- uted to the penetration of O2 due to animal activities. N. diversicolor had a greater influence than C. volutator on pore water chemistry, ammonium release, and active bacteria. As N. diversicolor burrowed deeper in the sediment than C. volutator, it irrigated a greater volume of sediment. The modes of sediment reworking and structure building, irrigation behaviour, and burrowing depths were factors sufficient to assign the 3 species into different functional groups.

Expansive covers of turf-forming algae on human-dominated coast: the relative effects of increasing nutrient and sediment loads

Abstract: Turf-forming algae form more extensive habitat on subtidal rock adjacent to urban than non-urban coast of South Australia. This pattern is frequently observed on the world’s temperate coasts and is variously considered to be a result of enhanced concentration of nutrients or rates of sediment deposition on urban coasts. We experimentally tested which of three components of environmental change (increased nutrients in water, increased nutrients in sediments and increased sediment deposition) best explain the expansive covers of turf-forming algae on urban coasts. All three treatments had independent and positive effects on the percentage cover of turf-forming algae. The addition of nutrients from the water column had the largest influence (ω 2=0.55), which was more than six times greater than the effect of nutrients added to sediments (ω 2=0.08). An increase in rate of deposition of sediments had substantial effects (ω 2=0.35), which were about one third less than those of water-borne nutrients. Importantly, the combined effect of all three treatments caused a 77% increase in percentage cover of turf-forming algae, which is comparable to the observed difference in covers between urban and non-urban coast in South Australia (93%). These results suggest that human activities that reduce water quality in both nutrient and sediment loads account for major change observed on human-dominated coasts. Despite this knowledge, we still lack complete information on the mechanisms that switch the primary subtidal habitat from canopy-forming algae to turf-forming algae on human-dominated coasts.

Stability of North Atlantic water masses in face of pronounced climate variability during the Pleistocene

Abstract: [1] Geochemical profiles from the North Atlantic Ocean suggest that the vertical δ13C structure of the water column at intermediate depths did not change significantly between glacial and interglacial time over much of the Pleistocene, despite large changes in ice volume and iceberg delivery from nearby landmasses. The most anomalous δ13C profiles are from the extreme interglaciations of the late Pleistocene. This compilation of data suggests that, unlike today (an extreme interglaciation), the two primary sources of northern deep water, Norwegian-Greenland Sea and Labrador Sea/subpolar North Atlantic, had different characteristic δ13C values over most of the Pleistocene. We speculate that the current open sea ice conditions in the Norwegian-Greenland Sea are a relatively rare occurrence and that the high-δ13C deep water that forms in this region today is geologically unusual. If northern source deep waters can have highly variable δ13C, then this likelihood must be considered when inferring past circulation changes from benthic δ13C records.

Habitat requirements for submerged aquatic vegetation in Chesapeake Bay: water quality, light regime, and physical-chemical factors

Abstract: We developed an algorithm for calculating habitat suitability for seagrasses and related submerged aquatic vegetation (SAV) at coastal sites where monitoring data are available for five water quality variables that govern light availability at the leaf surface. We developed independent estimates of the minimum light required for SAV survival both as a percentage of surface light passing though the water column to the depth of SAV growth (PLW min) and as a percentage of light reaching reaching leaves through the epiphyte layer (PLL min). Value were computed by applying, as inputs to this algorithm, statistically dervived values for water quality variables that correspond to thresholds for SAV presence in Chesapeake Bay. These estimates ofPLW min andPLL min compared well with the values established from a literature review. Calcultations account for tidal range, and total light attenuation is partitioned into water column and epiphyte contributions. Water column attenuation is further partitioned into effects of chlorophylla (chla), total suspended solids (TSS) and other substances. We used this algorithm to predict potential SAV presence throughout the Bay where calculated light available at plant leaves exceededPLL min. Predictions closely matched results of aerial photographic monitoring surveys of SAV distribution. Correspondence between predictions and observations was particularly strong in the mesohaline and polythaline regions, which contain 75–80% of all potential SAV sites in this estuary. The method also allows for independent assessment of effects of physical and chemical factors other than light in limiting SAV growth and survival. Although this algorithm was developed with data from Chesapeake Bay, its general structure allows it to be calibrated and used as a quantitative tool for applying water quality data to define suitability of specific sites as habitats for SAV survival in diverse coastal environments worldwide.

Variable ageing and storage of dissolved organic components in the open ocean

Abstract: Seawater dissolved organic matter (DOM) is the largest reservoir of exchangeable organic carbon in the ocean, comparable in quantity to atmospheric carbon dioxide. The composition, turnover times and fate of all but a few planktonic constituents of this material are, however, largely unknown. Models of ocean carbon cycling are thus limited by the need for information on temporal scales of carbon storage in DOM subcomponents, produced via the 'biological pump', relative to their recycling by bacteria. Here we show that carbohydrate- and protein-like substances in the open Atlantic and Pacific oceans, though often significantly aged, comprise younger fractions of the DOM, whereas dissolved lipophilic material exhibits up to approximately 90 per cent fossil character. In contrast to the millennial mean ages of DOM observed throughout the water column, weighted mean turnover times of DOM in the surface ocean are only decadal in magnitude. An observed size-age continuum further demonstrates that small dissolved molecules are the most highly aged forms of organic matter, cycling much more slowly than larger, younger dissolved and particulate precursors, and directly links oceanic organic matter age and size with reactivity.

Isotopic constraints on glacial/interglacial changes in the oceanic nitrogen budget

Abstract: early deglacial maximum in nitrate 15 N/ 14 N in suboxic zones and no significant glacialto-late Holocene change in global ocean nitrate 15 N/ 14 N. Consistent with the work of Brandes and Devol [2002], we find that the steady state 15 N/ 14 N of oceanic nitrate is controlled primarily by the fraction of total denitrification that occurs in the water column. Therefore a deglacial peak in the ratio of water column-to-sediment denitrification, caused by either a strong feedback between water column denitrification and the N reservoir or by an increase in sediment denitrification due to sea level rise, can explain the observed deglacial 15 N/ 14 N maximum in sediments underlying water column denitrification zones. The total denitrification rate and the mean ocean nitrate concentration are also important determinants of steady state nitrate 15 N/ 14 N. For this reason, modeling a realistic deglacial 15 N/ 14 N maximum further requires that the combined negative feedbacks from N2 fixation and denitrification are relatively strong, and N losses are relatively small. Our results suggest that the glacial oceanic N inventory was at most 30% greater than today’s and probably less than 10% greater. INDEX TERMS: 4267 Oceanography: General: Paleoceanography; 4805 Oceanography: Biological and Chemical: Biogeochemical cycles (1615); 4845 Oceanography: Biological and Chemical: Nutrients and nutrient cycling; 4870 Oceanography: Biological and Chemical: Stable isotopes; KEYWORDS: feedback, nitrogen isotopes, paleoceanography

Depth coenoclines and environmental considerations of western pacific larger foraminifera

Abstract: Symbiont-bearing benthic foraminifera are restricted to the euphotic zone of tropical and warm-temperate seas Species distribution is correlated with depth, and the continuous alteration of community structures represents a coenocline Since depth is a composite environmental gradient, the coenocline of larger foraminifera is not stable but alters with changes in primary limiting factors: temperature, light, water movement, substrate, and nutrients Temperature determines geographic distribution and affects the depth distribution of larger foraminifera by the development of a shallow thermocline that truncates the distribution of shallower species and excludes species adapted to the deepest euphotic zone Within these constraints, light is the most important primary factor because larger foraminifera are at least partly dependent upon photosynthesis by their algal endosymbionts for growth and calcification The microalgae show distinct intervals along the light gradient and the foraminiferal host develops various strategies for regulating light intensity First, well-structured environments in shallow waters allow shelter against irradiation by protecting in shadow areas Second, wall and test structures enable regulation of light penetration A range of mechanisms allows species to resist the highest energies in the breaker zone of the reef edge and crest, where foraminifera attach to inorganic or organic hard substrates Concentrations of dissolved and particulate organic matter in the water column, as well as sediments or other inorganic particles, influence depth distributions by changing water transparency and, therefore, photosynthesis Permanent or episodic elevations of concentrations therefore compress the coenocline upward Species adapted to hard substrates must compete for the reduced space, while species living in the deepest euphotic zone are at a disadvantage because they are insufficiently motile to surmount large depth differences Changing light transparencies due to nutrient input and different hydrodynamic conditions alter relations between the light coenocline and water depth Thus, paleodepth interpretations based on larger foraminiferal assemblages should be based not only on foraminiferal taxa and ecology, but also on environmental evidence for climate, terrigeneous influence, water transparency, and hydrodynamic conditions based on sedimentology, geochemistry, and associated fossil biota

Effects of sediment resuspension on phytoplankton production: teasing apart the influences of light, nutrients and algal entrainment

Abstract: Summary 1. Wind-induced sediment resuspension can affect planktonic primary productivity by influencing light penetration and nutrient availability, and by contributing meroplankton (algae resuspended from the lake bed) to the water column. We established relationships between sediment resuspension, light and nutrient availability to phytoplankton in a shallow lake on four occasions. 2. The effects of additions of surficial sediments and nutrients on the productivity of phytoplankton communities were measured in 300 mL gas-tight bottles attached to rotating plankton wheels and exposed to a light gradient, in 24 h incubations at in situ temperatures. 3. While sediment resuspension always increased primary productivity, resuspension released phytoplankton from nutrient limitation in only two of the four experiments because the amount of available nitrogen and phosphorus entrained from the sediments was small compared with typical baseline levels in the water column. In contrast, chlorophyll a entrainment was substantial compared with baseline water column concentrations and the contribution of meroplankton to primary production was important at times, especially when seasonal irradiance in the lake was high. 4. Comparison of the in situ light climate with the threshold of light-limitation of the phytoplankton indicated that phytoplankton in the lake were only likely to be light-limited at times of extreme turbidity (e.g. >200 nephelometric turbidity units), particularly when these occur in winter. Therefore, resuspension influenced phytoplankton production mainly via effects on available nutrients and by entraining algae. The importance of each of these varied in time. 5. The partitioning of primary productivity between the water column and sediments in shallow lakes greatly influences the outcome of resuspension events for water column primary productivity.

Degradation and mineralization of coral mucus in reef environments

Abstract: With in situ and laboratory chamber incubations we demonstrate that coral mucus, an important component of particulate organic matter in reef ecosystems, is a valuable substrate for microbial communities in the water column and sandy sediments of coral reefs. The addition of coral mucus to the water of benthic chambers placed on lagoon sands in the coral cay Heron Island, Aus- tralia, resulted in a rapid and significant increase in both O2 consumption and DIC production in the chambers. The permeable coral sands permitted the transport of mucus into the sediment with inter- facial water flows, resulting in the mucus being mainly (> 90%) degraded in the sediment and not in the water column of the chambers. A low ratio of 0.48 (in situ) to 0.64 (laboratory) for O2 consump- tion/DIC production after the addition of coral mucus, and high sulfate reduction rates (SRR) in natural sediments which were exposed to coral mucus, suggest a large contribution of anaerobic processes to the degradation of coral mucus. Oxygen penetrated less than 5 mm deep into these sediments. The microbial reaction to mucus addition was rapid, with a calculated in situ C turnover rate ranging from 7 to 18% h -1 . The degradation of coral mucus showed a dependency on the permeability of the carbonate sediments, with faster degradation and remineralization in coarse sands. This indicates the importance of permeable reef sediments for the trapping and degradation of organic matter. We suggest that coral mucus may have a function as a carrier of energy to the benthic microbial consumers.

Evaluation of salmon farming effects on marine systems in the inner seas of southern Chile: a large-scale mensurative experiment

Abstract: Summary The impact of salmon farming on the environment has been widely studied; there is, however, no consensus on the magnitude and quality of these effects and little information on their extent over large salmon farming areas. Forty-three salmon farm sites of which 29 are in full operation and grouped in nine locations in southern Chile were evaluated. Using statistical methods (two-way ANOVA), no effects were found on water column variables such as nitrate, ammonia, orthophosphate and chlorophyll, whereas they were significant on sediment variables such as nitrogen (N), phosphorus (P) and organic carbon (OC), among others. Natural variability evaluation of some parameters revealed that P in sediments had the lowest coefficient of variation (23.2%) when compared with N (65.2%), particulate organic matter (POM) (139%) and OC (39%), and showed more marked salmon farming effects with no locality effects. Four of the nine locations showed stronger effects on sediments, with average P values ranging from 150 to 230 mmol that were six to nine times higher than in control sites (25 mmol). Nitrogen and carbon in sediments, while showing significant effects on salmon farming, also showed locality effects that revealed other biogenic processes influencing sediment composition. Thus, P in sediments was proposed as a promising indicator of impact on salmon farming, although the relationship with fauna in sediments was not linear and somewhat variable; more research is therefore needed to understand such connections. Considering entire geographical locations, no relationship was found between sediment conditions under salmon cages and the condition of the water column at a farm. This may indicate the possibility of high dilution rates and recycling processes, which so far preclude the detection of more global impacts beyond the cages shadow.

The European coastal zone: characterization and first assessment of ecosystem metabolism

Abstract: The geomorphic, oceanographic, terrestrial and anthropogenic attributes of the European coastal zone are described and published data on ecosystem function (primary production and respiration) are reviewed. Four regions are considered: the Baltic Sea, Mediterranean Sea, Black Sea and the European Atlantic coast including the North Sea. The metabolic database (194 papers) suffers from a non-homogeneous geographical coverage with no usable data for the Black Sea which was therefore excluded from this part of our study. Pelagic gross primary production in European open shelves is, by far, the most documented parameter with an estimated mean of 41 mmol C m −2 d −1 , the lowest value is reported in the Mediterranean Sea (21 mmol C m −2 d −1 ) and the highest one in the Atlantic/North Sea area (51 mmol C m −2 d −1 ). Microphytobenthic primary production, mostly measured in shallow areas, is extrapolated to the entire 0–200 m depth range. Its contribution to total primary production is low in all regions (mean: 1.5 mmol C m −2 d −1 ). Although macrophyte beds are very productive, a regional production estimate is not provided in this study because their geographical distribution along the European coastline remains unknown. Measurements of pelagic community respiration are clearly too sparse, especially below the euphotic zone, to yield an accurate picture of the fate of organic matter produced in the water column. With a mean value of 17 mmol C m −2 d −1 , benthic community respiration consumes approximately 40% of the pelagic organic matter production. Estuaries generally exhibit high metabolic rates and a large range of variation in all parameters, except microphytobenthic primary production. Finally, the problem of eutrophication in Europe is discussed and the metabolic data obtained in the framework of the Land–Ocean Interactions in the Coastal Zone (LOICZ) project are compared with available direct measurements of net ecosystem production.

Temporal and Spatial Distribution of Dissolved Oxygen in the Pearl River Estuary and Adjacent Coastal Waters

Abstract: The Pearl River is one of the large rivers in the world and it discharges to the northern part of the South China Sea. There has been a concern about the deterioration of dissolved oxygen conditions in the Pearl River estuary and adjacent coastal waters. In this study, historical data on dissolved oxygen (DO) from 1980s, recent data from a summer cruise in 1999, and a 10-year time series in DO for 1990-2000 were used to examine spatial and temporal distribution of DO in the Pearl River estuary and adjacent coastal waters near Hong Kong. In the adjacent coastal waters, low oxygen waters 4 mg 1(-1) in the western part in August 1984. In July 1999, DO was 4 mg 1(-1). The 9-year time series showed that DO decreased periodically in summer, but rarely dropped to < 3 mg 1-1. There was no apparent trend of decreasing DO between 1990 and 2000. Compared to August 1984, DO decreased significantly during the summer of 1999 in the Pearl River estuary, but large scale hypoxia (< 2 mg 1-1) was not observed. The spatial distribution of low oxygen waters may be controlled by estuarine circulation because DO was significantly correlated with salinity in the summers of 1981 and 1984. Furthermore, the spatial distribution of DO in the bottom layer was parallel to the topography of the bottom, indicating the importance of benthic consumption of DO in the sediment and the subsequent flux of low DO waters from the sediment water interface resuspended by physical mixing. Relative to the high loading of nitrogen from the Pearl River, the present PO4 concentration is still low. It is possible that the lack of large areas of hypoxia in the region may be linked to phosphorus limitation as shown in the previous study. Phosphorus may also be a limiting factor for bacterial decomposition which has a strong control on total oxygen consumption in the water column and sediments. (C) 2004 Elsevier Ltd. All rights reserved.

Modern foraminiferal faunas of the southwestern to northern iceland shelf: oceanographic and environmental controls

Abstract: Surface sediment samples for analysis of modern foraminiferal assemblages were collected from 32 sites on the western and northern Iceland shelf during cruise B997 of the Icelandic ship Bjarni Saemundsson. The purpose of the study is to provide a basis for interpreting the variations in foraminiferal content in late glacial through Holocene sediments from cores at the same sites. Cluster analysis and principal components analysis of the benthic foraminiferal data and associated modern environmental parameters including hydrography, water depth, and sediment characteristics show that large progressive changes in the foraminiferal assemblages coincide with changes in the dominant oceanic surface currents and water masses around Iceland. The western areas of the shelf are overlain by relatively warm and saline Atlantic Water of the Irminger Current. The western shelf fauna is dominated by Cassidulina laevigata and other boreal species, whereas the planktic fauna is dominated by dextrally coiling Neogloboquadrina pachyderma. In Djupall, off the Northwest Peninsula, and in the shallow nearshore sites on the northern shelf, the boreal elements of the fauna diminish and arctic species become more important. However, the dominant species in these areas are Cibicides lobatulus and Astrononion gallowayi, attesting to winnowing of the shallow shelf areas by currents. In Isafjardardjup, the large fjord indenting the Northwest Peninsula, Cassidulina reniforme and Elphidium excavatum dominate the fauna as a result of cooler waters and more variable salinity conditions. A very large faunal shift is registered in the deep basins of the northern shelf. This faunal shift coincides with large changes in salinity and temperature stratification of the water column, reflecting the presence of the East Iceland Current. The fauna in the deep basins on the northern shelf are dominated by C. neoteretis, an indicator of modified Atlantic Water, and infaunal species that reflect organic fluxes to the seabed: Nonionellina labradorica and Melonis barleeanus, and the arctic species Islandiella norcrossi and I. helenae.

Nitrogen Isotope Dynamics of the Cariaco Basin, Venezuela

Abstract: [1] We report 15N/14N measurements of water column nitrate and ammonium, sinking particles, and sediments from the Cariaco Basin, an anoxic marine basin off the coast of Venezuela. Water column denitrification occurring in the basin has only a very small isotopic imprint on nitrate in the basin because nitrate consumption is nearly complete in the actively denitrifying water near the oxic/anoxic interface (∼275 m). Being free of a large denitrification signal, the δ15N of shallow thermocline nitrate is ∼3.5‰, significantly lower than the mean deep ocean nitrate δ15N of 5‰. This may be due to the nitrification of newly fixed N, whether it occurs within the basin or in open Atlantic waters that flow into the Cariaco over the sill. The 15N/14N of the sinking flux in the deepest trap (∼1250 m) is similar to that of thermocline nitrate, as expected given the complete consumption of nitrate in the surface layer. Moreover, the 15N/14N of the seafloor sediment is similar to that of the sinking flux, as is common in environments of high export production, low O2, and good organic matter preservation. Thus the modern Cariaco Basin records the 15N/14N of the thermocline nitrate, which, in turn, may record the input of newly fixed N to the upper ocean, be it local or more regional in origin.

Hydrographical variability on the French continental shelf in the Bay of Biscay, during the 1990s

Abstract: A synthesis of 9 years of hydrographic measurements, acquired during approximately 25 cruises (1992–2000) on the French continental shelf of the Bay of Biscay, is presented. The main focus is upon salinity distribution, as it is little known in this area. The whole of the data set covers 11 months of the year, with some sampling fields repeated several times a year, for multiple years. This timeseries allows temperature and salinity distributions to be studied together with their seasonal, interannual and mesoscale variability. The seasonal trend in temperature can be described in three stages, which are in agreement with past studies. Thermal stratification occurs between May and mid-September in a layer not, vert, similar50 m in thickness, whereas the water column remains homogenous from January to the beginning of April. The mid-September–December period corresponds to the progressive destruction of the thermocline.

Persistent organic pollutants at the base of the Antarctic marine food web.

Abstract: Various organochlorine pesticides and brominated diphenyl ethers (BDE-47, -99, and -100) were measured in sea ice algae, water column plankton, and juvenile and adult krill collected in the Palmer Long-Term Ecological Research (LTER) region west of the Antarctic Peninsula during late austral winter and midsummer, 2001-2002. BDEs were 100-1000 times higher in ice algae and 2-10 times higher in phytoplankton than the most abundant organochlorine pesticide, hexachlorobenzene (HCB), reflecting the current production and use of BDEs versus organochlorine pesticides. However, concentrations of HCB and BDEs were significantly lower in summer plankton than in ice algae indicating lower atmospheric inputs, removal from the water column, and/or biodilution of persistent organic pollutants at the base of the food web during summer. Concentrations of HCB (juvenile and adult krill) and BDEs (juvenile krill) were not significantly different from their primary food source (ice algae, phytoplankton), and BDEs were significantly lower in adult krill versus phytoplankton, indicating no biomagnification of HCB or BDEs during transfer from plankton to krill. The high concentrations of BDEs and HCB in ice algae and associated juvenile krill illustrate the importance of sea ice as a vector for entry of POPs into the Antarctic marine ecosystem.

Virus production and life strategies in aquatic sediments

Abstract: Aquatic sediments host much of the bacterial biomass and biodiversity and play a key role in biogeochemical cycles. However, the potential effect of viral infection remains unknown. We present estimates of virus production in a variety of benthic habitats of the Mediterranean Sea, characterized by different contamination levels and trophic states. Viriobenthos abundance in aquatic sediments ranged 108−109 ml−1 of sediment and was ~20 times higher than virioplankton abundance. Vertical profiles in sediment cores revealed large virus numbers at 1−m depth below the sediment surface. Virus production in both marine and freshwater sediments was 1–2 orders of magnitude higher than typical values reported for virioplankton (0.13–1.60 x 108 virus-like particles ml−1 sediment h−1), which indicates that virus turnover and infection rates can be higher in aquatic sediments than in the water column. Eutrophic and contaminated sediments displayed the highest virus production rates. Virus production was significantly correlated with the abundance of active bacteria and with bacterial cell production. We estimated an average burst size (BS) of 15–18 in marine and freshwater sediments. Viru-mediated bacterial mortality was high (on average, ≫40% of bacterial production) and increased from surface to deeper sediment levels, down to 100−cm depth. The fraction of bacteria with lysogenic infection (range, 0.0–1.8%) increased in deeper sediment layers (3.3% at 100−cm depth). Our results suggest that high benthic virus production rates can have a significant effect on benthic bacterial dynamics and indicate that virus production should be included in biogeochemical models of aquatic sediments.

Salinity, depth and the structure and composition of microbial mats in continental Antarctic lakes

Abstract: SUMMARY 1. Lakes and ponds in the Larsemann Hills and Bolingen Islands (East-Antarctica) were characterised by cyanobacteria-dominated, benthic microbial mats. A 56-lake dataset representing the limnological diversity among the more than 150 lakes and ponds in the region was developed to identify and quantify the abiotic conditions associated with cyanobacterial and diatom communities. 2. Limnological diversity in the lakes of the Larsemann Hills and Bolingen Islands was associated primarily with conductivity and conductivity-related variables (concentrations of major ions and alkalinity), and variation in lake morphometry (depth, catchment and lake area). Low concentrations of pigments, phosphate, nitrogen, DOC and TOC in the water column of most lakes suggest extremely low water column productivity and hence high water clarity, and may thus contribute to the ecological success of benthic microbial mats in this region. 3. Benthic communities consisted of prostrate and sometimes finely laminated mats, flake mats, epilithic and interstitial microbial mats. Mat physiognomy and carotenoid/ chlorophyll ratios were strongly related to lake depth, but not to conductivity. 4. Morphological-taxonomic analyses revealed the presence of 26 diatom morphospecies and 33 cyanobacterial morphotypes. Mats of shallow lakes (interstitial and flake mats) and those of deeper lakes (prostrate mats) were characterised by different dominant cyanobacterial morphotypes. No relationship was found between the distribution of these morphotypes and conductivity. In contrast, variation in diatom species composition was strongly related to both lake depth and conductivity. Shallow ponds were mainly characterised by aerial diatoms (e.g. Diadesmis cf. perpusilla and Hantzschia spp.). In deep lakes, communities were dominated by Psammothidium abundans and Stauroforma inermis. Lakes with conductivities higher than ±1.5 mS cm )1 became susceptible to freezing out of salts and hence pronounced conductivity fluctuations. In these lakes P. abundans and S. inermis were replaced by Amphora veneta. Stomatocysts were important only in shallow freshwater lakes. 5. Ice cover influenced microbial mat structure and composition both directly by physical disturbance in shallow lakes and by influencing light availability in deeper lakes, as well as indirectly by generating conductivity increases and promoting the development of seasonal anoxia.

Short residence time for iron in surface seawater impacted by atmospheric dry deposition from Saharan dust events : Results of the Meteor 55: Tropical SOLAS expedition

Abstract: Measurements of dissolved (DFe) and total iron (TFe) in the upper water column are presented from the German SOLAS (Surface Ocean - Lower Atmosphere Study) cruise (M55), along a west to east transect at 10°N, in the equatorial Atlantic in October/November 2002. Aerosol samples were collected simultaneously during this time and are used to estimate an iron flux to the surface waters. Resulting flux estimates combined with iron inventories in the near surface waters reveal extremely short fractional mean residence times (6-62 days) for total (dissolved and particulate) iron in waters directly under the path of Saharan dust plumes. These results suggest that individual dust storms can supply a significant amount of the present iron upper water column inventory which is subsequent rapidly removed by aggregation and sinking.