Biogeochemical investigation of the benthic nitrogen cycle in the Arabian Sea off Pakistan

A pronounced deficit of nitrogen (N) in the oxygen minimum zone (OMZ) of the Arabian Sea suggests the occurrence of heavy N-loss that is commonly attributed to pelagic processes. However, the OMZ water is in direct contact with sediments on three sides of the basin. Contribution from benthic N-loss to the total N-loss in the Arabian Sea remains largely unassessed. In October 2007, we sampled the water column and surface sediments along a transect cross-cutting the Arabian Sea OMZ at the Pakistan continental margin, covering a range of station depths from 360 to 1430 m. Benthic denitrification and anammox rates were determined by using 15N-stable isotope pairing experiments. Intact core incubations showed declining rates of total benthic N-loss with water depth from 0.55 to 0.18 mmol N m**-2 day**-1. While denitrification rates measured in slurry incubations decreased from 2.73 to 1.46 mmol N m**-2 day**-1 with water depth, anammox rates increased from 0.21 to 0.89 mmol N m**-2 day**-1. Hence, the contribution from anammox to total benthic N-loss increased from 7% at 360 m to 40% at 1430 m. This trend is further supported by the quantification of cd1-containing nitrite reductase (nirS), the biomarker functional gene encoding for cytochrome cd1-Nir of microorganisms involved in both N-loss processes. Anammox-like nirS genes within the sediments increased in proportion to total nirS gene copies with water depth. Moreover, phylogenetic analyses of NirS revealed different communities of both denitrifying and anammox bacteria between shallow and deep stations. Together, rate measurement and nirS analyses showed that anammox, determined for the first time in the Arabian Sea sediments, is an important benthic N-loss process at the continental margin off Pakistan, especially in the sediments at deeper water depths. Extrapolation from the measured benthic N-loss to all shelf sediments within the basin suggests that benthic N-loss may be responsible for about half of the overall N-loss in the Arabian Sea.

Biogenic silica in the coastal plume of the Mississippi River

The surface distributions of dissolved silicic acid, chlorophyll and diatom abundance were measured in the plume of the Mississippi River and adjacent waters during spring (late April and early May 1993) and summer (July 1992). In spring, the time of maximum river flow, there was an intense diatom bloom with a mean diatom abundance of 1.5 x 10**7 cells/l, more than an order of magnitude higher than in summer. Mixing curves of silicic acid concentration ([Si(OH)4]) versus salinity indicate that biological uptake within the river plume removed >99% of the Si(OH)4 supplied by the river in spring and 80 to 95% in summer. In spring [Si(OH)4] was occasionally depleted to <0.2 µM-among the lowest values ever reported from the ocean-with extensive depletion to >=0.5 µM over the shelf. In summer [Si(OH)4] was less severely depleted; the lowest measured was 0.93 µM and all others were >=2.4 µM. 30Si kinetic experiments were performed during both spring and summer to measure the degree to which the rate of Si uptake by the natural diatom assemblages was limited in situ by substrate availability. In spring the dependence of the specific uptake rate (V) on extracellular [Si(OH)4] conformed much more closely to the Michaelis-Menten saturation function than has been observed in past studies. Strong dependence of V on [Si(OH)4] was observed throughout the most Si(OH)4-depleted (<0.5 µM) region, where V was limited to 12 to 45% of the diatom assemblages' maximum uptake rate (Vmax). Half-saturation concentrations for Si uptake (Ks) averaged 0.85 uM (range = 0.48 to 1.71; n = 7) in spring, with the lowest values equal to the lowest previously reported for natural diatom assemblages. There was only 1 station in summer where V was limited by [Si(OH)4], and at that station Ks was 5.3 µM-quite high in comparison with previous studies. At stations where V was limited by [Si(OH)4], in both spring and summer, Chaetoceros spp. were numerically dominant; where there was no Si limitation other diatoms, usually Skeletonema costatum, dominated. The data thus indicate strong Si limitation in spring, with diatom assemblages well adapted to low [Si(OH)4], but little or no Si limitation in summer. Historical data suggest that coastal Si(OH)4 depletion and Si limitation may be recent phenomena in the northern Gulf of Mexico, resulting from increasing [NO3-] and decreasing [Si(OH)4] in the Mississippi River during the past 30 to 50 yr.

Physico-chemical characteristics and ikaite content of brine and ice samples taken during SIPEX and DDU campaigns in East Antarctica, 2007

Calcium carbonate precipitation in sea ice is thought to potentially drive significant CO2 uptake by the ocean. However, little is known about the quantitative spatial and temporal distribution of CaCO3 within sea ice, although it is hypothesized that high quantities of dissolved organic matter and/or phosphate (common in sea ice) may inhibit its formation. In this quantitative study of hydrous calcium carbonate as ikaite, sea ice cores and brine samples were collected from pack and land fast sea ice between September and December 2007 during two expeditions, one in the East Antarctic sector and the other off Terre Adélie. Samples were analysed for CaCO3, salinity, dissolved organic carbon/nitrogen, inorganic phosphate, and total alkalinity. No relationship between these parameters and CaCO3 precipitation was evident. Ikaite was found mostly in the uppermost layers of sea ice with maximum concentrations of up to 126 mg ikaite per litre melted sea ice being measured, although both the temporal and horizontal spatial distributions of ikaite were highly heterogeneous. The precipitate was also found in the snow on top of the sea ice at some of the sampling locations.

Part of the global DOC versus AOU (dissolved organic carbon/apparent oxygen utilization) data compilation, Ross Sea

Dissolved and particulate organic matter was measured during six cruises to the southern Ross Sea. The cruises were conducted during late austral winter to autumn from 1994 to 1997 and included coverage of various stages of the seasonal phytoplankton bloom. The data from the various years are compiled into a representative seasonal cycle in order to assess general patterns of dissolved organic matter (DOM) and particulate organic matter (POM) dynamics in the southern Ross Sea. Dissolved organic carbon (DOC) and particulate organic carbon (POC) were at background concentrations of approximately 42 and 3 µM C, respectively, during the late winter conditions in October. As the spring phytoplankton bloom progressed, organic matter increased, and by January DOC and POC reached as high as 30 and 107 µM C, respectively, in excess of initial wintertime conditions. Stocks and concentrations of DOC and POC returned to near background values by autumn (April). Approximately 90% of the accumulated organic matter was partitioned into POM, with modest net accumulation of DOM stocks despite large net organic matter production and the dominance of Phaeocystis antarctica. Changes in NO3 concentration from wintertime values were used to calculate the equivalent biological drawdown of dissolved inorganic carbon (DICequiv). The fraction of DICequiv drawdown resulting in net DOC production was relatively constant (ca. 11%), despite large temporal and spatial variability in DICequiv drawdown. The C : N (molar ratio) of the seasonally produced DOM had a geometric mean of 6.2 and was nitrogen-rich compared to background DOM. The DOM stocks that accumulate in excess of deep refractory background stocks are often referred to as "semi-labile" DOM. The "semi-labile" pool in the Ross Sea turns over on timescales of about 6 months. As a result of the modest net DOM production and its lability, the role DOM plays in export to the deep sea is small in this region.

Pore-water chemistry of abyssal Arabian Sea surface sediments

Benthic fluxes and pore-water compositions of silicic acid, nitrate and phosphate were investigated for surface sediments of the abyssal Arabian Sea during four cruises (1995-1998). Five sites located in the northern (NAST), western (WAST), central (CAST), eastern (EAST), and southern (SAST) Arabian Sea were revisited during intermonsoonal periods after the NE- and SW-Monsoon. At these sites, benthic fluxes of remineralized nutrients from the sediment to the bottom water of 36-106, 102-350 and 4-16 mmol/m**2/yr were measured for nitrate, silicic acid and phosphate, respectively. The benthic fluxes and pore-water compositions showed a distinct regional pattern. Highest fluxes were observed in the western and northern region of the Arabian Sea, whereas decreasing fluxes were derived towards the southeast. At WAST, the general temporal pattern of primary production, related to the NE- and SW-Monsoon, is reflected by benthic fluxes. In contrast, at sites NAST, SAST, CAST, and EAST a temporal pattern of fluxes in response to the monsoon is not obvious. Our results reveal a clear coupling between the general regional pattern of production in surface waters and the response of the benthic environment, as indicated by the flux of remineralized nutrients, though a spatially differing degree of decoupling during transport and remineralization of particulate organic matter and biogenic opal was observed. This has to be taken into account regarding budget calculations and paleoceanographic topics.

Hydrochemistry measured on water bottle samples during the METEOR International Indian Ocean Expedition 1964/65

The present volume gives the observed physical and chemical data obtained by R.V. "Meteor" in the Indian Ocean during cruise 1964/65. The tables are based on the computations made by the National Oceanographic Data Center (NODC) in Washington. In addition to the normally communicated data, the tables contain four chemical parameters: alkalinity, ammonia, fluoride, and calcium.

Physical, chemical and biological characteristics of disturbed and undisturbed Arctic lakes, Northwest Territories

1. Global warming is predicted to cause changes in permafrost cover and stability in the Arctic. Zones of high ion concentration in regions of ice-rich permafrost are a reservoir of chemicals that can potentially be transferred to fresh waters during thawing. Consequently, input of enriched runoff from the thaw and sediment and vegetation from the landscape could alter lakes by affecting their geochemistry and biological production. 2. Three undisturbed lakes and five lakes disturbed by retrogressive permafrost thaw slumps were sampled during late summer of 2006 to assess the potential effects of thermokarst shoreline slumping on water and sediment chemistry, the underwater light regime, and benthic macrophyte biomass and community structure. 3. Undisturbed lakes had sediments rich in organic material and selected micronutrients, while disturbed lakes had sediments richer in calcium, magnesium and strontium, greater transparency of the water column, and a well-developed submerged macrophyte community. 4. It is postulated that enriched runoff chemistry may alter nutrient availability at the sediment-water interface and also the degradation of organic material, thus affecting lake transparency and submerged macrophytes. The results suggest that retrogressive permafrost slumping can significantly affect food webs in arctic tundra lakes through an increase in macrophyte biomass and development of a more complex benthic habitat.

Barium in seawater and sediments from the Artic Ocean

As part of ongoing circulation studies in the Arctic, seawater samples for dissolved Ba concentrations were obtained during Sep.-Oct., 1992 at several locations in the Bering Strait, Eastern Chukchi and Southern Beaufort Seas. The results reveal a dynamic rang (10 to 150 nmol/kg) for this element in the Arctic equal to or greater than that in combined Atlantic, Indian and Pacific oceans. Lowest levels are observed in surface waters, with values tending to decrease northwards in the direction of currents generally flowing frorn the Bering Strait along the Alaskan coast. Low surfacc concentrations tend to be accompanied by relatively enriched near bottom levels. On the basis of these spatial distributions, hydrographic observations and a knowledge of its behavior in other marine settings, it appears that Ba can be significantly depleted from surface waters as a result of the highly seasonal biological aclivities over Arctic marginal shelves. Removal at the surface is counteracted to some extent by regeneration at depth or in the sediments and by riverine inputs. The biologically related drawdown is likely to enhance the contrast between 'background' surface Ba levels in the Arctic and waters imprinted by regeneration and/or rivers, These preliminary findings suggest that Ba holds particular promise for tracing river waters and the ventilation of halodine waters hy laterally sinking brines produced during ice formation over the shelves.