The oxygen minimum zone in the Arabian Sea during 1995

This paper focuses on the characteristics of the oxygen minimum zone (OMZ) as observed in the Arabian Sea over the complete monsoon cycle of 1995. Dissolved oxygen, nitrite, nitrate and density values are used to delineate the OMZ, as well as identify regions where denitrification is observed. The suboxic conditions within the northern Arabian Sea are documented, as well as biological and chemical consequences of this phenomenon. Overall, the conditions found in the suboxic portion of the water column in the Arabian Sea were not greatly different from what has been reported in the literature with respect to oxygen, nitrate and nitrite distributions. Within the main thermocline, portions of the OMZ were found that were suboxic (oxygen less than ∼4.5 μM) and contained secondary nitrite maxima with concentrations that sometimes exceeded 6.0 μM, suggesting active nitrate reduction and denitrification. Although there may have been a reduction in the degree of suboxia during the Southwest monsoon, a dramatic seasonality was not observed, as has been suggested by some previous work. In particular, there was not much evidence for the occurrence of secondary nitrite maxima in waters with oxygen concentrations greater than 4.5 μM. Waters in the northern Arabian Sea appear to accumulate larger nitrate deficits due to longer residence times even though the denitrification rate might be lower, as evident in the reduced nitrite concentrations in the northern part of the basin. Organism distributions showed string relationships to the oxygen profiles, especially in locations where the OMZ was pronounced, but the biological responses to the OMZ varied with type of organism. The regional extent of intermediate nepheloid layers in our data corresponds well with the region of the secondary nitrite maximum. This is a region of denitrification, and the presence and activities of bacteria are assumed to cause the increase in particles. ADCP acoustic backscatter measurements show diel vertical migration of plankton or nekton and movement into the OMZ. Daytime acoustic returns from depth were strong, and the dawn sinking and dusk rise of the fauna were obvious. However, at night the biomass remaining in the suboxic zone was so low that no ADCP signal was detectable at these depths. There are at least two groups of organisms, one that stays in the upper mixed layer and another that makes daily excursions. A subsurface zooplankton peak in the lower OMZ (near the lower 4.5 μM oxycline) was also typically present; these animals occurred day and night and did not vertically migrate.     

Dissolved iron(II) in the Pacific Ocean: Measurements from the PO2 and P16N CLIVAR/CO2 repeat hydrography expeditions

The redox speciation of dissolved iron in seawater was evaluated at 121 locations in the Pacific Ocean at depths of 15-1000 m, using the method of luminol chemiluminescence. The results indicate that reduced iron, Fe(II), is ubiquitous in surface seawater with a relatively consistent pattern of occurrence. Surface maxima were present in most profiles, with median concentrations of 25-30 pM representing 12-14% of the total dissolved iron. Concentrations decreased monotonically with depth to<12 pM within the upper euphotic zone. This pattern was observed during both day and nighttime sampling events, which suggests that non-photochemical production mechanisms can produce photochemical-like signatures. Further, if theoretical rates of Fe(II) oxidation are applicable to the open ocean, then the employed sampling methods precluded assessment of photochemically-produced Fe(II), regardless of ambient light conditions. For this and other reasons, the concentrations reported here for the upper water column likely represent lower limits of labile iron concentration, and suggest that dissolved iron may be more available for uptake than previously believed. Deeper in the water column, Fe(II) was also frequently detected, though it constituted a small fraction of the total dissolved iron. Possible source mechanisms at these depths include thermal (dark) reduction of Fe(III) organic complexes or remineralization of sinking biogenic particles containing Fe(II). In the northern Philippine Sea between the Japanese coast and the Izu-Bonin volcanic arc system, Fe(II) concentrations were found to be atypically high, possibly because of high atmospheric dust deposition near the surface and transport of sediment-derived iron at depth.     

Enhanced preservation of organic matter in sediments deposited within the oxygen minimum zone in the northeastern Arabian Sea

The presence of a strongly developed oxygen minimum zone (OMZ; [O₂]<2 μM) in the northeastern Arabian Sea affords the opportunity to investigate whether oxygen deficiency in bottom waters enhances the preservation of organic matter in the underlying sediments. We explored if the observed patterns of organic matter accumulation could be explained by differences in productivity, sedimentation rate, water depth, and mineral texture. The differences in the burial rates of organic matter in sediments deposited within or below the OMZ could not be explained on the basis of these factors. All collected evidence points to a coupling of low oxygen concentrations and enhanced organic matter preservation. Under more oxygenated conditions bioturbation as well as the presence of labile manganese and iron oxides are probably important factors for a more efficient microbially mediated degradation of organic matter. Pore water profiles of dissolved Mn²⁺ and Fe²⁺ show that reduction of manganese and iron oxides plays a minor role in sediments lying within the OMZ and a larger role in sediments lying below the OMZ.     

The habitat of mesopelagic scyphomedusae in Monterey Bay,California

In the mesopelagic zone, at depths of 200–1000 m in the Monterey Submarine Canyon, CA, medusae in three genera of scyphozoa, Atolla, Periphylla and Poralia, were observed, videotaped and collected over a 9-year period (1990–1998). Environmental data were obtained simultaneously using a remotely operated vehicle (ROV) with sensors for depth, temperature, salinity and dissolved oxygen. Shipboard measurements of these same properties at two reference stations in the region defined the local water masses and helped identify species niches using the metric of spiciness and oxygen levels of the waters in which medusae were visually “captured”. The most abundant genus of mesopelagic scyphomedusae was Atolla, found associated most strongly with the spicy (warm, salty) waters of the California Undercurrent, usually above the core of the oxygen minimum zone (OMZ; O₂>0.5 ml/l). The least abundant mesopelagic scyphomedusa was Periphylla, which occurred in more variable waters, including those with a greater contribution of fresher, colder (less spicy), subarctic water and, hence, most like those at the offshore California Current station in the most depleted oxygen zone (averaging O₂ <0.3 ml/l). Poralia was mostly confined to the densest, coldest water, with peak abundance at the lower boundary of the OMZ (i.e., 0.3< O₂<0.5 ml/l). These spiciness measures on local isopycnal surfaces within the mesopelagic zone, supported by data on dissolved oxygen concentrations, indicate highly significant but fine-scale habitat differences in species habitats in Central California waters. This in situ investigation appears to be one of only a few studies to document fine-scale, water mass affinities of mesopelagic zooplankton.     

Polychaete community structure in the South Eastern Arabian Sea continental margin (200–1000 m)

Macrofaunal polychaete communities (>500 µm) in the South Eastern Arabian Sea (SEAS) continental margin (200–1000 m) are described, based on three systematic surveys carried out in 9 transects (at ~200 m, 500 m and 1000 m) between 7°00′and 14°30′N latitudes. A total of 7938 polychaetes belonging to 195 species were obtained in 136 grab samples collected at 27 sites. Three distinct assemblages were identified in the northern part of the SEAS margin (10–14°30′N), occupying the three sampled depth strata (shelf edge, upper and mid-slope) and two assemblages (shelf edge and slope) in the south (7–10°N). Highest density of polychaetes and dominance of a few species were observed in the shelf edge, where the Arabian Sea oxygen minimum zone (OMZ) impinged on the seafloor, particularly in the northern transects. The resident fauna in this region (Cossura coasta, Paraonis gracilis, Prionospio spp. and Tharyx spp.) were characteristically of smaller size, and well suited to thrive in the sandy sediments in OMZ settings. Densities were lowest along the most northerly transect (T9), where dissolved oxygen (DO) concentrations were extremely low (<0.15 ml l⁻¹, i.e.<6.7 μmol l⁻¹). Beyond the realm of influence of the OMZ (i.e. mid-slope, ~1000 m), the faunal density decreased while species diversity increased. The relative proportion of silt increased with depth, and the dominance of the aforementioned species decreased, giving way to forms such as Paraprionospio pinnata, Notomastus sp., Eunoe sp. and lumbrinerids. Relatively high species richness and diversity were observed in the sandy sediments of the southern sector (7–9°N), where influence of the OMZ was less intense. The area was also characterized by certain species (e.g. Aionidella cirrobranchiata, Isolda pulchella) that were nearly absent in the northern region. The gradients in DO concentration across the core and lower boundary of the OMZ, along with bathymetric and latitudinal variation in sediment texture, were responsible for differences in polychaete size and community structure on the SEAS margin. Spatial and temporal variations were observed in organic matter (OM) content of the sediment, but these were not reflected in the density, diversity or distribution pattern of the polychaetes.     

Zooplankton associated with the oxygen minimum zone system in the northern upwelling region of Chile during March 2000

Zooplankton in the coastal upwelling region off northern Chile may play a significant biogeochemical role by promoting carbon flux into the subsurface OMZ (oxygen minimum zone). This work identifies the dominant zooplankton species inhabiting the area influenced by the OMZ in March 2000 off Iquique (20°S, northern Chile). Abundance and vertical distribution studies revealed 17 copepod and 9 euphausiid species distributed between the surface and 600 m at four stations sampled both by day and by night. Some abundant species remained in the well-oxygenated upper layer (30 m), with no evidence of diel vertical migration, apparently restricted by a shallow (40–60 m) oxycline. Other species, however, were found closely associated with the OMZ. The large-sized copepod Eucalanus inermis was found below the oxycline and performed diel vertical migrations into the OMZ, whereas the very abundant Euphausia mucronata performed extensive diel vertical migrations between the surface waters and the core of the OMZ (200 m), even crossing it. A complete assessment of copepods and euphausiids revealed that the whole sampled water column (0–600 m) is occupied by distinct species having well-defined habitats, some of them within the OMZ. Ontogenetic migrations were evident in Eucalanidae and E. mucronata. Estimates of species biomass showed a substantial (>75% of total zooplankton biomass) daily exchange of C between the photic layer and the OMZ. Both E. inermis and E. mucronata can actively exchange about 37.8 g C m⁻² d⁻¹ between the upper well-oxygenated (0–60 m) layer and the deeper (60–600 m) OMZ layer. This migrant biomass may contribute about 7.2 g C m⁻² d⁻¹ to the OMZ system through respiration, mortality, and production of fecal pellets within the OMZ. This movement of zooplankton in and out of the OMZ, mainly as a result of the migratory behavior of E. mucronata, suggests a very efficient mechanism for introducing large amounts of freshly produced carbon into the OMZ system and should, therefore, be considered when establishing C budgets for coastal upwelling systems.     

Meiofaunal distributions on the Peru margin:: relationship to oxygen and organic matter availability

A quantitative study of metazoan meiofauna was carried out on bathyal sediments (305, 562, 830 and 1210 m) along a transect within and beneath the oxygen minimum zone (OMZ) in the southeastern Pacific off Callao, Peru (12°S). Meiobenthos densities ranged from 1517 (upper slope, middle of OMZ) to 440–548 ind. 10 cm⁻² (lower slope stations, beneath the OMZ). Nematodes were the numerically dominant meiofaunal taxon at every station, followed by copepods and nauplii. Increasing bottom-water oxygen concentration and decreasing organic matter availability downslope were correlated with observed changes in meiofaunal abundance. The 300-m site, located in the middle of the OMZ, differed significantly in meiofaunal abundance, dominance, and in vertical distribution pattern from the deeper sites. At 305 m, nematodes amounted to over 99% of total meiofauna; about 70% of nematodes were found in the 2–5 cm interval. At the deeper sites, about 50% were restricted to the top 1 cm. The importance of copepods and nauplii increased consistently with depth, reaching ∼12% of the total meiofauna at the deepest site. The observation of high nematode abundances at oxygen concentrations <0.02 ml l⁻¹ supports the hypothesis that densities are enhanced by an indirect positive effect of low oxygen involving (a) reduction of predators and competitors and (b) preservation of organic matter leading to high food availability and quality. Food input and quality, represented here by chloroplastic pigment equivalents (CPE) and sedimentary labile organic compounds (protein, carbohydrates and lipids), were strongly, positively correlated with nematode abundance. By way of contrast, oxygen exhibited a strong negative correlation, overriding food availability, with abundance of other meiofauna such as copepods and nauplii. These taxa were absent at the 300-m site. The high correlation of labile organic matter (C-LOM, sum of carbon contents in lipids, proteins and carbohydrates) with CPE (Pearson's r=0.99, p<0.01) suggests that most of the sedimentary organic material sampled was of phytodetrital origin. The fraction of sediment organic carbon potentially available to benthic heterotrophs, measured as C-LOM/Total organic carbon, was on average 17% at all stations. Thus, a residual, refractory fraction, constitutes the major portion of organic matter at the studied bathyal sites.     

Occurrence and turnover of DMSP and DMS in deep waters of the Ross Sea,Antarctica

High concentrations of the phytoplankton metabolite dimethylsulfoniopropionate (DMSP) and its degradation product dimethylsulfide (DMS) are associated with blooms of Phaeocystis antarctica in the Ross Sea, Antarctica. Episodic and rapid vertical export of Phaeocystis biomass to deep water has been reported for the Ross Sea, therefore we examined the distribution and microbial consumption rates of DMSP and DMS throughout the sub-euphotic water column. Total DMSP (dissolved+particulate; DMSPt) was present at 0.5–22 nM at depths between 70 and 690 m during both the early bloom (November) and the late bloom (January). Sub-euphotic peaks of DMSP were sometimes associated with mid-water temperature maxima, and elevated DMSP below 70 m was found mainly in water masses characterized as Modified Circumpolar Deep Water or Antarctic Shelf Water. Overall, 50–94% of the integrated water-column DMSPt was found below the euphotic zone. At one station during the early bloom, local maxima of DMSPt (14 nM) and DMS (20 nM) were observed between 113 and 240 m and these maxima corresponded with high chlorophyll a concentrations, P. antarctica cell numbers, and Fv/Fm (the quantum yield of photosystem II). During the late bloom, a sub-euphotic maximum of DMSPt (15.8 nM) at 250 m cooccurred with peaks of chlorophyll a concentration, DMSP lyase activity, bacterial production and dissolved DMSP consumption rates. DMSP turnover contributed ～12% of the bacterial carbon demand between 200 and 400 m. DMS concentrations peaked at 286 m but the maximum concentration (0.42 nM) was far lower than observed during the early bloom, probably because of relatively rapid biological consumption of DMS (1–3 turnovers per day) which, in turn, contributed to elevated dissolved dimethylsulfoxide (DMSO) concentrations. Relatively stable DMSPt distributions at some sites suggest that rapid sinking of Phaeocystis biomass is probably not the major mechanism responsible for mesopelagic DMSP accumulations. Rather, subduction of near-surface water masses, lateral advective transport or trapping of slowly sinking P. antarctica biomass in intermediate water masses are more likely mechanisms. We found that a culture of P. antarctica maintained cellular integrity during 34 days of darkness, therefore the presence of intact cells (and DMSP) at depth can be explained even under a slow sinking/advection scenario. Whatever the mechanism, the large pools of DMSP and DMS below the euphotic zone suggest that export exerts a control on potential DMS emission from the surface waters of the Ross Sea.     

Biomass size-spectra of macrobenthic communities in the oxygen minimum zone off Chile

Estimates of macrofaunal secondary production and normalized biomass size-spectra (NBSS) were constructed for macrobenthic communities associated with the oxygen minimum zone (OMZ) in four areas of the continental margin off Chile. The presence of low oxygen conditions in the Humboldt Current System (HCS) off Chile was shown to have important effects on the size structure and secondary production of the benthic communities living in this ecosystem. The distribution of normalized biomass by size was linear (log₂–log₂ scale) at all stations. The slope of the NBSS ranged from −0.481 to −0.908. There were significant differences between the slopes of the NBS-spectra from the stations located in the OMZ (slope = −0.837) and those located outside the OMZ (slope = −0.463) (p < 0.05). The results of this study suggest that low oxygen conditions (<0.5 ml L⁻¹) appear to influence biomass size-spectra, because small organisms are better able to satisfy their metabolic demands. The annual secondary production was higher off central Chile (6.8 g C m⁻² y⁻¹) than off northern Chile (2.02 g C m⁻² y⁻¹) and off southern Chile (0.83 g C m⁻² y⁻¹). A comparison with other studies suggests that secondary production in terms of carbon equivalents was higher than in other upwelling regions.     

Aerobic ammonium oxidation in the oxycline and oxygen minimum zone of the eastern tropical South Pacific off northern Chile (∼20°S)

Aerobic NH₄⁺ oxidation rates were measured along the strong oxygen gradient associated with the oxygen minimum zone (OMZ) of the eastern tropical South Pacific off northern Chile (∼20°S) during 2000, 2003, and 2004. This process was examined by comparing NH₄⁺ rates of change during dark incubations, with and without the addition of allylthiourea, a classical inhibitor of the ammonia monooxygenase enzyme of ammonium-oxidizing bacteria. The contribution of aerobic NH₄⁺ oxidation in dark carbon fixation and NO₂⁻ rates of change were also explored. Thirteen samples were retrieved from the oxycline (252 to ⩽5 μM O₂; 15 to ∼65 m depth) and three from the oxygen minimum core (⩽5 μM O₂; 100–200 m depth). Aerobic NH₄⁺ oxidation rates were mainly detected in the upper part (15–30 m depth) of the oxycline, with rates ranging from 0.16 to 0.79 μM d⁻¹, but not towards the oxycline base (40–65 m depth). In the oxygen minimum core, aerobic NH₄⁺ oxidation was in the upper range and higher than in the upper part of the oxycline (0.70 and 1.0 μM d⁻¹). Carbon fixation rates through aerobic NH₄⁺ oxidation ranged from 0.18 to 0.43 μg C L⁻¹ d⁻¹ and contributed between 33% and 57% of the total dark carbon fixation, mainly towards the oxycline base and, in a single experiment, in the upper part of the oxycline. NO₂⁻ consumption was high (up to 10 μM d⁻¹) towards the oxycline base and OMZ core, but was significantly reduced in experiments amended with allylthiourea, indicating that aerobic NH₄⁺ oxidation could contribute between 8% and 76% of NO₂⁻ production, which in turn could be available for denitrifiers. Overall, these results support the important role of aerobic NH₄⁺ oxidizers in the nitrogen and carbon cycling in the OMZ and at its upper boundary.     

Speciation of Fe in the Eastern North Atlantic Ocean

In the Eastern North Atlantic Ocean iron (Fe) speciation was investigated in three size fractions: the dissolvable from unfiltered samples, the dissolved fraction (<0.2 μm) and the fraction smaller than 1000 kDa (<1000 kDa). Fe concentrations were measured by flow injection analysis and the organic Fe complexation by voltammetry. In the research area the water column consisted of North Atlantic Central Water (NACW), below which Mediterranean Overflow Water (MOW) was found with the core between 800 and 1000 m depth. Below 2000 m depth the North Atlantic Deep Water (NADW) proper was recognised. Dissolved Fe and Fe in the <1000 kDa fraction showed a nutrient like profile, depleted at the surface, increasing until 500–1000 m depth below which the concentration remained constant. Fe in unfiltered samples clearly showed the MOW with high concentrations (4 nM) compared to the overlying NACW and the underlying NADW, with 0.9 nM and 2 nM Fe, respectively. By using excess ligand (Excess L) concentrations as parameter we show a potential to bind Fe. The surface mixed layer had the highest excess ligand concentrations in all size fractions due to phytoplankton uptake and possible ligand production. The ratio of Excess L over Fe proved to be a complementary tool in revealing the relative saturation state of the ligands with Fe. In the whole water column, the organic ligands in the larger colloidal fraction (between 0.2 μm and 1000 kDa) were saturated with Fe, whereas those in the smallest fraction (<1000 kDa) were not saturated with Fe, confirming that this fraction was the most reactive one and regulates dissolution and colloid aggregation and scavenging processes. This regulation was remarkably stable with depth since the alpha factor (product of Excess L and K′), expressing the reactivity of the ligands, did not vary and was 10¹³. Whereas, in the NACW and the MOW, the ligands in the particulate (>0.2 μm) fraction were unsaturated with Fe with respect to the dissolved fraction, thus these waters had a scavenging potential.     

Anammox bacteria and the anaerobic oxidation of ammonium in the oxygen minimum zone off northern Chile

Anammox is the anaerobic oxidation of ammonium by nitrite or nitrate to yield N₂. This process, along with conventional denitrification, contributes to nitrogen loss in oxygen-deficient systems. Anammox is performed by a special group of bacteria belonging to the Planctomycetes phylum. However, information about the distribution, activity, and controlling factors of these anammox bacteria is still limited. Herein, we examine the phylogenetic diversity, vertical distribution, and activity of anammox bacteria in the coastal upwelling region and oxygen minimum zone off northern Chile. The phylogeny of anammox bacteria was studied using primers designed to specifically target 16S rRNA genes from Planctomycetes in samples taken during a cruise in 2004. Anammox bacteria-like sequences affiliated with Candidatus “Scalindua spp.” dominated the 16S rRNA gene clone library. However, 62% of the sequences subgrouped separately within this cluster and together with a single sequence retrieved from the suboxic zone of the freshwater Lake Tanganyika. The vertical distribution and activity of anammox bacteria were explored through CARD-FISH (fluorescence in situ hybridization with catalyzed reporter deposition) and ¹⁵N labeling incubations, respectively, at two different open-ocean stations during a second cruise in 2005. Anammox bacterial CARD-FISH counts (up to 3000 cells ml⁻¹) and activity (up to 5.75 nmol N₂ L⁻¹ d⁻¹) were only detected at the station subjected directly to the upwelling influence. Anammox cell abundance and activity were highest at 50 m depth, which is the upper part of the OMZ. In this layer, a high abundance of cyanobacteria and a marked nitrogen deficit were also observed. Thus, our results show the presence of a new subcluster within the marine anammox phylogeny and indicate high vertical variability in the abundance and activity of anammox bacteria that could be related to an intensification of carbon and nitrogen cycling in the upper part of the OMZ.     

Consumption and production on scales of a few days of inorganic carbon,nitrate and oxygen by the planktonic community: results of continuous measurements at the Dyfamed Station in the northwestern Mediterranean Sea (May 1995)

Continuous measurements between 0 and 200 m depth were performed every 2 h over two separate periods of four days at a station in the open northwestern Mediterranean Sea (Dyfamed Station) during the Dynaproc cruise in May 1995. Estimates of the daily variations in profiles of temperature, partial pressure of CO₂, oxygen, chlorophyll a and nutrients were obtained. The distributions of the various physical and chemical properties were clearly different during the two time series, which were separated by a period of 11 days during which a wind event occurred. The mean daily utilization or production due to biological processes of dissolved inorganic carbon (DIC), nitrate+nitrite and oxygen were calculated along isopycnals using a vertical diffusion model. Between the surface and about 20 m depth, DIC was consumed and O₂ released during the two time series while the nitrate+nitrite concentrations as well as supplies were zero. After the wind event, the O₂ : C : N ratios of consumption (or production) were, on average, near the Redfield ratios, but during the first time series, the C : N utilization ratio between 20 and 35 m was two to three times that of Redfield stoichiometry and the oxygen release was low. The integrated net community production (NCP) in terms of carbon was equivalent during the two time series, whereas the chlorophyll a biomass was twice as high, on average, during the first time series but did decrease. These results imply that the production systems were different during the two periods. The first time series corresponds to a period at the end of production, due to the nutrient depletion in the euphotic layer. The formation of degradation products of the living material in dissolved organic form is probably important as indicated by the high C : N utilization ratios. The second time series corresponds to a reactivation of the primary production due to the upward shift of nutrients after the wind event.     

Nitrous oxide and N-nutrient cycling in the oxygen minimum zone off northern Chile

Measurements of dissolved gases (O₂, N₂O), nutrients (NO₃⁻, NO₂⁻, PO₄³⁻), and oceanographic variables were performed off northern Chile (∼21°S) between March 2000 and July 2004, in order to characterize the existing oxygen minimum zone (OMZ) and identify processes involved in N₂O cycling. Both N₂O and NO₃⁻ displayed sharp, shallow peaks with concentrations of up to 124 nM (1370% saturation) and 26 μM, respectively, in association with a strong oxycline that impinges on the euphotic zone. NO₂⁻ accumulation below the oxycline's base reached up to 9 μM. The vertical distribution of physical and chemical parameters and the existing relationships between apparent oxygen utilization (AOU), apparent N₂O production (ΔN₂O), and NO₃⁻ revealed three main layers within the upper OMZ. The first layer, or the upper part of the oxycline, is located between the base of the mixed layer and the mid-point of the oxycline (around σ_t=25.5 kg m⁻³). There the O₂ declines from ∼250 to ∼50 μM, and strong (but opposing) O₂ and NO₃⁻ gradients and their associated AOU–ΔN₂O and AOU–NO₃⁻ relationships indicate that nitrification produces N₂O and NO₃⁻ in the presence of light. The second layer, or lower part of the oxycline, represents the upper OMZ boundary and is located between the middle and the base of the oxycline (25.9<σ_t<26.1 kg m⁻³). In this layer NO₃⁻ reduction begins at O₂ levels ranging from ∼50 to ∼11 μM and accumulation of 41–68% of the ΔN₂O pool occurs. The accumulation of N₂O (but not of NO₂⁻ or NH₄⁺) and the observed AOU–ΔN₂O and AOU–NO₃⁻ relationships (which are opposite to those of the overlying first layer) suggest that a coupling between nitrification and NO₃⁻ reduction is involved in N₂O cycling in this second layer. The third layer is the OMZ core, where the O₂ concentration remains constant (O₂<11 μM). It coincides with σ_t>26.2 kg m⁻³, which is typical of Equatorial Subsurface Water (ESSW). In this layer, N₂O and NO₃⁻ continue to decrease, but a large NO₂⁻ accumulation is observed. Considering all the data, a biogeochemical model for the upper OMZ off northern of Chile is proposed, in which nitrification and denitrification differentially mediate N₂O cycling in each layer.     

Deep penetrating benthic oxygen profiles measured in situ by oxygen optodes

For the first time in situ, deep penetrating O₂ profiles were measured in abyssal sediments in the western South Atlantic. Construction of deep penetrating O₂ optodes and adaptation to a benthic profiling lander are described. The opto-chemical oxygen sensors allow measurements to a depth of 55 cm in marine sediments. A vertical resolution of 0.5 cm was used to determine the O₂ dynamics in those oligotrophic deep sea sediments; the oxygen concentration across the sediment water interface was measured with a resolution of 100 μm. Oxygen penetration depth (OPD), diffusive oxygen uptake (DOU) and oxygen consumption rates were determined at four stations north of the Amazon fan and one at the Mid-Atlantic Ridge. Diffusive oxygen uptake rates ranged from 0.1 to 0.9 mmol m⁻² d⁻¹; the oxygen penetration depth ranged from 8 to 26 cm. Carbon consumption rates calculated from the diffusive oxygen uptake rates were in the range of 0.3–3.0 g C m⁻² a⁻¹. Comparison between in situ and laboratory DOU and OPD measurements confirmed previous findings that core recovery and warming have strong effects on the oxygen dynamics in deep sea sediments. Laboratory measurements yielded a decrease of 50–75% in OPD and consequently an increase in DOU by 1.5 and 18-times. Deep penetrating oxygen optodes provide a new tool to accurately determine oxygen dynamics (and thereby calculate carbon mineralization rates) in oligotrophic sediments. However, oxygen optodes as used in this study do not resolve the diffusive boundary layer (DBL). The data show that deep penetrating O₂ optodes in combination with high-resolution O₂ microelectrodes give a complete picture of the oxygen dynamics, including the DBL, in deep sea sediments.     

Benthic response to shellfish farming in Thau lagoon: Pore water signature

Vertical distributions of dissolved species across the sediment–water interface (SWI), including major cations (sodium, potassium, magnesium, calcium), minor cations (lithium, strontium, barium), redox sensitive species (dissolved manganese, iron, sulfate, sulfide, ammonium) and other chemical parameters (pH, alkalinity, soluble reactive phosphorous, dissolved silica) were studied in a Mediterranean lagoon used for intensive shellfish farming. In order to quantify the impact of this activity on diagenetic processes and the influence of seasonal changes, two stations contrasted with respect to organic carbon fluxes were sampled in Thau lagoon from March 2001 to August 2002 during four field campaigns in winter, spring, summer and fall. Well-defined layers enriched with redox sensitive species were observed following the conventional sequence of early diagenetic reactions. However, differences were observed between both stations in depths and thickness layers. Concentration gradients extended down to more than 92 cm depth at the central position of the lagoon (station C4 – 8 m depth) and down to 40 cm depth inside shellfish farming zones (station C5 – 9 m depth). Station C4 showed an unusual diagenetic signature: sharp dissolved oxygen, iron, nitrate and manganese gradients existed at the SWI but gradients of dissolved sulfide and alkalinity as well as other parameters (dissolved silica, Ba, etc.) were recorded only from 25 to 30 cm depth downward. Seasonal changes were observed in pore water composition as deep as 30–50 cm in station C4 (only 15 cm in station C5). The center of the lagoon is not directly subjected to biodeposits deriving from shellfish activity. Isotopic and bioturbation data allowed to rule out a reworking of the sediment deeper than a few centimeters. In addition to organic content of the sediment, physical parameters were likely to induce the 10–20 cm gap between dissolved iron and sulfide profile as well as the higher vertical extent of diagenetic sequence observed at station C4. Conversely to station C5, station C4 underwent stronger currents and wave effect probably generating advective transport of water through the sediment, but no permeability data were available to confirm this hypothesis. During summer, climatic conditions generated vertical stratification of the water column and transient suboxic conditions at the bottom. Such conditions drove the upward shift of redox fronts, compacting the diagenetic sequence. These effects were reinforced at station C5 by shellfish and its farm structures (mainly attenuation of current and increased heat absorption).     

Ocean oxygen minima expansions and their biological impacts

Climate models with biogeochemical components predict declines in oceanic dissolved oxygen with global warming. In coastal regimes oxygen deficits represent acute ecosystem perturbations. Here, we estimate dissolved oxygen differences across the global tropical and subtropical oceans within the oxygen minimum zone (200–700-dbar depth) between 1960–1974 (an early period with reliable data) and 1990–2008 (a recent period capturing ocean response to planetary warming). In most regions of the tropical Pacific, Atlantic, and Indian Oceans the oxygen content in the 200–700-dbar layer has declined. Furthermore, at 200 dbar, the area with O₂ <70 μmol kg^?1, where some large mobile macro-organisms are unable to abide, has increased by 4.5 million km². The tropical low oxygen zones have expanded horizontally and vertically. Subsurface oxygen has decreased adjacent to most continental shelves. However, oxygen has increased in some regions in the subtropical gyres at the depths analyzed. According to literature discussed below, fishing pressure is strong in the open ocean, which may make it difficult to isolate the impact of declining oxygen on fisheries. At shallower depths we predict habitat compression will occur for hypoxia-intolerant taxa, with eventual loss of biodiversity. Should past trends in observed oxygen differences continue into the future, shifts in animal distributions and changes in ecosystem structure could accelerate.     

Microbial degradation rates of small peptides and amino acids in the oxygen minimum zone of Chilean coastal waters

We found similar microbial degradation rates of labile dissolved organic matter in oxic and suboxic waters off northern Chile. Rates of peptide hydrolysis and amino acid uptake in unconcentrated water samples were not low in the water column where oxygen concentration was depleted. Hydrolysis rates ranged from 65 to 160 nmol peptide L⁻¹ h⁻¹ in the top 20 m, 8–28 nmol peptide L⁻¹ h⁻¹ between 100 and 300 m (O₂-depleted zone), and 14–19 nmol peptide L⁻¹ h⁻¹ between 600 and 800 m. Dissolved free amino acid uptake rates were 9–26, 3–17, and 6 nmol L⁻¹ h⁻¹ at similar depth intervals. Since these findings are consistent with a model of comparable potential activity of microbes in degrading labile substrates of planktonic origin, we suggest, as do other authors, that differences in decomposition rates with high and low oxygen concentrations may be a matter of substrate lability. The comparison between hydrolysis and uptake rates indicates that microbial peptide hydrolysis occurs at similar or faster rates than amino acid uptake in the water column, and that the hydrolysis of peptides is not a rate-limiting step for the complete remineralization of labile macromolecules. Low O₂ waters process about 10 tons of peptide carbon per h, double the amount processed in surface-oxygenated water. In the oxygen minimum zone, we suggest that the C balance may be affected by the low lability of the dissolved organic matter when this is upwelled to the surface. An important fraction of dissolved organic matter is processed in the oxygen minimum layer, a prominent feature of the coastal ocean in the highly productive Humboldt Current System.     

DOC dynamics in the meso and bathypelagic layers of the Mediterranean Sea

Seven years (2001–2008) of dissolved organic carbon (DOC) vertical profiles were examined in order to assess the main processes determining DOC concentration and distribution in the meso- and bathypelagic layers of the Mediterranean Sea. As expected, DOC showed high and highly variable concentrations in the surface layer of 57–68 μM (average values between 0 and 100 m), with a decrease to 44–53 μM between 200 and 500 m. Deep DOC distribution was strongly affected by deep-water formation, with a significant increase to values of 76 μM in recently ventilated deep waters, and low concentrations, comparable to those observed in the open oceanic waters (34–45 μM), where the oldest, deep waters occurred. In winter 2004/2005 a deep-water formation event was observed and the consequent DOC export at depth was estimated to range between 0.76–3.02 Tg C month^–1. In the intermediate layer, the main path of the Levantine Intermediate Water (LIW) was followed in order to estimate the DOC consumption rate in its core. Multiple regression between DOC, apparent oxygen utilization (AOU), and salinity indicated that 38% of the oxygen consumption was related to DOC mineralization when the effect of mixing was removed. In deep waters of the southern Adriatic Sea a DOC decrease of 6 μM, together with an AOU increase of 9 μM, was observed between the end of January 2008 and the end of June 2008 (5 months). These data indicate a rate of microbial utilization of DOC of about 1.2 μM C month⁻¹, with 92% of the oxygen consumption due to DOC mineralization. These values are surprisingly high for the deep sea and represent a peculiarity of the Mediterranean Sea.     

Spatial and seasonal evolution of dissolved oxygen and nutrients in the Southern Levantine Basin (Eastern Mediterranean Sea): chemical characterization of the water masses and inferences on the N : P ratios

The spatial and seasonal variability of nutrients and dissolved oxygen concentrations as well as the chemical characterization of the different water masses of the Southern Levantine Basin were determined in detail. In summer, the upper 150 m of the water body was stratified and the cross basin distribution of dissolved oxygen and nutrients was fairly constant. Surficial waters were saturated with dissolved oxygen, and a shallow oxygen maximum (oversaturated) was present at about 80 m depth. Oversaturation was attributed mainly to the physical process of rapid capping and trapping of oxygen in the Atlantic water (AW) mass, with only 28% of the excess oxygen originating from biological production. Nutrient concentrations were very low and showed an increase in the intermediate levels, coupled with a decrease in oxygen. The winter cross-section distribution showed an upper mixed layer of 100 m, with dissolved oxygen and nutrient concentrations fairly constant across the basin. The concentration of nitrate was higher than in summer, while phosphate was slightly lower and silicic acid similar. In winter, the influence of the physical features (gyres) could be detected up to the surface, and in summer they were detected by the chemical properties in the 150–600 m layer. In the transition layer between the Levantine intermediate water (LIW) and the deep water (DW) (400–700 m) there was a gradual decrease in dissolved oxygen and an increase in nutrient concentrations eastwards. The DW showed no seasonal variation, only spatial variability: dissolved oxygen decreased and silicic acid increased eastwards. No differences were found in nitrate and phosphate concentrations between the DW in the western and eastern provinces, indicating the oxidation of organic matter poor in N and P.N : P ratios in the upper water masses were seasonally dependent. The largest variation was found in the Levantine surface water (LSW), from an average of 52 in winter to 5 in summer. It is hypothesized that the gradual decrease from winter to summer values was due mainly to preferential atmospheric input of N in winter and P in summer, together with biological consumption and differential regeneration of N and P. In the DW, the N : P ratios were constant throughout the year (25.2±2.7, n=567), and higher than Redfield's ratio. It was speculated that the high N : P ratio in the DW was a result of oxidation of particulate organic matter deficient in P.The winter wet atmospheric input of N provided 12% of new N to the LSW. Average new production for the Southern Levantine Basin was estimated from the new N as 4.75 g C m⁻² yr⁻¹. The dry atmospheric contribution of P was estimated to significantly increase the P pool in the LSW. Dry deposition is not evenly distributed and occurs in episodic and localized events, which may have a large effect on productivity in the short periods when deposition occurs.There have been recently reported changes in the deep thermohaline circulation of the Eastern Mediterranean, with main contribution of the Aegean Sea as a source of DW. The data presented here can serve as a reference for assessing future changes in the chemical composition of the water masses in the Southern Levantine.