Louis A. Codispoti

The oxygen minimum zone in the Arabian Sea during 1995

Abstract 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 ∼4.5xa0μM) and contained secondary nitrite maxima with concentrations that sometimes exceeded 6.0xa0μM, 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 4.5xa0μM. 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 4.5xa0μM oxycline) was also typically present; these animals occurred day and night and did not vertically migrate.

Primary productivity and its regulation in the Arabian Sea during 1995

Abstract The annual cycle of monsoon-driven variability in primary productivity was studied in 1995 during the Arabian Sea Expedition as part of the United States Joint Global Ocean Flux Studies (US JGOFS). This paper describes the seasonal progression of productivity and its regulation on a section which ran from the coast of Oman to about 1000xa0km offshore in the central Arabian Sea at 65°E. During the SW Monsoon (June–mid-September), the coolest water and highest nutrient concentrations were close to the coast, although they extended offshore to about 800xa0km; during the January NE Monsoon, deep convective mixing provided nutrients to the mixed layer in the region 400 – 1000xa0km offshore. As expected, the SW Monsoon was the most productive season (123±9xa0mmol Cxa0m −2 xa0d −1 ) along the southern US JGOFS section from the coast to 1000xa0km offshore, but productivity in the NE Monsoon was surprisingly high (112±7xa0mmol C m −2 xa0d −1 ). There was no onshore/offshore gradient in primary productivity from 150 to 1000xa0km off the Omani coast in 1995, and there was no evidence of light limitation of either primary productivity or photosynthetic performance ( P opt B ) from deep convective mixing during the NE Monsoon, deep wind mixing during the SW Monsoon or offshore Ekman downwelling during the SW Monsoon. Productivity during the Spring Intermonsoon (86±6xa0mmol Cxa0m −2 xa0d −1 ) was much higher than in oligotrophic regions such as the tropical Pacific Ocean (29±2xa0mmol Cxa0m −2 xa0d −1 ) or the North Pacific gyre region (32±8xa0mmol Cxa0m −2 xa0d −1 ). The 1995 annual mean productivity (111±11xa0mmol Cxa0m −2 xa0d −1 ) along this section from the Omani coast to the central Arabian Sea was about equal to the spring bloom maximum (107±23xa0mmol Cxa0m −2 xa0d −1 ) during the 1989 North Atlantic Bloom Experiment (NABE) and the equatorial, 1°N–1°S wave guide maximum (95±6xa0mmol Cxa0m −2 xa0d −1 ) in the Pacific Ocean during the 1992 EqPac study. The 1995 SW Monsoon primary productivity was similar to the mean value observed in the same region in 1994 by the Arabesque Expedition (127±14xa0mmol Cxa0m −2 xa0d −1 ) and in 1964 by the ANTON BRUUN Expedition (115±27xa0mmol Cxa0m −2 xa0d −1 ). During the 1995 SW Monsoon, strong, narrow and meandering current filaments extended from the region of coastal upwelling to about 700xa0km offshore; these filaments had levels of biomass, primary productivity, chlorophyll-specific productivity and diatom abundance that were elevated relative to other locations during the SW Monsoon. The SW Monsoon was the most productive period, but SW Monsoon primary productivity values were lower than predicted because efficient grazing by mesozooplankton kept diatoms from accumulating the biomass necessary for achieving the high levels of primary productivity characteristic of other coastal upwelling regions. The high rates of chlorophyll-specific productivity ( P opt B >10xa0mmol C mg Chl −1 d −1 ) observed in the 1995 SW Monsoon, together with the observed dust flux and iron concentrations, indicate that the Arabian Sea was more iron replete than the equatorial Pacific Ocean or the Southern Ocean.

Biogeochemical regimes, net community production and carbon export in the Ross Sea, Antarctica

Abstract The net community production (NCP) of the Ross Sea, from the early austral spring (mid-October) to the austral summer (mid-February), has been estimated from the seasonal drawdown of CO 2 concentrations integrated over the top 100xa0m of the water column. The deficits in nutrients and CO 2 indicate three distinct biogeochemical regimes. The regime in the southwestern Ross Sea (Region I) had relatively shallow mixed layers and was dominated by diatom growth, as evidenced by a silicate (Def(Si)) to NCP removal ratio (0.11±0.04) that was similar to the silicate-to-carbon ratio found in diatoms growing in temperate regions. High NCP values (4.9–8.7xa0molxa0m −2 ) and low ratios of surplus total organic carbon (Surp(TOC)) to NCP (from 0.27 to 0.67) show high organic carbon export out of the upper 100xa0m of the water column. The second regime (Region II), located in the center of the southern Ross Sea polynya, also had high NCPs (4.4–10.8xa0molxa0m −2 ) but the mixed layers were deeper. The average Def(Si)/NCP ratio was 0.04±0.02, much lower than the southwestern sector and consistent with the observed growth of the haptophyte Phaeocystis antarctica . An increase in Def(Si) and the Def(Si)/NCP ratio during re-occupations of selected stations indicate the presence and persistence of diatom growth late into the summer. The third regime (Region III), in the northeastern Ross Sea, had shallow mixed layers and a wider range of Def(Si)/NCP ratios (0.10–0.31), indicating variations in silicate-to-carbon uptake by diatoms. The low NCPs (1.2–4.2xa0molxa0Cxa0m −2 ) that distinguished this area also may be due to micro-nutrient deficiencies in addition to prolonged ice coverage. NCP over the continental shelf of the Ross Sea (441,000xa0km 2 , defined by the 1000-m isopleth) is estimated to be 25±10xa0Tg of carbon per year, with a mean rate of 4.8±1.9xa0molxa0m −2 xa0yr. By mid-February, the productivity peak of the 1997 growing season had passed, and 19±7% of NCP remained in the upper 100xa0m as DOC, which presumably would not be exported but remineralized prior to the next growing season. During the same time period, 16±25% of NCP had already been removed from the upper 100xa0m as sinking biogenic particles, leaving 65% present in the POC fraction to be exported later or remineralized. High export of organic carbon (>50% of NCP) was shown in both diatom- and Phaeocystis -dominated regimes.

Linkages among runoff, dissolved organic carbon, and the stable oxygen isotope composition of seawater and other water mass indicators in the Arctic Ocean

[1]xa0Flow-weighted dissolved organic carbon (DOC) concentrations and δ18O values were determined from major arctic rivers, specifically the Ob, Yenisey, Lena, Kolyma, Mackenzie, and Yukon during 2003–2004. These data were considered in conjunction with marine data for DOC, δ18O values, nutrients, salinity, and fluorometric indicators of DOC obtained during sampling at the shelf-basin boundary of the Chukchi and Beaufort seas. On the basis of these data, freshwater in the sampled marine waters is likely derived from regional sources, such as the Mackenzie, the Bering Strait inflow, and possibly eastern Siberian rivers, including the Kolyma, or the Lena, but not rivers farther west in the Eurasian arctic. Freshwater from melted sea ice is insignificant over annual cycles, although melted sea ice was a locally dominant freshwater component following summer sea-ice retreat in 2002. DOC concentrations were correlated with the runoff fraction, with an apparent meteoric water DOC concentration of 174 ± 1 μM. This is lower than the flow-weighted concentrations measured at river mouths of the five largest Arctic rivers (358 to 917 μM), indicating removal of DOC during transport through estuaries, shelves and in the deep basin. Flow-weighted DOC concentrations in the two largest North American arctic rivers, the Yukon (625 μM) and the Mackenzie (358 μM), are lower than in the three largest Eurasian arctic rivers, the Ob (825 μM), the Yenesey (858 μM), and the Lena (917 μM). A fluorometer responding to chromophoric dissolved organic matter (CDOM) was not correlated with DOC concentrations in Pacific-influenced surface waters unlike previous observations in the Atlantic layer. Nutrient distributions, concentrations, and derived ratios suggest the CDOM fluorometer may be responding to the release of chromophoric materials from shelf sediments. Shipboard incubations of undisturbed sediment cores indicate that sediments on the Bering and Chukchi Sea shelves are a net source of DOC to the Arctic Ocean.

The US Southern Ocean Joint Global Ocean Flux Study: an introduction to AESOPS

Abstract The United States Southern Ocean Joint Global Ocean Flux Study (JGOFS), also known as AESOPS (Antarctic Environment and Southern Ocean Process Study), focused on two distinct regions. The first was the Ross-Sea continental shelf, where a series of six cruises collected a variety of data from October 1996 through February 1998. The second area was the southwest Pacific sector of the Southern Ocean, spanning the Antarctic Circumpolar Current (ACC) at ∼170°W. Data were collected within this region during five cruises from September 1996 through March 1998, as well as during selected transits between New Zealand and the Ross Sea. The first results of these cruses are described in this issue. The Ross-Sea investigation extensively sampled the area along 76°30′S to elucidate the temporal patterns and processes that contribute to making this one of the Antarctics most productive seas. Hydrographic distributions confirm that stratification is initiated early in October within the polynya, generating an environment that is favorable for phytoplankton growth. Significant spatial variations in mixed-layer depths, the timing of the onset of stratification, and the strength of the stratification existed throughout the growing season. Nutrient concentrations reflected phytoplankton uptake, and reached their seasonal minimal in early February. Chlorophyll concentrations were maximal in early January, whereas productivity was maximal in late November, which reflects the temporal uncoupling between growth and biomass accumulation in the region. Independent estimates of biogenic export suggest that majority of the flux occurred in late summer and was strongly uncoupled from phytoplankton growth. The ACC region exhibited seasonal changes that in some cases were greater than those observed in the Ross Sea. Sea ice covered much of the region south of the Polar Front in winter, and retreated rapidly in late spring and early summer. Mixed layers throughout the region shoaled in summer due to surface heating, while the addition of freshwater from melting sea ice enhanced stratification in the Seasonal Ice Zone, creating conditions favorable for phytoplankton growth. For example, silicic acid concentrations decreased from initial values as high as 65 to less than 2xa0μM within approximately 100xa0km (from 65.7 to 64.8°S). Fluorescence values, however, showed less than a two-fold variation over the same distance. The vertical flux of carbon in the Polar Front area is substantial, and marked variations in the composition of exported material exited over the region. The results provide a means whereby the controls of phytoplankton growth and organic matter flux and remineralization can be analyzed in great detail. Additional results of the AESOPS project are discussed.

Nutrient and carbon removal ratios and fluxes in the Ross Sea, Antarctica

Abstract Net community production (NCP) and nutrient deficits (Def( X )) were calculated using decreases in dissolved CO 2 and nutrient concentrations due to biological removal in the upper 200xa0m of the water column during four cruises in the Ross Sea, Antarctica along 76°30′S in 1996 and 1997. A comparison to excess dissolved and particulate organic carbon showed close agreement between surplus total organic carbon (TOC) and NCP during bloom initiation and productivity maximum; however, when TOC values had returned to low wintertime values NCP was still significantly above zero. This seasonal NCP, 3.9±1xa0mol Cxa0m −2 , must be equivalent to the particle export to depths greater than 200xa0m over the whole productive season. We estimate that the annual export was 55±22% of the seasonal maximum in NCP. The fraction of the seasonal maximum NCP that is exported through 200xa0m is significantly higher than that measured by moored sediment traps at a depth of 206xa0m. The removal of carbon, nitrate and phosphate (based on nutrient disappearance since early spring) and their ratios showed significant differences between regions dominated by diatoms and regions dominated by the haptophyte Phaeocystis antarctica . While the ΔC/ΔN removal ratio was similar (7.8±0.2 for diatoms and 7.2±0.1 for P. antarctica ), the ΔN/ΔP and ΔC/ΔP removal ratios for diatoms (10.1±0.3 and 80.5±2.3) were significantly smaller than those of P. antarctica (18.6±0.4 and 134.0±4.7). The similarity in ΔC/ΔN removal ratios of the two assemblages suggests that preferential uptake of phosphate by diatoms caused the dramatic differences in ΔC/ΔP and ΔN/ΔP removal ratios. In contrast to low ΔC/ΔP and ΔN/ΔP removal ratio in diatom-dominated areas early in the growing season, deficit N/P and C/P ratios in late autumn indicate that the elemental stochiometry of exported organic matter did not deviate significantly from traditional Redfield ratios. Changes in biologically utilized nutrient and carbon ratios over the course of the growing season indicated either a substantial remineralization of phosphate or a decrease in phosphate removal relative to carbon and total inorganic nitrogen over the bloom period. The species dependence in C/P ratios, and the relative constancy in the C/N ratios, makes N a better proxy of biological utilization of CO 2 .

Seasonal evolution of hydrographic properties in the Ross Sea, Antarctica, 1996–1997

Abstract This paper briefly describes hydrographic and chemical results from four seasonal process cruises in the Joint Global Ocean Flux Study (JGOFS) Antarctic Environment and Southern Ocean Process Study (AESOPS) in the Ross Sea. The data include temperature, salinity, oxygen, nutrients (NO 3 − , NO 2 − , NH 4 + , PO 4 −3 , Si(OH) 4 ), titration alkalinity (TA), and total inorganic CO 2 (TCO 2 ). In early spring (mid-October to early November 1996) ice cover was near 100%. The water column exhibited only small ranges of potential temperature, salinity, nutrients, TA and TCO 2 . These nearly uniform conditions observed during this cruise were used as initial conditions from which to evaluate seasonal changes in biogeochemical properties. Later in the spring (November/December) of the following year (1997), an expanded polynya was present. In the summer (January) 1997, the sea-ice cover was minimal. Meltwater dilution and warming of exposed surface waters were at their maximum. Finally, in early austral autumn (April 1997) rapid cooling and freezing of surface waters and intensified vertical mixing of the water column resulted in a return toward winter conditions, but with significant depletion of nutrients still evident in surface waters. Modified circumpolar deep water (MCDW) was observed at ∼175°E., varying in location zonally with time. Beneath the MCDW, near the eastern end of the section at ∼180°, northward intrusion of ice shelf water (ISW) was persistent though variable. Among the biogeochemical changes observed along this section were: (1) Significant ammonium accretion at about 100xa0m; (2) appreciable nitrate drawdowns throughout the upper 20–40xa0m of the water column; (3) silicic acid depletions during the summer in the surface waters, which were largest at the western end of the study area; (4) an increase in TA largely due to the loss of NO 3 − ; and (5) decreases in TCO 2 close to expected values for consumption of carbon and nitrogen in “Redfield” proportions. Consistent with previous observations, N, P and Si never approached limiting concentrations during any of the cruises.

Seasonal evolution of hydrographic properties in the Antarctic circumpolar current at 170°W during 1997-1998

Abstract This paper discusses the seasonal evolution of the hydrographic and biogeochemical properties in the Antarctic Circumpolar Current (ACC) during the US Joint Global Ocean Flux (JGOFS) Antarctic Environment and Southern Ocean Process Study (AESOPS) in 1997–1998. The location of the study region south of New Zealand along ∼170°W was selected based on the zonal orientation and meridional separation of the physical and chemical fronts found in that region. Here we endeavor to describe the seasonal changes of the macronutrients, fluorescence chlorophyll, particulate organic carbon (POC), and carbon dioxide (CO2) in the upper 400xa0m of the ACC during the evolution of the seasonal phytoplankton bloom found in this area. While the ACC has extreme variability in the meridional sense (due to fronts, etc.), it appears to be actually quite uniform in the zonal sense. This is reflected by the fact that a good deal of the seasonal zonal changes in nutrients distributions at 170°W follow a pattern that reflects what would be expected if the changes are associated with seasonal biological productivity. Also at 170°W, the productivity of the upper waters does not appear to be limited by availability of phosphate or nitrate. While there is a significant decrease (or uptake) of inorganic nitrogen, phosphate and silicate associated with the seasonal phytoplankton bloom, none of the nutrients, except perhaps silicate (north of the silicate front) are actually depleted within the euphotic zone. At the end of the growing season, nutrient concentrations rapidly approached their pre-bloom levels. Inspection of the ratios of apparent nutrient drawdown near 64°S suggests N/P apparent drawdowns to have a ratio of ∼10 and N/Si apparent drawdowns to have a ratio of >4. These ratios suggest a bloom that was dominated by Fe limited diatoms. In addition, the surface water in the Polar Front (PF) and the Antarctic Zone (AZ) just to the south of the PF take up atmospheric CO2 at a rate 2–3 times as fast as the mean global ocean rate during the summer season but nearly zero during the rest of year. This represents an important process for the transport of atmospheric CO2 into the deep ocean interior. Finally, the net CO2 utilization or the net community production during the 2.5 growing months between the initiation of phytoplankton blooms and mid-January increase southward from 1.5xa0mol Cxa0m−2 at 55°S to 2.2xa0mol Cxa0m−2 to 65°S across the Polar Frontal Zone (PFZ) into the AZ.

NITROGEN CYCLING IN THE SUBOXIC WATERS OF THE ARABIAN SEA

Arabian Sea contains one of the world’s three large oxygen deficient zones (ODZ). Within the ODZ oxygen concentration is vanishingly small between about 200 and 800 m depth and in this depth interval denitrification is the major mode or organic matter oxidation. This makes the Arabian Sea a globally important sink in the marine combined nitrogen cycle. Within the ODZ nitrate concentration profiles typically show a local minimum near 250 m and nitrate is depleted relative to phosphate. Stoichiometrically based estimates of the amount of nitrate removed from the water column, or the nitrate deficit, suggest maximum values of ∼12 μM. However, the amount of excess nitrogen gas, which is presumably a result of denitrification, has been estimated to be almost twice this value. Although the reason for this discrepancy between the nitrate deficit and the quantity of excess nitrogen gas remains unresolved, possible causes include nonRedfield stoichiometry for denitrification, contributions due to nitrogen fixation either in the Arabian Sea or in the ODZ source waters, anammox, or sedimentary denitrification. Nitrate deficit based estimates of overall denitrification rate are about 40 Tg N a−1, but if the larger excess nitrogen gas measurements are correct they suggest the rate could be correspondingly higher. Nitrate is enriched in the heavy N isotope within the denitrification zone due to denitrification. During upwelling this enriched N is brought into the euphotic zone where it is take up by phytoplankton. When remains of the phytoplankton sink and become incorporated into the sediment the resulting sediments are also enriched in the heavy isotope relative to other areas of the ocean. However, during glacial periods the N in the sediment was not enriched suggesting better ventilation of the ODZ waters and a lack of denitrification. Nitrous oxide, an important green-house gas and an intermediate in both denitrification and nitrification typically displays local maxima at the upper and lower boundaries of the ODZ, while it is found in very low concentrations within the ODZ. Low values in the ODZ are the result denitrification and this is evidenced by enrichment in the heavy isotopes of both N and O. Maxima at the boundaries of the ODZ may result from either

The Oceanic Nitrogen Cycle: A Double-Edged Agent of Environmental Change?

This chapter is based on a lecture delivered at a International Year of the Ocean meeting held at Simon Fraser University during November 1998. This meeting had a strong element of environmental concern and action and thus was a departure from the “pure” science meetings that I normally attend. The meeting reminded me of my younger days in the Pacific Northwest when I was active in the local Sierra Club Chapter. Overall, I was most impressed with the meeting, and I hope that I can do justice to the confidence that the meeting organizers placed in me by convincing you that the oceanic nitrogen cycle cannot be neglected when we consider environmental change. As is generally the case, wise use of our knowledge vis-a-vis this cycle could be helpful whereas misuse or neglect could be harmful.