Net Community Production Rates Research Articles

Temporal variability of nitrogen fixation and particulate nitrogen export at Station ALOHA

AbstractWe present nearly 9 yrs (June 2005–December 2013) of measurements of upper‐ocean (0 m to 125 m) dinitrogen (N2) fixation rates, coupled with particulate nitrogen (PN) export at 150 m, from Station ALOHA (22° 45′N, 158°W) in the North Pacific Subtropical Gyre. Between June 2005 and June 2012, N2 fixation rates were measured based on adding the 15N2 tracer as a gas bubble. Beginning in August 2012, 15N2 was first dissolved into filtered seawater and the 15N2‐enriched water was subsequently added to N2 fixation incubations. Direct comparisons between methodologies revealed a robust relationship, with the addition of 15N2‐enriched seawater resulting in twofold greater depth‐integrated rates than those derived from adding a 15N2 gas bubble. Based on this relationship, we corrected the initial period of measurements, and the resulting rates of N2 fixation averaged 230 ± 136 μmol N m−2 d−1 for the full time series (n = 71). Analysis of the 15N isotopic composition of sinking PN, together with an isotope mass balance model, revealed that N2 fixation supported 26–47% of PN export during calendar years 2006–2013. The N export derived from these fractional contributions and measured N2 fixation rates ranged between 502 and 919 μmol N m−2 d−1, which are equivalent to rates of net community production (NCP) of 1.5 to 2.7 mol C m−2 yr−1, consistent with previous independent estimates of NCP at this site.

Net community production at Ocean Station Papa observed with nitrate and oxygen sensors on profiling floats

AbstractSix profiling floats equipped with nitrate and oxygen sensors were deployed at Ocean Station P in the Gulf of Alaska. The resulting six calendar years and 10 float years of nitrate and oxygen data were used to determine an average annual cycle for net community production (NCP) in the top 35 m of the water column. NCP became positive in February as soon as the mixing activity in the surface layer began to weaken, but nearly 3 months before the traditionally defined mixed layer began to shoal from its winter time maximum. NCP displayed two maxima, one toward the end of May and another in August with a summertime minimum in June corresponding to the historical peak in mesozooplankton biomass. The average annual NCP was determined to be 1.5 ± 0.6 mol C m−2 yr−1 using nitrate and 1.5 ± 0.7 mol C m−2 yr−1 using oxygen. The results from oxygen data proved to be quite sensitive to the gas exchange model used as well as the accuracy of the oxygen measurement. Gas exchange models optimized for carbon dioxide flux generally ignore transport due to gas exchange through the injection of bubbles, and these models yield NCP values that are two to three time higher than the nitrate‐based estimates. If nitrate and oxygen NCP rates are assumed to be related by the Redfield model, we show that the oxygen gas exchange model can be optimized by tuning the exchange terms to reproduce the nitrate NCP annual cycle.

Dynamics of carbonate chemistry, production, and calcification of the Florida Reef Tract (2009–2010): Evidence for seasonal dissolution

AbstractOcean acidification is projected to lower the Ωar of reefal waters by 0.3–0.4 units by the end of century, making it more difficult for calcifying organisms to secrete calcium carbonate while at the same time making the environment more favorable for abiotic and biotic dissolution of the reefal framework. There is great interest in being able to project the point in time when coral reefs will cross the tipping point between being net depositional to net erosional in terms of their carbonate budgets. Periodic in situ assessments of the balance between carbonate production and dissolution that spans seasonal time scales may prove useful in monitoring and formulating projections of the impact of ocean acidification on reefal carbonate production. This study represents the first broad‐scale geochemical survey of the rates of net community production (NCP) and net community calcification (NCC) across the Florida Reef Tract (FRT). Surveys were performed at approximately quarterly intervals in 2009–2010 across seven onshore‐offshore transects spanning the upper, middle, and lower Florida Keys. Averaged across the FRT, the rates of NCP and NCC were positive during the spring/summer at 62 ± 7 and 17 ± 2 mmol m−2 d−1, respectively, and negative during the fall/winter at −33 ± 6 and −7 ± 2 mmol m−2 d−1. The most significant finding of the study was that the northernmost reef is already net erosional (−1.1 ± 0.4 kg CaCO3 m−2 yr−1) and midreefs to the south were net depositional on an annual basis (0.4 ± 0.1 kg CaCO3 m−2 yr−1) but erosional during the fall and winter. Only the two southernmost reefs were net depositional year‐round. These results indicate that parts of the FRT have already crossed the tipping point for carbonate production and other parts are getting close.

Assessing net community production in a glaciated Alaskan fjord

Abstract. The impact of deglaciation in Glacier Bay has been observed to seasonally influence the biogeochemistry of this marine system. The influence from surrounding glaciers, particularly tidewater glaciers, has the potential to affect the efficiency and structure of the marine food web within Glacier Bay. To assess the magnitude and the spatial and temporal variability in net community production in a glaciated fjord, we measured dissolved inorganic carbon, inorganic macronutrients, dissolved oxygen, and particulate organic carbon between July 2011 and July 2012 in Glacier Bay, Alaska. High net community production rates were observed across the bay (~ 54 to ~ 81 mmol C m−2 d−1) between the summer and fall of 2011. However, between the fall and winter, as well as between the winter and spring of 2012, air–sea fluxes of carbon dioxide and organic matter respiration made net community production rates negative across most of the bay as inorganic carbon and macronutrient concentrations returned to pre-bloom levels. The highest organic carbon production occurred within the west arm between the summer and fall of 2011 with ~ 4.5 × 105 kg C d−1. Bay-wide, there was carbon production of ~ 9.2 × 105 g C d−1 between the summer and fall. Respiration and air–sea gas exchange were the dominant drivers of carbon chemistry between the fall and winter of 2012. The substantial spatial and temporal variability in our net community production estimates may reflect glacial influences within the bay, as meltwater is depleted in macronutrients relative to marine waters entering from the Gulf of Alaska in the middle and lower parts of the bay. Further glacial retreat will likely lead to additional modifications in the carbon biogeochemistry of Glacier Bay, with unknown consequences for the local marine food web, which includes many species of marine mammals.

High-resolution view of the spring bloom initiation and net community production in the Subantarctic Southern Ocean using glider data

AbstractIn the Southern Ocean, there is increasing evidence that seasonal to subseasonal temporal scales, and meso- to submesoscales play an important role in understanding the sensitivity of ocean primary productivity to climate change. This drives the need for a high-resolution approach to resolving biogeochemical processes. In this study, 5.5 months of continuous, high-resolution (3 h, 2 km horizontal resolution) glider data from spring to summer in the Atlantic Subantarctic Zone is used to investigate: (i) the mechanisms that drive bloom initiation and high growth rates in the region and (ii) the seasonal evolution of water column production and respiration. Bloom initiation dates were analysed in the context of upper ocean boundary layer physics highlighting sensitivities of different bloom detection methods to different environmental processes. Model results show that in early spring (September to mid-November) increased rates of net community production (NCP) are strongly affected by meso- to submesoscale features. In late spring/early summer (late-November to mid-December) seasonal shoaling of the mixed layer drives a more spatially homogenous bloom with maximum rates of NCP and chlorophyll biomass. A comparison of biomass accumulation rates with a study in the North Atlantic highlights the sensitivity of phytoplankton growth to fine-scale dynamics and emphasizes the need to sample the ocean at high resolution to accurately resolve phytoplankton phenology and improve our ability to estimate the sensitivity of the biological carbon pump to climate change.

Dissolved oxygen dynamics during a phytoplankton bloom in the Ross Sea polynya

AbstractThe Ross Sea polynya is one of the most productive regions in the Southern Ocean. However, limited access and high spatio-temporal variability of physical and biological processes limit the use of conventional oceanographic methods to measure early season primary productivity. High-resolution observations from two Seagliders provide insights into the timing of a bloom in the southern Ross Sea polynya in December 2010. Changes in chlorophyll and oxygen concentrations are used to assess bloom dynamics. Using a ratio of dissolved oxygen to carbon, net primary production is estimated over the duration of the bloom showing a sensitive balance between net autotrophy and heterotrophy. The two gliders, observing spatially distinct regions during the same period, found net community production rates of -0.9±0.7 and 0.7±0.4 g C m-2 d-1. The difference highlights the spatial variability of biological processes and is probably caused by observing different stages of the bloom. The challenge of obtaining accurate primary productivity estimates highlights the need for increased observational efforts, particularly focusing on subsurface processes not resolved using surface or remote observations. Without an increased observational effort and the involvement of emerging technologies, it will not be possible to determine the seasonal trophic balance of the Ross Sea polynya and quantify the shelf’s importance in carbon export.

Estimates of net community production in the Southern Ocean determined from time series observations (2002–2011) of nutrients, dissolved inorganic carbon, and surface ocean pCO2 in Drake Passage

In remote regions such as the open Southern Ocean, satellite observations often provide the only available tool with which to evaluate large-scale biogeochemical processes. However, these observations need to be carefully evaluated with in situ measurements. With an average of 20 crossings per year from 2002 to 2011, the Drake Passage Time-series (DPT) represents one of the most complete datasets of biogeochemical measurements in the open Southern Ocean. This dataset offers a unique opportunity to validate satellite-based productivity algorithms and to improve understanding of the role of this region in the global carbon cycle. Net community production (NCP) was estimated using discrete measurements of total dissolved inorganic carbon (TCO2) and phosphate (PO43−), and high-frequency underway measurements of the partial pressure of carbon dioxide in the surface ocean (pCO2surf) from the DPT, combined with estimates of gas exchange, Ekman transport, wind stress curl, and vertical entrainment. We estimate annual NCP using seasonal PO43− (NCPPO43−) and TCO2 (NCPTCO2) budgets of 1.2±0.7 and 1.6±0.4molCm−2yr−1, respectively. Budget terms for gas exchange, entrainment, and advective supply indicate that a closed system seasonal-drawdown approach that does not consider additional terms may underestimate NCP in this region by nearly 35%. NCP estimates are compared to satellite algorithms commonly used to estimate both net primary production (NPP) and organic carbon export. Budget-based NCP approaches indicate high rates of NCP during austral spring with little additional NCP over austral summer. In contrast, satellite approaches suggest a more gradual increase and decline in NCP rates over the growing season with approximately 40% of NCP accumulating during austral summer.

Environmental and ecological controls of coral community metabolism on Palmyra Atoll

Accurate predictions of how coral reefs may respond to global climate change hinge on understanding the natural variability to which these ecosystems are exposed and to which they contribute. We present high-resolution estimates of net community calcification (NCC) and net community production (NCP) from Palmyra Atoll, an uninhabited, near-pristine coral reef ecosystem in the central Pacific. In August–October 2012, we employed a combination of Lagrangian and Eulerian frameworks to establish high spatial (~2.5 km2) and temporal (hourly) resolution coral community metabolic estimates. Lagrangian drifts, all conducted during daylight hours, resulted in NCC estimates of −51 to 116 mmol C m−2 h−1, although most NCC estimates were in the range of 0–40 mmol C m−2 h−1. Lagrangian drift NCP estimates ranged from −7 to 67 mmol C m−2 h−1. In the Eulerian setup, we present carbonate system parameters (dissolved inorganic carbon, total alkalinity, pH, and pCO2) at sub-hourly resolution through several day–night cycles and provide hourly NCC and NCP rate estimates. We compared diel cycles of all four carbonate system parameters to the offshore surface water (0–50 m depth) and show large departures from offshore surface water chemistry. Hourly Eulerian estimates of NCC aggregated over the entire study ranged from 14 to 53 mmol C m−2 h−1, showed substantial variability during daylight hours, and exhibited a diel cycle with elevated NCC in the afternoons and depressed, but positive, NCC at night. The Eulerian NCP range was very high (−55 to 177 mmol C m−2 h−1) and exhibited strong variability during daylight hours. Principal components analysis revealed that NCC and NCP were most closely aligned with diel cycle forcing, whereas the NCC/NCP ratio was most closely aligned with reef community composition. Our analysis demonstrates that ecological community composition is the primary determinant of coral reef biogeochemistry on a near-pristine reef and that reef biogeochemistry is likely to be responsive to human behaviors that alter community composition.

Net community production and export from Seaglider measurements in the North Atlantic after the spring bloom

AbstractMean rates of net community production (NCP) and particulate organic carbon (POC) export were estimated from sensor measurements of dissolved oxygen (O2), chlorophyll fluorescence (chl F), and particulate backscatter (bbp700) collected from three Seagliders that surveyed a 20 × 20 km area in the North Atlantic subsequent to a large diatom bloom. Since the Seagliders sampled geographically fixed patterns, care was taken in the calculation of all terms applicable to the Eulerian reference frame, including local rate of change, vertical mixing, air‐sea exchange, and horizontal advection. Although similar studies of NCP in the open ocean have generally assumed advection to be insignificant, we have found that this term cannot be ignored when dealing with temporal scales of ≤1 month and/or spatial scales ≤20 km. The overlapping sampling pattern of the Seagliders was sufficiently rapid such that 4–5 day time scales observed in the O2 and POC data were adequately resolved and variations were not a consequence of aliasing spatial variability. During the study period, ratios of chlorophyll fluorescence‐to‐particulate backscatter (chl:bbp700) were lower than values encountered during the spring diatom bloom, suggesting the phytoplankton community was predominantly composed of smaller cells (picoplankton and nanoplankton) and/or coccolithophorids. Coupled budgets of oxygen and POC indicated a net community production of 1.0 mol C m−2 and carbon export of 0.6 mol C m−2, respectively, over a period of 23 days. Thus, the production and export of carbon that occurred over the month‐long experiment period was comparable to that encountered during the spring bloom.

Rates of summertime biological productivity in the Beaufort Gyre: A comparison between the low and record-low ice conditions of August 2011 and 2012

The Arctic Ocean is changing rapidly as the global climate warms but it is not well known how these changes are affecting biological productivity and the carbon cycle. Here we study the Beaufort Gyre region of the Canada Basin in August and use the large reduction in summertime sea ice extent from 2011 to 2012 to investigate potential impacts of climate warming on biological productivity. We use the gas tracers O2/Ar and triple oxygen isotopes to quantify rates of net community production (NCP) and gross oxygen production (GOP) in the gyre. Comparison of the summer of 2011 with the summer of 2012, the latter of which had record low sea ice coverage, is relevant to how biological productivity might change in a seasonally ice-free Arctic Ocean. We find that, in the surface waters measured here, GOP in 2012 is significantly greater than in 2011, with the mean basin-wide 2012 GOP=38±3mmolO2m−2d−1 whereas in 2011, mean basin GOP=16±5mmolO2m−2d−1. We hypothesize that this is because the lack of sea ice and consequent increase in light penetration allows photosynthesis to increase in 2012. However, despite the increase in GOP, NCP is the same in the two years; mean NCP in 2012 is 3.0±0.2mmolO2m−2y−1 and in 2011 is 3.1±0.2mmolO2m−2y−1. This suggests that the heterotrophic community (zooplankton and/or bacteria) increased its activity as well and thus respired the additional carbon produced by the increased photosynthetic production. In both years, stations on the shelf had GOP 3 to 5 times and NCP 2 to 10 times larger than the basin stations. Additionally, we show that in 2011, the NCP/GOP ratio is smallest in regions with highest ice cover, suggesting that the microbial loop was more efficient at recycling carbon in regions where the ice was just starting to melt. These results highlight that although satellite chlorophyll records show, and many models predict, an increase in summertime primary production in the Arctic Basin as it warms, the net amount of carbon processed by the biological pump during summer may not change as a function of ice cover. Thus, a rapid reduction in summertime ice extent may not change the net community productivity or carbon balance in the Beaufort Gyre.

Marginal impacts of rising temperature on Arctic benthic microalgae production based on in situ measurements and modelled estimates

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 501:25-40 (2014) - DOI: https://doi.org/10.3354/meps10688 Marginal impacts of rising temperature on Arctic benthic microalgae production based on in situ measurements and modelled estimates Jana Woelfel1,2,*, Anja Eggert3, Ulf Karsten2 1Leibniz Institute for Baltic Sea Research, Department of Marine Chemistry, Trace Gases, 18119 Rostock, Germany 2University of Rostock, Institute of Biological Sciences, Applied Ecology and Phycology, 18059 Rostock, Germany 3Leibniz Institute for Baltic Sea Research, Department of Physical Oceanography and Instrumentation, Theoretical Oceanography and Numerical Modelling, 18119 Rostock, Germany *Corresponding author: jana.woelfel@io-warnemuende.de ABSTRACT: Arctic microphytobenthic in situ net community production (NCP) was measured via the oxygen exchange rate in benthic chambers equipped with optical sensor spots at 3 representative sandy sites in Kongsfjorden (Svalbard, 79° N, 12° E) at 3 to 11 m water depth during June 2008. No significant differences were detected between stations and depths in either photoautotrophic biomass or primary production. All sites showed low but variable rates of NCP, ranging from -0.3 to +0.6 mmol O2 m-2 h-1 (-9 to +18 mg O2 m-2 h-1). The numerical model of Walsby (1997) was applied to estimate seasonal and regional rates of nearshore NCP for 2008 using in situ P/E curve parameters, solar global radiation, wind data and satellite-derived sea surface temperatures. The highest daily NCP rates of 15 to 18 mmol O2 m-2 d-1 were found throughout the Arctic spring and summer seasons (1 April until 31 August 2008) at shallow depths <15 m. The differences between modelled stratified and fully mixed water column conditions were small and in the range of 10%. Depth- and time-integrated NCP of the entire Kongsfjorden coastline during June 2008 yielded 9 t of O2. A predicted increase in the sea surface temperature by 2°C would have only a marginal impact on the NCP (<3%). Our model sensitivity analysis of the uncertainties associated with P/E curve parameters and Q10 values clearly showed a crucial dependence of NCP on the Q10 values. Thus, further studies on temperature dependence are required. KEY WORDS: Arctic benthic diatoms · Respiration and net production · Biomass · Sediment characteristics · Benthic chambers · Kongsfjorden Full text in pdf format PreviousNextCite this article as: Woelfel J, Eggert A, Karsten U (2014) Marginal impacts of rising temperature on Arctic benthic microalgae production based on in situ measurements and modelled estimates. Mar Ecol Prog Ser 501:25-40. https://doi.org/10.3354/meps10688 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 501. Online publication date: March 31, 2014 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2014 Inter-Research.

High‐resolution carbon budgets on a Palau back‐reef modulated by interactions between hydrodynamics and reef metabolism

Diel trends in carbon chemistry and hydrodynamic regimes were measured on a coral‐dominated back‐reef community in the Republic of Palau, western Pacific. Observed diel ranges over 5 d were pH 7.92–8.09, partial pressure of carbon dioxide 33.4–52.7 Pa, measured total alkalinity 2080–2272 µmol kg−1, total dissolved inorganic carbon 1763–1939 µmol kg−1, aragonite saturation state 2.84–3.98, and calcite saturation state 4.44–5.67. The combination of reef metabolism and hydrodynamic regimes drives this variability. We report net community calcification (NCC) and net community production (NCP) rates of −20.4 (± 7.8) to 41.2 (± 37.7) mmol CaCO3 m−2 h−1 and 77.3 (± 95) to 64 (± 22) mmol C m−2 h−1, respectively. Using a control volume approach, we show that NCC and NCP rate estimates are quite sensitive to the effective control volume dimensions. At this site, tracking vertical mixing of water masses within the control volume shows that active water mixing is often confined to a region 2–3 m above the corals. Vertical carbon mass fluxes into and out of the benthic environment were unexpectedly large for the slow current flow rates observed, suggesting that coral morphology or reef rugosity may affect mass transport of carbon and nutrients. The ranges of NCC and NCP values highlight the need for additional field work on various reefs to tune and optimize the control volume approach to achieve its full potential for reef metabolism monitoring.

The influence of net community production and phytoplankton community structure on CO2 uptake in the Gulf of Alaska

AbstractBiological productivity is a key factor controlling the ocean's ability to take up carbon dioxide from the atmosphere. However, the ecological dynamics that drive regions of intense productivity and carbon export are poorly understood. In this study, we present high‐spatial‐resolution estimates of air‐sea CO2 flux, net community production (NCP) rates calculated from O2/Ar ratios, and phytoplankton population abundances determined by continuous underway measurements on a cruise across the Gulf of Alaska in May 2010. The highest rates of NCP (79.2 ± 13 mmol C m‐2 d‐1) and oceanic CO2 uptake (air‐sea flux of −14.7 ± 2.4 mmol C m‐2 d‐1) were observed across a transition zone between the high‐nitrate low‐chlorophyll (HNLC) waters of the Alaskan Gyre and the coastal waters off the Aleutian Islands. While the transition zone comprises 20% of the total area covered in crossing the Gulf of Alaska, it contributed 49% of the total NCP and 48% of the total CO2 uptake observed along the cruise track. A corresponding transition zone phytoplankton bloom was dominated by two small‐celled (&lt;20 µm) phytoplankton communities, which were distinct from the phytoplankton communities in the surrounding Alaskan Gyre and coastal waters. We hypothesize that mixing between iron‐rich coastal waters and iron‐limited Alaskan Gyre waters stimulated this bloom and fueled the high NCP and CO2 export observed in the region.

Global net community production and the putative net heterotrophy of the oligotrophic oceans

Reconciling rates of organic carbon export from the euphotic zone with the consumption of organic material in the dark ocean remains one of the major quantitative uncertainties of the ocean carbon cycle. Euphotic zone net community production (NCP) provides one broad constraint on export flux and potential carbon drawdown. However, in vitro measurements of NCP consistently suggest that oligotrophic oceans are net heterotrophic, which is inconsistent with evidence of their carbon export to depth. Further, we have been unable to identify organic inputs on a scale to supplement the purported net heterotrophy. Here, we calculate global NCP rates using empirical relationships between in vitrophotosynthesis (P) and respiration (R) and a satellite‐based productivity model. A low value for global NCP (∼139 ± 325 Tmol C a−1) is found when a single P versus R (PvR) relation is derived from all in vitro data, with areas of net heterotrophy occupying 52% of the surface ocean. If a set of PvR relationships are instead derived by segregating the in vitro data into broad latitudinal zones associated with differing nutrient dynamics, we find a global NCP distribution in better agreement with independent model estimates of particulate carbon export, except in the 10°–40° latitudinal band where negative NCP values remain. Consistency between NCP and particulate export across all latitudes is achieved by applying a single PvR relationship derived using all in vitro data collected outside the 10°–40° latitudinal band. With this model, global NCP is estimated at ∼781 ± 393 Tmol C a−1and modeled values at well‐characterized field sites are in good agreement with non‐incubation based in situ measurements. We infer from our results thatin vitro NCP data from oligotrophic sites are too low, and suggest that this error is more likely the result of underestimated photosynthesis than overestimated respiration, although the precise physiological nature of the problem remains to be demonstrated.

High‐resolution estimates of net community production and air‐sea CO2 flux in the northeast Pacific

Rates of net community production (NCP) and air‐sea CO2 flux in the Northeast Pacific subarctic, transition zone and subtropical regions (22°N–50°N, 145°W–152°W) were determined on a cruise in August–September 2008 by continuous measurement of surface values of the ratio of dissolved oxygen to argon (O2/Ar) and the partial pressure of CO2 (pCO2). These estimates were compared with simultaneous measurements of sea surface temperature (SST), chlorophyll‐a (chl‐a), flow cytometry, and discrete surface nutrient concentrations. NCP and CO2 influx were greatest in the subarctic (45°N–50°N, 25.8 ± 4.6 and 4.1 ± 0.9 mmol C m−2 d−1) and northern transition zone (40°N–45°N, 17.1 ± 4.4 and 2.1 ± 0.5 mmol C m−2 d−1), with mean NCP ∼6–8× greater than mean CO2 invasion (error estimates reflect 1 σ confidence intervals). Contrastingly, the southern transition zone (32°N–40°N) and subtropics (22°N–32°N) had lower mean NCP (5.4 ± 1.8 and 8.1 ± 2.1 mmol C m−2 d−1, respectively) and mean CO2 efflux (3.0 ± 0.5 and 0.1 ± 0.0 mmol C m−2 d−1, respectively). In the subarctic and transition zone, NCP was highly correlated with surface chl‐a and CO2 influx, indicating strong coupling between the biological pump and CO2 uptake. Meridional trends in our NCP estimates in the transition zone and subtropics were similar to those for integrated summertime NCP along the cruise track determined using an upper ocean climatological carbon budget.

Primary production and carbon export rates across the subpolar N. Atlantic Ocean basin based on triple oxygen isotope and dissolved O2 and Ar gas measurements

Gross photosynthetic O2 production (GOP) rates in the subpolar North Atlantic Ocean were estimated using the measured isotopic composition of dissolved oxygen in the surface layer on samples collected on nine transits of a container ship between Great Britain and Canada during March 2007 to June 2008. The mean basin‐wide GOP rate of 226 ± 48 mmol O2 m−2 d−1 during summer was double the winter rate of 107 ± 41 mmol O2 m−2 d−1. Converting these GOP rates to equivalent 14C‐based PP (14C‐PPeqv) yielded rates of 1005 ± 216 and 476 ± 183 mg C m−2 d−1 in summer and winter, respectively, that generally agreed well with previous 14C‐based PP estimates in the region. The 14C‐PPeqv estimates were 1–1.6× concurrent satellite‐based PP estimates along the cruise track. A net community production rate (NCP) of 87 ± 12 mmol O2 m−2 d−1 (62 ± 9 mmol C m−2 d−1) and NCP/GOP of 0.35 ± 0.06 in the mixed layer was estimated from O2/Ar and 17Δ measurements (61°N 26°W) during spring bloom conditions in May 2008. Contrastingly, a much lower long‐term annual mean NCP or organic carbon export rate of 2.8 ± 2.7 mol C m−2 yr−1 (8 ± 7 mmol C m−2 d−1) and NCP/GOP of 0.07 ± 0.06 at the winter mixed layer depth was estimated from 15 years of surface O2 data in the subpolar N. Atlantic collected during the CARINA program.

Experimental assessment of the effect of UVB radiation on plankton community metabolism along the Southeastern Pacific off Chile

Abstract. The potential effects of UV on community metabolism (NCP, GPP and R) were assessed along the Southeast Pacific off the Chilean coast during the Humbold-2009 cruise (54.80° S–23.85° S) on board R/V Hespérides from 5 to 15 March 2009. Estimates of community metabolism were performed at eight stations, including three stations on Patagonian fjords and five stations on the Humboldt Current System. The effect of UVB radiation on net community production (NCP) was evaluated at the stations in the Humboldt Current system by comparing metabolic rates derived using quartz bottles, largely transparent to UVB, and borosilicate glass, which is opaque to UVB and part of UVA, incubated under the ambient solar radiation. Autotrophic planktonic communities with variable NCP prevailed along the area, with the highest NCP rates (7.1–11.1 mmol O2 m−3 d−1) observed in the Patagonian fjords and the northernmost station. All five experiments showed significantly different NCP rates between communities incubated under the full ambient radiation and those incubated under reduced UVB. One of the experiments showed elevated NCP when the community was exposed to the full solar radiation, while four experiments showed a significantly lower NCP in the presence of UVB. These results suggest that the intense UVB radiation in this area, partly inhibits NCP in the Southwest Pacific off Chile.

Hydrographic controls on net community production and total organic carbon distributions in the eastern Bering Sea

In order to assess spatial and temporal variability of net community production (NCP) in shelf areas of the eastern Bering Sea, seawater samples for dissolved inorganic carbon (DIC) and total organic carbon (TOC) were collected during BEST-BSIERP cruises in the spring, summer, and fall of 2009 and compared to prior measurements made in 2008. DIC and TOC data were used to estimate seasonal changes in rates of NCP and the balance of net autotrophy versus heterotrophy in different shelf areas. In 2009, springtime surface layer DIC concentrations were generally uniform across the shelf and averaged ∼2100μmolkg−1, although concentrations in northern shelf areas (under sea-ice cover) were slightly higher (∼2130μmolkg−1). Subsequently, surface layer DIC (∼1950μmolkg−1) decreased significantly by summertime with the largest drawdown of DIC observed in the Middle Domain between 57° and 61°N. In this area, high NCP rates of up to 92mmolCm−2d−1 were observed and were higher than those reported in 2008. Comparing 2008 and 2009, the shelfwide average drawdown of DIC in the upper 30m between spring and summer was greater by ∼16μmolkg−1. In both spring and summer of 2008 and 2009, concentrations of TOC generally decreased from the coast. TOC concentrations were tightly coupled to salinity, particularly in spring, and largely influenced by the discharge of the Yukon and Kuskokwim Rivers. TOC accumulation between spring and summer was relatively small. In nearshore regions of the shelf, negative rates of NCP observed in 2009 were indicative of net heterotrophy with remineralization of labile organic carbon from rivers likely contributing to the observed net respiration signal in this region. In contrast, net heterotrophy was not observed in 2008, when river discharge rates were 30% lower (likely with lower river transport of TOC). While 2009 rates of production were higher outside the coastal domain than those observed in 2008, integrated annual production over the shelf was fairly comparable between the two years (2008: 103TgCyr−1; 2009: 97.2TgCyr−1). DOC accumulation in the surface layer was also equivalent between the two years (∼12μmolkg−1), and in both years shelfwide export production was estimated to be ∼75% of total NCP.

Carbon dynamics and community production in the Mississippi River plume

Dissolved inorganic carbon (DIC), total alkalinity (TAlk), pH, and dissolved oxygen (DO) were determined in the Mississippi River plume during five cruises conducted in the spring, summer, and fall. In contrast to many other large rivers, both DIC and TAlk were higher in river water than in seawater. Substantial losses of DIC, relative to TAlk, occurred within the plume, particularly at intermediate salinities. DIC removal was accompanied by high DO, high pH, and nutrient depletion, and was attributed to high phytoplankton production. As a result, the carbonate saturation in the plume became much higher than in ocean and river waters. A mixing model was used to determine DIC removal. We provide evidence that the use of a two—end‐member (river and ocean) mixing model was valid during late summer and fall (low discharge period). However, for other periods we used salinity and TAlk to delineate a mixing model that included two river end members and an ocean end member. Net community production rates in the plume, estimated using a box model, peaked in the summer and were among the highest reported to date for large river plumes. In the summer and fall, biological production in the river plume consumed a majority of the available nutrients, whereas during the spring only a small fraction of the available nutrients were consumed in the plume. Biological production was the dominant process influencing pH and carbonate saturation state along the river—ocean gradient, whereas physicochemical dynamics of mixing played an important role in controlling the TAlk and DIC distributions of this large river plume.

Vertical structure, seasonal drawdown, and net community production in the Ross Sea, Antarctica

[1] We calculate net community production (NCP) during summer 2005–2006 and spring 2006 in the Ross Sea using multiple approaches to determine the magnitude and consistency of rates. Water column carbon and nutrient inventories and surface ocean O2/Ar data are compared to satellite-derived primary productivity (PP) estimates and 14C uptake experiments. In spring, NCP was related to stratification proximal to upper ocean fronts. In summer, the most intense C drawdown was in shallow mixed layers affected by ice melt; depth-integrated C drawdown, however, increased with mixing depth. ΔO2/Ar-based methods, relying on gas exchange reconstructions, underestimate NCP due to seasonal variations in surface ΔO2/Ar and NCP rates. Mixed layer ΔO2/Ar requires approximately 60 days to reach steady state, starting from early spring. Additionally, cold temperatures prolong the sensitivity of gas exchange reconstructions to past NCP variability. Complex vertical structure, in addition to the seasonal cycle, affects interpretations of surface-based observations, including those made from satellites. During both spring and summer, substantial fractions of NCP were below the mixed layer. Satellite-derived estimates tended to overestimate PP relative to 14C-based estimates, most severely in locations of stronger upper water column stratification. Biases notwithstanding, NCP-PP comparisons indicated that community respiration was of similar magnitude to NCP. We observed that a substantial portion of NCP remained as suspended particulate matter in the upper water column, demonstrating a lag between production and export. Resolving the dynamic physical processes that structure variance in NCP and its fate will enhance the understanding of the carbon cycling in highly productive Antarctic environments.