Variability Of Net Community Production Research Articles

Exploring Five Methods for Estimating Net Community Production on the Siberian Continental Shelf and Slope of the Arctic Ocean

The loss of sea ice and changes to vertical stratification in the Arctic Ocean are altering the availability of light and nutrients, with significant consequences for net community production (NCP) and carbon export. However, a general lack of quality data, particular during winter months, inhibits our ability to quantify such change. As a result, two parameters necessary for calculating annual NCP, integration depth (Zint) and pre-bloom nitrate concentration (Npre), are often either assigned or estimated from summer measurements. Vertical profiles of temperature, salinity, nitrate, and dissolved oxygen were collected during three cruises conducted between August and October of 2013, 2015, and 2018 in a data-sparse region of the Arctic Ocean along the Siberian continental slope. Estimates of NCP were calculated from these data using five different methods that either assigned constant values for Zint and/or Npre or estimated these parameters from summer observations. The five methods returned similar mean values of Zint (44–54 m), Npre (5.4–5.7 mmol m–3), and NCP (12–16 g C m–2) across the study region; however, there was considerable variability among stations/profiles. It was determined that the NCP calculations were particularly sensitive to Npre. Despite this sensitivity, mean NCP estimates calculated along four transects re-occupied during the three cruises generally agreed across the five methods with two important exceptions. First, methods with pre-assigned Zint and/or Npre underestimated the NCP when the nitracline shoaled in the Laptev Sea and when high-nutrient shelf waters were advected northward from the East Siberian Sea shelf in 2015. In contrast, the methods that directly estimated both Zint and Npre did not suffer from this bias. These results suggest that assignment of Npre and/or Zint provides reasonable estimates of NCP, particularly averaged over larger spatial scales and/or longer time scales, but these approaches are not suitable for evaluating interannual variability in NCP, particularly in dynamic regions. Combining all methods across the three cruise years indicates NCP in the Laptev Sea and Lomonosov Ridge areas (10–11 g C m–2) was slightly lower than that north of Severnaya Zemlya (13 g C m–2) and in the East Siberian Sea (16 g C m–2).

Estimation of ocean export production ratios based on mixed layer depth and satellite chlorophyll observations

We introduced a simple methodology to determine basin-wide marine export production ratios during spring phytoplankton bloom periods by combining monthly varying mixed layer depths (MLD) derived from observed temperature depth profiles, satellite time series of chlorophyll-a (Chl-a) concentrations, and hypothetical critical depth. The MLD-Chl-a based approach yielded monthly export ratios reflecting seasonal variability of net community production in correlation with the MLD variations. Regional averages of export ratio in the spring for the southern part of the East Sea (36–39°N, 130–134°E), North Pacific Ocean (42–45°N, 151–155°E), and North Atlantic Ocean (57–60°N, 17–21°W) are 0.44 ± 0.22, 0.42 ± 0.16, and 0.73 ± 0.29, respectively, which compare favorably with prior field data. This suggests that the approach enables estimation of export production ratios with high spatial and temporal resolutions, complementing the methods that rely on extrapolation of limited field measurements to larger space and time scales.

Ocean Circulation Causes Strong Variability in the Mid‐Atlantic Bight Nitrogen Budget

AbstractUnderstanding of nitrogen cycling on continental shelves, a critical component of global nutrient cycling, is hampered by limited observations compared to the strong variability on a wide range of time and space scales. Numerical models have the potential to partially alleviate this issue by filling spatiotemporal data gaps and hence resolving annual area‐integrated nutrient fluxes. In this study, a three‐dimensional biogeochemical‐circulation model was implemented to simulate the Mid‐Atlantic Bight (MAB) nitrogen budget. Model results demonstrate that, on average, MAB net community production (NCP) was positive (+0.27 Tg N/year), indicating net autotrophy. Interannual variability in NCP was strong, with annual values ranging between 0.19 and 0.41 Tg N/year. Along‐shelf and across‐shelf horizontal transport fluxes were the other dominant terms in the nitrogen budget and were primarily responsible for this NCP variability. The along‐shelf current transported nitrogen from the north (0.65 Tg N/year) into the MAB, supplementing the nitrogen entering from terrestrial inputs (0.27 Tg N/year). However, NCP was highest in the year when total water volume transport and inorganic nitrogen input was strongest across the continental slope in the southern MAB, rather than when terrestrial inputs were greatest. Interannual variability of NCP appears to be linked to changes in the positions of the Gulf Stream and Slope Water Gyre. Overall, the strong spatiotemporal variability of the nitrogen fluxes highlights the importance of observations throughout all seasons and multiple years in order to accurately resolve the current status and future changes of the MAB nitrogen budget.

Interannual variability in net community production at the Western Antarctic Peninsula region (1997–2014)

AbstractIn this study, we examined the interannual variability of net community production (NCP) in the Western Antarctic Peninsula (WAP) using in situ O2/Ar‐NCP estimates (2008–2014) and satellite data (SeaWiFS and MODIS‐Aqua) from 1997 to 2014. We found that NCP generally first peaks offshore and follows sea‐ice retreat from offshore to inshore. Annually integrated NCP (ANCP) displays an onshore‐to‐offshore gradient, with coastal and shelf regions up to 8 times more productive than offshore regions. We examined potential drivers of interannual variability in the ANCP using an Empirical Orthogonal Function (EOF) analysis. The EOF's first mode explains ∼50% of the variance, with high interannual variability observed seaward of the shelf break. The first principal component is significantly correlated with the day of sea‐ice retreat (R = −0.58,p < 0.05), as well as the Southern Annular Mode (SAM) and El Niño Southern Oscillation (ENSO) climate indices in austral spring. Although the most obvious pathway by which the day of sea‐ice retreat influences NCP is by controlling light availability early in the growing season, we found that the effect of day of sea‐ice retreat on NCP persists throughout the growing season, suggesting that additional controls, such as iron availability, are preconditioned or correlated to the day of sea‐ice retreat.

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.

Satellite estimates of net community production indicate predominance of net autotrophy in the Atlantic Ocean

There is ongoing debate as to whether the oligotrophic ocean is predominantly net autotrophic and acts as a CO2 sink, or net heterotrophic and therefore acts as a CO2 source to the atmosphere. This quantification is challenging, both spatially and temporally, due to the sparseness of measurements. There has been a concerted effort to derive accurate estimates of phytoplankton photosynthesis and primary production from satellite data to fill these gaps; however there have been few satellite estimates of net community production (NCP). In this paper, we compare a number of empirical approaches to estimate NCP from satellite data with in vitro measurements of changes in dissolved O2 concentration at 295 stations in the N and S Atlantic Ocean (including the Antarctic), Greenland and Mediterranean Seas. Algorithms based on power laws between NCP and particulate organic carbon production (POC) derived from 14C uptake tend to overestimate NCP at negative values and underestimate at positive values. An algorithm that includes sea surface temperature (SST) in the power function of NCP and 14C POC has the lowest bias and root-mean square error compared with in vitro measured NCP and is the most accurate algorithm for the Atlantic Ocean. Nearly a 13year time series of NCP was generated using this algorithm with SeaWiFS data to assess changes over time in different regions and in relation to climate variability. The North Atlantic subtropical and tropical Gyres (NATL) were predominantly net autotrophic from 1998 to 2010 except for boreal autumn/winter, suggesting that the northern hemisphere has remained a net sink for CO2 during this period. The South Atlantic sub-tropical Gyre (SATL) fluctuated from being net autotrophic in austral spring-summer, to net heterotrophic in austral autumn–winter. Recent decadal trends suggest that the SATL is becoming more of a CO2 source. Over the Atlantic basin, the percentage of satellite pixels with negative NCP was 27%, with the largest contributions from the NATL and SATL during boreal and austral autumn–winter, respectively. Variations in NCP in the northern and southern hemispheres were correlated with climate indices. Negative correlations between NCP and the multivariate ENSO index (MEI) occurred in the SATL, which explained up to 60% of the variability in NCP. Similarly there was a negative correlation between NCP and the North Atlantic Oscillation (NAO) in the Southern Sub-Tropical Convergence Zone (SSTC), which explained 90% of the variability. There were also positive correlations with NAO in the Canary Current Coastal Upwelling (CNRY) and Western Tropical Atlantic (WTRA) which explained 80% and 60% of the variability in each province, respectively. MEI and NAO seem to play a role in modifying phases of net autotrophy and heterotrophy in the Atlantic Ocean.

Spatial and temporal variation of net community production and its regulating factors in the Amundsen Sea, Antarctica

We observed ΔO2/Ar in the surface waters of the Amundsen Sea, Antarctica, during the austral summers in 2011 and 2012 to investigate the variability of net community production (NCP). Corresponding to the typical peak phytoplankton bloom period, the ΔO2/Ar of the Amundsen Sea Polynya (ASP) reached 30% in early January 2011 and had a strong positive correlation with the sea surface temperature (SST) and chlorophyll-a (Chl-a). In contrast, ΔO2/Ar decreased to −10% in the sea ice zone (SIZ), which was likely associated with either net O2 consumption in the unlit area or the entrainment of deep water containing low dissolved oxygen. Near the terminal stage of the phytoplankton bloom in late February 2012, we observed the same contrasting ΔO2/Ar features between the ASP and SIZ. However, the ΔO2/Ar in the ASP was not >10%, which corresponded with the overall reduction in Chl-a, solar radiation, and SST compared with the corresponding values in 2011. The average net community production in the ASP was 119 ± 79 mmol O2 m−2 d−1 in January 2011, and 23 ± 14 mmol O2 m−2 d−1 in February 2012. The strong correlations of NCP with SST and mixed layer depth (MLD) indicate that the ASP phytoplankton bloom is likely initiated by a combination of increased light availability and SST in early summer. Low SST and variable fluorescence to maximum florescence ratio (Fv/Fm) in February indicate that decreased solar radiation and Fe availability are likely responsible for the phytoplankton bloom demise.

Pelagic community production and carbon-nutrient stoichiometry under variable ocean acidification in an Arctic fjord

Abstract. Net community production (NCP) and carbon to nutrient uptake ratios were studied during a large-scale mesocosm experiment on ocean acidification in Kongsfjorden, western Svalbard, during June–July 2010. Nutrient depleted fjord water with natural plankton assemblages, enclosed in nine mesocosms of ~ 50 m3 in volume, was exposed to pCO2 levels ranging initially from 185 to 1420 μatm. NCP estimations are the cumulative change in dissolved inorganic carbon concentrations after accounting for gas exchange and total alkalinity variations. Stoichiometric coupling between inorganic carbon and nutrient net uptake is shown as a ratio of NCP to a cumulative change in inorganic nutrients. Phytoplankton growth was stimulated by nutrient addition half way through the experiment and three distinct peaks in chlorophyll a concentration were observed during the experiment. Accordingly, the experiment was divided in three phases. Cumulative NCP was similar in all mesocosms over the duration of the experiment. However, in phases I and II, NCP was higher and in phase III lower at elevated pCO2. Due to relatively low inorganic nutrient concentration in phase I, C : N and C : P uptake ratios were calculated only for the period after nutrient addition (phase II and phase III). For the total post-nutrient period (phase II + phase III) ratios were close to Redfield, however they were lower in phase II and higher in phase III. Variability of NCP, C : N and C : P uptake ratios in different phases reflects the effect of increasing CO2 on phytoplankton community composition and succession. The phytoplankton community was composed predominantly of haptophytes in phase I, prasinophytes, dinoflagellates, and cryptophytes in phase II, and haptophytes, prasinophytes, dinoflagellates and chlorophytes in phase III (Schulz et al., 2013). Increasing ambient inorganic carbon concentrations have also been shown to promote primary production and carbon assimilation. For this study, it is clear that the pelagic ecosystem response to increasing CO2 is more complex than that represented in previous work, e.g. Bellerby et al. (2008). Carbon and nutrient uptake representation in models should, where possible, be more focused on individual plankton functional types as applying a single stoichiometry to a biogeochemical model with regard to the effect of increasing pCO2 may not always be optimal. The phase variability in NCP and stoichiometry may be better understood if CO2 sensitivities of the plankton's functional type biogeochemical uptake kinetics and trophic interactions are better constrained.

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.

Interannual variability of net community production and air-sea CO2 flux in a naturally iron fertilized region of the Southern Ocean (Kerguelen Plateau)

AbstractThe interannual variability of net community production (NCP) and air-sea CO2 flux in a naturally iron fertilized and productive area of the Southern Ocean (Kerguelen plateau) was investigated using a 1D biogeochemical model driven by satellite chlorophyll, sea surface temperature and wind speed data for the 1997–2007 period. The model simulates the low fCO2 and dissolved inorganic carbon (DIC) measured during summers 2004–05, 2005–06, 2006–07 and the high NCP derived from a seasonal carbon budget in the surface waters of these blooms. Although satellite data show high interannual variability in the dynamics and magnitude of the bloom during the 1997–2007 decade, the simulated interannual variability of the NCP was only ± 14%. This unexpected result could be due to the combined effect of both the duration and the start date of the bloom, the latter determining the depth of the mixed layer used to compute the NCP. In the productive area, the interannual variability of air-sea CO2 flux (± 13%) was not only driven by the biological effect but also by the solubility effect. Our results contrast with previous studies in the high nutrient, low chlorophyll regions of the Southern Ocean.

Production and respiration control the marine microbial metabolic balance in the eastern North Atlantic subtropical gyre

Two main contrasted hypotheses have arisen during the last decades about the factors controlling the planktonic net metabolic balance in oligotrophic waters: gross primary production controls net community production vs. variability of net community production is also influenced by changes in microbial respiration. This work discusses both hypotheses analyzing the variability of metabolic rates along a gradient from the margin to the centre of the North Atlantic oligotrophic gyre, i.e. from relatively productive to more oligotrophic conditions. Net community production (NCP) was close to zero (between −3.34 and −11.77 mmol O 2 m −2 d −1) at the margin of the gyre and tended towards net heterotrophy (−44.03 mmol O 2 m −2 d −1) to the centre of the gyre as both gross primary production (GPP) and community respiration (CR) decreased. The strong relationships found between nutrient availability and both NCP and GPP suggest that factors controlling GPP are prevalent in determining NCP variability in this biogeographic region. However implementation of existing models to predict NCP from the measured GPP indicates that the precise estimation of NCP in different oligotrophic systems requires consideration of the magnitude and variability of microbial respiration rates.

Seasonal compensation of microbial production and respiration in a temperate sea

Gross oxygen production (GP), dark respiration (DR) and net community production (NCP) were studied for 16 mo in the euphotic layer of 3 stations through the coastal transitional zone of the southern Bay of Biscay, and related to hydrographic and nutrient conditions, phytoplankton biomass and C incorporation. Microbial O 2 fluxes exhibited seasonal patterns linked to the seasonal cycle of water column stratification and mixing, with positive NCP during the spring, negative throughout the summer and close to zero in winter. This pattern was altered at coastal regions, where productive periods were linked to coastal upwelling, whereas in winter persistent net heterotrophy was measured, presumably in relation to increases in organic matter discharge of continental origin. The comparison of NCP with O 2 anomaly and NO 3 concentration in the euphotic zone, the spatial and temporal scales studied and the prevalence of steady-state conditions offshore support the conclusion that the maintenance of summer heterotrophy in the region was based upon the consumption of the surplus of organic matter produced in spring. The uncoupling in the microbial auto- and heterotrophic metabolisms, based on the accumulation and delayed consumption of dissolved organic matter as a consequence of the processes controlling phytoplankton growth and microbial heterotrophic activity in temperate seas, would explain such a pattern. The close relationship observed between the seasonal variability in NCP and the magnitude of spring net production and predictions derived from the seasonal cycles of O 2 anomaly in middle latitudes and atmospheric O 2 led us to conclude that the seasonal compensation of production and respiration processes is a characteristic of the dynamics of the pelagic ecosystem, at least in coastal temperate seas. The implications of this conclusion are of great relevance for the interpretation of new production and the estimation of the trophic status of the ocean from direct measurements of plankton net production.