Net Community Production In Mixed Layer Research Articles

Impact of Vertical Mixing on Summertime Net Community Production in Canadian Arctic and Subarctic Waters: Insights From In Situ Measurements and Numerical Simulations

AbstractWe present ΔO2/Ar‐based estimates of mixed layer net community production (NCP) from three summer cruises in the North American Arctic and Subarctic oceans. Coupling shipboard underway and discrete observations with output from an ocean circulation model, we correct the NCP estimates for vertical mixing fluxes impacting the surface O2 budget. Large positive mixing fluxes, exceeding 100 mmol O2 m−2 d−1, were derived in regions of strong wind‐driven mixing, such as the Labrador Sea (LS), and in the physically‐dynamic Canadian Arctic Archipelago. In contrast, flux corrections were small (<10 mmol O2 m−2 d−1, on average) in the density‐stratified Baffin Bay, where mixing was low, and parts of the well‐mixed Hudson Strait (HS), where vertical O2 gradients were weak. The distribution of corrected NCP was highly heterogenous across the study region, reflecting varying contributions of nutrient supply, freshwater input and sea ice dynamics. Elevated NCP was apparent in the LS, HS, and nearshore regions influenced by glacial meltwater and recent ice retreat. Low NCP and localized net heterotrophy occurred in Baffin Bay, and near strong freshwater and organic matter sources in Hudson Bay and the Queen Maud Gulf. A multiple linear regression model developed using available oceanographic data explained ∼58% of the observed NCP variability. Our work demonstrates the spatially explicit influence of vertical mixing on ΔO2/Ar‐based NCP calculations across varied hydrographic conditions, and presents a novel approach to account for this process. This study contributes new knowledge of biological productivity distributions in under‐sampled, rapidly changing, high‐latitude waters.

ΔO2/N2′ as a New Tracer of Marine Net Community Production: Application and Evaluation in the Subarctic Northeast Pacific and Canadian Arctic Ocean

We compared field measurements of the biological O2 saturation anomalies, ΔO2/Ar and ΔO2/N2, from simultaneous oceanographic deployments of a membrane inlet mass spectrometer and optode/gas tension device (GTD). Data from the Subarctic Northeast Pacific and Canadian Arctic Ocean were used to evaluate ΔO2/N2 as an alternative to ΔO2/Ar for estimates of mixed layer net community production (NCP). We observed strong spatial coherence between ΔO2/Ar and ΔO2/N2, with small offsets resulting from differences in the solubility properties of Ar and N2 and their sensitivity to vertical mixing fluxes. Larger offsets between the two tracers were observed across hydrographic fronts and under elevated sea states, resulting from the differential time-response of the optode and GTD, and from bubble dissolution in the ship’s seawater lines. We used a simple numerical framework to correct for physical sources of divergence between N2 and Ar, deriving the tracer ΔO2/N2′. Over most of our survey regions, ΔO2/N2′ provided a better analog for ΔO2/Ar, and thus more accurate NCP estimates than ΔO2/N2. However, in coastal Arctic waters, ΔO2/N2 and ΔO2/N2′ performed equally well as NCP tracers. On average, mixed layer NCP estimated from ΔO2/Ar and ΔO2/N2′ agreed to within ∼2 mmol O2 m–2 d–1, with offsets typically smaller than other errors in NCP calculations. Our results demonstrate a significant potential to derive NCP from underway O2/N2 measurements across various oceanic regions. Optode/GTD systems could replace mass spectrometers for autonomous NCP derivation under many oceanographic conditions, thereby presenting opportunities to significantly expand global NCP coverage from various underway platforms.

ΔO2/N2′ as a tracer of mixed layer net community production: Theoretical considerations and proof‐of‐concept

AbstractWe present a method to estimate net community production (NCP) from underway ΔO2/N2 observations. This approach, based on high‐resolution measurements using relatively low‐cost O2‐optode and gas tension device instrumentation, provides an alternative to NCP derivation from mass spectrometric analysis of ΔO2/Ar. To fully exploit ΔΟ2/Ν2 as an NCP tracer, it is necessary to account for differences in surface water argon (Ar) and nitrogen (Ν2) saturation states. Using a one‐dimensional mixed layer model, evaluated against field observations, we examined the divergence between ΔO2/Ar and ΔΟ2/Ν2 resulting from various physical processes. Results show that changes in sea surface temperature, bubble injection and vertical mixing can decouple mixed layer ΔO2/Ar and ΔΟ2/Ν2, while biological N2‐fixation has a negligible effect. Based on readily available environmental data, we developed a framework to correct for Ar and N2 solubility differences, yielding a new tracer, ΔΟ2/Ν2′, that is a near‐analog of ΔO2/Ar. Through model simulations and field measurements, we show that ΔΟ2/Ν2′ provides an excellent approximation to ΔO2/Ar, and that uncertainty and biases in ΔO2/N2′ are small relative to other errors in NCP calculations. This work demonstrates the potential for ΔO2/N2′ measurements to significantly expand NCP estimates from a variety of sampling platforms.

Annual Mixed Layer Carbon Budget for the West Antarctic Peninsula Continental Shelf: Insights From Year‐Round Mooring Measurements

AbstractFor the first time the annual carbon budget on the West Antarctic Peninsula shelf was studied with continuously measured CO2 system parameters (pH and pCO2) from a subsurface mooring. The temporal evolution of the mixed layer dissolved inorganic carbon (DIC) is investigated via a mass balance. The annual mixed layer DIC inventory change was 1.1 ± 0.4 mol m−2 yr−1, which was mainly regulated by biological drawdown (−2.8 ± 2.4 mol m−2 yr−1), diapycnal eddy diffusion (2.6 ± 1.3 mol m−2 yr−1), entrainment/detrainment (0.9 ± 0.4 mol m−2 yr−1), and air‐water gas exchange (0.4 ± 2.1 mol m−2 yr−1). Significant carbon drawdown was observed in the spring and summer, which was replenished by the physical processes mentioned above. These observations suggest this area is an annual atmosphere CO2 sink with a mixed layer net community production of 2.8 ± 2.4 mol m−2 yr−1. These results highlight the significant seasonality in the DIC mass balance and the necessity of year‐round continuous observations for robust assessments of biogeochemical cycling in this region.

Estimating net community production in the Southern Ocean based on atmospheric potential oxygen and satellite ocean color data

The seasonal cycle of atmospheric potential oxygen (APO ∼ O2 + 1.1 CO2) reflects three seasonally varying ocean processes: 1) thermal in‐ and outgassing, 2) mixed layer net community production (NCP) and 3) deep water ventilation. Previous studies have isolated the net biological seasonal signal (i.e., the sum of NCP and ventilation), after using air‐sea heat flux data to estimate the thermal signal. In this study, we resolve all three components of the APO seasonal cycle using a methodology in which the ventilation signal is estimated based on atmospheric N2O data, the thermal signal is estimated based on heat flux or atmospheric Ar/N2 data, and the production signal is inferred as a residual. The isolation of the NCP signal in APO allows for direct comparison to estimates of NCP based on satellite ocean color data, after translating the latter into an atmospheric signal using an atmospheric transport model. When applied to ocean color data using algorithms specially adapted to the Southern Ocean and APO data at three southern monitoring sites, these two independent methods converge on a similar phase and amplitude of the seasonal NCP signal in APO and yield an estimate of annual mean NCP south of 50°S of 0.8–1.2 Pg C/yr, with corresponding annual mean NPP of ∼3 Pg C/yr and a mean growing season fratio of ∼0.33. These results are supported by ocean biogeochemistry model simulations, in which air‐sea O2 and N2O fluxes are resolved into component thermal, ventilation and (for O2) NCP contributions.

The seasonal pCO2 cycle at 49°N/16.5°W in the northeastern Atlantic Ocean and what it tells us about biological productivity

A 2‐year record of mixed layer measurements of CO2 partial pressure (pCO2), nitrate, and other physical, chemical, and biological parameters at a time series site in the northeast Atlantic Ocean (49°N/16.5°W) is presented. The data show average undersaturation of surface waters with respect to atmospheric CO2 levels by about 40 ± 15 μatm, which gives rise to a perennial CO2 sink of 3.2 ± 1.3 mol m−2 a−1. The seasonal pCO2 cycle is characterized by a summer minimum (winter maximum), which is due to the dominance of biological forcing over physical forcing. Our data document a rapid transition from deep mixing to shallow summer stratification. At the onset of shallow stratification, up to one third of the mixed layer net community production during the productive season had already been accomplished. The combination of high prestratification productivity and rapid onset of stratification appears to have caused the observed particle flux peak early in the season. Mixed layer deepening during fall and winter reventilated CO2 from subsurface respiration of newly exported organic matter, thereby negating more than one third of the carbon drawdown by net community production in the mixed layer. Chemical signatures of both net community production and respiration are indicative of carbon overconsumption, the effects of which may be restricted, though, to the upper ocean. A comparison of the estimated net community production with satellite‐based estimates of net primary production shows fundamental discrepancies in the timing of ocean productivity.