An official website of the United States government
Abstract Uncertainties in the temporal and spatial patterns of marine primary production and respiration limit our understanding of the ocean carbon (C) cycle and our ability to predict its response to environmental changes. Here we present a comprehensive time‐series analysis of plankton metabolism at the Hawaii Ocean Time‐series program site, Station ALOHA, in the North Pacific Subtropical Gyre. Vertical profiles of gross oxygen production (GOP) and community respiration (CR) were quantified using the18O‐labeled water method together with net changes in O2to Ar ratios during dawn to dusk in situ incubations. Rates of14C‐bicarbonate assimilation (14C‐based primary production [14C‐PP]) were also determined concurrently. During the observational period (April 2015 to July 2020), euphotic zone depth‐integrated (0–125 m) GOP and14C‐PP ranged from 35 to 134 mmol O2m−2d−1and 18 to 75 mmol C m−2d−1, respectively, while CR ranged from 37 to 187 mmol O2m−2d−1. All biological rates varied with depth and season, with seasonality most pronounced in the lower portion of the euphotic zone (75–125 m). The mean annual ratio of GOP to14C‐PP was 1.7 ± 0.1 mol O2(mol C)−1. While previous studies have reported convergence of GOP and14C‐PP with depth, we find a less pronounced vertical decline in the GOP to14C‐PP ratios, with GOP exceeding14C‐PP by 50% or more in the lower euphotic zone. Variability in CR was higher than for GOP, driving most of the variability in the balance between the two.
more »
« less
Vertical nitrate flux fuels new production over summertime Northeast U.S. Shelf
Abstract In aquatic ecosystems, allochthonous nutrient transport to the euphotic zone is an important process that fuels new production. Here, we use high‐resolution physical and biogeochemical observations from five summers to estimate the mean vertical nitrate flux, and thus new production over the Northeast U.S. Shelf (NES). We find that the summertime nitrate field is primarily controlled by biological uptake and physical advection–diffusion processes, above and below the 1% light level depth, respectively. We estimate the vertical nitrate flux to be 8.2 ± 5.3 × 10−6 mmol N m−2 s−1for the mid‐shelf and 12.6 ± 8.6 × 10−6 mmol N m−2 s−1for the outer shelf. Furthermore, we show that the new production to total primary production ratio (i.e., the f‐ratio), consistently ranges between 10% and 15% under summer conditions on the NES. Two independent approaches—nitrate flux‐based new production and O2/Ar‐based net community production—corroborate the robustness of the f‐ratio estimation. Since ~ 85% of the total primary production is fueled by recycled nutrients over sufficiently broad spatial and temporal scales, less than 15% of the organic matter produced in summer is available for export from the NES euphotic zone. Our direct quantification of new production not only provides more precise details about key processes for NES food webs and ecosystem function, but also demonstrates the potential of this approach to be applied to other similar datasets to understand nutrient and carbon cycling in the global ocean.
more »
« less
- PAR ID:
- 10573225
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Limnology and Oceanography
- Volume:
- 70
- Issue:
- 2
- ISSN:
- 0024-3590
- Format(s):
- Medium: X Size: p. 360-376
- Size(s):
- p. 360-376
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract The highly stratified, oligotrophic regions of the oceans are predominantly nitrogen limited in the surface ocean and light limited at the deep chlorophyll maximum (DCM). Hence, determining light and nitrogen co-limitation patterns for diverse phytoplankton taxa is crucial to understanding marine primary production throughout the euphotic zone. During two cruises in the deep-water Gulf of Mexico, we measured primary productivity (H13CO3−), nitrate uptake (15NO3−) and ammonium uptake (15NH4+) throughout the water column. Primary productivity declined with depth from the mixed layer to the DCM, averaging 27.1 mmol C m−2 d−1. The fraction of growth supported by NO3− was consistently low, with upper euphotic zone values ranging from 0.01 to 0.14 and lower euphotic zone values ranging from 0.03 to 0.44. Nitrate uptake showed strong diel patterns (maximum during the day), whereas ammonium uptake exhibited no diel variability. To parameterize taxon-specific phytoplankton nutrient and light utilization, we used a data assimilation approach (Bayesian Markov Chain Monte Carlo) including primary productivity, nutrient uptake and taxon-specific growth rate measurements. Parameters derived from this analysis define distinct niches for five phytoplankton taxa (Prochlorococcus, Synechococcus, diatoms, dinoflagellates and prymnesiophytes) and may be useful for constraining biogeochemical models of oligotrophic open-ocean systems.more » « less
-
Abstract The southern Benguela upwelling system (SBUS) supports high rates of primary productivity that sustain important commercial fisheries. The exceptional fertility of this system is reportedly fueled not only by upwelled nutrients but also by nutrients regenerated on the broad and shallow continental shelf. We measured nutrient concentrations and the nitrogen (N) and oxygen (O) isotope ratios (δ15N and δ18O) of nitrate along four zonal lines in the SBUS in late summer and early winter to evaluate the extent to which regenerated nutrients augment the upwelled nutrient reservoir originating offshore. During summer upwelling, a decrease in on‐shelf nitrate δ18O revealed that 0–48% of the subsurface nutrients derived from in situ remineralization. The nitrate regenerated on‐shelf in the more quiescent winter (0–63% of total nitrate) extended further offshore along the mid‐shelf. A shoreward increase in subsurface nitrate δ15N and a greater N deficit in on‐shelf bottom waters further indicated N loss to benthic (and at times, watercolumn) denitrification coincident with the on‐shelf remineralization. Our data show that remineralized nutrients get trapped on the SBUS shelf in summer through early winter, enhancing the nutrient pool that can be upwelled to support surface production. We hypothesize that this process is aided by a number of equatorward‐flowing hydrographic fronts that impede the lateral exchange of surface waters. The extent to which nutrients remain trapped on the shelf has implications for the occurrence of hypoxic events in the SBUS.more » « less
-
Abstract. Significant rates of primary production occur in the oligotrophic ocean, without any measurable nutrients present in the mixed layer, fueling a scientific paradox that has lasted for decades. Here, we provide a new determination of the annual mean physical supply of nitrate to the euphotic zone in the western subtropical North Atlantic. We combine a 3-year time series of measurements of tritiugenic 3He from 2003 to 2006 in the surface ocean at the Bermuda Atlantic Time-series Study (BATS) site with a sophisticated noble gas calibrated air–sea gas exchange model to constrain the 3He flux across the sea–air interface, which must closely mirror the upward 3He flux into the euphotic zone. The product of the 3He flux and the observed subsurface nitrate–3He relationship provides an estimate of the minimum rate of new production in the BATS region. We also apply the gas model to an earlier time series of 3He measurements at BATS in order to recalculate new production fluxes for the 1985 to 1988 time period. The observations, despite an almost 3-fold difference in the nitrate–3He relationship, yield a roughly consistent estimate of nitrate flux. In particular, the nitrate flux from 2003 to 2006 is estimated to be 0.65 ± 0.14 mol m−2 yr−1, which is ~40 % smaller than the calculated flux for the period from 1985 to 1988. The difference in nitrate flux between the time periods may be signifying a real difference in new production resulting from changes in subtropical mode water formation. Overall, the nitrate flux is larger than most estimates of export fluxes or net community production fluxes made locally for the BATS site, which is likely a reflection of the larger spatial scale covered by the 3He technique and potentially also by the decoupling of 3He and nitrate during the obduction of water masses from the main thermocline into the upper ocean. The upward nitrate flux is certainly large enough to support observed rates of primary production at BATS and more generally in the oligotrophic subtropical ocean.more » « less
-
Abstract Net community production (NCP) was estimated from nitrate profiles measured via biogeochemical Argo floats drifting in the Argentine Basin. Two criteria were tested for defining hydrographic fronts used to separate the study area into five zones: potential density anomaly at 450 m and potential temperature at 100 m. The latter definition was preferred as it minimized overlapping among zones. Float profiles within each zone were used to construct monthly median profiles of nitrate. Monthly nitrate inventories were calculated for each zone by integrating the median profiles between the surface and a depth of 100 or 200 m. Three methods were utilized to estimate NCP from the nitrate drawdown. The resulting mean NCP estimates indicated a decline in NCP from 3 to 4 mol C m−2 yr−1south of ∼40°S to ≤1 mol C m−2 yr−1north of ∼40°S. The monthly median profiles suggested 20%–100% of drawdown occurred by the end of December; however, chlorophyll fluorescence indicated phytoplankton activity persisted through austral summer. We speculate that primary production during these summer months was supported by regenerated nitrogen sources (not nitrate), despite replete concentrations, likely due to the relative scarcity of bioavailable iron known to persist in the region. While a northward advective flux of nitrate was strongly suggested by meridional nitrate gradients over the upper 0–300 m, vertical mixing was apparently necessary to stimulate new production, indicating both processes are important for NCP in the Argentine Basin. This work highlights the potential for floats in studying biogeochemical cycles in hydrographically complex regions.more » « less
