James K. B. Bishop

Iron supply and demand in the upper ocean

Iron is hypothesized to be a limiting micronutrient for ocean primary production. This paper presents an analysis of the iron budget in the upper ocean. The global distribution of annual iron assimilation by phytoplankton was estimated from distributions of satellite-derived oceanic primary production and measured (Fe:C)(cellular) ratios. The distributions of iron supply by upwelling/mixing and aeolian deposition were obtained by applying (Fe:NO3)(dissolved) ratios to the nitrate supply and by assuming the soluble fraction of mineral aerosols. A lower bound on the rate of iron recycling in the photic zone was estimated as the difference between iron assimilation and supply. Global iron assimilation by phytoplankton for the open ocean was estimated to be 12 x 10(9) mol Fe yr(-1). Atmospheric deposition of total Fe is estimated to be 96 x 10(9) mol Fe yr(-1) in the open ocean, with the soluble Fe fraction ranging between 1 and 10 percent (or 1-10 x 10(9) mol Fe yr(-1)). By comparison, the upwelling/entrainment supply of dissolved Fe to the upper ocean is small, similar to 0.7 x 10(9) mol Fe yr(-1). Uncertainties in the aeolian flux and assimilation may be as large as a factor of 5-10 but remain difficult to quantify, as informationmorexa0» is limited about the form and transformation of iron from the soil to phytoplankton incorporation. An iron stress index, relating the (Fe:N) demand to the (Fe:N) supply, confirms the production in the high-nitrate low-chlorophyll regions is indeed limited by iron availability.«xa0less

The continental margin is a key source of iron to the HNLC North Pacific Ocean

Here we show that labile particulate iron and manganese concentrations in the upper 500m of the Western Subarctic Pacific, an iron-limited High Nutrient Low Chlorophyll (HNLC) region, have prominent subsurface maxima between 100-200 m, reaching 3 nM and 600 pM, respectively. The subsurface concentration maxima in particulate Fe are characterized by a more reduced oxidation state, suggesting a source from primary volcagenic minerals such as from the Kuril/Kamchatka margin. The systematics of these profiles suggest a consistently strong lateral advection of labile Mn and Fe from redox-mobilized labile sources at the continental shelf supplemented by a more variable source of Fe from the upper continental slope. This subsurface supply of iron from the continental margin is shallow enough to be accessible to the surface through winter upwelling and vertical mixing, and is likely a key source of bioavailable Fe to the HNLC North Pacific.

Comparison of algorithms for estimating ocean primary production from surface chlorophyll, temperature, and irradiance

[1]xa0Results of a single-blind round-robin comparison of satellite primary productivity algorithms are presented. The goal of the round-robin exercise was to determine the accuracy of the algorithms in predicting depth-integrated primary production from information amenable to remote sensing. Twelve algorithms, developed by 10 teams, were evaluated by comparing their ability to estimate depth-integrated daily production (IP, mg C m−2) at 89 stations in geographically diverse provinces. Algorithms were furnished information about the surface chlorophyll concentration, temperature, photosynthetic available radiation, latitude, longitude, and day of the year. Algorithm results were then compared with IP estimates derived from 14C uptake measurements at the same stations. Estimates from the best-performing algorithms were generally within a factor of 2 of the 14C-derived estimates. Many algorithms had systematic biases that can possibly be eliminated by reparameterizing underlying relationships. The performance of the algorithms and degree of correlation with each other were independent of the algorithms’ complexity.

The dynamic ocean biological pump: Insights from a global compilation of particulate organic carbon, CaCO3, and opal concentration profiles from the mesopelagic

[1]xa0We have compiled a global data set of 62 open ocean profiles of particulate organic carbon (POC), CaCO3, and opal concentrations collected by large volume in situ filtration in the upper 1000 m over the last 30 years. We define concentration-based metrics for the strength (POC concentration at depth) and efficiency (attenuation of POC with depth in the mesopelagic) of the biological pump. We show that the strength and efficiency of the biological pump are dynamic and are characterized by a regime of constant and high transfer efficiency at low to moderate surface POC and a bloom regime where the height of the bloom is characterized by a weak deep biological pump and low transfer efficiency. The variability in POC attenuation length scale manifests in a clear decoupling between the strength of the shallow biological pump (e.g., POC at the export depth) and the strength of the deep biological pump (POC at 500 m). We suggest that the paradigm of diatom-driven export production is driven by a too restrictive perspective on upper mesopelagic dynamics. Indeed, our full mesopelagic analysis suggests that large, blooming diatoms have low transfer efficiency and thus may not export substantially to depth; rather, our analysis suggests that ecosystems characterized by smaller cells and moderately high %CaCO3 have a high mesopelagic transfer efficiency and can have higher POC concentrations in the deep mesopelagic even with relatively low surface or near-surface POC. This has negative implications for the carbon sequestration prospects of deliberate iron fertilization.

234Th as a tracer of particulate organic carbon export in the subarctic northeast Pacific Ocean

Abstract Profiles of particulate and dissolved 234 Th ( t 1/2 =24.1xa0d) were collected during the Canadian JGOFS program (February, May, and August 1996 and February 1997) along a transect from 125°W to 145°W at 50°N (Ocean Station Papa – OSP). Persistent features included a deficit of 234 Th relative to its parent 238 U in the upper ∼75xa0m and occasionally an excess of total 234 Th (particulate+dissolved) at ∼150–200xa0m attributed to redissolution of the particulate-bound nuclide. To calculate POC export fluxes, a steady-state model of 234 Th export was combined with measurements of the POC/ 234 Th ratio determined on particles >1-μm collected by in situ filtration. Along the transect, e-ratios (POC export/primary production) calculated for the 0.1% light level were similar during February 1997 and May 1996, 0.06–0.08 (avg. POC flux =2.8–7.1xa0mmolxa0Cxa0m −2 xa0d −1 ). During August 1996, e-ratio s reached a maximum, averaging 0.13 (avg. POC flux=7.6xa0mmolxa0Cxa0m −2 xa0d −1 ). Particulate 234 Th fluxes measured by drifting cylindrical sediment traps were 3-fold greater than the flux calculated for the water column 234 Th deficit. Finally, to study changes in 234 Th and POC export on short time-scales, some stations were sampled twice within 36xa0hours. During February 1997, 234 Th increased nearly 50% from 60–100xa0m in a 36 h period. Hydrographic data suggest that this change was due to advective processes.

Transformations of biogenic particulates from the pelagic to the deep ocean realm

This overview compares and contrasts trends in the magnitude of the downward Particulate Organic Carbon (POC) #ux with observations on the vertical proles of biogeochemical parameters in the NE subarctic Pacic. Samples were collected at Ocean Station Papa (OSP, 503N, 1453W), between 18}22 May 1996, on pelagic stocks/rate processes, biogenic particle #uxes (drifting sediment traps, 100}1000 m), and vertical proles of biogeochemical parameters from MULVFS (Multiple Unit Large Volume Filtration System) pumps (0}1000 m). Evidence from thorium disequilibria, along with observations on the relative partitioning of particles between the 1}53 lm and ’53 lm classes in the 50 m mixed layer, indicate that there was little particle aggregation within the mixed layer, in contrast to the 50}100 m depth stratum where particle aggregation predominated. Vertical proles of thorium/uranium also provided

Suspended particle organic composition and cycling in surface and midwaters of the equatorial Pacific Ocean

Abstract In this study we relate spatial and temporal variation in the organic composition of suspended particles to current conceptual models of open-ocean particle cycling. Suspended particles in surface (0–200xa0m) and midwaters (200–1000xa0m) of the equatorial Pacific Ocean were collected during the 1992 US JGOFS Equatorial Pacific (EqPac) program. Samples collected during El Nino (Survey I) and normal conditions (Survey II) were analyzed for pigment, amino acid, fatty acid, and neutral lipid concentrations and compositions. Principal Components Analysis (PCA) and other statistical methods were used to assess changes in particulate organic composition between Surveys I and II, over 24° of latitude, from 15 to 850xa0m depth, and to compare our compositional data with previously published data from EqPac sinking particles. These analyses indicated that surface suspended particles (0–200xa0m) were similar in composition to surface ocean phytoplankton and were less degraded than particles sinking out of the euphotic zone (105xa0m). The organic composition of suspended particles in surface waters varied with latitudinal and El-Nino-induced changes in phytoplankton assemblages. Midwater suspended particles (200–1000xa0m) contained labile phytodetrital material derived from particles exiting the euphotic zone (105xa0m). However, labile organic constituents of midwater suspended particles were increasingly degraded by microbes or consumed by midwater metazoans with depth. The increase in degradation state observed for midwater suspended particles may also have been caused by dilution of deeper (450–850xa0m) suspended particle pools with more refractory material originating from fast-sinking particles, e.g., fecal pellets. However, the mechanism controlling midwater particle degradation state varied with flux regime; dilution of midwater suspended particles dominated only in the higher flux regime found at equatorial latitudes (5°N–5°S) during Survey II (normal conditions). In summary, it is apparent that organic matter alteration in midwaters, and not cycling within the euphotic zone, has the larger effect on organic composition of suspended particles in the deep equatorial Pacific Ocean.

The downward flux of biogenic material in the NE subarctic Pacific: importance of algal sinking and mesozooplankton herbivory

In the present study we examine factors that a!ect the downward #ux of biogenic carbon in the NE subarctic Pacic, one of the important high-nutrient-low-chlorophyll (HNLC) regions in the open ocean. We focus on the role of mesozooplankton, since their seasonal peaks in biomass and growth are in phase with the seasonal variations in the downward POC #uxes, whereas phytoplankton biomass is more-or-less uniform year-round. The relative importance of mesozooplankton and algal sinking was examined using the pigment composition of material accumulated in short-term free-drifting sediment traps positioned just below the upper stratied surface layer (ca. 100}200 m). This was compared with the phytoplankton composition in the surface waters, and with the grazing activity (gut pigments and fecal pellet production rates) of the most abundant large copepods. We also examined whether the relationships between the downward #ux of carbon and pelagic processes were similar in the coastal, continental margin and o!shore HNLC regions of the NE subarctic Pacic, the latter represented by Ocean Station Papa (OSP). Our results show that grazing had a variable impact on the downward #ux of biogenic carbon. Carbon-transformed pheopigments (particularly pyropheophorbide a, frequently associated with copepod grazing) represented up to 13% of the total downward POC #ux inshore (in May 1996) and 8}9% at OSP in May and February 1996, respectively. This #ux of pheopigments was accompanied by a large potential input of fecal pellets from large copepods (as estimated from defecation rates of freshly collected animals) only in May 1996 at OSP, suggesting that pheopigments came from other sources (other herbivores, senescing algae) in

Year‐round observations of carbon biomass and flux variability in the Southern Ocean

[1]xa0Three Carbon Explorer (CE) floats profiling to kilometer depths in the Southern Ocean tracked dawn-dusk variations of mixing and stratification, particulate organic carbon, and light scattering and sedimentation at 100, 250, and 800 m continuously from January 2002 to April 2003. Data were analyzed in conjunction with contemporaneous satellite winds and chlorophyll and derived subsurface light fields. The CE deployed at 66°S, 172°W operated in the ice edge zone in absence of light. Two CEs deployed at 55°S, 172°W recorded wintertime mixing to ∼400 m yet observed very different bloom dynamics and sedimentation the following spring. Four hypotheses are explored. The strongest hypothesis is that shallow transient stratification of the deep winter mixed layer to shallower than photosynthetic critical depth occurred more frequently in the nonbloom, higher-sedimentation case. The lower particle export to 800 m under the bloom was hypothesized to be due to higher interception of sinking carbon by a relatively starved overwintering zooplankton population. In the Southern Ocean, surface phytoplankton biomass may counterindicate particle flux at kilometer depths.

Controls on solute concentration‐discharge relationships revealed by simultaneous hydrochemistry observations of hillslope runoff and stream flow: The importance of critical zone structure

We investigated controls on concentration-discharge relationships of a catchment underlain by argillite by monitoring both groundwater along a hillslope transect and stream chemistry. Samples were collected at 1-3-day intervals over four years (2009-2013) in Elder Creek in the Eel River Critical Zone Observatory in California. Runoff at our study hillslope is driven by vadose zone flux through deeply weathered argillite (5-25 m thick) to a perched, seasonally dynamic groundwater that then drains to Elder Creek. Low flow derives from the slowly draining deepest perched groundwater that reaches equilibrium between primary and secondary minerals and saturation with calcite under high subsurface pCO2. Arriving winter rains pass through the thick vadose zone, where they rapidly acquire solutes via cation exchange reactions (driven by high pCO2), and then recharge the groundwater that delivers runoff to the stream. These new waters displayed lower solute concentrations than the deep groundwater by less than a factor of 5 (except for Ca). Up to 74% of the total annual solute flux is derived from the vadose zone. The deep groundwaters Ca concentration decreased as it exfiltrates to the stream due to CO2 degassing and this Ca loss is equivalent of 30% of the total chemical weathering flux of Elder Creek. The thick vadose zone in weathered bedrock and the perched groundwater on underlying fresh bedrock result in two distinct processes that lead to the relatively invariant (chemostatic) concentration-discharge behavior. The processes controlling solute chemistry are not evident from stream chemistry and runoff analysis alone. This article is protected by copyright. All rights reserved.