Subarctic Northeast Pacific Ocean Research Articles

Small particles and heterotrophic protists support the mesopelagic zooplankton food web in the subarctic northeast Pacific Ocean

AbstractA quantitative understanding of the mesopelagic zooplankton food web is key to development of accurate carbon budgets and geochemical models in marine systems. Here we use compound specific nitrogen stable isotope analysis of amino acids to quantify the trophic structure of the microzooplankton and mesozooplankton community during summer in the subarctic northeast Pacific Ocean during the EXport Processes in the Ocean from Remote Sensing (EXPORTS) field campaign. Source amino acid values in particles and zooplankton provide strong evidence that basal resources for the mesopelagic zooplankton food web were primarily small (), suspended or slow‐sinking particles, but that surface organic matter delivered by vertically migrating zooplankton may have also been important. Comparisons of values of source and trophic amino acids provide estimates of food web length, which decrease significantly with depth and suggest that protistan microzooplankton are key components of the food web from the surface to at least 500. These results emphasize the importance of small particles as a source of carbon and nitrogen to mesopelagic communities in this region, support observations of an inverse relationship between zooplankton vertical migration and small particles as sources of carbon to deep‐sea food webs in low productivity environments, and document the role of heterotrophic protists as key trophic intermediaries in the mesopelagic zone at this location.

The Outsized Role of Salps in Carbon Export in the Subarctic Northeast Pacific Ocean.

Periodic blooms of salps (pelagic tunicates) can result in high export of organic matter, leading to an "outsized" role in the ocean's biological carbon pump (BCP). However, due to their episodic and patchy nature, salp blooms often go undetected and are rarely included in measurements or models of the BCP. We quantified salp-mediated export processes in the northeast subarctic Pacific Ocean in summer of 2018 during a bloom of Salpa aspera. Salps migrated from 300 to 750m during the day into the upper 100m at night. Salp fecal pellet production comprised up to 82% of the particulate organic carbon (POC) produced as fecal pellets by the entire epipelagic zooplankton community. Rapid sinking velocities of salp pellets (400-1,200md-1) and low microbial respiration rates on pellets (<1% of pellet C respired day-1) led to high salp pellet POC export from the euphotic zone-up to 48% of total sinking POC across the 100m depth horizon. Salp active transport of carbon by diel vertical migration and carbon export from sinking salp carcasses was usually <10% of the total sinking POC flux. Salp-mediated export markedly increased BCP efficiency, increasing by 1.5-fold the proportion of net primary production exported as POC across the base of the euphotic zone and by 2.6-fold the proportion of this POC flux persisting 100m below the euphotic zone. Salps have unique and important effects on ocean biogeochemistry and, especially in low flux settings, can dramatically increase BCP efficiency and thus carbon sequestration.

Prokaryotic responses to a warm temperature anomaly in northeast subarctic Pacific waters

Recent studies on marine heat waves describe water temperature anomalies causing changes in food web structure, bloom dynamics, biodiversity loss, and increased plant and animal mortality. However, little information is available on how water temperature anomalies impact prokaryotes (bacteria and archaea) inhabiting ocean waters. This is a nontrivial omission given their integral roles in driving major biogeochemical fluxes that influence ocean productivity and the climate system. Here we present a time-resolved study on the impact of a large-scale warm water surface anomaly in the northeast subarctic Pacific Ocean, colloquially known as the Blob, on prokaryotic community compositions. Multivariate statistical analyses identified significant depth- and season-dependent trends that were accentuated during the Blob. Moreover, network and indicator analyses identified shifts in specific prokaryotic assemblages from typically particle-associated before the Blob to taxa considered free-living and chemoautotrophic during the Blob, with potential implications for primary production and organic carbon conversion and export.

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.

Bacterial responses to background organic pollutants in the northeast subarctic Pacific Ocean.

Thousands of man-made synthetic chemicals are released to oceans and compose the anthropogenic dissolved organic carbon (ADOC). Little is known about the effects of this chronic pollution on marine microbiome activities. In this study, we measured the pollution level at three sites in the Northeast Subarctic Pacific Ocean (NESAP) and investigated how mixtures of three model families of ADOC at different environmentally relevant concentrations affected naturally occurring marine bacterioplankton communities' structure and metabolic functioning. The offshore northernmost site (North) had the lowest concentrations of hydrocarbons, as well as organophosphate ester plasticizers, contrasting with the two other continental shelf sites, the southern coastal site (South) being the most contaminated. At North, ADOC stimulated bacterial growth and promoted an increase in the contribution of some Gammaproteobacteria groups (e.g. Alteromonadales) to the 16 rRNA pool. These groups are described as fast responders after oil spills. In contrast, minor changes in South microbiome activities were observed. Gene expression profiles at Central showed the coexistence of ADOC degradation and stress-response strategies to cope with ADOC toxicities. These results show that marine microbial communities at three distinct domains in NESAP are influenced by background concentrations of ADOC, expanding previous assessments for polar and temperate waters.

Fecal pellet production by mesozooplankton in the subarctic Northeast Pacific Ocean

AbstractMesozooplankton fecal pellets sinking from the sea‐surface constitute an important component of the biological carbon pump, which is variable in part due to differences in zooplankton community composition on various scales. Particulate organic carbon production as fecal pellets by the entire mesozooplankton community has, until now, not been directly measured. We measured fecal pellet carbon (FPC) production rates of the mesozooplankton community separated into five size classes (0.2 to &gt; 5.0 mm), and rates of key taxa, in the upper 100 m of the subarctic Northeast Pacific Ocean, August–September, 2018. We scaled up FPC production rates to total FPC production in the epipelagic zone, which was 2.8‐fold higher at night than day due to diel vertical migration. Despite its lower total biomass, the smallest size class (0.2–0.5 mm) contributed more FPC than other size classes (up to 81.6%), due to high weight‐specific FPC production rates (3.0 μgFPC mgDW−1 h−1). The pelagic tunicate Salpa aspera had the highest individual FPC production rate (7.5 μgFPC ind−1 h−1) and when present, contributed most to community FPC production (up to 82.3%). While the large copepod Neocalanus cristatus dominated the 2.0–5.0 mm fraction (up to ~ 100% by biomass), its FPC production rate was low (0.4 μgFPC mgDW−1 h−1), likely due to minimal feeding and gut shrinkage preceding diapause. Comparison of FPC production with export recorded in sediment traps suggests high attenuation of smaller pellet classes in the epipelagic (86%). Our results provide direct measurements of mesozooplankton FPC production in a “low‐production, low‐export” region.

Allometry and the calculation of zooplankton metabolism in the subarctic Northeast Pacific Ocean

Abstract Using measurements of respiration and dissolved organic carbon (DOC) excretion from the subarctic Northeast Pacific Ocean (August 2018), we explore the efficacy of pre-existing allometric relationships to predict metabolic rates of diel vertically migrating zooplankton, and to test taxon-specific influences on these calculations at our study site. Non-taxon-specific allometric equations were associated with our best predictive model, and they underestimated measured respiratory values by ~10%. The best prediction of DOC release from estimates of biomass used taxon-specific coefficients and overestimated DOC production by 12%. There is a distinct allometric relationship for DOC excretion that varies between taxa, and slightly higher DOC production in more carnivorous groups. This study provides uncertainty estimates for zooplankton active flux analyses in the region, and identifies important research directions for allometry in biogeochemical studies.

Modeling the seasonal variability of phytoplankton in the subarctic northeast Pacific Ocean

The subarctic northeast Pacific Ocean (SNEP) is a high-nitrate, low-chlorophyll region where, due to iron (Fe) limitation, chlorophyll a (chl a) concentration is persistently low throughout the year despite abundant nitrate. Observations from a Biogeochemical-Argo float (BGC-Argo) deployed in the SNEP in 2012 show prominent variations in chl a in the upper ocean (0-150 m). Chl a peaks in late summer and autumn at a value &gt;3�times that in winter; these observations at a fine resolution can help us disentangle the mechanisms driving the full seasonal cycle of phytoplankton. A one-dimensional physical-biogeochemical coupled model was applied at OSP to investigate the factors driving phytoplankton dynamics in the SNEP. Seasonal variability of chl a in the upper layer is reproduced by the model; results are reasonably comparable to the BGC-Argo observations in 2012 near OSP. Phytoplankton growth is limited in winter by the deep mixed layer, which reduces light availability; light gradually increases later in the year due to shoaling of the mixed layer and increasing solar radiation. In spring and early summer, Fe limitation restricts phytoplankton growth, especially diatoms, and grazing pressure on small phytoplankton from overwintering microzooplankton inhibits accumulation of phytoplankton biomass. Chl a accumulates throughout the summer, peaking in late summer and autumn as mesozooplankton consume microzooplankton, which reduces microzooplankton grazing on phytoplankton. Although the model study was restricted to the seasonal pattern of BGC-Argo observations in 2012, it nevertheless fully describes seasonal variability in chl a in the SNEP and is crucial to understanding mechanisms underlying phytoplankton dynamics in high-nitrate, low-chlorophyll regions.

Glacial discharge into the subarctic Northeast Pacific Ocean during the last glacial

Abstract Understanding the response of the climate to abrupt changes in the Earth system represents a key objective in paleoclimatology. Heinrich events in the last glacial, during which significant amounts of glacial discharge entered the North Atlantic Ocean, triggered the development of colder conditions across much of the globe. Despite widespread documentation of these events, including their occurrence and global significance, the impact of Heinrich events on the North American Cordilleran Ice Sheet and subarctic North Pacific Ocean remains relatively unconstrained. Here, records of diatom oxygen isotopes are used to show that significant amounts of glacial discharge from the Cordilleran Ice Sheet were released into the open waters of the northeast Pacific Ocean throughout the last glacial. Based on the available age model, many of these episodes and calculated changes in sea surface salinity coincide with Heinrich events. If accurate, these findings would confirm that ocean-atmospheric teleconnections linked the North Atlantic and North Pacific Oceans during intervals of abrupt change in the last glacial, as well as indicating the wider susceptibility of regional ice-sheets to global alterations in the climate system.

Satellite‐Detected Ocean Ecosystem Response to Volcanic Eruptions in the Subarctic Northeast Pacific Ocean

AbstractVolcanic eruptions in the Aleutian Archipelago during the summer of 2008 deposited large quantities of iron‐laden ash to the subarctic North Pacific Ocean. The surface ocean ecosystem response has been previously characterized using limited in situ and autonomous field measurements and numerical modeling, but basin‐scale satellite remote sensing has been limited to a simple description of chlorophyll variability. Here, we show that the ecosystem response is a complex combination of phytoplankton biomass and physiology that can be described with satellite ocean color diagnostics such as the chlorophyll‐to‐carbon biomass ratio (Chl:Cphyto) and chlorophyll fluorescence yield. Together, these quantities outline a more complete picture of ecological responses spanning unique signals of iron stress (and relief from), photoacclimation, changes in phytoplankton growth rate, increases in biomass, and timescales of decay for these processes.

Phytoplankton responses to the 2014–2016 warming anomaly in the northeast subarctic Pacific Ocean

AbstractSignificant changes in the abundance and composition of phytoplankton were observed along Line P in the northeast subarctic Pacific as a result of a rapid warming of surface waters in 2014–2015. This feature, labeled “the blob,” reached ~ 4°C above normal and restricted winter ocean–surface nutrient renewal due to increased stratification. As a result, surface nutrients were the lowest observed and nitrate depletion in summer extended farther offshore than in the last three decades. Within this nitrate‐depleted region, there was unusually low phytoplankton biomass and a dramatic increase in the dominance of cyanobacteria, including Prochlorococcus, which had not been previously observed in this region. Farther offshore, in the iron‐limited region, phytoplankton biomass and the abundance of haptophytes and chlorophytes increased during “the blob.” By 2016, surface nutrient and phytoplankton concentrations were still low, but at most of the stations, phytoplankton composition was similar to that observed before the warming occurred, except for an increase in diatoms farther offshore. These changes at the base of the food web could have ecosystem‐wide implications.

Optically-derived estimates of phytoplankton size class and taxonomic group biomass in the Eastern Subarctic Pacific Ocean

We evaluate several algorithms for the estimation of phytoplankton size class (PSC) and functional type (PFT) biomass from ship-based optical measurements in the Subarctic Northeast Pacific Ocean. Using underway measurements of particulate absorption and backscatter in surface waters, we derived estimates of PSC/PFT based on chlorophyll-a concentrations (Chl-a), particulate absorption spectra and the wavelength dependence of particulate backscatter. Optically-derived [Chl-a] and phytoplankton absorption measurements were validated against discrete calibration samples, while the derived PSC/PFT estimates were validated using size-fractionated Chl-a measurements and HPLC analysis of diagnostic photosynthetic pigments (DPA). Our results showflo that PSC/PFT algorithms based on [Chl-a] and particulate absorption spectra performed significantly better than the backscatter slope approach. These two more successful algorithms yielded estimates of phytoplankton size classes that agreed well with HPLC-derived DPA estimates (RMSE = 12.9%, and 16.6%, respectively) across a range of hydrographic and productivity regimes. Moreover, the [Chl-a] algorithm produced PSC estimates that agreed well with size-fractionated [Chl-a] measurements, and estimates of the biomass of specific phytoplankton groups that were consistent with values derived from HPLC. Based on these results, we suggest that simple [Chl-a] measurements should be more fully exploited to improve the classification of phytoplankton assemblages in the Northeast Pacific Ocean.

Fine-scale spatial and interannual cadmium isotope variability in the subarctic northeast Pacific

We present dissolved cadmium (Cd) concentrations, [Cd], and stable isotope compositions, ε112/110Cd, in high-resolution depth profiles from five stations along the Line P transect in the subarctic northeast Pacific Ocean. In addition to profiles collected in 2012, subsurface isopycnal samples and surface samples were collected in 2013 and 2014 respectively, providing both temporal and spatial coverage. Surface waters are characterized by Cd depletion relative to phosphate (PO43−) compared to deepwater Cd:PO43−, and high inferred remineralization ratios in the nutricline (0.45nmolμmol−1) are observed, consistent with Cd enrichment relative to phosphorus (P) in surface-derived biogenic particles. The correlation between Cd and PO43− weakens at depths where oxygen is highly depleted as shown by local minima in dissolved [Cd] and the tracer Cd⁎. The decoupling, which is driven by a deficit of Cd relative to PO43−, appears consistent with the recent hypothesis of dissolved Cd removal in oxygen-depleted regions by insoluble metal sulfide formation.Dissolved ε112/110Cd indicates a biologically driven fractionation in surface waters with more positive (heavy) values in the upper water column and lower (light) values in deeper waters. The highest ε112/110Cd observed in our sample set (5.19±0.23) is comparable to observations from the Southern Ocean but is significantly lighter than maximum reported surface values from the subtropical North Pacific of ε112/110Cd≥15. A global compilation of low [Cd] surface water shows similar differences in maximum ε112/110Cd. A surface water intercalibration should be prioritized to help determine if these differences at low [Cd] reflect true physical or biological variability or are due to analytical artefacts. Surface samples from the 2012 sampling campaign fit a closed-system Rayleigh fractionation model; however, surface waters sampled in 2014 had much lower [Cd] with relatively constant ε112/110Cd that cannot be explained by a closed-system Rayleigh model. These results correspond with a warm water surface anomaly found along Line P in 2014 and demonstrate that there is interannual variability in the biogeochemical cycling of Cd and its isotopes in the subarctic North Pacific. In contrast to other ocean basins where vertical variability in ε112/110Cd is observed at depth, deep and intermediate waters in the North Pacific have a near-uniform ε112/110Cd value (mean of 1.14±0.37, n=43, 2SD) representative of nearly all samples at or below 1000 m depth. Imprinted upon this nearly homogeneous intermediate and deep North Pacific ε112/110Cd signature are fine-scale spatial trends, with heavier values observed toward the coastal end of Line P than the oceanic end at intermediate depths, and with slightly heavier values in subtropical North Pacific deep water compared to the subarctic North Pacific. The nearly constant Cd isotopic composition of North Pacific deep waters is consistent with the inflow of Circumpolar Deep Water at depth in the Pacific basin, along with deep remineralization, and supports the potential of ε112/110Cd as a tracer of global deepwater circulation.

Dissolved low-molecular weight thiol concentrations from the U.S. GEOTRACES North Atlantic Ocean zonal transect

Low-molecular weight thiols, including cysteine and glutathione, are biomolecules involved in a variety of metabolic pathways and act as important antioxidant and metal buffering agents. In this last capacity, they represent a potential mechanism for modulating the bioavailability and biogeochemistry of many trace elements in the ocean, particularly for chalcophilic elements (e.g., Cu, Zn, Cd, Ag and Hg). For this reason, and in the context of the international GEOTRACES program that seeks to understand the biogeochemistry of trace elements in the ocean, we measured the concentration of individual dissolved low-molecular weight thiols during the U.S. GEOTRACES North Atlantic Zonal Transect (USGNAZT). Only two thiols were identified, cysteine and glutathione, in contrast to results from the northeast subarctic Pacific Ocean, where the dipeptides glycine-cysteine and arginine-cysteine were also present and γ-glutamylcysteine was dominant. Concentrations of cysteine and glutathione in the North Atlantic Ocean were lower than in the Pacific and ranged from below detection (~0.01nM) to 0.61nM of cysteine and up to 1.0nM of glutathione, with cysteine generally more abundant than glutathione. Vertical profiles of cysteine and glutathione were broadly consistent with their biological production, being more abundant in surface water and usually below detection at depths greater than about 200m. Subsurface concentration maxima, often co-incident with the deep chlorophyll maximum, were frequently observed but not universal. We conclude that cysteine and glutathione do not make up significant portions of complexation capacity for Cu and Zn in the upper open ocean but could be important for Cd, Hg, and potentially other chalcophiles. Extremely low concentrations of cysteine and glutathione in deep water suggest that higher molecular-weight thiols are a more important ligand class for chalcophiles in that portion of the ocean.

Using 67Cu to Study the Biogeochemical Cycling of Copper in the Northeast Subarctic Pacific Ocean

Microbial copper (Cu) nutrition and dissolved Cu speciation were surveyed along Line P, a coastal to open ocean transect that extends from the coast of British Columbia, Canada, to the high-nutrient-low-chlorophyll (HNLC) zone of the northeast subarctic Pacific Ocean. Steady-state size fractionated Cu uptake rates and Cu:C assimilation ratios were determined at in situ Cu concentrations and speciation using a 67Cu tracer method. The cellular Cu:C ratios that we measured (~30 µmol Cu mol C-1) are similar to recent estimates using synchrotron x-ray fluorescence (SXRF), suggesting that the 67Cu method can determine in situ metabolic Cu demands. We examined how environmental changes along the Line P transect influenced Cu metabolism in the sub-microplankton community. Cellular Cu:C assimilation ratios and uptake rates were compared with net primary productivity, bacterial abundance and productivity, total dissolved Cu, Cu speciation, and a suite of other chemical and biological parameters. Total dissolved Cu concentrations ([Cu]d) were within a narrow range (1.46 to 2.79 nM), and Cu was bound to a ~5-fold excess of strong ligands with conditional stability constants ( ) of ~1014. Free Cu2+ concentrations were low (pCu 14.4 to 15.1), and total and size fractionated net primary productivity (NPPV; µg C L-1 d-1) were negatively correlated with inorganic Cu concentrations ([Cu′]). We suggest this is due to greater Cu′ drawdown by faster growing phytoplankton populations. Using the relationship between [Cu′] drawdown and NPPV, we calculated a regional photosynthetic Cu:C drawdown export ratio between 1.5 and 15 µmol Cu mol C-1, and a mixed layer residence time (2.5 to 8 years) that is similar to other independent estimates (2-12 years). Total particulate Cu uptake rates were between 22 and 125 times faster than estimates of Cu export; this is possibly mediated by rapid cellular Cu uptake and efflux by phytoplankton and bacteria or the effects of grazers and bacterial remineralization on dissolved Cu. These results provide a more detailed understanding of the interactions between Cu speciation and microorganisms in seawater, and present evidence that marine phytoplankton modify Cu speciation in the open ocean.

Iron-copper interactions in iron-limited phytoplankton in the northeast subarctic Pacific Ocean

In August 2010, iron (Fe) and Fe and copper (Cu) addition incubation experiments were conducted at two low Fe stations (P20 and P26) along Line P, off the western coast of British Columbia, to investigate Cu physiology in Fe- and Fe-light co-limited phytoplankton. Chlorophyll a concentrations ([Chl a]), maximum variable fluorescence yield (Fv/Fm), and Fe uptake rates by the Cu-dependent high-affinity Fe transport system (HAFeTS) were measured. Additions of Fe resulted in an increase in [Chl a] and Fv/Fm at both stations compared with the controls, regardless of light availability, and confirmed that the phytoplankton communities were Fe-limited. Uptake of Fe by the HAFeTS in both incubations increased with the addition of Fe, and likely reflects luxury Fe uptake and storage. While the in situ inorganic Cu concentrations were similar to those that can induce Cu-limitation in laboratory cultures, increasing Cu availability had no effect on biomass accumulation during both incubations, regardless of Fe availability or light regime. At P26, additions of 1 nmol L−1 CuSO4 resulted in a short-term increase in Fv/Fm of the phytoplankton community, and an increase in Fe uptake rates by large phytoplankton (>5 μm), but only when light was not limiting. These data confirm a complex interaction between light, Fe and Cu physiology in indigenous phytoplankton communities, and suggest that these interactions may be both spatially heterogeneous and different for different phytoplankton size classes.

Evaluation of functional trait diversity for marine zooplankton communities in the Northeast subarctic Pacific Ocean

Functional trait diversity relates variation in the structure of biological communities to function and ecosystem processes. Zooplankton occupy a central position in marine food webs, modulating energy availability to higher trophic levels, while themselves affected by environmental variation. The use of traits with functional diversity potentially enables a more mechanistic understanding of variation and regulation of zooplankton communities than is possible with taxonomic diversity alone. Traits for 42 zooplankton species from the northeast subarctic Pacific were assembled from the literature and applied to a 16-year time series from the oceanographic Line P. We assembled traits on body size, ontogeny, habitat and feeding behaviours. Six major functional groupings were identified via a trait-based cluster dendrogram. Several functional diversity indices were also calculated, and compared with analogous taxonomic diversity indices. Analogue diversity indices were significantly correlated. Both types of diversity indices revealed some year-specific “anomalies” which were associated with broad-scale oceanographic and climatic regime shifts. We propose that the functional diversity approach may represent an additional ecological tool with which we can gain further understanding of zooplankton function and trophic linkages in a changing ocean, in part because functional traits are often based on easily measured morphological characters.

Estimates of micro-, nano-, and picoplankton contributions to particle export in the northeast Pacific

Abstract. The contributions of micro-, nano-, and picoplankton to particle export were estimated from measurements of size-fractionated particulate 234Th, organic carbon, and phytoplankton indicator pigments obtained during five cruises between 2010 and 2012 along Line P in the subarctic northeast Pacific Ocean. Sinking fluxes of particulate organic carbon (POC) and indicator pigments were calculated from 234Th–238U disequilibria and, during two cruises, measured by a sediment trap at Ocean Station Papa. POC fluxes at 100 m ranged from 0.65 to 7.95 mmol m−2 d−1, similar in magnitude to previous results at Line P. Microplankton pigments dominate indicator pigment fluxes (averaging 69 ± 19% of total pigment flux), while nanoplankton pigments comprised the majority of pigment standing stocks (averaging 64 ± 23% of total pigment standing stocks). Indicator pigment loss rates (the ratio of pigment export flux to pigment standing stocks) point to preferential export of larger microplankton relative to smaller nano- and picoplankton. However, indicator pigments do not quantitatively trace particle export resulting from zooplankton grazing, which may be an important pathway for the export of small phytoplankton. These results have important implications for understanding the magnitude and mechanisms controlling the biological pump at Line P in particular, and more generally in oligotrophic gyres and high-nutrient, low-chlorophyll (HNLC) regions where small phytoplankton represent a major component of the autotrophic community.

Iron(II) variability in the northeast subarctic Pacific Ocean

Distributions of dissolved iron (<0.2μm, dFe) and its reduced form, Fe(II), were measured during 3 cruises along Line P, a transect from the continental slope to the high nitrate, low chlorophyll (HNLC) northeast subarctic Pacific Ocean. Concentrations of Fe(II) ranged from below the detection limit (4.3pM or less) to 330pM, and dFe concentrations were as high as 3.6nM. Maximum concentrations for both Fe(II) and dFe occurred in waters over the continental slope, with Fe(II) consistently increasing towards the bottom, consistent with Fe(II) supply from benthic sources on the continental shelf and slope. Low oxygen concentrations (~10μM) and pH (~7.5) in the North Pacific oxygen deficient zone (ODZ) likely serve to stabilize Fe(II) and may contribute to enhanced dFe release from shelf and slope sediments via reductive dissolution. Concentration gradients along isopycnal surfaces indicate that Fe(II) is transported several hundred kilometers from the continental slope at depth, and there is evidence that episodic events may advect shelf-derived Fe(II) similar distances near the surface. Comparison of transport times and Fe(II) half-lives suggests that it is unlikely for pre-formed Fe(II) to be transported these distances, but that sedimentary particles advected off the shelf and slope may constitute a continuous source of Fe(II) both at depth and near the surface. At the offshore stations, the Fe(II) time series reveals deep local maxima that are transient in time and space and are consistent with a sporadic Fe(II) source, such as remineralization of sinking particles.

Acquisition of organically complexed copper by marine phytoplankton and bacteria in the northeast subarctic Pacific Ocean

Copper (Cu) is an essential micronutrient for marine phytoplankton, but can cause toxicity at elevated intracellular concentrations. The majority of Cu (>99.9%) in oceanic surface waters is bound to strong organic ligands, presumably produced by prokaryotes to detoxify Cu. Although laboratory studies have demonstrated that organically complexed Cu may be bioavailable to marine eukaryotic phytoplankton, the bioavailability of Cu organic complexes to indigenous marine phytoplankton has not been examined in detail. Using the carrier free radioisotope 67Cu at an iron limited station in the northeast subarctic Pacific Ocean, we performed size fractionated short-term Cu uptake assays with three Cu(II)-chelates, and 67Cu bound to the strong in situ ligands, with or without additions of weak Cu(I) ligands. Estimates of the maximum supply of inorganic Cu (Cu′) to the cell surface of eukaryotic phytoplankton were unable to account for the observed Cu uptake rates from the in situ ligands and two of the three added Cu(II)-chelates. Addition of 10nM weak organic Cu(I) ligands enhanced uptake of Cu bound to the in situ ligands. Thus, Cu within the in situ and strong artificial Cu(II) organic ligands was accessible to the phytoplankton community via various possible Cu uptake strategies, including: cell surface enzymatically mediated reduction of Cu(II) to Cu(I), the substrate of the high-affinity Cu transport system in eukaryotes; or ligand exchange between weak Cu-binding ligands and the cellular Cu transporters. During a 14-hour uptake assay, particulate Cu concentrations reached a plateau in most treatments. Losses were observed in some treatments, especially in the small size fractions (<5μm), corresponding with faster initial Cu uptake rates. This may indicate that Cu cycling is rapid between particulate and dissolved phases due to cellular efflux or remineralization by micrograzers. The acquisition of Cu from the strong in situ ligands puts into question the historic role attributed to Cu binding ligands in decreasing Cu bioavailability.