NE Subarctic Pacific Research Articles

Sperm whale acoustic ecology investigations using Jeffrey Nystuen‘s Passive Aquatic Listeners

When Jeffrey Nystuen invented the Passive Aquatic Listener (PAL) for quantifying rainfall and wind speed using underwater sound spectra, he considered all biological sounds in the recordings as noise. Using this noise data, I built a doctoral thesis, comprised of three published papers. None of this would have happened without Jeff’s continuous encouragement and support. Here, we present results from analyzing (1) five years (2007–2012) of PAL recordings from Ocean Station PAPA (OSP) in the offshore Gulf of Alaska; and (2) 19 months of PAL data from two sites, Pylos and Athos Stations (in the Hellenic Trench and North Aegean Trough respectively), in the Greek Seas, investigating the acoustic ecology of sperm whales (Physeter macrocephalus). Results of the bioacoustic analysis revealed the year-round presence of sperm whales at OSP and the Ionian Sea, with higher detections during the warm seasons. The sperm whale time series from OSP was correlated with in situ and remotely-sensed oceanographic variables to improve our understanding of global-warming-driven changes in the pelagic ecosystem of the NE Subarctic Pacific. Results from the Hellenic Trench emphasize the risk from increased shipping noise and contribute to the conservation efforts for the small, endangered sperm whale population in an understudied region.

Drivers of variation in crustacean zooplankton production rates differ across regions off the west coast of Vancouver Island and in the subarctic NE Pacific

Abstract The subarctic NE Pacific is comprised of several oceanographic regimes, in which regional variability in sea surface temperature (SST), satellite chlorophyll a, and crustacean zooplankton biomass influence the production rates of crustacean zooplankton. Traditional methods for estimating zooplankton production rates are labour/time intensive and restricted to select copepod species. A practical field alternative is the ‘chitobiase method’, which yields community-level biomass production rates (BPR) analogous to traditional moulting rate methods. BPR was measured along the west coast of Vancouver Island and in the subarctic NE Pacific during 2005, 2009–2011, and 2015–2018. Generalized additive modelling identified SST and the developing crustacean zooplankton biomass as key drivers of BPR variability. BPR varied positively with the proportion of developing copepod biomass relative to that of non-copepod crustacean zooplankton biomass. Our analysis indicates that BPR variation is associated with zooplankton community composition, but that high zooplankton biomass is not necessarily predictive of high BPR. Specifically, higher BPR is associated with a higher relative biomass of large-bodied, cold-water indicator species, and a lower biomass of non-copepod crustaceans.

Consistent Relationships Among Productivity Rate Methods in the NE Subarctic Pacific

AbstractPhytoplankton photosynthesize in surface waters, exporting organic carbon to depth through the biological pump. Quantifying productivity and the export of carbon is important to understanding the global carbon cycle and predicting its future changes. An issue in quantifying rates is that the many existing methods are not all equivalent, making comparisons between studies using different methods challenging. Our goal is to compare in situ and in vitro methods in order to identify where methods agree in the NE subarctic Pacific. During this study, we measured productivity using two in situ methods (oxygen/argon ratio and triple oxygen isotope mass balance approaches) and four in vitro methods (13C, , , and H218O uptake rates through incubations), and compared the results with one satellite‐based productivity estimate. The in situ carbon export method was consistently higher than the in vitro method, likely due to dissolved organic matter release not included in our incubation measurements. Upwelling bringing low‐O2 water to the surface and the interaction between bloom dynamics and a method's inherent time of integration cause outliers from the relationship. In contrast, in situ and in vitro methods for estimating gross primary production were consistent across a wide range in rates. We find that chlorophyll‐a concentration is strongly related to many of our measured rates. Satellite estimates of primary production are consistently different from 13C incubations. Our identification of consistent trends and causes for disagreement will allow observations from one method to be converted to another, facilitating future comparisons across studies.

High-frequency observations from a deep-sea cabled observatory reveal seasonal overwintering of Neocalanus spp. in Barkley Canyon, NE Pacific: Insights into particulate organic carbon flux

Many submarine canyons are known hotspots of pelagic and benthic biodiversity and productivity. Despite a very limited knowledge of the ecology, biodiversity and ecosystem function of Canada’s West Coast canyons, Barkley Canyon is becoming a relatively well studied system, particularly after the installation of the NEPTUNE seafloor cabled observatory in 2009. Video observations of large densities of overwintering calanoid copepods (likely a combination of Neocalanus plumchrus, N. cristatus, and a small contribution of N. flemingeri) drifting near the bottom at 970 m in the axis of Barkley Canyon motivated our interest in investigating the temporal dynamics of their ontogenetic migration cycle. Particularly, since these large calanoid copepods, and especially Neocalanus plumchrus, comprise up to 50% of of the mesozooplankton biomass in the subarctic NE Pacific, being considered a keystone species in the trophodynamics of pelagic ecosystems in the region. Here we used ∼20-months (May 2013–Jan 2015) of seafloor video imagery combined with acoustic Doppler current and backscatter time-series data from the NEPTUNE observatory to identify the precise timing and seasonal and inter-annual variability in the deep ontogenetic migration of Neocalanus spp. in Barkley Canyon. A total of 33,486 still images were extracted from 1674 × 5-min segment videos, captured at two-hour intervals, and used in a computer-automated image analysis protocol designed to estimate Neocalanus spp. densities near the seafloor. The results from the entire time-series revealed close correspondence with the described developmental and reproductive cycle for Neocalanus spp., with the highest densities of copepodite-5 (C5) and adult individuals present at depth from the late fall and through the winter. The concomitant high-frequency (2 MHz) ADCP backscatter time-series nearly mirrored the patterns obtained from the video imagery, and also highlighted a clear inter-annual variability, with higher copepod densities in 2013 relative to 2014. Such inter-annual variability was also evidenced by ground-truth net tow casts from Line P and La Perouse monitoring stations in the vicinity of Barkley Canyon. The low and high-frequency ADCP (75 KHz and 2 MHz) current data demonstrated an along axis mean flow near the bottom and an up-canyon mean subsurface flow from 70 to 300 m above the seabed, suggesting a recirculation cell at this segment of the canyon. Based on this circulation pattern and on our video and backscatter data, we propose a conceptual model describing how the topographically-constrained currents along the canyon axis and the up-canyon flow may help trap the seasonally migrating biomass of Neocalaus spp. near the core of its overwintering depth at mid-canyon (∼1000 m). Based on a previously calculated 25-yr mean of carbon export flux attributed to N. plumchrus in the NE Pacific (i.e., 5 g C m2 yr−1; min. 1.44, max 8.82C m2 yr−1 – Bradford-Grieve et al., 2001), which considers respiration and mortality at the overwintering depth throughout winter after spawning, we estimated that 35–215 tons of carbon reach Barkley Canyon’s seafloor yearly over an area of approximately 24 km2. Future studies should aim to further constrain the role of submarine canyons in transporting and concentrating deep zooplankton migrating biomass, as there are at least another 230 submarine canyons in the subarctic NE Pacific, a region where zooplankton biomass is heavily dominated by deep ontogenetically migrating calanoid copepods.

Euphotic zone nitrification in the NE subarctic Pacific: Implications for measurements of new production

Ammonium (NH4+) oxidation rates, nitrate (NO3−) uptake rates and dissolved nutrient concentrations were measured throughout the euphotic zone along the Line P oceanographic transect in the NE subarctic Pacific, on three cruises (winter, spring and late-summer) in 2009. NH4+ oxidation was found to proceed throughout the euphotic zone and rates ranged from undetectable to 97nmolL−1d−1. Evidence was found to suggest that both light and NH4+ concentrations played a role in regulating NH4+ oxidation rates, with rates increasing as light decreased and NH4+ concentrations increased. The observation that NH4+ oxidation was a ubiquitous process throughout the euphotic zone demonstrates that not all NO3− present in this region of the water column should be considered new nitrogen, and has important implications for estimates of new production in the NE subarctic Pacific. Depth-integrated NO3− uptake rates ranged from 0.65 to 6.0mmolm−2d−1, and we found that euphotic zone NH4+ oxidation could have supported between 3 and 100% of the phytoplankton NO3− demands along Line P. Taking potential euphotic zone NO3− regeneration into account resulted in new production estimates being reduced by an average of ~50%. During spring and late-summer, the degree by which new production was reduced, when euphotic zone NO3− regeneration was taken into consideration, was tightly anti-correlated to total NO3− uptake rates. This indicates that the implications of euphotic zone nitrification for estimates of new production are greatest in the less productive regions of the oceans.

Real-time estimation of pH and aragonite saturation state from Argo profiling floats: Prospects for an autonomous carbon observing strategy

[1] We demonstrate the ability to obtain accurate estimates of pH and carbonate mineral saturation state (Ω) from an Argo profiling float in the NE subarctic Pacific. Using hydrographic surveys of the NE Pacific region, we develop empirical algorithms to predict pH and Ω using observations of temperature (T) and dissolved O2. We attain R2 values greater than 0.98 and RMS errors of 0.018 (pH), 0.052 (Ωarag), and 0.087 (Ωcalc) for data between 30–500 m, σθ < 27.1. After calibrating optode-based O2 data, we apply the algorithms to T and O2 data from an Argo profiling float to produce a 14 month time-series of estimated pH and Ωarag in the upper water column of the NE subarctic Pacific. Comparison to independent data collected nearby in 2010 indicates pH and Ωarag estimates are robust. Although the method will not allow detection of anthropogenic trends in pH or Ωarag, this approach will provide insight into natural variability and the key biogeochemical controls on these parameters. Most importantly, this work demonstrates that an assemblage of well-calibrated regional algorithms and Argo float data can be used as a low-cost, readily-deployable component of a global ocean carbon observing strategy.

Fine-scale spatial and temporal variability of surface water dimethylsufide (DMS) concentrations and sea–air fluxes in the NE Subarctic Pacific

We report the results of a 2 year (6 cruises) survey of surface water dimethylsulfide (DMS) concentrations in the Subarctic Pacific Ocean using membrane inlet mass spectrometry for high spatial resolution measurements. Our results document strong variability in surface water DMS concentrations along a repeated coastal–oceanic transect. DMS concentrations were consistently low (~ 1–2 nM) in February, and increased to summertime mean values of ~ 8 nM. For both survey years, we observed higher DMS concentrations in near shore waters during early summer (June), but a reversal of this spatial gradient in August when the highest DMS concentrations were observed in open ocean regions. Sea–air DMS fluxes ranged from ~ 5 to 25 μmol m − 2 day − 1 and did not show the consistent spatial and temporal patterns observed for surface water concentrations. Fine-scale spatial variability was present for both surface water concentrations and sea–air fluxes. Some of this variability could be attributed to underlying physical and biologically heterogeneity, including the presence of localized phytoplankton blooms at the coastal–oceanic transition boundary. Using a simple statistical approach, we estimate that the mean length scale of DMS variability along our sampling transects was < 10 km, which was significantly shorter than that observed for physical variables (temperature and salinity), but somewhat larger than the variability length scale of chlorophyll a (chl a). Using multiple linear regression analysis, we were unable to produce a single consistent empirical relationship predicting surface DMS concentrations from a suite of biological and hydrographic variables. However, DMS did show significant correlations with some variables including calcite, chlorophyll a and mixed layer depth (MLD). Mean DMS concentrations across cruises were related to chl a/MLD ratios and the solar radiation dose, in a manner that was consistent with previously published algorithms. However, these algorithms failed to predict the mesoscale and sub-mesoscale variability along the cruise track. High frequency measurements of DMS during an extended occupation of Ocean Station Papa demonstrated large variability over periods ranging from ~ 12 to > 24 h. This variability could not be explained by simple empirical analysis, but showed some correspondence with daily cycles of incoming solar irradiance. Simple calculations suggest that biological turnover was likely a significant source of temporal variability in DMS concentrations at Ocean Station P. Our observations demonstrate the importance of high-resolution measurement techniques in furthering our understanding of ocean DMS dynamics.

Volcanic ash fuels anomalous plankton bloom in subarctic northeast Pacific

Using multiple lines of evidence, we demonstrate that volcanic ash deposition in August 2008 initiated one of the largest phytoplankton blooms observed in the subarctic North Pacific. Unusually widespread transport from a volcanic eruption in the Aleutian Islands, Alaska deposited ash over much of the subarctic NE Pacific, followed by large increases in satellite chlorophyll. Surface ocean pCO2, pH, and fluorescence reveal that the bloom started a few days after ashfall. Ship‐based measurements showed increased dominance by diatoms. This evidence points toward fertilization of this normally iron‐limited region by ash, a relatively new mechanism proposed for iron supply to the ocean. The observations do not support other possible mechanisms. Extrapolation of the pCO2 data to the area of the bloom suggests a modest ∼0.01 Pg carbon export from this event, implying that even large‐scale iron fertilization at an optimum time of year is not very efficient at sequestering atmospheric CO2.

Seasonal and interannual variability in phytoplankton and nutrient dynamics along Line P in the NE subarctic Pacific

We analysed mixed-layer seasonal and interannual variability in phytoplankton biomass and macronutrient (NO 3 and Si(OH) 4) concentrations from three decades of observations, and nitrogen uptake rates from the 1990s along Line P in the NE subarctic Pacific. Chlorophyll a concentrations near 0.35 mg m −3 were observed year-round along Line P except at the nearshore station (P4) where chlorophyll a concentrations in spring were on average 2.4 times the winter values. In contrast, the temporal variability in carbon-to-chlorophyll ratios at the two main end members of Line P (P4 and OSP) was high. Large seasonal and interannual variability in NO 3 and Si(OH) 4 concentration were observed along Line P. Highest upper mixed-layer (top 15 m) nutrient concentrations occurred on the continental shelf in late summer and early fall due to seasonal coastal upwelling. Beyond the shelf, maximum nutrient concentrations increased gradually offshore, and were highest in late winter and early spring due to mixing by winter storms. Interannual variations in upper mixed-layer nutrient concentrations beyond the shelf (>128°W) were correlated with E–W winds and the PDO since 1988 but were not correlated with either climate index between 1973 and 1981. Despite differences in nutrient concentration, nutrient utilization (ΔNO 3 and ΔSi(OH) 4) during the growing season were about 7.5 μM at all offshore stations. Variations in ΔNO 3 were correlated with those of ΔSi(OH) 4. The annual cycle of absolute NO 3 uptake (ρNO 3) and NH 4 uptake (ρNH 4) rates by phytoplankton in the upper mixed-layer showed a weak increasing trend from winter to spring/summer for the period 1992–1997. Rates were more variable at the nearshore station (P4). Rates of ρNO 3 were low along the entire line despite abundant NO 3 and low iron (Fe), at the offshore portion of Line P and sufficient Fe at the nearshore station (P4). As a result, new production contributed on average to only 32 ± 15% of the total nitrogen (N) uptake along Line P. NO 3 utilization in the NE subarctic Pacific is probably controlled by a combination of environmental variables, including Fe, light and ambient NH 4 levels. Elevated ambient NH 4 concentrations seem to decrease the rates of new production (and f-ratios) in surface waters of the oceanic subarctic NE Pacific. Contrary to expectation, phytoplankton biomass, nutrient utilization (ΔNO 3 and ΔSi(OH) 4), and nitrogen uptake (ρNO 3 + ρNH 4) varied relatively little along Line P, despite significant differences in the factors controlling phytoplankton composition assemblages and production. Future studies would benefit from including other variables, especially light limitation, to improve our understanding of the seasonal and interannual variability in phytoplankton biomass and nutrients in this region.

Phytoplankton processes during a mesoscale iron enrichment in the NE subarctic Pacific: Part III—Primary productivity

Abstract As part of the Canadian SOLAS program, a large scale iron (Fe) enrichment experiment (Subarctic Ecosystem Response to Iron Enrichment Study; SERIES) was performed in the NE subarctic Pacific in July of 2002. Dissolved Fe was added to a 77 km 2 patch of seawater and the evolution of the subsequent phytoplankton bloom was monitored for 26 days. Particulate organic primary productivity (OPP) inside the patch began to increase in all phytoplankton size-fractions (picophytoplankton, nanophytoplankton and microphytoplankton) relative to outside the patch within 48 h. After day 10, microphytoplankton (>20 μm) were responsible for the vast majority of both OPP and phytoplankton biomass. Maximum OPP of ca. 15 mmol C m −3 d −1 was achieved on day 15, representing a 20-fold increase from average OPP measured outside the patch. Water-column integrated, biomass (chl a )-specific OPP ( P b int ) of the total phytoplankton assemblage peaked twice, once following the first Fe infusion on day 4 (2.9 mmol C mg chl a −1 d −1 ) and then coinciding with maximum OPP on day 15 (2.6 mmol C mg chl a −1 d −1 ). Maximum P b int achieved on day 4 represented a 5-fold increase relative to P b int measured outside the patch. Water-column integrated OPP also peaked on day 15 at ca. 251 mmol C m −2 d −1 , and coincided with a rapid decline in silicic acid (Si(OH 4 )) concentrations. At this time, microphytoplankton accounted for ca. 90% of total OPP. Patch-averaged chlorophyll a (chl a ) concentrations were maximal (∼5 mg m −3 , >16 times the outside patch) on day 18, during which time microphytoplankton OPP had begun to decline. In addition to OPP, particulate inorganic primary productivity (IPP) also increased due to an elevated coccolithophore abundance, reaching a maximum of 0.25 mmol C m −3 d −1 achieved 9 days after the initial Fe enrichment, which then decreased back to rates similar to those measured outside of the patch. Changes in primary productivity were also assessed using pulse amplitude-modulated (PAM) fluorometry. Relative electron transport rates (ETR) obtained by PAM fluorometry were significantly correlated ( p r 2 =0.82) with the 14 C-based primary production rates during the Fe enrichment experiment. The increase in all measured photosynthetic parameters with Fe enrichment provides compelling evidence that primary productivity in the NE subarctic Pacific is regulated by Fe availability during the summer.

Evolution of phytoplankton pigments in an in-situ iron enrichment experiment in the subarctic NE Pacific

Abstract Phytoplankton pigments were analysed using high performance liquid chromatography (HPLC) analysis following an in-situ iron (Fe) enrichment in the subarctic NE Pacific. Profiles of pigment concentrations in the upper 100 m were measured both inside and outside the Fe-enriched patch over the first 14 days of the experiment. In the upper 50 m, all phytoplankton pigments measured, with the exception of zeaxanthin, were significantly higher inside the Fe-enriched patch than in surrounding waters. Chlorophyll a increased strongly in the Fe patch, with concentration in the upper 20 m reaching 1–1.3 mg m −3 between 7 and 14 days after enrichment, an order of magnitude higher than outside (0.1–0.15 mg m −3 ). Depth integrated chlorophyll a (mg m −2 ) in the upper 50 m inside the patch revealed two major peaks at around 7–9 and 12–14 days. During the first peak, 19′-hexanoyloxyfucoxanthin was the other major pigment, suggesting that prymnesiophytes were abundant at this time, and diagnostic pigment indices confirmed that nanoplankton dominated the phytoplankton community. Degradation pigments phaeophytin a 1 and a 2 , and phaeophorbide a 2 also peaked during this nanoplankton bloom. During the second peak in chlorophyll a (12–14 days), the other major pigment was fucoxanthin suggesting that diatoms were abundant, and diagnostic pigment indices confirmed that this later bloom was dominated by microplankton. Phaeophytin a 1 and a 2 and phaeophorbide a 1 also strongly increased in the Fe patch during this second peak. The only phytoplankton pigment not significantly higher in the upper 50 m of the Fe-enriched patch was zeaxanthin, and diagnostic pigment indices confirmed that picoplankton were relatively insensitive to Fe enrichment. The potential for pigments as markers for other biogeochemical consequences of the Fe fertilisation is briefly discussed.

Phytoplankton processes during a mesoscale iron enrichment in the NE subarctic Pacific: Part II—Nutrient utilization

Abstract The subarctic Pacific is one of the three main regions in which phytoplankton productivity is limited by the availability of iron (Fe). During the Subarctic Ecosystem Response to Iron Enrichment (SERIES) experiment, the response of phytoplankton to the addition of Fe and the consequential effects on chemical and physical water properties were monitored. Over the duration of the Fe-induced phytoplankton bloom, macronutrient concentrations (nitrate (NO 3 ), silicic acid (Si(OH) 4 ) and phosphate (PO 4 )) were drawn down with Si(OH) 4 being depleted to low concentrations ( 4 : NO 3 ratio varied between two phases of the bloom. From days 0 to 10 (phase I), when all phytoplankton size classes increased in biomass, the dissolved Si(OH) 4 : NO 3 ratio of the seawater in the patch increased as a result of the greater drawdown of NO 3 . After day 10 (phase II), when diatoms dominated the patch, a rapid decline in Si(OH) 4 concentrations resulted in a sharp decrease in the Si(OH) 4 : NO 3 ratio of the seawater. Increases in the suspended particulate biogenic silica (BSi) and particulate nitrogen (PN) resulted in a BSi: PN ratio of ca. 2 in the later stages of the Fe-induced bloom. The uptake of NO 3 was enhanced due to the Fe enrichment. In the patch, absolute NO 3 uptake rates increased in both large (⩾5 μm) and small ( 3 uptake over the duration of the experiment. Biomass-specific NO 3 uptake rates also increased, but, in the small cells, the extent of the increase was largely dependent on the proxy for biomass used (PN or chlorophyll a ). The photosynthetic efficiency of the phytoplankton assemblage was assessed at various stages of the bloom through the use of pulse amplitude-modulated (PAM) fluorometry. The trends in maximal and operational photochemical yields measured in the patch suggest that bloom termination resulted from a combination of Fe-stress and Si-stress. The observed changes in nutrient utilization during SERIES demonstrate the crucial role of Fe in regulating macronutrient inventories and NO 3 uptake rates by phytoplankton in Fe-limited regions such as the NE subarctic Pacific.

Mesozooplankton community response during the SERIES iron enrichment experiment in the subarctic NE Pacific

The response of the mesozooplankton community was examined during the development and early decline of the Subarctic Ecosystem Response to Iron Enrichment Study diatom bloom in July 2002. No significant changes in species composition were observed during this period. In terms of abundance, the community was dominated by small copepods (Oithona spp. and Pseudocalanus spp.). The abundance of large herbivorous calanoids of the genus Neocalanus (N. plumchrus, N. cristatus and N. flemingeri) was already low at the start of the experiment as most of these animals had already descended to their overwintering depth. Mesozooplankton biomass was dominated by the calanoids Eucalanus bungii, Calanus pacificus, Metridia pacifica and, to a lesser extent, late stage copepodites (CIV, CV) of Neocalanus spp. In general, the abundance of all copepod species tended to increase inside the patch (relative to conditions outside the patch) during the experiment. However, mean species-specific abundances were not significantly different inside and outside of the patch, except for a significant increase of E. bungii (p<0.05) that occurred between Days 10 and 13 inside the iron-fertilized patch. By Day 8, most of the E. bungii population had shifted its vertical distribution into the surface mixed layer. We hypothesize that a wind event displacing the surface mixed layer coincident with continued immigration from below the thermocline facilitated this increase in abundance. Here we address this higher trophic level response to pulsed primary production in terms of potential mechanisms and their potential impact on the diatom bloom.

Phytoplankton community response to Fe and temperature gradients in the NE (SERIES) and NW (SEEDS) subarctic Pacific Ocean

Abstract Ship-board iron enrichment bottle experiments were carried out with samples collected at the mesoscale iron fertilization experimental site (SERIES) in the subarctic NE Pacific in the summer of 2002. Samples were collected on Day 14 of the experiment outside the patch that was in a typical high nitrate and low chlorophyll (HNLC) condition. The iron concentration in the incubation bottles ranged from 0.1 to 2.0 nM by adding FeCl3 solution. The increase in chlorophyll-a (chl-a) in the micro (>10 μm) and nano-sized (2–10 μm) fraction was observed as a function of the added iron. Chl-a in the pico-sized fraction (0.7–2 μm) showed no increase with time. Nitrate and silicate were exhausted in the Fe-amended bottles, while those in the control bottle remained at the end of incubation. The relative consumption ratio of silicate to nitrate for the control bottles was significantly higher than that for the Fe-amended bottles. As a hyperbolic relation was found between iron concentration and the rate of increase in Chl-a (specific growth rate) for the micro and nano-sized fraction, the Monod equation was fit to obtain a maximum growth rate (μmax) and a half-saturation constant for iron (KFe). The μmax values were 0.72 and 0.48 d−1 for the micro and nano-sized fraction, respectively. The KFe values were 0.10 and 0.08 nM for the micro and nano-sized fraction, respectively. The μmax agreed with the rate of increase in Chl-a observed in situ for the mesoscale iron fertilization experiment. The μmax value for micro-sized fraction at 12 °C was half of that in the western subarctic Pacific Ocean (SEEDS experiment in 2001), indicating the Chl-a increase rate (potential growth rate) after iron enrichment was much higher in SEEDS than that in SERIES. The KFe values were much lower than that in SEEDS, suggesting that the phytoplankton community in the NE subarctic Pacific Ocean acclimates to a lower ambient Fe concentration. This difference in KFe between SERIES (NE) and SEEDS (NW) may reflect the previously suggested gradient in Fe flux to the subarctic Pacific Ocean. A temperature gradient was also applied to investigate the effect of temperature on the growth response of the phytoplankton community. No obvious effect of temperature increase to 16 °C was found in SERIES, while μmax and KFe changed significantly with temperature in SEEDS.

Mesozooplankton response to iron enrichment during the diatom bloom and bloom decline in SERIES (NE Pacific)

Abstract A mesoscale iron-fertilization experiment was carried out in the eastern subarctic Pacific during summer 2002. The iron patch was traced for 26 days after the enrichment, and the abundance and behavior of mesozooplankton was compared with those outside of the patch during the first half of the experiment (days 2–18) by Sastri and Dower [2006. Mesozooplankton community response during the SERIES iron enrichment experiment in the subarctic NE Pacific. Deep-Sea Research Part II.) and during the post-enrichment diatom bloom and its period of decline (days 15–26; this paper). The surface chlorophyll-a concentration in the patch was high between days 15 and 17 (6 mg m−3) and decreased to 1.4 mg m−3 at the end of the observation. Dominant zooplankton species in the upper 200 m were copepods: Eucalanus bungii, Pseudocalanus spp., Neocalanus plumchrus, N. cristatus, and Metridia pacifica. Species composition did not change significantly in the patch over the observation period. However, shallower distribution depths of E. bungii, N. cristatus and M. pacifica were observed in the patch during and after the diatom bloom. Especially, E. bungii was mainly distributed in the subsurface layer outside of the patch, but it was mainly in the surface mixed layer inside the patch, where it also had an enhanced development rate and increased biomass. We also propose the accumulation mechanism of zooplankton in the patch due to the upward immigration. Moreover, the abundance of the first copepodite stage of E. bungii and calyptopis larvae of euphausiids increased several fold in the patch compared to the densities outside the patch. The increases in both species are considered to be due to lowered mortality during the egg and naupliar stages, which was caused by lowered relative importance of eggs and nauplii in the diets of the suspension-feeding omnivores in the patch due to increased diatom abundance during the diatom bloom. Gut-pigment contents of dominant copepods in the patch increased 6–8 times, and the maximum values were observed during the bloom peak. The grazing impact on phytoplankton was low during the bloom period, but increased in the declining period of the diatom bloom.

Modelling the ecosystem response to iron fertilization in the subarctic NE Pacific: The influence of grazing, and Si and N cycling on CO2 drawdown

Abstract We have employed a coupled one-dimensional mixed layer /ecosystem /carbon cycle model to simulate both the normal annual cycle and the iron-fertilization experiment in the subarctic NE Pacific Ocean near Ocean Station P (50°N, 145°W) during summer 2002. We considered two size classes of phytoplankton, the larger representing diatoms, where each size class has a different degree of iron limitation, and compartments for nitrate, ammonium, microzooplankton, sinking detritus, and a prescribed annual cycle in mesozooplankton. The base ecosystem model is formulated in terms of nitrogen, but is coupled to sub-models of silicon and carbon. Diatoms formed aggregates during blooms that sink rapidly from the surface ocean, and diatoms also can be grazed by microzooplankton, consistent with observations. Using the same parameter set as for the base ecosystem model, we reproduce the basic responses to fertilization: an initial bloom of small phytoplankton (including calcifying coccolithophorids), followed rapidly by an increase of microzooplankton biomass; a continuing increase in diatoms that peak as silicate becomes limiting; and a later rapid sinking event of both carbon and silica particulates. Generally this sequence proceeds more rapidly in simulations than in situ. Simulations of the fertilization response show little sensitivity to the assumed fraction of small phytoplankton that are calcifiers, but a strong sensitivity to the assumed diatom uptake ratio Si:N. With an uptake ratio of 2.5, silicate is rapidly exhausted after fertilization, and 8 months later the regional p CO 2 was 14 μatm higher than in the case with no fertilization (assuming no exchange with surrounding waters): for all other simulations the p CO 2 anomaly is negative (indicating increased CO 2 exchange from the atmosphere) but small, 2–5 μatm, suggesting a persistence for a single large-scale fertilization of less than 1 year. Simulated mixed layer Si:N drawdown ratios for different fixed diatom uptake ratios of Si:N illustrate the dangers of interpreting uptake ratios from drawdown ratios: (i) for a fixed uptake ratio the drawdown ratio varies with time as the ratio of small to large phytoplankton changes, and (ii) simulated drawdown ratios are always higher than uptake ratios because of the more rapid recycling (according to the model structure) of N relative to Si in the surface layer. Sensitivity simulations, with the diatom uptake ratio for Si:N varying as an inverse function of iron limitation and with a higher remineralization rate for detrital Si, delayed the onset of silica limitation after fertilization by several days. The magnitude and timing of the diatom peak was unchanged, indicating that in the model the (early) termination of the diatom bloom following fertilization resulted from formation and sinking of aggregates, and not Si limitation.

Climatic effect on DMS producers in the NE sub-Arctic Pacific: ENSO on the upper ocean

We examined dimethylsulphide (DMS) data of two locations (P26 and P20) 1996–2001 in sub-Arctic NE Pacific with emphasis on 2 yr of DMS production measured in June and September of 1999 and 2000. At Station P26, integrated DMS to mixed layer depth (MLD) in June during 1999–2001 La Ni˜na period averaged 102 ± 25 μmol m-2 or only 42% of the average of 242 ± 27 μ mol m-2 measured during 1997–1998 El Ni˜no period. The summer integrated DMS to MLD of 246 μmol m-2 in the year 2000 was 61% of summer average in the period 1996–1998 of 402 ± 115 μmol m-2. At Station P20 where the mean DMS level of the MLD in June during the La Ni˜na years of 1999–2001 was 77 ± 7 μmol m-2, about 35% of 232 ± 22 μmol m-2 during the 1997–1998 El Ni˜no. Phytoplankton species rich in DMSP (an algal precursor to DMS) were also more abundant in 1998 than in the years 1999–2001 with low DMS. The ENSO event was characterized by warmer, more stratified waters with MLD shallowing to about 19 m from 1996 to early 1998, compared to the following years. These results provide a first example of how climate fluctuations, through altering the physical and chemical properties of the upper ocean, may influence the structure of the phytoplankton assemblage and hence DMS concentrations in the open ocean.

Physical processes that enhance nutrient transport and primary productivity in the coastal and open ocean of the subarctic NE Pacific

Abstract In comparison to the open ocean, several additional processes including coastal upwelling, river discharge, tidal mixing, estuarine circulation and benthic remineralization enhance nutrient supply to the surface waters of the continental shelf. In general, coastal waters become nitrate-limited during the phytoplankton growing season, whereas iron and dissolved silicate limit phytoplankton growth in the less productive oceanic waters of the Gulf of Alaska. If coastal processes supply ample amounts of macro and micro nutrients during the growing season, diatom communities dominated by species such as Skeletomema costatum, Chaetoceros spp. and Thalassiosira spp. will bloom. Growth rates of these bloom populations typically range from 0.5 to >1.5 doublings per day, which place a high demand on nutrients. Any transport of coastal waters away from the shelf will enhance productivity in oceanic waters. The general circulation of the eastern subarctic Pacific does not allow for offshore transport except in special circumstances. These include anticyclonic mesoscale eddy formation, which can export as much as 5000 km 3 of nutrient-rich waters from the shelf in a single eddy, and recirculation of waters away from southwestern Alaska due to the cyclonic circulation around the western edge of the Alaskan Gyre. Recirculation can carry nutrient-rich water from the coast to the vicinity of Ocean Station Papa (50°N, 145°W) within a few months. For both eddies and gyre recirculation, much of the water being carried into the open ocean lies below the euphotic zone. Iron enrichment occurs to a depth of at least 1000 m in the Gulf of Alaska as a result. Periods of enhanced eddy formation or recirculation may supply iron that enriches the open ocean for several years. Damming of the Columbia River and human uses of its waters have resulted in more winter and less summer discharge of fresh water and dissolved Si. Coastal currents in this area flow north in winter and south in summer. As a consequence, there has been a net temporal change in Si inputs to the ocean since the mid 1970s, with about 4×10 9 mol more Si flowing north in winter and 9×10 9 mol less Si flowing south in summer compared with the early 1900s. Since silicate limits diatom growth and diatoms are responsible for most carbon export in phytoplankton, implications for ocean productivity are significant.

Seasonal and interannual trends in heterotrophic bacterial processes between 1995 and 1999 in the subarctic NE Pacific

Abstract Heterotrophic bacterial abundance and productivity were measured in the euphotic zone over 4 years during winter, late spring, and late summer in the subarctic NE Pacific from September 1995 to February 1999. Sampling took place during 11 cruises at five hydrographic stations spaced along the east/west line- P transect from slope waters at P4 (1200 m depth) to the open-ocean waters at Ocean Station Papa (OSP) (4250 m depth). P4 exhibits a spring bloom and summer nutrient depletion, whereas OSP exhibits no spring bloom, consistently replete macronutrients, and iron limitation of primary productivity. Bacterial biomass, derived from cell abundance (assuming 10 fg C cell −1 ), ranged from an average of ∼6 μg C l −1 in the winter to 10 and 14 μg C l −1 in the spring and summer all along line- P . Factors for converting the uptake of radiolabeled substrate to bacterial productivity were determined empirically from dilution experiments. The empirically derived conversion factors for both thymidine and leucine were positively correlated to in situ cell abundance. Bacterial productivity estimated from [ 3 H]-thymidine and [ 14 C]-leucine incorporation rates was lowest in the winter (∼0.2 μg C l −1 d −1 ), with little spatial variability. Bacterial productivity increased 4–10-fold in spring at P4 compared with 0–4-fold at the more oceanic stations. Between 1995 and 1998, the average summer bacterial abundance all along line- P decreased by ∼50%, while bacterial productivity decreased by ∼90%. These decreases reduced summer bacterial productivity and growth rates to nearly winter levels in both the open-ocean and slope waters during late summer 1998. The average bacterial productivity at OSP is similar to other open-ocean regions while bacterial abundance is slightly higher providing for relatively lower growth rates and inferring lower associated loss rates.

Time-series study of the biogeochemistry of the North East subarctic Pacific: reconciliation of the Corg/N remineralization and uptake ratios with the Redfield ratios

Abstract Ten years of a time-series study at Station P or OSP (50°N, 145°W) in the NE subarctic Pacific are presented in this paper. Measurements of nutrients, primary production, dissolved inorganic carbon (DIC), total alkalinity (TA), dissolved organic carbon and nitrogen (DOC and DON), and sinking organic and inorganic particulate fluxes (derived from free-drifting sediment traps positioned every 100 m in the upper 1000 m) were made. One-dimensional C and N budgets were calculated in the mixed layer and in 4 layers below 50 m down to 1000 m. The remineralization and uptake stoichiometric ratios were determined, with an emphasis on the C org /N ratios. Three independent approaches were used: (1) nutrient vertical gradients in the 100–1000 m, (2) nutrient seasonal drawdown in the 0–50 m upper layer, and (3) nutrient regeneration from sinking particles. Analysis of the nutrient vertical gradients showed that the P/N/C org /−O 2 remineralization molar ratios were 1/16.6/107.9/−170.7 and that the average C org /N was about 6.5 in the upper 400 m. In contrast, the seasonal DIC:NO 3 − drawdown molar ratio varied widely from 6.9 to 12.1 and averaged 9.3. However, when corrections were made for CO 2 gas exchange, dissolved organic matter (DOM) production and CaCO 3 production, the true average C org /N uptake ratio was estimated to be 6.1. This study suggested that the elevated DIC:NO 3 − depletion ratios were mostly due to calcification, and that the net effect of gas exchange and DOM production was small at station P. Furthermore, whereas the POC/PON flux (particulate organic C and N fluxes) ratio ranged from 8 to 12 (mole ratio) and increased with depth, there was a linear relationship between the POC and PON fluxes, with a slope of 7.2±0.2. This indicated that the C org /N remineralization ratio from sinking particles was relatively low and that there was a methodological problem with free-drifting sediment traps.