Subarctic Northeast Pacific Research Articles

Characterizing dietary variability and trophic positions of coastal calanoid copepods: insight from stable isotopes and fatty acids

The spring zooplankton community in the Strait of Georgia (British Columbia, Canada) is characterized by the presence of several calanoid copepod species which collectively make up ~90% of the mezozooplankton biomass. Here, we investigate interspecific, interannual, and geographic variability in the diets and trophic positions of these copepods using a combination of fatty acids and stable isotopes. To characterize geographic variability in diet, we compare our findings from the Strait of Georgia with similar data from Ocean Station P in the subarctic northeast Pacific. Both fatty acid and stable isotope signatures indicate the existence of three trophic levels, even within the limited size range of these copepods: Neocalanus plumchrus and Calanus marshallae are primarily omnivorous, while Euchaeta elongata is carnivorous and Eucalanus bungii is herbivorous. Fatty acid markers of trophic position (e.g., DHA/EPA, 18:1n-9/18:1n-7) correlate significantly with δ15N, while markers indicating the proportion of diatoms to flagellates in the diet (e.g., 16PUFA/18PUFA and DHA/EPA) correlate significantly with δ13C, after the effect of lipid concentration on δ13C is accounted for. Despite the general correlation between stable isotopes and fatty acids, the former are not sensitive enough to capture the range of interannual variability observed in the latter, and can only capture substantial shifts in the diet over geographic scales. However, regardless of variability in food quality, the relative trophic positions of these copepods do not change significantly either spatially or temporally.

Effects on zooplankton of a warmer ocean: Recent evidence from the Northeast Pacific

The consequences for pelagic communities of warming trends in mid and high latitude ocean regions could be substantial, but their magnitude and trajectory are not yet known. Environmental changes predicted by climate models (and beginning to be confirmed by observations) include warming and freshening of the upper ocean and reduction in the extent and duration of ice cover. One way to evaluate response scenarios is by comparing how “similar” zooplankton communities have differed among years and/or locations with differing temperature. The subarctic Pacific is a strong candidate for such comparisons, because the same mix of zooplankton species dominates over a wide range of temperature climatologies, and observations have spanned substantial temperature variability at interannual-to-decadal time scales. In this paper, we review and extend copepod abundance and phenology time series from net tow and Continuous Plankton Recorder surveys in the subarctic Northeast Pacific. The two strongest responses we have observed are latitudinal shifts in centers of abundance of many species (poleward under warm conditions), and changes in the life cycle timing of Neocalanus plumchrus in both oceanic and coastal regions (earlier by several weeks in warm years and at warmer locations). These zooplankton data, plus indices of higher trophic level responses such as reproduction, growth and survival of pelagic fish and seabirds, are all moderately-to-strongly intercorrelated (∣ r∣ = 0.25–0.8) with indices of local and basin-scale temperature anomalies. A principal components analysis of the normalized anomaly time series from 1979 to 2004 shows that a single “warm-and-low-productivity” vs. “cool-and-high-productivity” component axis accounts for over half of the variance/covariance. Prior to 1990, the scores for this component were negative (“cool” and “productive”) or near zero except positive in the El Niño years 1983 and 1987. The scores were strongly and increasingly positive (“warm” and “low productivity”) from 1992 to 1998; negative from 1999 to 2002; and again increasingly positive from 2003-present. We suggest that, in strongly seasonal environments, anomalously high temperature may provide misleading environmental cues that contribute to timing mismatch between life history events and the more-nearly-fixed seasonality of insolation, stratification, and food supply.

Variations in nutrients, carbon and other hydrographic parameters related to the 1976/77 and 1988/89 regime shifts in the sub-arctic Northeast Pacific

For time series at Station P (50°N, 145°W) and stations along Line P, long term changes in eight oceanographic and chemical parameters (sea surface temperature (SST), salinity, oxygen, phosphate (PO 4), silicate (SiO 4), nitrate (NO 3), dissolved inorganic carbon (DIC), and apparent oxygen utilisation (AOU)) were influenced by climate regime shifts with a “step change” of anomalies for nutrients and carbon in the sub-arctic Pacific during the 1976/77 and 1988/89 regime shifts. The presence of regime shifts in the data in the late 1970s and the late 1980s was supported by the statistical test of [Rodionov, S.N., 2004. A sequential algorithm for testing climate regime shifts. Geophysical Research Letters 31, L09204, doi:10.1029/2004GL019448], based on the Student t-test. The response of nutrients and carbon to the regime shifts was more intensive in 1976/77 than in 1988/89. Salinity, PO 4, SiO 4, NO 3, oxygen and DIC showed positive anomalies during 1950–1975 and negative ones during 1976–1995. The effect of La Niña on nutrients and carbon was larger than that of El Niño. Strong La Niña events (e.g. 1988/89) caused a sudden increase in nutrients and carbon. Two regime shifts (1976/77 and 1988/89) occurred just after two strong La Niña events in 1976/77 and 1988/89. At Station P, upwelling of nutrient-poor subsurface water tended to decrease the nutrients at the surface.

Assessing microbial responses to iron enrichment in the Subarctic Northeast Pacific: Do microcosms reproduce the in situ condition?

Abstract A microcosm experiment was conducted in the NE Pacific in July 2002 to compare the microbial response between microcosms and the Subarctic Ecosystem Response to Iron-Enrichment Study (SERIES) in situ iron-enrichment experiment. Seawater microcosms (20 L) were incubated aboard ship under natural light using three treatments: (1) low-iron seawater amended with 4 nmol l−1 FeSO4 (+Fe); (2) low-iron seawater amended with 4 nmol l−1 FeSO4 and 86 nmol l−1 GeO2 (+Fe+Ge); (3) seawater collected from the in situ Fe-enriched patch (PW). The +Fe+Ge treatment used germanium to control diatom growth to assess the role of diatoms in dimethylsulfoniopropionate (DMSP) production. The following variables were measured in the microcosms and in situ: chlorophyll a (chl a), nitrate ( NO 3 - ), silicic acid (Si(OH)4), phytoplankton abundance and species identification, bacterial abundance (including estimates of low- and high-DNA bacteria), bacterial production, bacterial specific growth rate, particulate and dissolved DMSP and dimethylsulfide (DMS) concentrations. There was little or no significant difference (ANCOVA) in the response of most variables between the +Fe and PW microcosms, but large differences were observed between both these treatments and the in situ data from the enriched patch. Chl a in all microcosms increased from ambient levels (approx. 0.5–1 μg l−1) to approx. 4.5–6.2 μg l−1 after 11 d incubation, when NO 3 - was fully depleted from all microcosms. During this same period, in situ chl a increased more slowly to a maximum of 2.9 μg l−1 on day 11. Nanophytoplankton and picophytoplankton were more abundant in the microcosms relative to the in situ community, which became dominated by large diatoms. Bacterial abundance was similar in the microcosms and in situ, but bacterial production was significantly higher in the microcosms. While neither DMSPd nor DMS accumulation showed significant differences between the microcosms and in situ, particulate DMSP concentrations increased significantly faster in the +Fe and PW treatments. These differences represent bottle effects resulting from the containment of the microcosms, which suppresses grazing, alters community and food web structure, enhances iron and nutrient regeneration, and isolates the community from physical transport and export processes including sinking. Thus during this experiment, the microcosms were not a good model for the in situ system in terms of the effects of iron on the phytoplankton biomass, nutrient uptake, bacterial dynamics and DMSPp production. In the germanium-amended treatment, the inhibition of diatom growth resulted in enhanced growth of other taxa and a suppression of bacterial production, leading to increased production of DMSP and DMS and strong correlations between DMSP, DMS and non-diatom phytoplankton taxa. Diatoms did not contribute significantly to particulate DMSP concentrations.

Microbial response to a mesoscale iron enrichment in the NE Subarctic Pacific: Bacterial community composition

Changes in microbial community composition were determined during the subarctic ecosystem response to iron enrichment study (SERIES), a mesoscale Fe enrichment conducted in a high-nutrient low-chlorophyll (HNLC) region of the Northeast Subarctic Pacific, in July 2002. Phylogenetic composition using fluorescence in situ hybridization (FISH), relative DNA content using flow cytometry (FCM), and cellular morphometrics (shape and volume) of heterotrophic bacteria were used to characterize community composition from samples collected within and below the mixed layer, inside and outside the Fe-patch. The proportion of total cells detected as members of the Cytophaga-Flavobacterium cluster increased in a log-linear manner from 16 (±1.0)% to 47 (±1.9)% in samples within the mixed layer, inside the Fe-enriched patch, while outside the patch, the proportion remained ⩽21 (±2.2)%. Temporal changes in the proportion of cells in the mixed layer with high DNA content (% HDNA) were significantly different inside and outside the Fe-enriched patch, where inside the patch % HDNA increased 2-fold after a week, reaching 93% towards the end of the observation period. Coupling in situ observations with the results of manipulation experiments allowed us to determine the relative contributions of bottom-up (nutrient limitation) and top-down (grazing) processes on heterotrophic bacterial abundance and community composition. Shifts in heterotrophic bacterial community composition inside the Fe-enriched patch were mainly controlled by top-down processes and moderately controlled by bottom-up controls (organic substrate limitation).

DMSP and DMS dynamics during a mesoscale iron fertilization experiment in the Northeast Pacific–Part II: Biological cycling

Dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) biological cycling rates were determined during SERIES, a mesoscale iron-fertilization experiment conducted in the high-nutrient low-chlorophyll (HNLC) waters of the northeast subarctic Pacific. The iron fertilization resulted in the rapid development of a nanoplankton assemblage that persisted for 11 days before abruptly crashing. The nanoplankton bloom was followed by a diatom bloom, accompanied by an important increase in bacterial abundance and production. These iron-induced alterations of the plankton assemblage coincided with changes in the size and biological cycling of the DMSP and DMS pools. The initial nanoplankton bloom resulted in increases in particulate DMSP (DMSPp; 77–180 nmol L−1), dissolved DMSP (DMSPd; 1–24 nmol L−1), and biological gross (0.11–0.78 nmol L−1 h−1) and net (0.04–0.74 nmol L−1 h−1) DMS production rates. During the nanoplankton bloom, DMSPd consumption by bacteria exceeded their sulfur demand and the excess sulfur was probably released as DMS, consistent with the high gross DMS production rates observed during that period. The crash of the nanoplankton bloom was marked by the rapid decline of DMSPp, DMSPd, and gross DMS production to their initial values. Following the crash of the nanoplankton bloom, bacterial production and estimated sulfur demand reached transient maxima of 9.3 μg C L−1 d−1 and 14.2 nmol S L−1 d−1, respectively. During this period of high bacterial production, bacterial DMSPd consumption was also very high (6 nmol L−1 h−1), but none of the consumed DMSPd was converted into DMS and a net biological DMS consumption was measured. This transient period initiated a rapid decrease in DMS concentrations inside the iron-enriched patch, which persisted during the following diatom bloom due to low biological gross and net DMS production that prevented the replenishment of DMS. Our results show that the impact of Fe fertilization on DMS production in HNLC waters result from a complex interplay between the dynamics of the algal blooms and their influence on bacterial DMSP and DMS metabolism.

INTERACTIVE EFFECTS OF IRRADIANCE AND TEMPERATURE ON THE PHOTOSYNTHETIC PHYSIOLOGY OF THE PENNATE DIATOM PSEUDO‐NITZSCHIA GRANII (BACILLARIOPHYCEAE) FROM THE NORTHEAST SUBARCTIC PACIFIC1

This study examined how light and temperature interact to influence growth rates, chl a, and photosynthetic efficiency of the oceanic pennate diatom Pseudo‐nitzschia granii Hasle, isolated from the northeast subarctic Pacific. Growth rates were modulated by both light and temperature, although for each irradiance tested, the growth rate was always the greatest at ∼14°C. Chl a per cell was affected primarily by temperature, except at the maximum chl a per cell (at 10°C) where the effects of light were noticeable. At both ends of the temperature gradient, cells displayed evidence of chlorosis even at low light intensities. Chl fluorescence data suggested that cells at 8°C were significantly more efficient in their photosynthetic processes than cells at 20°C, despite having comparable concentrations of chl. Cells at low temperature showed photosynthetic characteristics similar to high‐irradiance‐adapted cells. The decline of growth rates beyond the optimum growth temperature coincided with the cell's inability to accumulate chl in response to increasing temperature. The decline in photosynthetic ability at 20°C was likely due to a combination of high‐temperature stress on cellular membranes and a decline in chl. Our results highlight the important interactions between light and temperature and the need to incorporate these interactions into the development of phytoplankton models for the subarctic Pacific.

The evolution and termination of an iron-induced mesoscale bloom in the northeast subarctic Pacific

We initiated and mapped a diatom bloom in the northeast subarctic Pacific by concurrently adding dissolved iron and the tracer sulfur hexafluoride to a mesoscale patch of high-nitrate, low-chlorophyll waters. The bloom was dominated by pennate diatoms and was monitored for 25 d, which was sufficiently long to observe the evolution and termination of the bloom and most of the decline phase. Fast repetition–rate fluorometry indicated that the diatoms were iron-replete until day 12, followed by a 4–5-d transition to iron limitation. This transition period was characterized by relatively high rates of algal growth and nutrient uptake, which pointed to diatoms using intracellularly stored iron. By days 16–17, the bloom was probably limited simultaneously by both iron and silicic acid supply, because low silicic acid concentrations were evident. Modeling simulations, using data from our study, provided an estimate of the critical threshold for algal aggregation. Observed diatom abundances during the bloom exceeded this threshold between days 13 and 17. Mass sedimentation of diatoms and diatom aggregates was recorded in surface-tethered free-drifting sediment traps at 50 m in depth on day 21. Although the termination of the bloom was probably controlled by the availability of both iron and silicic acid, we cannot rule out the role of algal aggregation. The bloom decline was likely triggered by the onset of mass sedimentation. During our study, evidence of both diatom species succession and species-specific aggregation point to important links between algal nutrient stress and the initiation of algal aggregation.

Macronutrient dynamics in an anticyclonic mesoscale eddy in the Gulf of Alaska

Abstract Each winter, long-lived anticyclonic eddies are spawned off the southern tip of the Queen Charlotte Islands off the British Columbia coast. An anticyclonic eddy that formed in winter 2000 (Haida-2000) was sampled six times over 20 months as it traveled from the coast into High Nitrate-Low Chlorophyll waters of the Subarctic northeast Pacific. Repeated shipboard observations coupled with satellite radar altimetry and ocean colour suggest that Haida-2000 underwent a phytoplankton bloom early in life while still in coastal waters. This bloom caused a near depletion in eddy surface nutrients (nitrate, phosphate, silicic acid). While nitrate concentrations were restored to initial levels during winter 2001, the silicic acid inventory within Haida-2000 remained lower than initial observations. Below the euphotic zone, deep nutrient concentrations were altered by eddy decay, interactions with bathymetric features, and by the coalescence of a second, younger eddy that restored coastal characteristics within the core of Haida-2000. Estimates of new production (3–3.5 mmol NO3− m−2 d−1) derived from seasonal changes in nitrate inventories fell between values previously reported for coastal and mid-gyre environments for both years studied. In contrast, removal of silicic acid was twice as high (7.0 mmol Si(OH)4 m−2 d−1) as nitrate during the first year, but less than half as high in Year 2 (1.3 mmol Si(OH)4 m−2 d−1). Changes in the timing of nutrient drawdown accompanied the shift from high to low Si(OH)4:NO3− drawdown ratios, with the maximum changing from spring to autumn, similar to long-term observations at Ocean Station P (50°N, 145°W). Relative to the local environment, the eddy evolved from a nutrient-rich to a nutrient-poor body of water, indicating that the path of these anticyclonic eddies determines their role in nutrient supply and distribution in the Gulf of Alaska.

Survey of bottom-up controls of Emiliania huxleyi in the Northeast Subarctic Pacific

In contrast to the North Atlantic, there are few studies of the coccolithophore Emiliania huxleyi in the Northeast Subarctic Pacific. Yet, recent observations in this area suggest that during spring E. huxleyi is more important than previously thought. To survey potential bottom-up controls of E. huxleyi, seawater samples were collected from the surface layer in the Northeast (NE) Pacific, mainly during June 2001. Average surface mixed layer irradiance, temperature, salinity, nutrient concentrations (nitrate, phosphate, silicate), and abundances of E. huxleyi were determined. Data were partitioned among high nutrient-low chlorophyll (HNLC) waters and mesoscale eddies that carry waters enriched with macronutrients and iron. No significant differences existed between HNLC waters and mesoscale eddies, with respect to relationships among E. huxleyi abundance and physical and chemical parameters. All chemical and physical parameters, except underwater irradiance, were significantly inter-correlated . E. huxleyi represented up to 67%, but generally <10%, of autotrophic carbon. Abundances were highest at lower temperatures and higher nutrient concentrations, underwater irradiance, and salinity.

Impacts of atmospheric variability on a coupled upper‐ocean/ecosystem model of the subarctic Northeast Pacific

The biologically‐mediated flux of carbon from the upper ocean to below the permanent thermocline (the biological pump) is estimated to be ∼10 PgC/yr [Houghton et al., 2001], and plays an important role in the global carbon cycle. A detailed quantitative understanding of the dynamics of the biological pump is therefore important, particularly in terms of its potential sensitivity to climate change and its role in this change via feedback processes. Previous studies of coupled upper‐ocean/planktonic ecosystem dynamics have considered models forced by observed atmospheric variability or by smooth annual and diurnal cycles. The second approach has the drawback that environmental variability is ubiquitous in the climate system, and may have a nontrivial impact on the (nonlinear) dynamics of the system, while the first approach is limited by the fact that observed time series are generally too short to obtain statistically robust characterizations of variability in the system. In the present study, an empirical stochastic model of high‐frequency atmospheric variability (with a decorrelation timescale of less than a week) is estimated from long‐term observations at Ocean Station Papa in the northeast subarctic Pacific. This empirical model, the second‐order statistics of which resemble those of the observations to a good approximation, is used to produce very long (1000‐year) realizations of atmospheric variability which are used to drive a coupled upper‐ocean/ecosystem model. It is found that fluctuations in atmospheric forcing do not have an essential qualitative impact on most aspects of the dynamics of the ecosystem when primary production is limited by the availability of iron, although pronounced interannual variability in diatom abundance is simulated (even in the absence of episodic iron fertilization). In contrast, the impacts of atmospheric variability are considerably more significant when phytoplankton growth is limited in the summer by nitrogen availability, as observed closer to the North American coast. Furthermore, the high‐frequency variability in atmospheric forcing is associated with regions in parameter space in which the system alternates between iron and nitrogen limitation on interannual to interdecadal timescales. Both the mean and variability of export production are found to be significantly larger in the nitrogen‐limited regime than in the iron‐limited regime.

Seasonal changes in the distribution of dissolved organic nitrogen in coastal and open-ocean waters in the North East Pacific: sources and sinks

The distribution of dissolved organic nitrogen (DON) and nitrate were determined seasonally (winter, spring and summer) during three years along line P, i.e. an E-W transect from the coast of British Columbia, Canada, to Station P (50degreesN, 145degreesW) in the subarctic North East Pacific Ocean. In conjunction, DON measurements were made in the Straits of Juan de Fuca and Georgia within an estuarine system connected to the NE Pacific Ocean. The distribution of DON at the surface showed higher values of 4-17 muM in the Straits relative to values of 4-10 muM encountered along line P, respectively. Along line P, the concentration of DON showed an inshore-offshore gradient at the surface with higher values near the coast. The equation for the conservation of DON showed that horizontal transport of DON (inshore-offshore) was much larger than vertical physical mixing. Horizontal advection of DON-rich waters from the coastal estuarine system to the NE Pacific Ocean was likely the cause of the inshore-offshore gradient in the concentration of DON. Although the concentration of DON was very variable in space and time, it increased from winter to summer, with an average build up of 4.3 muM in the Straits and 0.7 muM in the NE subarctic Pacific. This implied seasonal DON sources of 0.3 mmol N m(-2) d(-1) at Station P and 1.5 mmol N m(-2) d(-1) in the Straits, respectively. These seasonal DON accumulation rates corresponded to about 15-20% of the seasonal nitrate uptake and suggested that there was a small seasonal build up of labile DON at the surface. However, the long residence times of 180-1560 d indicated that the most of the DON pool in surface waters was refractory in two very different productivity regimes of the NE Pacific. (C) 2002 Elsevier Science Ltd. All rights reserved.

Total dissolved zinc in the upper water column of the subarctic North East Pacific

Abstract Total dissolved Zn concentrations are reported for winter and summer along the E–W line P transect and for summer along S–N line Z transect extending from Ocean Station Papa (OSP; 50°N 145°W) to the respective shelf waters. Surface-water (10 m) concentrations ranged from 0.04 nM in the open ocean to 0.9 nM at the Canadian shelf station. A clear gradient is evident, with dissolved Zn concentrations decreasing with increasing distance from shore along the E–W transect in both summer and winter. No consistent trend was observed along the S–N transect. Low concentrations of Zn (0.05–0.07 nM) were observed in near-surface water at OSP in winter. The vertical distribution of dissolved Zn is oceanographically consistent, showing a silicon-like vertical profile. Concentrations of silicic acid and dissolved Zn at depth are higher than those at equivalent depths in the Atlantic, as expected from deep-water circulation patterns and build-up in concentration of these elements. Dissolved Zn/Si ratios in the upper 200 m indicate a trend of decreasing with distance from shore, which infers a decoupling between dissolved Zn and silicic acid in the upper ocean. We hypothesise that the silicon-like profile of dissolved Zn is a result of recycling from relatively biologically resistant organic particulate phases, that leads to profiles very similar to those of dissolved silicon. Other than at two stations where there is very high primary production, there is no apparent horizontal relationship between total dissolved Zn and chlorophyll a or production.

Iron cycling and nutrient-limitation patterns in surface waters of the World Ocean

Abstract A global marine ecosystem mixed-layer model is used to study iron cycling and nutrient-limitation patterns in surface waters of the world ocean. The ecosystem model has a small phytoplankton size class whose growth can be limited by N, P, Fe, and/or light, a diatom class which can also be Si-limited, and a diazotroph phytoplankton class whose growth rates can be limited by P, Fe, and/or light levels. The model also includes a parameterization of calcification by phytoplankton and is described in detail by Moore et al. (Deep-Sea Res. II, 2002). The model reproduces the observed high nitrate, low chlorophyll (HNLC) conditions in the Southern Ocean, subarctic Northeast Pacific, and equatorial Pacific, and realistic global patterns of primary production, biogenic silica production, nitrogen fixation, particulate organic carbon export, calcium carbonate export, and surface chlorophyll concentrations. Phytoplankton cellular Fe/C ratios and surface layer dissolved iron concentrations are also in general agreement with the limited field data. Primary production, community structure, and the sinking carbon flux are quite sensitive to large variations in the atmospheric iron source, particularly in the HNLC regions, supporting the Iron Hypothesis of Martin (Paleoceanography 5 (1990) 1–13). Nitrogen fixation is also strongly influenced by atmospheric iron deposition. Nitrogen limits phytoplankton growth rates over less than half of the world ocean during summer months. Export of biogenic carbon is dominated by the sinking particulate flux, but detrainment and turbulent mixing account for 30% of global carbon export. Our results, in conjunction with other recent studies, suggest the familiar paradigm that nitrate inputs to the surface layer can be equated with particulate carbon export needs to be expanded to include multiple limiting nutrients and modes of export.

Nitrogen isotope variations in the subarctic northeast Pacific: relationships to nitrate utilization and trophic structure

The isotopic composition of dissolved nitrate, size-fractionated suspended particulate organic matter (SPOM) and zooplankton was determined on a transect (Line P) between a coastal upwwelling domain and Station Papa in the subarctic northeast Pacific. Station Papa lies in one of the three extensive high-nutrient low-chlorophyll (HNLC) domains of the open ocean, where plankton standing stocks are seasonally uniform and production is lower than the potential production based on available nutrients. In spring 1993, surface water δ 15NO 3 − decreased from east to west along the transect, from ca 12%. in the coastal domain to 8%. at Station Papa, while nitrate concentration ([N0 3 −) increased from 3 to 12 μM in the same direction. Concurrently, δ 15NO 3 − at 400 m depth showed a much smaller change, from 5.3 to 3.1%., indicating a larger δ 15N difference between deep and surface waters at the coastal end of the transect. The isotopic trend for SPOM was similar, from approximately 11 to 3%. for bulk SPOM, 8.5 to 3%. for the < 5 μm fraction, 10 to 4%. for the 50–253 μm fraction, and 12 to 5%. for the < 253 μm fraction. Zooplankton had δ 15N values ranging from 12%. off the coast to 8%. at Station Papa. The fractionation factor for the nitrogen isotopes from these field observations was estimated to be 5%. from the change in δ 15NO 3 − between surface and deep water and from the difference between δ 15N SPOM and δ 15NO 3 − and assuming the SPOM represents an instantaneous product. Seasonal transects of δ 15N SPOM and [N0 3 −] show that there are different relationships between these two parameters depending on the availability and relative utilization of N0 3 −; δ 15N SPOM is more or less constant in the open ocean where [NO 3 −] > 7 μM, it increases rapidly where nitrate utilization is high (that is, where biological uptake is greater than physical supply), and it decreases during upwelling events when physical supply of nutrients overwhelms the biological uptake rate. Finally, the nitrogen isotopes show the expected trophic enrichment 15N, with bulk zooplankton being isotopically heavier than SPOM by 3.9%. at Station Papa and 2.2%. in the coastal domain. This difference possibly reflects the existence of a relatively short food chain in the coastal domain and a longer food chain, involving microzooplanktonic grazers, at Station Papa. The range of δ 15N among seven zooplankton groups was 3.7%., the data suggesting an increasing trophic hierarchy: pteropods, salps, euphausiids, copepods-medusaeamphipods, chaetognaths.

EDTA suppresses the growth of oceanic phytoplankton from the Northeast Subarctic Pacific

The effect of EDTA on the growth and biochemical composition of two oceanic phytoplankton isolates from the Northeast Subarctic Pacific was examined. Free metal ion concentrations were kept similar for all EDTA treatments (1, 10, 100 μM EDTA), and were present in replete amounts. In both the oceanic coccolithophore and the oceanic diatom, 100 μM EDTA inhibited cell division, causing a >50% and a 30% decrease in maximum growth for the coccolithophore and diatom, respectively. Cell volume, chlorophyll a and nitrogen quotas were not affected by EDTA concentrations, but carbon quotas were, with EDTA concentrations of 100 μM causing a significant increase in cellular carbon for the diatom, and coccolith carbon for the coccolithophore. The observed detrimental effects of EDTA on cell division may help explain why oceanic species have been difficult to culture. Caution is warranted when large concentrations of EDTA are used in laboratory studies, especially when working with newly isolated oceanic species.

SOFAR propagation conditions in the Subarctic Northeast Pacific Ocean

An analysis of two extensive sound speed profile surveys (winter and summer) of the subarctic region (above 40° N) in the Northeast Pacific Ocean shows that the depth of the deep sound channel axis is symmetric about the Alaskan Gyre (centered at 53° N, 153° W). The axis depth ranges from 100 m or less in the Gyre to 400 m or more along the coast of North America. A model for axis depth contours is developed which has some interesting results. For example, a north-south track along 140° W has little change of axis depth with latitude, while an east-west track along 53° N would have a significant change in axis depth. An estimate of the yearly variability of the model is made from long-term oceanographic data obtained at weather station “Papa” (50° N, 145° W). Sound propagation loss is computed for selected tracks using a range-dependent acoustic prediction model and these results are compared to experimental data.

Nitrate distribution in the subarctic Northeast Pacific Ocean

Data are presented on the distribution of nitrate in the subarctic Northeast Pacific Ocean. Over most of the area nitrate is present at the surface throughout the year at a concentration above that limiting phytoplankton growth. However, within coastal regions and south of about 45°N, nitrate may become depleted during the spring and summer. It is suggested that the maintenance of high nitrate concentrations over much of the area is due to relatively intensive entrainment of deep water into the upper zone coupled with a slow rate of removal of nitrate by the primary producers. Nitrate depletion in the southern area may be explained by less intensive entrainment of deep water and the utilization of nutrients by a subsurface phytoplankton maximum beneath the seasonal thermocline.