Comparison of factors controlling phytoplankton productivity in the NE and NW subarctic Pacific gyres

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Abstract

The subarctic North Pacific is one of the three major high nitrate low chlorophyll (HNLC) regions of the world. The two gyres, the NE and the NW subarctic Pacific gyres dominate this region; the NE subarctic Pacific gyre is also known as the Alaska Gyre. The NE subarctic Pacific has one of the longest time series of any open ocean station, primarily as a result of the biological sampling that began in 1956 on the weathership stationed at Stn P (50°N, 145°W; also known as Ocean Station Papa (OSP)). Sampling along Line P, a transect from the coast (south end of Vancouver Island) out to Stn P has provided valuable information on how various parameters change along this coastal to open ocean gradient. The NW subarctic Pacific gyre has been less well studied than the NE gyre. This review focuses mainly on the NE gyre because of the large and long term data set available, but makes a brief comparison with the NW gyre. The NE gyre has saturating NO3 concentrations all year (winter = about 16 μM and summer = about 8 μM), constantly very low chlorophyll (chl) (usually <0.5 mg m−3) which is dominated by small cells (<5 μm). Primary productivity is low (about 300–600 mg C m−2 d−1 and varies little (2 times) seasonally. Annual primary productivity is 3 to 4 times higher than earlier estimates ranging from 140 to 215 g C m−2 y−1. Iron limits the utilization of nitrate and hence the primary productivity of large cells (especially diatoms) except in the winter when iron and light may be co-limiting. There are observations of episodic increases in chl above 1 mg m−3, suggesting episodic iron inputs, most likely from Asian dust in the spring/early summer, but possibly from horizontal advection from the Alaskan Gyre in summer/early fall. The small cells normally dominate the phytoplankton biomass and productivity, and utilize the ammonium produced by the micrograzers. They do not appear to be Fe-limited, but are controlled by microzooplankton grazers. The NW Subarctic Gyre has higher nutrient concentrations and a shallower summer mixed depth and photic zone than Stn P in the NE gyre. Chl concentrations tend to be higher (0.5 to 1.5 μg L−1) than Stn P, but primary productivity in the summer is similar to Stn P (∼600 mg C m−2 d−1). There are no seasonal data from this gyre. Iron enrichment experiments in October, resulted in an increase in chl (mainly the centric diatom Thalassiosira sp.) and a draw down of nitrate, suggesting that large phytoplankton are Fe-limited, similar to Stn P.

Introduction

The subarctic North Pacific Ocean is one of the three major HNLC (high nitrate low chlorophyll) regions of the world, which are now estimated to cover 20–30% of the world's oceans. The study of the NE subarctic Pacific has mainly taken place during three eras; 1) the Canadian weathership station (1956–81), 2) SUPER (Subarctic Pacific Ecosystem Research) program (1984–88) and WOCE (World Ocean Circulation Experiment; 1988–98), and 3) the Canadian JGOFS program (Joint Global Ocean Flux Study; 1992–97).

Much of the research on lower trophic levels in the NE subarctic Pacific has been focused on one Station, Station P (Stn P; also known as Ocean Station Papa (OSP) 50°N and 145°W, 4200 m deep) where a Canadian weathership was positioned from 1956–1981. This ship-of-opportunity provided a floating platform to begin one of the longest open ocean time series to date. From 1964–81 a series of 13 stations along a transect between the south end of Vancouver Island and Stn P were sampled approximately every 6 weeks (Whitney & Freeland, 1999). These stations were increased to 26 stations in 1981 and this transect has subsequently been referred to as Line P (Fig. 1). Since the removal of the weathership in 1981, Wong and colleagues at the Institute of Ocean Sciences, Sidney, B.C., have maintained some seasonal sampling at Stn P and along Line P.

Line P was the one repeat section in the NE subarctic Pacific for the World Ocean Circulation Experiment (WOCE) and from 1992 to 1997 it was the focus of the Canadian Joint Global Ocean Flux Studies (CJGOFS). A brief history of Stn P and Line P are given in Whitney and Freeland (1999). Sizeable data sets appear in various reports; hydrographic data from 1959–1990 in Tabata and Weichselbaumer (1992); nutrients, chlorophyll and primary productivity for the Alaska Gyre in Anderson, Lam, Booth and Glass (1977); and in a thesis (Parslow, 1981).

This review will focus on data from Stn P, but will include Line P data to illustrate the spatial changes in nutrients and phytoplankton from the coast to the open ocean (Stn P). The physical oceanography off the coast of Japan is complex and is influenced by the Kuroshio and Oyashio currents. The Kuroshio current is subtropical in nature, while the Oyashio is a southward flowing coastal current containing warm core rings propagated from the Kuroshio. Even though most of the research off the coast of Japan is from these two regions, we have excluded them from this review because they are not directly comparable to the Alaska Gyre. However, the structure of the lower trophic levels in the Oyashio region is covered by Taniguchi (1999). This review will compare the Alaska Gyre (NE subarctic Pacific gyre) and the much less well studied NW subarctic Pacific Gyre in terms of factors controlling primary productivity in the two gyres. In comparing the eastern and western subarctic Pacific, care should be taken to specify precisely which regions (especially in the western Pacific) are being compared.

Section snippets

Physical setting

The Alaska Gyre is located in the Gulf of Alaska and extends westward to near the dateline. It is bounded to the south by the slow (∼10 cm s−1) eastward-flowing Subarctic Current at 45–50°N, to the east by the highly variable Alaska Current, and to the north and west by the strong (>30 cm s−1) southwestward-flowing Alaska Stream along the continental slope of Alaska (Dodimead, Favorite & Hirano, 1963Favorite, Dodimead & Nasu, 1976Bograd, Thomson, Rabinovich & Leblond, 1999) (Fig. 1). The Alaska

Western Subarctic Gyre

The Western Subarctic Gyre (WSG), lies to the northeast of Japan and southeast of the Kamchatka Peninsula at about 155–165°E to 45–50°N (Fig. 1; Favorite et al., 1976). The WSG is located just north of the transition front and the subarctic Current. The WSG has been much less studied than the Alaska Gyre (AG). A long term time series station, Kyodo North Pacific Time Series (KNOT), at 44°N 155°E (along the southwest edge of the gyre) has been selected for future routine sampling.

A summary of

Conclusions

Our knowledge of the NE subarctic Gyre (Alaska Gyre) has mainly been derived from the long term data set obtained at Stn P on the southern edge of the gyre. In this area small phytoplankton are controlled by microzooplankton grazing (top down control) and the fast growth rates of the microzooplankton allows them to respond rapidly to any changes in the growth rate of the small cells. The large phytoplankton (>5 μm) are controlled by iron concentrations (bottom up control) and the sources of

Future directions

The long term time series along Line P and at Stn P must be maintained. Interesting decadal oscillations are now apparent. At Stn P, a deep-sea mooring with a fluorometer will allow better temporal coverage (similar to the weathership period) of chlorophyll in order to determine the relationship between dust events from Asia (now observed on the SeaWifs satellite) and phytoplankton growth. The Posiedon/Topex satellite will allow us to track eddies as they move offshore from the south end of the

Acknowledgements

This research was supported by the Natural Sciences and Engineering Research Council of Canada and the Department of Fisheries and Oceans and is part of the Canadian JGOFS project. We thank Nelson Sherry for his valuable comments on this manuscript. In particular, we thank Dr. C.S. Wong, Institute of Ocean Sciences for having the foresight to continue this valuable time series when the Canadian weathership left Stn P in 1981.

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    Present address: NIWA, Department of Chemistry, Box 6414, University of Dunedin, Dunedin, New Zealand.

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