We used temperature and depth data from 25 archival tags carried by Chinook salmon Oncorhynchus tshawytscha at sea to explore whether and how these fish alter their patterns of habitat use in response to variable oceanographic conditions off the coasts of Oregon and California. The Chinook salmon persistently used a narrow range of thermal habitats (8 to 12°C) during all months of the year, irrespective of location, time or year of release. In general, individuals appeared to adjust their vertical position in the water column to maintain this persistent thermal experience. There was noticeable individual and seasonal variation in the depths used, with the deepest habitats being used during winter. The patterns of depth use were related to the annual cycles of surface temperatures and surface productivity. Chinook salmon synchronously responded to anonymously warm surface temperatures in August 2003 by using relatively deeper habitats. Declines in surface productivity during autumn were accompanied by an apparent switch from relatively shallow habitats, to deeper, presumably benthic, habitats. The persistent use of a narrow range of temperatures suggests that variation in oceanographic conditions do not necessarily correspond to variation in the temperatures that Chinook salmon use. The effects of environmental variability on their growth and maturation in the California Current may, therefore, be relatively independent of temperature-mediated physiological responses. Rather, it seems relatively more important to understand how variable ocean conditions affect the food-web topology in the thermal habitats that Chinook salmon use.
Historical nitrate, phosphate, and dissolved oxygen data from the central Arctic Ocean are examined with particular emphasis on the conservative parameters NO (9 * NOz + O•. ) and PC (135 * POt + O•.). The NO/PC ratio is shown to increase with depth in the Canada Basin, being ~0.78 in Surface and Upper Halocllne Waters and ~1.0 in the Atlantic Layer and Deep Waters. Lower Halocline Water is marked by NO and PC mlnlma and intermediate NO/PC. NO/PC ratios from the Arctic sheri seas are examined to determine possible source regions for the various water masses. The NO/PC ratio of Canada Basin Deep Water implies an upper bound of ~11% shelf water contribution to this water mass. A slight oxygen maximum core in the Lower Halocline Water is identified at a salinity of 5=34.5 in the vicinity of the Alpha Ridge. This core appears to be diminished by diapycnal mlxing and does not extend into the Beaufort Gyre. BACKGROUND The distribution of chemicals in the ocean can provide insight into the locations, processes and rates of water mass formation. Direct observation of water mass formation is particularly difficult in the Arctic; hence Arctic oceanographers have focused extensively on deriving such information from chemical oceanographic measurements. In this paper we examine the available historical data from the Arctic Ocean using perhaps the most basic chemical tracers: nutrients and oxygen. Shelf Inputs to Halocline Water The Arctic Ocean's strong halocline separates a mixed surface layer from a deeper, relatively warm Atlantic-derived layer. Chemical oceanographers have sub-divided the halocline into Upper and Lower Halocline Water types (UHW and LHW [Jones and Anderson, 1986]). The UHW is identilled by a nutrient maximum which was originally explained as a signature of Pacific-derived waters coming in through the Bering Strait. These waters were recogniled to be altered in the Chukchi Sea [Coachman and Barnes, 1961; Kinnelt et al., 1970a]. Moore et al. [1983] attributed the high levels of all nutrients primarily to the Bering Sea source; however, they also suggested that "new" halocline waters formed over continental shelves might become enriched in nutrients through contact with sediments, and that the high nutrient content of halocline waters might therefore have a more widespread origin on other Arctic continental shelves. This idea was developed by Jones and Anderson [1986] who argued that nutrient regeneration within the confines of the Arctic Basin was primarily responsible for the maintenance of the nutrient maximum layer. Their reasoning implied that high nitrate and phosphate levels were less a function of proximity to the Bering Sea, but were more dependent on the physical nature of the continental shelf over which the UHW is formed. These authors attributed UHW to a Chukchi Sea origin on the basis of high silicate concentrations (attributed to a Bering Sea source) and hypothesiled longer shelf residence times in this region which permit a greater contribution of bottom-regenerated nutrients. Kinney et al. ...
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