No abstract

Seasonal development of dissolved-oxygen deficits (hypoxia) represents an acute system-level perturbation to ecological dynamics and fishery sustainability in coastal ecosystems around the globe. Whereas anthropogenic nutrient loading has increased the frequency and severity of hypoxia in estuaries and semi-enclosed seas, the occurrence of hypoxia in open-coast upwelling systems reflects ocean conditions that control the delivery of oxygen-poor and nutrient-rich deep water onto continental shelves. Upwelling systems support a large proportion of the world's fisheries, therefore understanding the links between changes in ocean climate, upwelling-driven hypoxia and ecological perturbations is critical. Here we report on the unprecedented development of severe inner-shelf (<70 m) hypoxia and resultant mass die-offs of fish and invertebrates within the California Current System. In 2002, cross-shelf transects revealed the development of abnormally low dissolved-oxygen levels as a response to anomalously strong flow of subarctic water into the California Current System. Our findings highlight the sensitivity of inner-shelf ecosystems to variation in ocean conditions, and the potential impacts of climate change on marine communities.

No abstract

Data from the Coastal Transition Zone (CTZ) experiment axe used to describe the velocity fields and water properties associated with cold filaments in the California Current. Combined with previous field surveys and satellite imagery, these show seasonal vaxiability with maximum dynamic height ranges and velocities in summer and minimum values in late winter and early spring. North of Point Arena (between 39øN and 42øN) in spring-summer the flow field on the outer edge of the cold water has the chaxacter of a meandering jet, carrying fresh, nutrient-poor water from farther north on its offshore side and cold, salty, nutrient-rich water on its inshore side. At Point Arena in midsummer, the jet often flows offshore and continues south without meandering back onshore as strongly as it does faxther north. The flow field south of Point Arena in summer takes on more of the chaxacter of a field of mesoscale eddies, although the meandering jet from the north continues to be identifiable. The conceptual model for the May-July period between 36 øN and 42øN is thus of a surface jet that meanders through and interacts with a field of eddies; the eddies are more dominant south of 39øN, where the jet broadens and where multiple jets and filaments axe often present. At the surface, the jet often separates biological communities and may appeax as a barrier to cross-jet transport, especially north of Point Arena early in the season (March-May). However, phytoplankton pigment and nutrients are carried on the inshore flank of the jet, and pigment maxima axe sometimes found in the core of the jet. The biological effect of the jet is to define a convoluted, 100 to 400-km-wide region next to the coast, within which much of the richer water is contained, and also to carry some of that richer water offshore in meanders along the outer edge of that region.

Seasonal cycles of coastal wind stress, adjusted sea level (ASL), shelf currents, and water temperatures off the west coast of North America (35øN to 48øN) are estimated by fitting annual and semiannual harmonics to data from 1981-1983. Longer records (9-34 years) of monthly ASL indicate that these two harmonics adequately represent the long-term monthly average seasonal cycle and that the current measurement period is long enough to estimate the seasonal cycles. We characterize the differences between fall/winter and spring/summer as follows: For fall/winter, monthly mean winds north of 35øN are northward for 3-6 months (longer in the north than in the south); south of 35øN, the mean winds are near zero or weakly southward; monthly mean alongshore currents are northward over midshelf and shelf break at all locations sampled at depths of 35 m and deeper and are associated with high coastal sea levels and relatively warm water temperatures. For spring/summer, monthly mean wind stresses are southward at all latitudes for 3-6 months (longer in the south than in the north), sea levels are low, and water temperatures are relatively cool; monthly mean currents at 35 m depth over the shelf are southward for 1-6 months (longer at the shelf break than over midshelf and longer in the north than in the south), while the deeper currents are less southward or northward. The magnitudes of the seasonal cycles of all variables are maximum between approximately 38øN and 43øN, generally decreasing slightly to the north and greatly to the south. At each location the seasonal cycle of the alongshore current from 35 m depth at midshelf leads the sea level slightly and both lead the wind stress and temperatures by 1-2 months. The seasonal cycles of all variables show a south-to-north progression (south leads north by 1-2 months). At 48øN, annual mean currents at 50 m depth over the shelf break oppose the annual mean wind (northward wind and southward current). Similarly, at 35øN, annual mean currents at 35 m depth over both midshelf and shelf break are opposed to the annual mean wind (southward wind and northward current). From 35øN to 43øN, both summer and winter regimes are dominated by strongly fluctuating currents. 1507 1508 STRUB ET AL.: SEASONAL CYCLES OVER THE WESTERN U.S. CONTINENTAL SHELF