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Oceanography of the Pacific Northwest Coastal Ocean and Estuaries with Application to Coastal Ecosystems

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Oceanography of the Pacific Northwest Coastal Ocean and Estuaries with Application to Coastal Ecosystems Oceanography of the Pacific Northwest Coastal Ocean and Estuaries with Application to Coastal Ecosystems Barbara M. Hickey1 and Neil S. Banas School of Oceanography Box 355351 University of Washington, Seattle, WA 98195-7940 Tel.: 206 543 4737 e-mail: [email protected] Submitted to Estuaries May 2002 1Corresponding author. Hickey and Banas 2 Abstract This paper reviews and synthesizes recent results on both the coastal zone of the U.S. Pacific Northwest (PNW) and several of its estuaries, as well as presenting new data from the PNCERS program on links between the inner shelf and the estuaries, and smaller-scale estuarine processes. In general ocean processes are large-scale on this coast: this is true of both seasonal variations and event-scale upwelling-downwelling fluctuations, which are highly energetic. Upwelling supplies most of the nutrients available for production, although the intensity of upwelling increases southward while primary production is higher in the north, off the Washington coast. This discrepancy is attributable to mesoscale features: variations in shelf width and shape, submarine canyons, and the Columbia River plume. These and other mesoscale features (banks, the Juan de Fuca eddy) are important as well in transport and retention of planktonic larvae and harmful algae blooms. The coastal-plain estuaries, with the exception of the Columbia River, are relatively small, with large tidal forcing and highly seasonal direct river inputs that are low-to-negligible during the growing season. As a result primary production in the estuaries is controlled principally not by river-driven stratification but by coastal upwelling and bulk exchange with the ocean. Both baroclinic mechanisms (the gravitational circulation) and barotropic ones (lateral stirring by tide and wind) contribute to this bulk exchange, though tidal circulations appear to dominate during the low-riverflow growing season on ~monthly scales. Because estuarine hydrography and ecology are so dominated by ocean signals, the coast estuaries, like the coastal ocean, are largely synchronous on seasonal and event time scales, though intrusions of the Columbia River plume can cause strong asymmetries between Washington and Oregon estuaries during spring downwelling conditions. Property coherence increases between spring and summer as wind forcing becomes more spatially coherent along the coast. Estuarine habitat is structured not only by large scale forcing but also by fine scale processes in the extensive intertidal zone, such as differential solar heating or differential advection by tidal currents. Hickey and Banas 3 Introduction Recent results on the physical oceanography of the U.S. Pacific Northwest (PNW) coastal region are integrated in this paper to provide a framework for understanding ecosystem variability. The coastal region important to the regional ecosystem includes both the nearshore zone and the coastal estuaries. Many species utilize both these regions at different life stages. For example, Dungeness crab frequently utilize coastal estuaries for the first year of their life, re-entering the ocean to become part of the fishery as juveniles (Gunderson et al. 1990). Salmon, on the other hand, utilize the estuary at both the beginning and end of their life cycles and the coastal zone as adults. As we will demonstrate, ocean variability in nearshore regions of the U.S. West Coast and, in particular, its coastal estuaries, is distinctly different from that in estuaries and nearshore regions of the U.S. East Coast. The West Coast is embedded within an Eastern Boundary Current System; the East Coast is embedded within a Western Boundary System. Thus, whereas the West Coast is dominated by upwelling, the East Coast is not; whereas upwelling provides plentiful nutrients to the West Coast and its estuaries, on the East Coast nutrients are more commonly supplied by river outflow. Ocean variability along the West Coast is generally very large scale (> 500 km), a result of large-scale atmospheric systems (Halliwell and Allen 1987). Nevertheless, we will show that significant alongshore gradients occur in coastal productivity in the PNW. Moreover, we will demonstrate that mesoscale features such as banks and submarine canyons play important and even critical roles in ecosystem function. In many ways, at least during the summer growing season, coastal estuaries in the PNW may be considered as extensions of the coastal ocean: as we will discuss, flushing rates are on the order of a few days and property variability is controlled by changes at the ocean end of the estuary rather than by riverflow at its head. Thus, like the ocean processes, both the several-day and seasonal fluctuations that occur over the growing season occur nearly simultaneously across the PNW coast estuaries. However, actual values of water properties such as temperature, salinity and velocity will differ depending on the specific estuary configuration. In the following, the large-scale processes acting on the PNW coastal zone are first described. This is followed by a discussion of nutrient variability (Section 2). With this setting the effects of important mesoscale features such as submarine banks, canyons and river plumes are presented (Section 3). Following the description of coastal processes, processes and variability within the coastal estuaries is described (Section 4), with particular focus on the estuaries studied in the PNCERS program. The interaction of the coastal ocean with these estuaries and the Hickey and Banas 4 similarity of water property variation in the several estuaries are demonstrated using time series and survey data collected in the PNCERS program. 1. Large scale processes in the Pacific Northwest coastal ocean a) Seasonal variability The U.S. Pacific Northwest coastal zone is embedded within the California Current System (CCS), a system of currents with strong interannual, seasonal and several-day (event) scale variability (Fig. 1) (Hickey 1998). The California Current System includes the southward California Current, the wintertime northward Davidson Current, the northward California Undercurrent, which flows over the continental slope beneath the southward upper layers, as well as "nameless" shelf and slope currents with primarily shorter-than-seasonal time scales. The PNW includes one major river plume (the Columbia), several smaller estuaries, and (primarily in the north) numerous submarine canyons. The dominant scales and dynamics of the circulation over much of the CCS are set by several characteristics of the physical environment; namely, 1) strong alongshore winds; 2) large alongshore scales for both the winds and the bottom topography (Halliwell and Allen 1987); and 3) a relatively narrow and deep continental shelf. Because of these characteristics, coastal-trapped waves (disturbances that travel northward along the shelf and slope) are efficiently generated and propagate long distances along the continental margins of much of western North America. Thus, much of the variability in the PNW is caused by processes occurring southward of the region (i.e., “remote forcing”). Because of the generally southward alongshore wind stress in spring and summer, coastal upwelling is the dominant process controlling water property variability (see review in Smith 1995). The California Current flows southward year-round offshore of the U.S. West Coast from the shelf break to a distance of 1000 km from the coast (Hickey 1979, Hickey 1998) (Fig. 1). The current is strongest at the sea surface, and generally extends over the upper 500 m of the water column. Seasonal mean speeds are ~10 cm s-1. The California Undercurrent is a relatively narrow feature (~10-40 km) flowing northward over the continental slope of the CCS at depths of about 100-400 m as a nearly continuous feature, transporting warmer, saltier Southern water northward along the coast. The Undercurrent has a jet-like structure, with the core of the jet located just seaward of and just below the shelf break and with peak speeds of ~30-50 cm s-1. The Undercurrent provides a possible northward transport route for larvae, larval fish and even phytoplankton seed stock. Because of its proximity to the Hickey and Banas 5 shelf break, the Undercurrent is the source of much of the water supplied to the shelf during coastal upwelling. The onshore transport of this water during upwelling offers a mechanism for onshore transport of plankton entrained in the Undercurrent. A southward undercurrent (the “Washington Undercurrent”) occurs over the continental slope in the winter season in the PNW (Werner and Hickey 1983). This undercurrent occurs at deeper depths than the northward undercurrent (~300-500 m). The existence of this undercurrent, like that of the northward undercurrent, likely depends on the co-occurrence of opposing wind stress and alongshore pressure gradient forces. The Davidson Current flows northward in fall and winter north of Point Conception. This northward flow is generally broader (~100 km in width) and sometimes stronger than the corresponding subsurface northward flow in other seasons (the "Undercurrent") and extends seaward of the slope. Currents and water properties of the CCS both over the shelf and in the region offshore of the shelf undergo large seasonal fluctuations. The California Current and Undercurrent are strongest in summer to early fall and weakest in winter. The Davidson Current is strongest in winter. Seasonal mean shelf currents are generally southward in the upper
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