Water Column Dynamics in the Pacific Northwest
Jan Newton, Applied Physics Lab, University of Washington
Abstract
Pacific Northwest marine waters represent some of the more diverse and dynamic aquatic systems in the world, due to strong influences from tides and weather, as well as to a variety of geological formations ranging from shallow blind-end bays to deep, well-flushed channels. Although the marine water column is a fluid habitat, seawater has both structure and movement that directly determine the biotic living conditions and water quality of these systems. The outer Pacific coastal waters are highly influenced by weather (e.g., wind direction, drought) as well as climate dynamics (e.g., El Niño, climate change) that drive ocean circulation and affect surface water properties. Within the inland and estuarine Pacific Northwest waters, these atmospheric forcings continue to affect the water column and are met by additional influences from the watershed (river runoff) as well as from the geomorphologic features of the basin itself. Thus, local water column characteristics vary on many spatial and temporal scales. River-mouth estuaries, such as Willapa Bay and Grays Harbor, are shallow, well flushed and typically dominated by ocean conditions in summer and river in winter. The fjord-like Puget Sound has areas that are deep, with well-mixed waters as well as areas that are shallow, with waters that are stratified or capped with freshwater. Wind and tides may be sufficient to mix the waters in some times of the year, yet not when rivers are flowing. Both vertical mixing and horizontal exchange affect the physical, chemical, and biological nature of the water column. Vertical mixing or lack thereof (strong stratification) will define an organism’s exposure to pelagic habitat characteristics such as light and nutrients. The horizontal exchange (or flushing) of water defines how long it will stay in the basin, along with it’s dissolved and suspended particulate components. Because of the diversity of water column conditions in the Pacific Northwest, assessing these characteristics is essential to interpreting growing conditions and water quality.