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399 N NEAR-SHORE PHYSICAL PROCESSES, EFFECTS OF MARGARET ANNE MCMANUS AND ANNA PFEIFFER-HOYT University of Hawaii Oceanographers have long known that organisms are not distributed homogeneously in the water column. Distributions of plankton vary both horizontally and vertically across a continuum of temporal and spatial scales. These distribution patterns result from a combination of physical circulation patterns as well as planktonic growth, behavior, and mortality. In the coastal environment, some of the physical processes important for structuring distributions of planktonic organisms include water column stratification, fronts, internal waves, and tidal bores. VERTICAL STRATIFICATION The Pycnocline A pycnocline is a region of the water column with a rapid vertical change in density (Fig. ). In coastal waters, the pycnocline separates water at the sea’s surface, which is mixed by wind and waves, from water near the sea floor, which is mixed by tidal action. When a pycnocline exists, the water column is vertically stratified. The depth and intensity of the pycnocline depend on factors such as wind strength, the input of low-salinity water, and the degree of tidal mixing. When the density difference between the layers of water on either side of the pycnocline is great, the pycnocline can act as a barrier to vertical transport of dissolved and particulate materials. This barrier has signi ficant consequences for living organisms. Thin Layers Recent advances in optical and acoustical technology have allowed oceanographers to examine the distribution of plankton at fine vertical scales. These advances have led to the discovery of thin layers of plankton below the sea surface (Fig. ). A thin layer is an aggregation of plankton in which the concentration is at least three times greater than the concentration of these organisms outside the layer, with a vertical thickness of a few centimeters to a few meters and a horizontal extent on the scale of kilometers . Over % of thin phytoplankton layers observed in the coastal ocean have been found within the pycnocline FIGURE 1 Density of a stratified water column (red line) and variation in chlorophyll concentration (green line) with depth. Chlorophyll is a light-harnessing pigment that is common to phytoplankton. A thin layer, indicated by the peak in chlorophyll concentration, is most often found at the base of the pycnocline. of a stratified water column. Phytoplankton require both light and nutrients to grow. Light is often ample in the upper reaches of the water column above the pycnocline, whereas nutrients are often ample in the lower reaches of the water column below the pycnocline. The combination of suitable light and the diffusion of nutrients at the pycnocline may create an optimal location for the growth of phytoplankton. The pycnocline is also an area where currents are generally low and turbulence is suppressed. It has been shown that organisms in thin layer structures decrease their transport distances because they are associated with these regions of reduced current flow. NEAR-SHORE FRONTS Fronts are features in the ocean where physical, chemical, and biological properties change rapidly over a narrow horizontal area. Fronts often occur where two water masses with distinct properties meet (Fig. ). The spatial scale of oceanic fronts ranges from to meters in depth, from meters to kilometers across the front, and from meters to , kilometers along the front. Fronts that are commonly found in the coastal zone include tidal fronts, upwelling fronts, fronts associated with geomorphic features (such as headlands, islands, and canyons), shelf break fronts, plume fronts, and estuarine fronts. the front. In the convergent zone, plankton will stay at the surface if their buoyancy or swimming speed is great enough to overcome the downward currents, which are typically less than . millimeters per second. Indeed, many plankton can swim faster than vertical currents in the ocean, allowing them to maintain a preferred depth in the water column by responding to cues such as light and pressure. A second mechanism for enhanced biomass at fronts is enhancement of phytoplankton growth within the frontal region. As previously mentioned, phytoplankton need both light and nutrients to grow. Consider an example of a front between a water mass that is well mixed throughout the water column and a second water mass that is highly stratified. In the well-mixed water mass, although nutrients will be adequate for growth, phytoplankton productivity will be limited by light because phytoplankton in the well-mixed water mass are mixed from the surface to the bottom, spending reduced amounts of time in the photic zone. In the strati...
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