In 1996 three cruises were conducted to simultaneously quantify the fine-scale optical and physical structure of the water column. Data from 120 profiles were used to investigate the temporal occurrence and spatial distribution of thin layers of phytoplankton as they relate to variations in physical processes. Thin layers ranged in thickness from a few centimeters to a few meters. They may extend horizontally for kilometers and persist for days. Thin layers are a recurring feature in the marine environment; they were observed and measured in 54% of our profiles. Physical processes are important in the temporal and spatial distribution of thin layers. Thin layer depth was closely associated with depth and strength of the pycnocline. Over 71% of all thin layers were located at the base of, or within, the pycnocline. The strong statistical relationships between thin layers and physical structure indicate that we cannot understand thin layer dynamics without understanding both local circulation patterns and regional physical forcing.

No abstract

Two sets of observations were made on the distribution of Pseudo-nitzschia taxa in a fjord in the San Juan Islands, Washington, USA. From May 21 to 31, 1996, we observed the spatiotemporal distribution of a dense bloom of P. fraudulenta. Microscopic observations of live material were compared to physical-optical water-column structure, currents and wind. At the start of the study, dense concentrations of Pseudo-nitzschia spp. were observed directly at the surface. Optical profiles indicated that most cells were concentrated in a thin layer at ~5 m depth, which appeared to be contiguous throughout the sound. Several days later, sustained winds forced a plume of lighter water over the surface of the sound, displacing the original water mass, with its entrained flora, to depth. The resulting near-bottom thin layer persisted for several days, and contained >106 Pseudonitzschia spp. cells l -1. Microscopic examination of live cells from the deep layer revealed that colonies were alive and motile. In 1996 and again in 1998, we observed P. pseudodelicatissima living within colonies of Chaetoceros socialis. Water-column thin layers, near-bottom thin layers and populations of Pseudo-nitzschia spp. within C. socialis colonies could easily escape detection by routine monitoring procedures, and may be a potential source of unexplained toxicity events. KEY WORDS: Pseudo-nitzschia · Chaetoceros socialis · Thin layers · Physical forcingResale or republication not permitted without written consent of the publisher

A size-structured, time and vertically-dependent model was used to investigate the effects of water column structure on the distribution of larvae of the oyster Crassostrea virginica. Formulations used to model larval growth and behavior are based upon laboratory studies. Simulated vertical larval distributions obtained for conditions representative of a well-mixed, partially stratified and strongly stratified water column illustrate the effect that salinity and temperature gradients have on moderating larval swimming and hence on larvae vertical location. For well-mixed conditions, smaller larvae are dispersed throughout most of the water column. For strongly stratified conditions, the smaller-sized larvae cluster within the region of strong salinity change. Intermediate-sized larvae cluster within or directly below the region of strong salinity change. The oldest larvae are found near the bottom for all salinity conditions since their location is determined primarily by sinking rate. Additional simulations show that diurnal salinity changes interact with larval behavioral responses to create patchy larval distributions Finally, simulations show that the inclusion of an upwelling or downwelling velocity can overwhelm the behavioral responses of smaller larvae and result in much different vertical distributlons. The simulated vertical larval distributions show that changes in larval migratory b e h a v~o r which are brought about by changes in the vertical sal~nity gradient can significantly alter larval distribution patterns. These, when combined with horizontal advective flows, have important implications for larval dispersal.

No abstract