Many of the organisms which form harmful algal blooms are known to occur in thin layers. In some cases, layer formation is due to behavior of the organism. Alternatively, it may result from biological-physical interactions. Recent innovations in optical/physical instrumentation and techniques for collection of high-resolution data have greatly improved our ability to define the temporal and spatial extent of layers, and to investigate the role of biological-physical interactions in controlling their dynamics. This may be critical to the understanding of some kinds of HABs, because the dynamics and impacts of a population concentrated into a thin layer may be very different from that of one spread throughout the water column.

The first step to predicting the occurrence of thin layers is documentation of how often, and under what circumstances they occur. As part of an effort to develop and field test instruments designed to quantify thin layers, we collected a data base of more than 150 physical-optical profiles over a three month period in the summer of 1996 in East Sound, Washington, USA. Thin layers are a recurrent phenomenon in this fjord, and they can be dominated by, or contain harmful algae. Thin layers occurred in over 54% of our profiles. Their depth and intensity was closely associated with the depth and strength of the pycnocline. Over 72% were located at the base of, or, above and in the pycnocline. Of the remaining layers sampled, roughly 14% were associated with water masses being advected into the system below the primary pycnocline, while the final 14% occurred when the pycnocline was diffuse. More than half were found in regions of significant current shear. When the horizontal and vertical patterns of layer distribution are coupled with measurements of currents, it becomes clear that the patterns are strongly influenced by basin-scale circulation.