There is growing evidence that a variety of phytoplankton and zooplankton can form highly concentrated layers that are substantially thinner than can be detected using sample bottles or nets. The application of new optical and acoustic sampling techniques that can resolve down to scales of a few centimeters has shown that these "thin layers" can range in thickness from a few tens of centimeters to a few meters, yet extend for kilometers and persist for more than 24 hours. In addition, siphon samples collected from within these thin layers have demonstrated that they can contain enhanced concentrations of harmful algae or even be dominated by harmful algae. For example, in East Sound, WA, we have detected thin layer blooms of a variety of HABs including Pseudo-nitzschia spp., Alexandrium catenella, Dinophysis acuminata, D. norvegica, Noctluca scintillans and Chaetoceros concavicornis/convolutus.

The repeated observation of thin layers raises the question of the role of physical processes in their formation and dissipation. As part of an effort address this question, we coupled our measurements of fine biological and physical structure with simultaneous measurements of currents and current shear. These measurements indicate that while there are cases where the thin layer occurs in a region of no shear and minimal flow (and thus dynamics must dominated by biological processes), in many cases the thin layers are found in regions where currents will affect their dynamics by controlling their retention in the system, by shearing them into thinner layers, or by inducing turbulence that dissipates the layers. These results indicate that we need to consider more than just biological processes in trying to understand the dynamics and impacts of HABs that occur in thin layers.