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The biological and physical processes contributing to planktonic thin layer dynamics were examined in a multidisciplinary study conducted in East Sound, Washington, USA between June 10 and June 25, 1998. The temporal and spatial scales characteristic of thin layers were determined using a nested sampling strategy utilizing 4 major types of platforms: (1) an array of 3 moored acoustical instrument packages and 2 moored optical instrument packages that recorded distributions and intensities of thin layers; (2) additional stationary instrumentation deployed outside the array comprised of meteorological stations, wave-tide gauges, and thermistor chains; (3) a research vessel anchored 150 m outside the western edge of the array; (4) 2 mobile vessels performing basin-wide surveys to define the spatial extent of thin layers and the physical hydrography of the Sound. We observed numerous occurrences of thin layers that contained locally enhanced concentrations of material; many of the layers persisted for intervals of several hours to a few days. More than one persistent thin layer may be present at any one time, and these spatially distinct thin layers often contain distinct plankton assemblages. The results suggest that the species or populations comprising each distinct thin layer have responded to different sets of biological and/or physical processes. The existence and persistence of planktonic thin layers generates extensive biological heterogeneity in the water column and may be important in maintaining species diversity and overall community structure.

We studied the biomass and production of heterotrophic bacteria on several types of marine snow including those composed predominantly of &atoms, fecal pellets, larvacean house mucus, or miscellaneous detrital components at 8 stations in the Pacific Ocean off California. We concurrently measured photoautotrophic biomass and production, and particulate organic carbon (POC) on marine snow to examine the quantitative significance of bacterial processes in carbon flow pathways in different types of marine snow Although a typical marine snow floc contained about 106 bacteria, the bacterial carbon (BOC) and phytoplankton carbon (PhytoC) were each < 13 % of total POC. Most of the floc carbon therefore consisted of detritus, unlike whole seawater samples (in other studies) where BOC + PhytoC generally comprises about 50 % of the POC. PhytoC on marine snow and in the surrounding water was generally similar to BOC (except at one station). Floc POC turnover time based on bacterial carbon demand. assuming a 30 O/ O carbon assimilation efficiency, was 20 to 100 d. This was comparable to the POC residence time reported for the Southern California Bight. Bacterial specific growth rates on flocs varied greatly between stations (0.06 to 0.96 d-l) with highest values at stations with diatom flocs where they were comparable to growth rates of free-living bacteria in mesotrophic waters. The ratio of bacterial carbon production to primary production on marine snow was typical of seawater samples from other studies. Bacterial carbon production was measured by the leucine incorporation method while bacterial cell production (and growth rate) was measured by the thymidlne incorporation method simultaneously on the same marine snow floc. We could thus calculate bacterial carbon per-new-cell (C,) and compare it with the microscopy-based average carbon per cell (C,,) to test the hypothesis that new cells reflect the average carbon content of the parent assemblage (bacteria on marine snow were much larger than those in the surrounding water). In 73 % of our samples the carbon content of the new daughter cells was within a factor of 1.5 of the carbon content of the parent cells.

The dynamics and formation mechanisms of marine snow aggregates from abandoned larvacean houses were examined by laboratory experiments and field sampling during a spring diatom bloom in a shallow fjord on the west coast of the USA. Intact aggregates were sampled both from sediment traps and directly from the water column by divers. All aggregates were composed of 1 abandoned house of the larvacean Oikopleura dioica to which numerous diatoms, fecal pellets, c~liates, and amorphous detritus were attached. High vertical flux rates (20000 to 120000 houses m-2 d") and settling velocities (average 120 m d-') imply a rapid turnover of suspended larvacean houses, and concentrations of diatoms and fecal pellets in the aggregates exceeding ambient concentrations by 3 to 5 orders of magnitude suggest their potential importance in driving the vertical flux of particles. Identical particle assemblages were observed in aggregates collected in the water column and in sediment traps. Most of the fecal pellets found in the houses were most likely produced by the larvaceans themselves. Numbers of diatoms per house corresponded with the diatom concentrations in the ambient water and, on average, each aggregate contained diatoms in abundances corresponding to those found in 4.5 ml of ambient water. Laboratory measurements showed that larvacean houses scavenge diatoms and fecal pellets while sinklng, and observed scavenging rates were similar to those predicted from theory. However, both predicted and observed scavenging rates In experiments were orders of magnitude too low to account for the particle content observed on aggregates from the water column. Based on models, shear coagulation was also assessed to be insignificant in forming aggregates. It is concluded that most of the particles become attached to the incurrent filters of the larvacean house while it is still inhabited, and that shear and sinking insignificantly contribute to particle collisions and adhesions on the abandoned house

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