Davis Report

### VPR Sampling during the GLOBEC Stratification Study

    INVESTIGATORS:

    Cabell S. Davis
    Woods Hole Oceanographic Institution
    Woods Hole, MA

    Scott M. Gallager
    Woods Hole Oceanographic Institution
    Woods Hole, MA

#### OBJECTIVES:

- To determine the fine scale (cm) vertical distribution of zooplankton prey species, salinity, temperature, phytoplankton and particulate concentrations over the course of the development of spring stratification.

- To determine mechanisms controlling aggregations at the pycnocline, i.e, to determine how swimming behaviors of individual prey organisms interact with microscale turbulence and food level as a function of water column stratification.

#### METHODS

- Repetitive profiling (e.g. slow towyoing) with the VPR to quantify the fine scale vertical distribution of the copepods, hydrography, fluorescence, and light transmission while following a patch of fish larvae. This was done on each of the zooplankton process cruises in to examine the changes in vertical distributions before, during, and after the onset of stratification.

- Direct quantification of individual copepod swimming behaviors, small scale turbulence, and phytoplankton concentrations using the VPR mounted on the ROV JASON. This was done on the last zooplankton process cruise (June 8-20, 1995) after the onset of stratification to examine swimming and turbulence in relation to position in the stratified water column. These data are being used in numerical and analytical models of patch formation in a stratified fluid to determine the underlying causes of the prey aggregations at the pycnocline.

#### RESULTS

The series of cruises was highly successful. We were able to make a series of over 100 VPR towyos during the zooplankton process cruises to quantify the fine-microscale horizontal and vertical distribution of plankton, including the target zooplankton species Calanus and Pseudocalanus. We found that the plankton do in fact aggregate in the pycnocline, and we were able to use the VPR on JASON to directly observe swimming behaviors as well as microscale turbulence. In addition, contrary to prior studies, we found delicate particulate matter including gelatinous organisms and marine snow, which are not observed in net tows or bottle samples, are the dominant material in the water column. Video is being processed using our automatic focus-detection system. Parallel development (ONR grant) of an automatic taxonomic identification system will allow rapid analysis of the extracted in-focus images. In the meantime, we are using a point-and-click user interface to sort the images. Using this approach we have obtained data for several publications (listed below). We will continue to analyze these data over the course of next year.

#### PUBLICATIONS RESULTING FROM THIS GRANT:

Davis, C. S., S. M. Gallager, M. Marra, and W. K. Stewart. Rapid visualization of plankton abundance and taxonomic composition using the Video Plankton Recorder. Deep Sea Res. (in press)

Benfield, M. C., C. S. Davis, P. H. Wiebe, S. M. Gallager, R. G. Lough, and N. J. Copley. Comparative distributions of calanoid copepods, pteropods and larvaceans estimated from concurrent Video Plankton Recorder and MOCNESS tows in the stratified region of Georges Bank. Deep Sea Res. (in press)

Gallager, S. M., C. S. Davis, and A. Epstein. High-resolution spatial distributions of plankton correlated with hydrography in Great South Channel using the Video Plankton Recorder. Deep Sea Res. (in press)

Norrbin, M. F., C. S. Davis, and S. M. Gallager. Differences in structure and composition of zooplankton between mixed and stratified regions of George's Bank. Deep Sea Res. (in press)