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)