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Data Report for Video Plankton Recorder Cruise Data Report for
Video Plankton Recorder Cruise
R/V Peter W. Anderson, March 12-14, 1998
Massachusetts Water Resources Authority
Environmental Quality Department
ENQUAD Report 1998-22


Data Report for Video Plankton Recorder Cruise
R/V Peter W. Anderson, March 12-14, 1998







Cabell S. Davis
and
Scott M. Gallager


Consultants to:
SeaScan Inc., 346 Gifford St., Falmouth, MA 02540
(Under Subcontract with Battelle Ocean Sciences,
397 Washington St., Duxbury, MA 02332)







Submitted to

Massachusetts Water Resources Authority
Environmental Quality Department
100 First Avenue
Charlestown Navy Yard
Boston, MA 02129 Davis CS and Gallager SM. 1998. Data Report for Video Plankton Recorder Cruise R/V
Peter W Anderson, March 12-14, 1998. Boston: Massachusetts Water Resources Authority.
Report 1998-22. 118 p. EXECUTIVE SUMMARY

Plankton population abundance varies over a broad range of temporal and spatial scales. Due to
technological limitations, this variability has been largely undersampled, and as a result, we have a
limited understanding of both the variations in population size as well as the underlying processes
controlling it. In Massachusetts and Cape Cod Bays, there is an important societal need to understand
the variations in the plankton, as this region is becoming increasingly impacted by anthropogenic
sources of contaminants. The Bays provide a rich source of seafood and serve as a principal feeding
habitat for the endangered northern right whale.

In preparation for the relocation of the Boston Harbor Sewer Outfall 15 km offshore into western
Massachusetts Bay, considerable effort has been mounted to monitor many ecological components of
Bays system including the plankton, benthos, and fishes. The zooplankton monitoring has consisted
of 12 stations sampled 6 times per year, and there now exists several years of baseline data. Although
the sampling is sparse by design, these data are thought to provide adequate baseline information for
determining possible changes in the zooplankton populations after the new outfall comes on line.
Detecting change however will not necessarily provide insights into the causes of the change or what
remedial action may be needed.

In order to augment the baseline plankton data with high-resolution distributional data on plankton
and associated environmental variables, we conducted a Video Plankton Recorder survey covering
the entire region of Massachusetts and Cape Cod Bays during March 12-14, 1998. These data will
help provide insights into how and why the plankton is distributed in the Bays over a broad range of
scales. This report describes the methods used during the cruise as well as the post-cruise data
processing, visualization, and analysis.

Data were collected continuously for 58 hours covering an along track distance of 350 nautical miles,
with a spatial resolution of centimeters. Data were obtained for temperature, salinity, fluorescence,
beam attenuation, downwelling light, and abundance of dominant zooplankton and large
phytoplankton taxa (rod-shaped diatoms, Chaetoceros chains, Oithona, Oithona w/ eggs, unidentified
copepods, ostracod/barnacle cyprids, larvaceans, barnacle nauplii, and pteropods). Plankton data
were obtained from automatic identification of ~300,000 high-magnification images of plankton
captured during the survey.

Two main water types were found in the Bays, and we have termed these Massachusetts Bay Bottom
Water (MBBW) and Cape Ann Plume Water (CAPW). The colder-fresher CAPW was present in the
upper part of the water column throughout the northern and western parts of the Bays, while the
warmer-saltier MBBW was present throughout the lower part of the water column in Massachusetts
Bay and in northern central Cape Cod Bay. The T-S diagram indicates that the water in Cape Cod
Bay was a mixture of these two main water types. The T-S plot also suggested that a pronounced
warm plume, which was observed in eastern Cape Cod Bay, was a result of local heating in this area.
It is speculated that the warm surface plume was being driven out of Cape Cod Bay around
Provincetown by strong wind forcing.

Characteristic spatial distributions were found for both phytoplankton and zooplankton taxa. The
different planktonic taxa were found to have different affinities for the different water types, thus
providing some insights into their origins. Dominant features of the phytoplankton included an
intense bloom of rod-shaped diatoms in eastern Cape Cod Bay, which corresponded to very large
fluorescence values and was associated with the warm surface plume in this region. Fluorescence
was high in Cape Cod Bay as determined from both the VPR fluorometery data as well as from
SeaWifs data. The region of the CAPW had lower fluorescence and plankton abundance values. By

2 contrast a less intense but broader distribution of chain-forming diatoms of the genus Chaetoceros
was found throughout southern Massachusetts Bay and was associated with the MBBW, suggesting
an offshore origin.

For zooplankton, the dominant copepod was Oithona, which, like Chaetoceros, was distributed
throughout southern Massachusetts Bay, but it extended further into northern Massachusetts Bay.
Egg-bearing adult female Oithona were found to be more abundant and more patchy at depth.
Another group that was dominant in this region was ostracods/cyprids, which were substantially
more abundant at depth. These groups were associated with the warmer-saltier MBBW. Other
zooplankton groups were associated with both the Cape Cod Bay waters and the MBBW.
Zooplankton abundance was generally much lower in the CAPW.

The correlation length scales for the various taxa and for the environmental variables were also
computed. The correlograms (normalized covariance of abundance values plotted versus lagged
distance) revealed several interesting features. First, all of the plankton taxa, except for rod-shaped
diatoms and unidentified copepods, exhibited a sharp loss in correlation over very short distances (~2-
4 km). Such a decline was not observed in the physical variables or in fluorescence or attenuation,
implying taxa-specific small-scale patchiness. Second, for the environmental variables, as well as for
most plankton taxa, a large negative correlation was found at the largest length scales (40-60 km).
This trend was due to the large-scale gradient in values across the entire region due to the intrusions
of the CAPW and MBBW. The shapes of the correlograms varied among the different taxa and
environmental variables, reflecting their different distributional patterns.

The data provide insights into the characteristic spatial distribution of dominant phytoplankton and
zooplankton taxa as a function of their physical environment. The data suggest that the CAPW serves
to dilute the plankton in the northern and western parts of the Bays but also contributes significantly
to the formation of Cape Cod Bay water. This CAPW water flows from Cape Ann along the western
side of the Bays and into Cape Cod Bay. Cape Cod Bay appears to act as a cul-de-sac in which local
surface heating may be important in initiating phytoplankton blooms during spring. The dominant
hydrographic patterns observed together with the associated plankton distributions can be viewed as
characteristic of the spring period in the bays. These characteristic patterns yield large scale
correlations that are likely to be observable during any spring period. Dense small-scale copepod
patches were not observed, nor were feeding right whales This was likely due to the high winds,
which act to dissipate plankton patches.

Further study of heat flux, wind forcing, and moored T-S data, prior to and during our cruise, is
needed to support these concepts. Analysis of longer time series (eg. moored T-S data) is needed to
place this March 1998 data in a seasonal and interannual context. Further statistical analysis of
subsets of VPR data, e.g., treating each towyo as a separate plankton net tow, is needed for
quantification of the underlying statistical distributions and for comparison with the MWRA net tow
data. Comparison of the data presented in this report with other VPR data from March, 1996, March,
1997, and June 1998 is currently in progress.

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TABLE OF CONTENTS



EXECUTIVE SUMMARY ...................................................................................................... 2
LIST OF TABLES.................................................................................................................... 6
LIST OF FIGURES .................................................................................................................. 6
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