Abstract:
A key question in biological oceanography is how plankton populations maintain
themselves in regions of favorable growth and survival in the face of horizontal transport
by ocean currents. Plankton are thought to be retained on the highly productive Georges
Bank by the clockwise flow, which intensifies with vernal warming. The extent to which
plankton are transported off the bank to the southwest or transported northward and
retained on the bank remains poorly understood. This thesis examined the relationship
between plankton and physical properties in the southwest corner of the bank, the
retention-loss region (RLR). Analysis of field data (Video Plankton Recorder, Acoustic
Doppler Current Profiler, and satellite-tracked drifters) and modeling results was
performed to quantify the relationships between plankton, hydrography, and currents and
the fluxes through the RLR. Temperature-salinity-plankton diagrams and factor analysis
revealed that most plankton taxa had characteristic relationships to the hydrography, with
the exception of copepods which were everywhere abundant. The flux of plankton
during a complete tidal cycle and in the de-tided current data indicated this region was
not retentive to plankton, since the bulk of the flow remained to the southwest, despite the
presence of a vernally warmed surface layer. A Lagrangian particle trajectory model was
used to further examine transport of plankton through the RLR during late spring /early
summer (June) when vernal stratification was established. Passive particles were used,
since no die1 vertical migration by plankton was found in the data. The model revealed
that the bulk of the plankton was carried out of the RLR through the southern and western
boundaries. The modeling and data analysis show clearly that the plankton were lost
from the bank to the southwest rather than being re-circulated to the north. These results
have important implications for the plankton populations on Georges Bank and can be
used in future modeling efforts that examine the factors controlling plankton populations
in this region.
