The first two analyses will characterize between year differences in the overall development of the gadid year classes. The second two will address the within year variability. By knowing the date of hatch and by collaborating with the GLOBEC modelling group, the time and location of spawning for the surviving larvae in each survey will be estimated. Comparing these distributions to the estimated initial larval production at that time, the time and space distribution of mortality will be estimated. From this the degree to which mortality is temporally and spatially uniform (chronic), on the one hand, and spatially dependent and/or episodic, on the other, will be determined. For the non-uniform mortality, investigation of the other GLOBEC data sets (in collaboration with other GLOBEC investigators) will determine if the mortality was associated with variations in water properties, physical forcing, prey characteristics, or predators. The otolith increment histories will be analyzed to identify if survivors have common patterns early in life that set them apart from the rest of the population at that earlier time. This analysis will determine if, in situations of chronic mortality, faster growth (wider increments) is associated with survival.
Nutrients and Phytoplankton (David W. Townsend)
Funding is requested to measure distributions of phytoplankton
chlorophyll a and inorganic nutrients on Georges Bank on each of the
six two-week Broad-scale survey cruises to Georges Bank as part of
the Phase II GLOBEC program. Cruises are scheduled in each month
from January to June in 1997 and 1998. The PI has designed the
study about questions related to the nature of secondary production
on Georges Bank as related to inorganic nutrient fluxes. Primary
production on Georges Bank is among the highest of any region of the
world ocean, exceeding 400 gC m-2 y-1 in the central portion
(O'Reilly et al., 1987), but the ratio of secondary production of
zooplankton and benthos to primary production by phytoplankton on
the Bank is lower than would normally be expected when it is
compared with other marine areas (Cohen and Grosslein, 1987; Sherman
et al., 1987). Reasons proposed by earlier workers to explain this
discrepancy between primary and secondary production have dealt
primarily with advective losses of phytoplankton from the Bank
(Mountain and Schlitz, 1987; Cohen and Grosslein, 1987).
Calculations of cross-Bank nutrient fluxes by Townsend and Pettigrew
(1996) suggest that Georges Bank is too large a geographic feature
to function efficiently on nutrients delivered via fluxes from
deeper waters around its periphery and therefore much of the primary
production is recycled. Thus it follows that production of higher
trophic level biomass, from zooplankton to fish, is limited by the
frequency and intensity of processes that drive fluxes of deep water
nitrogen onto the Bank. Data collected as part of this study will
allow more refined estimates of nutrient fluxes onto and about
Georges Bank -- including those based on modeling experiments with
the Lynch et al., GLOBEC modeling team.