U.S. GLOBEC: Broad-Scale Ichthyoplankton, Hydrography and Nutrient Studies on Georges Bank

Hydrography (David Mountain)

Hydrographic data will be collected at all stations on the Broad-Scale survey cruises in 1997, 1998 and 1999, processed and reported to the rest of the program. Analyses of the data will be conducted to 1) document the evolution of the water properties through each field season, 2) determine the interannual variability of water properties during the program (1995-1999), and 3) identify the spatial and temporal extent of intrusions of waters onto the Bank. In addition a collaborative analysis will be done with Beardsely et al. (in a separate proposal) of the Slope Water intrusion observed in 1995, using all available data.

Ichthyoplankton Studies
(Wallace Morse, Peter Berrien, John Green)

Bank-wide ichythyoplankton studies will be conducted to make growth and mortality estimates of the gadid (cod and haddock) egg and larval populations. Vertically discrete samples will be collected by MOCNESS on the 39 standard Broad-Scale stations and by double oblique, vertically integrated bongo net tows at these standard stations and at approximately 40 additional locations on each survey. One net from each Bongo tow will be preserved in 5% buffered formalin and used for quantifying fish egg abundance. The other bongo and MOCNESS ichthyoplankton nets will be preserved in 95% alcohol and used for determining larval abundance and for analysis of larval fish otoliths. Analyses of the samples collected in 1995 and 1996 (and then subsequently the samples from 1997, 1998 and 1999) will be done to determine 1) the spawning production curve for each species, 2) the seasonally averaged length/age dependent mortality rate for each population, 3) the estimated month- to-month average mortality rates and 4) by analysis of the otoliths, the hatch date and the increment width history for each larvae.

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.