Project Summary

Mark D. Ohman and Ted Durbin

U.S. GLOBEC: Egg production, growth, and mortality, and the role of frontal processes in copepod population dynamics on Georges Bank

Proposal Objectives include:

1. On broad-scale survey cruises determine spatial and temporal changes in abundance of nauplii of Calanus, Pseudocalanus, and of other copepods on Georges Bank during the winter/spring period from a plankton pump.
2. On process cruises
   1. determine the spatial, and seasonal variations in Calanus and Pseudocalanus egg viability, egg and naupliar mortality rates, naupliar development rates, and copepodite development and growth rates. Determine whether egg and naupliar mortality rates are correlated with abundances and consumption rates of potential predators.
   2. determine adundance, growth rates and physiological condition of the target copepod species, as well as the abundance of other zooplankton species, over fine spatial scales across the crest-southern flank tidal mixing front during late April-early May.
   3. provide the modelling and synthesis/retropsective groups with copepod distributions, recruitment, growth and mortality rates. In collaboration with other groups, determine whether there are distinct and persistent centers of recruitment for each of the dominant species. Determine the spatial, seasonal and interannual variation in egg production, birth rates, and early mortality rates for the dominant species including Calanus and Pseudocalanus.

Cruise Plans:

1. late February-early March on the NE Peak
2. late April-early May on the S Flank.

The general approach will be to deploy clusters of three drifters and track them for 7-9 days while we measure growth, egg production and mortality rates of the target species. Drifters (drouged at 15 m) will be deployed in regions of low horizontal gradients of T/S/density to reduce the chances that drifters will diverge markedly. If they do, we will recover and re-deploy them. Deployment of drifters in triads will also permit us to estimate the rates of surface divergence/convergence. We will sample copepods by pumping from the entire water column every 24 h (5 replicates), all at the same time of day. Three bongo hauls with 333 m mesh nets will also be made at the same station because the pump appears to undersample older copepodite stages (>C4) and adult female Calanus finmarchicus. At the end of each station animals and water will be collected for physiological experiments. Total station time should not be more than about 3 hours.

The spring cruise will include a series of pump transects across the Southern Flank-Crest tidal mixing front to determine small-scale spatial variabilty in abundance and physiological condition of the target copepod species and other zooplankton. With the pump we will obtain taxonomic and stage information on all target copepod species developmental stages, as well as other smaller zooplankton. Stations will be 5-7 km apart and 5-6 stations will occupied on each transect. Transects will be carried out in phase with the tidal cycle with back-to-back transects in each direction. The direction of each transect will be the same as that of the tidal flow so that features will not become compressed as we sample. Time of day that samples are collected will not be a factor since broad-scale survey results (1995 and 1996) show no evidence of any significant day-night differences in depth distribution of Calanus on Georges Bank, only in the GOM and in Slope waters.

At each station, CTD, pump and bongo casts will be carried out. Station time should be no longer than 30 minutes which will allow one station to be occupied each hour. Depth stratified samples (10 m depth bins) will be collected with a plankton pump. The pump will provide small spatial scale information of all stages of the target copepods and other small zooplankton (e.g. hydroids and omnivorous copepods). These samples will be used by Sullivan for small predator distributions and Bucklin to look at small scale differences in distribution of P. moultoni and P. newmani across the front. The plankton pump has considerable advantages over the 1 m² MOCNESS for this small-scale sampling because samples can be collected much more rapidly (a pump cast will take about 20 minutes), and it quantitatively collects all developmental stages of the target copepods. The bongo samples will provide information on horizontal distribition of larger predators for Madin/Sullivan, and will provide us with animals for measurement of RNA/DNA, C, N, female reproductive index and size of the target copepod species. The MOCNESS at either end of the transect will provide depth information of the larger predators for Madin/Sullivan.