Stratification Summary Talk: Biological Results

Jeff A. Runge

Three hypotheses about the effects of stratification on zooplankton and ichthyoplankton productivity were put forward in the Implementation Plan for the NW Atlantic/Georges Bank Study (1992: U.S. Globec Report #6). The following is a report on the status of these hypotheses, based on results from cruises in 1993-1994, as well as 1995 process and broadscale cruises:

Hypothesis I: Seasonal density stratification over the southern flank of the Bank causes prey aggregation in the pycnocline and increased survival of predator populations.

The abundance of Calanus copepodites and Pseudocalanus females during VPR transects in January, March and June, 1995 has been determined and color-contoured in two dimensional, x-z plots. The water column was well-mixed during the January and March transects; correspondingly, there was no vertical structure in the copepod distributions. Horizontal and vertical structure was observed in some other taxa (for example, larvaceans on the southern half of the bank in January and Rhizoselenia mats near the shelf break in March), however. The June, 1995 transect began in thermally stratified, southern flank water and ended over the slope in a streamer of shelf water extruded by a proximate warm core ring. Calanus was found principally in the surface layer above the thermocline whereas Pseudocalanus females, interestingly, were present primarlily below the thermocline. A fluorescence maximum on the shelf followed vertical fluctuations in the thermocline and was well correlated with the distribution of the colonial diatom, Chaetoceros socialis. These plots served to illustrate the capability of the VPR to resolve vertical biological structure and show how some taxa may be aggregated in the thermocline, whereas others may be concentrated in the mixed layer or in deeper water below the thermo- or pycnocline.

At present, the most complete, recent set of data on the distribution of cod and haddock larvae and their prey on the southern flank derives from observations made in May of 1993 and 1994. The data illustrate that the southern flank is a dynamic and complex environment. Observations show that many possible situations may occur in stratifed water, from aggregation of fish larvae and copepod nauplii in the vicinity of the thermocline to almost uniform dispersion of fish larve and their prey, with no apparent correlation in time and space. Given the range of situations, modeling will be essential for the evaluation of the importance of stratification and vertical distribution of predator and prey on fish growth rates.

Hypothesis II: Turbulent mixing, generated by wind and tidal forcing, has a significant impact on rates of ingestion, respiration, and predation; the processes of turbulent mixing and seasonal density stratification influence predator-prey encounter rates and thus growth and survival of individual organisms.

This subject is under active investigation during Phase II of the Georges Bank/NW Atlantic study. Observations taken during a Jason/VPR deployment in June, 1995, show organisms concentrated in a narrow layer at approximately 40 m, where in situ estimates of turbulence were lowest. This depth zone was also the only region in the water column where copepod swimming speed was greater than the rate of turbulent dissipation. These data provide evidence that the vertical structure of turbulence influences the distribution and aggregation of organisms in the water column.

Hypothesis III: Differences in phytoplankton abundance and species composition mediated by differences in water column stability result in measurable differences in copepod recruitment and growth rates. This leads to greater abundances in one region over another, due solely to high growth rates in situ.

The test of this hypothesis focussed on the relationship between an index of stratification and the growth and reproductive rates of C. finmarchicus. Stratification was estimated from the difference between sigma-t at 30 m and sigma-t at 1-2m. There was no apparent relationship between the stratification index and either C. finmarchicus egg production rate (in terms of %body carbon d-1, which takes into account seasonal variation in female body size) or molting rate (an estimate of development rate in units of d-1) of stages C1-C4 during 1995 process cruises. There was no apparent relationship between the reproductive index (an estimate of the daily spawning rate, directly related to egg production rate) and the stratification index , either at stations > 75 m in April-June or at stations on the southern flank in May and June during 1995 broadscale cruises. In general, carbon mass-specific egg production rates varied between approximately 11% d-1 across regions and seasons (January-June) in 1995, regardless of stratification. Feeding experiments showed that older stages of Calanus are ominvores and at times primarily carnivores on Georges Bank. This may explain why Calanus is able to sustain moderate to high egg production even in low chlorophyll and unstratified water.

Contributing PIs: C. Ashjian, L. Buckley, E.Calderone, R. Campbell, C. Davis, E. Durbin, S. Gallager, D. Gifford, L. Incze, G. Lough, J. Runge, M. Sieracki