Bollens Report

### Georges Bank Zooplankton and Storm Effects: Historical Data Analysis and Comparison with Model Results

    INVESTIGATORS:

    Stephen M. Bollens
    Woods Hole Oceanographic Institution
    Woods Hole, MA 02543
    508-289-3213
    sbollens@whoi.edu

    Cabell S. Davis
    Biology Department
    Woods Hole Oceanographic Institution
    Woods Hole, MA 02543
    508-289-2333
    cdavis@whoi.edu

    Andrew R. Solow
    Marine Policy Center
    Woods Hole Oceanographic Institution
    Woods Hole, MA 02543
    508-289-2746
    asolow@whoi.edu

    Grant Period: July 1, 1993 - June 30, 1995

#### STATEMENT OF OBJECTIVES

There are three major objectives of this research.

1. Analysis of Atlantis data from 1939-1941. These largely unpublished data have recently been uncovered in WHOI archives and will be analyzed for changes in the distribution (3-D) and abundances of several species, including Calanus, Pseudocalanus, Centropages, Metridia, larval cod, Gadus morhua, and larval haddock, Melanogrammus aeglefinus. (It is worth noting that the Atlantis data are the only 3-D, bank-wide extant zooplankton data from Georges Bank.) We will quantify the temporal (weeks to years) and spatial (3-D) variability in abundances of these species, and test for longer-term (i.e., inter-decadal) changes by comparing these data with those from the MARMAP and ICNAF programs.

2. Investigation of Storm Effects. We will use extant data on the distribution and abundance of zooplankton, in conjunction with hydrographic, wind, and commercial fisheries recruitment data, to test for adverse effects of storms, either direct or indirect, on the survival and recruitment of copepods and larval fishes on Georges Bank.

3. Comparison with Model Results. The extant data will be assimilated into a series of coupled biological/physical models currently being developed by a GLOBEC modeling project that will allow for model validation and refinement of predictions of how and why storm events affect different populations of zooplankton and larval fish on Georges Bank.

#### STATEMENT OF WORK

1. Our initial efforts focused on acquiring the hardware and data to achieve the above objectives. Specifically, we installed a SUN workstation in S. Bollens' laboratory and transferred all of the Atlantis data and some of the MARMAP and meteorological data to the workstation.

2. Our analyses to date have focused primarily on the Atlantis data, and have included:
   - examination of temporal variability in abundance of each taxa in 15 districts of the Georges Bank region;

   - examination of horizontal distribution and abundance, including the use of semi-variograms to estimate correlation length scales (CLS) for each taxa on each cruise;

   - examination of vertical distributions and diel vertical migration (DVM) patterns of each taxa and developmental stage on each cruise, including calculating daytime and nighttime weighted mean depths and testing statistically for differences between these as an indicator of DVM behavior;

   - obtained wind data from the NODC for 1939-1941, digitized the data, calculated a number of different measures of storminess (e.g., average daily wind speed, wind speed cubed, and Ekman displacement) and compared these to zooplankton CLSs and population growth rates to look for the effects of storm events.

3. The results of our analyses of the Atlantis data have begun to be compared with parallel analyses of the MARMAP data, e.g., comparison of bank-wide abundances of key species and developmental stages to test for inter-decadal (1939-41 vs. 1977-87) changes in the zooplankton.

4. The analyses of the Atlantis data for zooplankton spatial variability and CLSs have been compared with similar analyses that one of us (ARS) have been undertaking on zooplankton data from the North Sea. This has led to a consideration of estimation precision in zooplankton sampling and the optimal design of zooplankton surveys.

#### SUMMARY OF KEY FINDINGS

1. Seasonal abundances and life histories of the dominant zooplankton on Georges Bank exhibited qualitatively similar patterns a half-century ago to those of more recent sampling periods (e.g. MARMAP in the 1980's). For instance, both Calanus and Pseudocalanus exhibited late spring maxima in abundance, whereas Centropages and Sagitta peaked in abundance later in the year. Cod and haddock larvae also showed peak abundances in spring, with cod preceding haddock by several weeks.

2. Bank-wide horizontal distributions of the dominant species were also similar to those described from more recent (e.g., MARMAP) data sets. Regions of higher than average abundance for the dominant species were as follows: Sagitta and Centropages hamatus <60 m; Calanus and C. typicus between 60-100m; Pseudocalanus <100m; Metridia > 100m.

3. Vertical distributions of zooplankton on the bank during 1939-41 varied enormously - between species, between developmental stages, between cruises, and between regions of the bank - with changes in DVM behavior (e.g., reverse vs. normal) occurring in several species. For instance, daytime depth of Calanus increased with developmental stage such that early copepodid stages were typically non-migratory in the surface but later stages were undertaking normal DVM, although a statistically significant reverse migration in adult Calanus was observed on at least one date. For Pseudocalanus, changes in DVM behavior occurred between seasons (e.g., during spring 1940, reverse DVM in March-April, non-migratory in April-May, and normal DVM in June) and between regions (reverse DVM in waters < 100 m and normal DVM in waters >100m during May-June 1941).

4. Horizontal patchiness (e.g., correlation length scales [CLS]) of zooplankton on Georges Bank appear to be affected by storms (wind events). For instance, daily wind speed cubed (q3), a common proxy for mixing intensity, averaged over 30 days prior to each cruise, was positively significantly correlated with the CLSs of Calanus, Centropages, and all species combined. The five other abundant taxa considered all showed positive but insignificant correlations. Thus storminess would appear to break down (homogenize) small-scale patchiness and thereby increase the length scales at which zooplankton abundances are correlated.

5. Population growth rates of zooplankton on Georges Bank also appear to be affected by storms (wind events), with those species dominant in the shoal regions of the bank being most strongly influenced. For instance, realized rate of population increase between cruises was significantly negatively correlated with the magnitude of average daily wind speed calculated over the same periods for Pseudocalanus and C. hamatus (both shoally distributed on or near the peak of the bank); species more deeply distributed off-bank (e.g., Calanus and Metridia) showed no such dependence. However, specific events might be expected to effect all species - a comparison of the May-June inter-cruise period for 1940 and 1941 shows that in the high wind year (1940; 13 m s-1) population growth rates for the 10 dominant taxa were in all cases lower than in the low wind year (1941; 8 m s-1). Thus storminess would appear to negatively affect zooplankton abundances on Georges Bank, most likely by advecting animals off the bank.

#### PUBLICATIONS RESULTING FROM THIS GRANT:

Bollens, S. M., M. C. Landsteiner, C. S. Davis, and A. R. Solow. In Preparation. Three-dimensional distribution and abundance of zooplankton on Georges Bank, 1939-1941, with comments on the effects of storm events. J. Mar. Res.

Solow, A. R., S. M. Bollens, C. S. Davis, and P. H. Wiebe. In Preparation. Patchiness, estimation precision, and the optimal design of zooplankton surveys. J. Plankton Res.

#### PRESENTATIONS RESULTING FROM THIS GRANT:

Bollens, S. M., M. C. Landsteiner, C. S. Davis, and A. R. Solow. Three-dimensional distribution and abundance of zooplankton on Georges Bank, 1939-1941. The Oceanography Society Meeting, Newport, RI, April 1994.