Durbin#2 Report

### US GLOBEC: Recruitment and production rates of Calanus finmarchicus and Pseudocalanus spp. on Georges Bank

    INVESTIGATORS:

    Dr Edward Durbin
    Graduate School of Oceanography
    University of Rhode Island
    South Ferry Rd
    Narragansett, RI, 02882
    Telephone: (401) 792-6695
    Facsimile: (401) 792-6240
    email: edurbin@gsosun1.gso.uri.edu

    Dr Ann Durbin
    Graduate School of Oceanography
    University of Rhode Island
    South Ferry Rd
    Narragansett, RI, 02882

    Dr Jeffrey Runge
    Institut Maurice-Lamontagne
    850 route de la Mer
    C. P. 1000
    Mont Joli, PQ G5H 3Z4
    Canada
    (418) 775-0676
    (418) 775-0542 fax
    email: j_runge@iml3.iml.dfo.ca

    GRANT PERIOD:  September 1, 1993 - August 31, 1996

#### STATEMENT OF OBJECTIVES:

As part of the broad scale surveys:

- To measure abundance and stage distribution of Calanus and Pseudocalanus nauplii and early copepodites at selected stations on Georges Bank and in adjacent waters.

As part of the stratification study:

- To experimentally measure growth and molting rates of the dominant stages of Calanus and Pseudocalanus at sites in the different hydrographic regimes. Growth is being measured both as changes in carbon and nitrogen and from RNA/DNA ratios. Molting rates are being determined in the lab as well as from changes in stage frequency distribution from plankton pump samples at drifter sites.

- To measure egg production rates of adult female Calanus and Pseudocalanus at these sites using both shipboard incubation and egg ratio techniques.

#### STATEMENT OF WORK:

##### Broad Scale Survey Plankton Pump Sampling

Plankton pump samples are being collected at 18 of the 38 broad scale survey stations to provide information on copepod nauplii and other small zooplankters that are not sampled with the 1 m2 MOCNESS net. Samples are collected on a 50 um mesh net over the same depth ranges as the 1 m2 MOCNESS zooplankton samples. These are 0-15 m, 15-40 m, 40-100 m or bottom if it is less than 100 m, and 100-bottom where the depth is greater than 100 m.

Six Broad Scale survey cruises are planned per year. The cruise dates for 1995 are: 10 - 21 Feb, 13 - 24 March, 11 - 23 April, 8 - 19 May, 5 - 17 June, and 11 - 22 July. Additional samples have been collected during the Albatross IV training cruise to Georges Bank (AL 9410) between 7 - 18 Nov, 1994, and on the first process cruise (EN259) 10 - 22 Jan, 1995. These additional samples help provide a better seasonal coverage of the population dynamics of the target zooplankton species on Georges Bank. During 1996 a similar series of cruises is planned. However, during this year we are planning to move the July cruise to January to provide information on the early winter recruitment of zooplankton on the bank.

In the laboratory the samples are split to provide a subsample of about 1200 animals. The two target species are enumerated to stage. Other copepods are identified to species and grouped as nauplii, copepodites and adults.

The data for each cruise will be presented in a cruise report when all samples for each cruise are completed. In addition as individual samples are completed they will be available on the data base management system. This has not yet been set up but will be done by the end of July.

##### Growth and egg production rates during process cruises

Growth and egg production rate studies were carried out at sites in the different hydrographic regimes on each of the five zooplankton process cruises in 1995. At each site ARGOS/GPS drifters were drogued at 15 m for 2 to 3 days. Every 12 hrs a CTD and plankton pump cast was made. From the latter we will try to determine in situ molting and mortality rates. 1 m2 MOCNESS samples were collected during the first day at each site. Copepods were collected for growth and molting rate experiments on the first day at each drifter site. Individual stages of Calanus were sorted and incubated for two days in deck incubators. Initial and final size, C and N content, and RNA/DNA ratios were measured. Artificial cohorts of nauplii were created by sieving and these were incubated in large deck mesocosms containing water collected in the chlorophyll a maximum with a diaphragm pump. Changes in the stage distribution measured every 12 hrs enable us to calculate stage durations. Similar experiments were carried out during the pilot process cruise on the Columbus Iselin 25 th May - 16 June 1994. In addition, Calanus copepodites were sorted from samples collected at other stations on the bank and in adjacent waters for length, C and N, and RNA/DNA.

In situ egg production rates of Calanus finmarchicus were measured at a number of sites, including each drifter site, by sorting individual females and incubating them in filtered sea water (FSW) in petri dishes for 24 h at in situ temperatures under a 12:12 light cycle. Eggs were counted and removed each 8 h to prevent cannibalism and then preserved for future egg viability and quality assessment. Females were preserved at the end of the experiment for C and N analysis. In some experiments a portion of the females at the end of the experiment were preserved for RNA/DNA measurement. At some stations, when time was limiting or weather too rough, females were individually incubated in 45 ml flasks and then both eggs and animal preserved in the container at the end of incubation period.

This latter method was also used for egg production measurements with other species (Centropages, and Temora) as time permitted. Egg counts from the pump samples will be used to estimate egg production of Pseudocalanus. Comparison of Calanus population egg production rates from lab measures of egg production and field measures of adult female abundance with egg abundance measures in pump samples will provide an estimate of egg mortality. Egg production rates of Calanus are being compared with the stage of gonad development to calibrate the Reproductive Index for the Georges Bank region.

#### SUMMARY OF KEY FINDINGS:

##### Broad Scale Survey Plankton Pump Sampling

The first five cruises have been successfully completed with one remaining to be carried out. It is still too early in the project to provide many results. However, initial results show a much earlier initiation of reproduction by Calanus finmarchicus on the bank than expected. A few animals were reproducing on the bank in November while by mid-January about half the Calanus were adult females and a cohort of young nauplii was developing. Surveys in future years will show whether this observation in 1995 is a normal occurance.

Initial comparisons of pump and MOCNESS samples indicate quite similar abundance numbers for the copepodites. Because the pump samples provide a complete age structure (nauplii to adults) within the same sample we feel that these samples are particularly useful for population analysis.

Also they provide measurements of abundance of nauplii which are the primary food of young fish larvae. Advantages of the MOCNESS samples are that they integrate a larger volume, and provide samples from depths greater than 100 m.

##### Growth and Egg production measurements

Results from the 1995 process cruises are not yet available (three months at sea between Jan and June have not left much time for sample analysis). Data from the May-June 1994 pilot process cruise are presently being written up for a paper. Some of the major findings include:

1. The water column was well mixed on the north-east peak (visited early during the cruise) and in the shallow portions of the bank. On the southern flank stratification was developing during the cruise.

2. The drifters moved in a clockwise direction around the bank between 5 and 20 cm sec-1.

3. There were differences in the zooplankton between the shallow (< 60 m) well-mixed regions of the bank and the deeper regions. Calanus was present in very low numbers in the shallow regions with a low proportion of nauplii, indicating low recruitment and/or high egg mortality. In contrast there were very high numbers of Centropages and Temora nauplii present in this region. Hydroids, and polychaete larvae were very abundant in this region as well. In deeper waters Calanus, Pseudocalanus, Temora and Centropages were all abundant. At the upstream drifter site on the northeast peak there were very few Calanus nauplii and they constituted only 30% of the total Calanus population while downstream on the southern flank 95% of the Calanus population were nauplii. Temora and Centropages showed a similar pattern, while there was a large proportion of Pseudocalanus nauplii present at both stations.

4. In mesocosm experiments molting rates of C4 Calanus and C5 Pseudocalanus were slow to very slow. The two species showed different responses at each station. Although development rates were not enhanced by enrichment of the mesocosms, short term growth as evidenced by weight gain was. In contrast older stage nauplii molted at rates close to maximum at all sites.

5. Egg production rates of Calanus varied between about 17 and 64 eggs female-1 day-1. Highest rates were observed at the stratified site on the southern flank. Variations in egg production rates may be affected by food (which will affect not only clutch size, but time between clutches), temperature (which explains some of the variability observed on this cruise), and female mortality rates (where reproductive rates differ with female age, differences in mortality rates will affect egg production rates). Pseudocalanus similarly showed highest egg production rates at the stratified site, although relative differences between stations were less.

A preliminary report of this cruise was presented as a poster at the 1995 TOS meeting in Newport, RI. Copies of figures and tables are available from E. Durbin. Below is a copy of the abstract.

Recruitment and Growth of Calanus finmarchicus and Pseudocalanus sp on Georges Bank during late spring

E. Durbin, J. Runge, A. Durbin, R. Campbell, P. Garrahan, M. Bemis, S. Plourde.
Graduate School of Oceanography, University of Rhode Island, South Ferry Rd, Narragansett, RI, 02882. edurbin@gsosun1.gso.uri.edu OR Institut Maurice-Lamontagne, 850 Route de la Mer, C.P. 1000, Mont Joli, PQ G5h3Z4, Canada, j_runge@iml3.iml.dfo.ca

Population dynamics of the dominant copepods on Georges Bank were investigated during late May-early June 1994 as part of the US-GLOBEC Georges Bank Program. ARGOS/GPS drifters were deployed on the northeast peak, in the shallow well-mixed region, and on the southern flank of Georges Bank. These were used to track zooplankton populations for up to three days at each site. MOCNESS and plankton pump sampling provided a complete description of the age structure of the dominant copepods at each site. Differences in egg production rates and age structure of Calanus finmarchicus between the northeast peak and the southern flank, coupled with the drifter tracks, suggest a pattern of advection of copepods onto the bank, increased egg laying as they encounter higher food levels, and then the subsequent advection of this developing cohort along the southern flank. Stage durations measured in mesocosms on board ship varied with stage at a given site. Nauplii and young copepodites generally developed quite rapidly, and in some cases near estimated maximum rates, at all sites. In contrast, late stage copepodites of Calanus and Pseudocalanus were developing very slowly. Results indicate that environmental conditions for growth and production on Georges Bank vary significantly for different developmental stages of the dominant copepods.