U.S. GLOBEC: Phytoplankton and Protozoa in the Diets of Copepods on Georges Bank

INVESTIGATORS:

Dian J. Gifford
University of Rhode Island

Michael E. Sieracki
Bigelow Laboratory for Ocean Science

STATEMENT OF OBJECTIVES:

Our objectives are (1) to describe quantitatively the nano- and microplankton prey fields on Georges Bank and (2) to measure consumption of nano- and microplankton organisms (i.e., plant and animal prey 2-200 um in size) by all copepodid life history stages of the biomass dominant copepod, Calanus finmarchicus, under varying conditions of water column mixing and stratification on Georges Bank. We focused on Calanus because it is the dominant member of the holozooplankton community on Georges Bank during late winter, spring and early summer when its various life history stages constitute major items in the diet of the pelagic stages of cod and haddock. Our GLOBEC co-investigator, Scott Gallager, performed complementary feeding studies with early stage cod and haddock larvae. These studies are described in a separate report on the GLOBEC WWW site.

SCIENTIFIC BACKGROUND:

Definitions. Nano- and micro-phytoplankton and zooplankon are defined operationally on the basis of size as consisting of organisms 2-20 um and 20-200 um respectively (sensu Sieburth, et al. 1978). These operational categories contain a diversity of taxa including diatoms, flagellates and dinoflagellates in the nano- and microphytoplankton, and ciliates, hererotrophic flagellates, heterotrophic dinoflagellates, sarcodines, and metazoan nauplii in the nano- and microzooplankton.These organisms are ubiquitous and abundant in the euphotic zone of marine waters (Beers 1978; Conover 1982). Heterotrophic flagellates are typically the most abundant nanozooplankton organisms, while ciliates and heterotrophic dinoflagellates usually dominate the microzooplankton.

Omnivory in copepod diet. Planktonic protozoa function as important trophic intermediaries in pelagic food webs by repackaging small bacterial and algal cells into food items which are accessible to larger consumers. Although it has long been obsberved that calanoid copepods which feed in the euphotic zone tend to be omnivorous, they are often treated as if they are exclusively herbivorous in both mathematical models and manipulative experiments. Studies with a number of species in a diversity of marine environments indicate that calanoid copepods consume nano- and microzooplankton in laboratory experiments done under controlled conditions and in the field when presented with natural assemblages of prey. Further, consumption of protozoan prey affects copepod condition and, ultimately, production (reviewed in Gifford 1991).

METHODS:

Distribution and abundance of chlorophyll, nano- and microplankton. Samples for size-fractionated chlorophyll, nanoplankton and microplankton were collected during hydrocasts from the top, middle and bottom of the water column at stations where the water column was well mixed. At stations where the water column was stratified, samples were collected from the top, middle and bottom of the mixed layer, and more intensively in stratified layers and around features such as the chlorophyll maximum and the pycnocline. Bulk seawater was collected using teflon-lined 10-L Go-flo bottles mounted on the CTD rosette. Chlorophyll samples were collected onto GF/F filters and analyzed by fluorometry. Nanplankton samples were fixed with glutaraldehyde, stained with fluorochrome, and collected on black nuclepore filters. They were analyzed using Color Image Analyzed Fluorescence Microscopy (Sieracki and Viles 1990). Microplankton samples were preserved with 10% acid Lugols solution, concentrated by settling, and analyzed by inverted microscopy (Gifford 1993a). Samples for analysis of POC and major nutrients were collected from selected hydrocasts.

Copepod feeding experiments. Manipulative experiments were done using natural prey assemblages in which consumption of phytoplankton and protozoa was measured directly by monitoring the disappearance of prey. Feeding was measured at each time series station under prey conditions representative of the mixed layer and/or the pycnocline, as appropriate. Ingestion rates of consumers were measured directly in shipboard experiments using natural prey assemblages. Experiments were done with all life history stages of Calanus sufficiently abundant to be sorted: stages C1-C6 females. The detailed protocol for these experiments is described in Gifford (1993a; b). In brief, the design onsists of 2 treatments, an experimental treatment consisting of the nano- and microplankton prey assemblage incubated in bottles with copepods added and a control treatment in which bottles contain only the prey assemblage. Treatment bottles are incubated for 24 hours on-deck in a water-cooled incubator equipped with a rotating wheel. Samples for size-fractionated chlorophyll, nanoplankton and microplankton are collected at the beginning and end of an experiment and analyzed as described above. Per capita and carbon-specific rates of copepod clearance and ingestion are calculated according to Frost (1972) and Marin, et al. (1986).

ACTIVITIES DURING 1994-1995:

We participated in one cruise in 1994 and 5 cruises in 1995 (Table 1). On the 1994 cruise, a month-long shake-down effort during May-June, we performed 12 experiments with stage C4, C5 and adult female Calanus finmarchicus, as stratification developed (Table 2). Preliminary results from these experiments are discussed below. During the 5 cruises in 1995, we performed a total of 39 experiments with stages C1, C2, C3, C4, C5 and adult female Calanus. Experimental work was done between January and June, as the copepod population proceeded through its ontogenetic development and the water column evolved from well-mixed to stratified conditions. Samples from these experiments are presently under analysis, and will not be discussed further here.

Table 1.  GLOBEC Georges Bank process cruises 1994-1995.

                Cruise          Research Vessel         Date                             

                CI9407          Columbus Iselin         25 May-16 June 1994              
                EN259           Endeavor                9-21 January 1995
                EN262           Endeavor                23 February-10 March 1995
                EN264           Endeavor                27 March-8 April 1995
                EN266           Endeavor                26 April-9 May 1995
                EN267B          Endeavor                7-21 June 1995


Table 2.  Experiments with Calanus finmarchicus during cruise CI9407.


                Experiment Number         Date          Copepodid Stage

                        Cal-1           05-29-94        C6 Female
                        Cal-2           05-29-94        C5
                        Cal-3           05-29-94        C4
                        Cal-4           05-31-94        C6 Female
                        Cal-5           05-31-94        C5
                        Cal-6           05-31-94        C5
                        Cal-7           06-07-94        C6 Female
                        Cal-8           06-07-94        C5
                        Cal-9           06-10-94        C6 Female
                        Cal-10          06-10-94        C6 Female
                        Cal-11          06-11-94        C5
                        Cal-12          06-11-94        C5

RESULTS TO DATE:

Cruise CI9407 on R.V. Columbus Iselin occupied two time series stations on Georges Bank during May-June 1994: the MIXED SITE on the bank crest, and the STRATIFIED SITE in deeper water on the southern flank (Figure 1). On the map of the study area, Stations 21, 41, and 43 are the STRATIFIED SITE; Stations 22 and 42 are the MIXED SITE. Each site was marked with a drogued ARGOS drifter, which was followed for the duration that the site was occupied. During the duration of the cruise, both drifters were entrained in the tidal ellipse. The STRATIFIED SITE drifter moved to the south and west between the 60 and 100 m isobaths. The MIXED SITE drifter moved to the south and west within the 60 m isobath. Previous cruises had reported well developed stratification on the southern flank of the Bank. However, by the time CI9407 arrived on 29 May, a series of storms had completely mixed the water colum at all on-Bank locations. Stratification developed gradually between 29 May and 14 June, producing a strongly stratified water cloumn characterized by a chlorophyll maximum at ~20m at the STRATIFIED site.

The prey field. Total chlorophyll-a was generally less than 2ug/L at both the STRATIFIED and MIXED sites. At least half of the chlorophyll standing stock was <5 um in size in all experiments. The nanoplankton was dominated by cyanobacteria and heterotrophic flagellates. The microphytoplankton consisted primarily of large centric diatoms. The microzooplankton assemblage was dominated by aloricate choreotrich ciliates >5 um. Heterotrophic dinoflagellates of the genus Protoperidinium were abundant, as was the autOtrophic ciliate Mesodinium rubra.

Feeding rates. Ingestion of chlorophyll, cyanobacteria, heterotrophic flagellates and ciliates from a subset of the experiments is shown in Figure 2. As expected, feeding effort was concentrated on particles >5 um, and cyanobacteria were not consumed. Clearance rates (not shown) of ciliates and heterotrophic flagellates greatly exceeded clearance of bulk chlorophyll except in Experiment 10, where the prey assemblage was collected from the chlorophyll maximum. The majority of carbon ingested came from phytoplankton and heterotrophic flagellate prey.

Overall, Calanus finmarchicus females and C5s were food-limited in May 1994 before the onset of water column stratification, ingesting <10% of their body carbon/d. Only after stratification developed, and only when they were fed the microplankton assemblage from the chlorophyll maximum (Experiment Cal 10) were the copepods able to consume more than 20% of their body carbon/d. Our most surprising finding to date is the importance of heterotrophic flagellates in the diet of Calanus prior to development of the chlorophyll maximum.

BIBLIOGRAPHY:

Beers, J.R. 1978. About microzooplankton. pp. 288-290 in: A. Sournia (ed.) Phytoplankton Manual, UNESCO, Paris.

Conover, R.J. 1982. Interrelatuons between microzooplankton and other plankton organisms. Ann. Inst. Oceanogr. Paris 59S: 31-46.

Frost, B.W. 1972. Effects of size and concentration of food on the feeding behavior of the marine planktonic copepod Calanus pacificus. Limnol. Oceanogr. 17: 805-815.

Gifford, D.J. 1991. The protozoan-metazoan link in pelagic ecosystems. J. Protozool. 38: 81-86.

Gifford, D.J. 1993a. Protozoa in the diets of Neocalanus spp. in the oceanic subarctic Pacific Ocean. Progr. Oceanogr. 32: 223-237.

Gifford, D.J. 1993b. Consumption of planktonic protozoa by suspension feeding copepods. pp. 723-727 in: P.F. Kemp, et al. (eds.), Handbook of Methods in Aquatic Microbial Ecology. Lewis Publishers, Boca Raton, FL.

Marin, V. et al. 1986. Measuring feeding rates of pelagic herbivores: analysis of experimental design and methods. Mar. Biol. 93: 49-58.

Sieburth, J. McN. et al. 1978. Pelagic ecosystem structure: heterotrophic compartments of the plankton and their relationship to plankton size fraction. Limnol. Oceanogr. 23: 1256-1263.

Sieracki, M.E. and C. L. Viles 1990. Color image-analyzed fluorescence microscopy: a new tool for marine microbial ecology. Oceanography: 3: 30-36.

[NOT AVAILABLE YET] Figure 1. Drifter station location during May and June 1994. Stations 21, 41 and 43 are the STRATIFIED SITE. Stations 22 and 42 are the MIXED SITE.

[NOT AVAILABLE YET] Figure 2. Ingestion (ugC/copepod/d) of cyanobacteria, ciliates, chlorophyll and heterotrophic flagellates by Calanus finmarchicus during development of stratification on Georges Bank in May-June 1994.