Spatial Distributions of Plankton in Shark Bay, Western Australia - 1983

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During June 1983, the abundance and diversity of zooplankton were sampled at a series of locations though Shark Bay, Western Australia. High-salinity experiments were also conducted on certain zooplankton to determine abundance patterns for
individual species.

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Statement: Sample stations (see thumbnail) were selected to provide a series from the ocean to Hamelin Pool. Most sites were visited only once during the cruise, but several stations were visited more than once. An additional series of samples for salinity and zooplankton abundance data was taken while at anchor in a channel on Faurk Sill (station 17), between 1630 h on 16 June and 0830 h on 17 June. Except for this series and a single night sample in Hamelin Pool (station 23), all samples were taken by day. Temperature was measured with a laboratory thermometer. Salinity was measured on board ship with a hand-held refractometer and, on samples returned to Perth, with a conductivity meter. Because of the large salinity gradient, the refractometer precision (0.5 mg per litre) was sufficient for this survey. Several water samples were collected for filtration and analysis of particulate matter. Samples were filtered onto GF/C glass-fibre filters and frozen in acetone for later fluorometric analysis for chlorophyll (Strickland and Parsons 1972). Particulate matter collected on precombusted glass-fibre filters was dried in a desiccator and later analysed for carbon and nitrogen with a CHN analyser, and for particulate phosphorus by persulfate oxidation (Menzel and Corwin 1965). Zooplankton samples were collected by oblique tows from near the bottom to the surface with a ring net of 150-um mesh and 0.5-m mouth fitted with a flow meter. Qualitative net phytoplankton samples were taken with a net of 10-um mesh and 0.5-m mouth. All samples were immediately preserved in 2-4% (v/v) formaldehyde. Aliquots of the zooplankton samples were later taken to provide 200-500 animals, which were identified to as low a taxonomic level as practicable. Copepods were identified to species. Phytoplankton samples were similarly subsampled and taxa identified, in most cases, to genus. Abundance data for adult copepods from the daytime spatial series of samples were transformed (to log[N+ 11) and examined by cluster analysis (BMDP Program PKM) to detect clusters of stations. We conducted two experiments on salinity tolerance of zooplankton, using the most abundant copepod species in Shark Bay, Bestiola similis. Copepods for both experiments were collected at 43 mg per litre salinity, and size fractionated to obtain late juveniles and adults. Copepods were incubated in 4-litre plastic jars suspended in the bay to maintain water motion and temperature. In experiment 1, copepods were incubated in water of six salinities from 43 to 63 mg per litre, and aliquots were removed several times during incubation of up to 50 h. In the second experiment, phytoplankton collected by tow with a net of 10 umm mesh was added to the high-salinity water, then the copepods were incubated in replicate samples of either high or low salinity for 21 h. Mortality was determined by staining copepods with neutral red, a vital stain (Dressel et al. 1972). The feeding rates of two species of Calanopia, the most abundant calanoid copepods in Hamelin Pool, were estimated. Four batches of 15 females of Calanopia sp. 2 and one batch of Calanopia sp. 1 were placed in surface water in 4-litre plastic jars and incubated in the dark for 18 h. Parallel incubations without copepods served as controls. The samples were preserved after incubation and the copepods and phytoplankton in the samples counted. **References** Dressel, D. M., Heinle, D. R., and Grote, M. C.(1972). Vital staining to sort dead and live copepods. Chesapeake Sci. 13, 156-9. Menzel, D. W., and Corwin, N. (1965). The measurement of total phosphorus in seawater based on the liberation of organically bound fractions by persulfate oxidation. Limnol. Oceanogr. 10, 280-2. Strickland, J. D. H., and Parsons, T. R. (1972). 'A Practical Handbook of Seawater Analysis.' (Fisheries Research Board of Canada: Ottawa.)