C.5.2.5 Grazing Rates and Food-web Transfer of Toxins:(PIs: Turner, Doucette)
Rationale: Zooplankton interact with toxic phytoplankton through grazing impacts and by vectoring toxins through pelagic food webs. The role of zooplankton in both processes depends on community composition and abundance. Alexandrium toxins affect consumers through ingestion and/or exposure to excreted toxins. Accumulation of toxins in zooplankton facilitates vectorial intoxication of zooplanktivorous fish (White, 1981; Tester et al. 1996). Mortalities of whales, which feed on large copepods, have been attributed to PSP toxins (Geraci et al., 1989). We propose to assess toxicity in algal and zooplankton size fractions in the WMCC and EMCC regions to identify primary routes of toxin transfer within food webs. Since PSP toxins can occur extracellularly (White, 1981; Doucette, unpub. data), possibly affecting the micro-zooplankton community (Smayda, 1992), some of the first measurements of dissolved toxins associated with Alexandrium will also be obtained.
Numerous grazing studies have used cultured toxic phytoplankton, but few have employed mixed natural assemblages (Turner and Anderson, 1983; Turner and Tester, 1989). In this study, grazing rates of key species co-occurring with Alexandrium will be determined, which, together with zooplankton composition and abundance, will provide estimates of community grazing impacts. This information is needed to explain Alexandrium distributions and bloom dynamics, and is essential for development of coupled physical/biological models simulating Alexandrium bloom dynamics.
Community composition: During Years 1 and 3, zooplankton samples will be collected at 12 and 15 stations/cruise in the WMCC and EMCC regions, respectively. Sampling and counting techniques developed previously during Alexandrium bloom studies in Massachusetts Bay will be used. Vertical tows will be taken with 102 mm-mesh nets equipped with flowmeters to quantify grazer community composition. Non-quantitative samples collected concurrently will be partitioned into 20-64, 64-200, 200-500, and >500 mm fractions to determine relative taxon proportions in each.
Food Web PSP Toxin Transfer: Measurements of toxin in size-fractioned samples will be obtained by collecting material on a filter and extracting toxins. Samples for dissolved toxin will be filtered at 0.2 mm. Both size-fractioned and dissolved PSP toxins will be determined with a saxitoxin receptor binding assay (Doucette et al., in press), which has detection limits of 5-50 nM STX equiv. The receptor assay has yielded reliable quantitative estimates of toxin in cultured as well as field populations of Alexandrium (Doucette et al., in press), and in zooplankton-dominated size fractions from Massachusetts Bay (Doucette and Turner, unpubl. data). To further validate field-based use of the receptor assay, selected samples will be analyzed using HPLC (Anderson et al., 1994).
Grazing Impact: Experimentally-determined, individual-animal grazing rates on Alexandrium, performed with field abundances of both Alexandrium and zooplankton, will permit calculation of grazing impact on bloom dynamics (e.g., Turner and Anderson, 1983; Uye, 1986). Grazing experiments will be conducted during ‘process studies’ in the WMCC region, and during the May-June bloom season at Bigelow Laboratory using material from Casco Bay. All experiments will be conducted daily over 4-5 days and use protocols adapted from Turner and Anderson (1983) and Turner et al. (1997). Known numbers of known stages and copepod species (or other abundant zooplankters) will be allowed to feed on Alexandrium-containing natural communities at ambient temperatures. Grazers used in experiments will vary with the abundance of local taxa, likely to include copepods, cladocerans, meroplankters, and large tintinnids. Spring/summer zooplankton assemblages in the WMCC region and Casco Bay are likely to be similar to, and will thus be representative of, the EMCC region. Upon termination of experiments, phytoplankton taxa will be preserved and enumerated. Grazing rates will be calculated using the Frost (1972) equations, and will aid in assessing whether there is selection for or against toxic Alexandrium cells. Field abundances will be used with individual animal grazing rates to yield estimates of grazer community impact as a percentage of initial Alexandrium concentration. Note that use of the most abundant grazers in experiments reflects the likelihood that these taxa will exert the highest grazing impact. Since less abundant, larger animals may be the predominant consumers of Alexandrium, as noted previously (Doucette and Turner, unpubl. data), their grazing rates will also be determined.