Zooplankton potential

One example for a chemically defended zooplankton species is the Antarctic pteropod Clione antarctica. This shell-less pelagic mollusk offers a potentially rich source of nutrients to planktivorous predators. Nonetheless fish do not prey on this organism, due to its efficient chemical defense. In a bioassay-guided structure elucidation, pteroenone 37 could be isolated and characterized as the main defensive principle of C. antarctica [82,83]. If embedded in alginate, this compound is a feeding-deterrent in nanomolar concentrations. This unusual metabolite is likely to be produced by C. antarctica itself and not accumulated from its food, since its major food sources did not contain any detectable quantities of 37. 

Thus chitin is abunckmt in the sea, in diatom blooms and in the zooplankton, most notably in the shoals of krill and on the land, in invertebrates and in fungi in the soil. Potential industrial sources are wastes from shrimps and crabs, krill, squid, clams and oysters, and fungal fermentations (13). The krUl fishery alone produces 3000 tons per year, currently going to waste. 

Analyses of carbon isotopic composition were performed as an independent measure of potential food sources for zooplankton. The 813C values of terrestrial detritus and of littoral zone emergents such as Carex ranged from -26.0 to -29.2%o (Table I 49, 92). By comparison, the zooplankton were quite depleted in 13C their 813C values were more similar to phytoplankton and ranged from -32.1%o in Daphnia to -41.0%o in Cyclops (Table I). 

Thomann et al. (1992) and Morrison et al. (1997) have developed kinetic models employing rate constants to assess the extent of chemical bioaccumulation in zooplankton, as Tables 9.1 and 9.2 summarize. Thomann et al. (1992) list relationships which incorporate organism physiology, bioenergetics, and chemical characteristics to estimate uptake and elimination rate constants which are used to estimate bioaccumulation. Morrison et al. (1997) rely on physiological information to estimate bioaccumulation. Both models provide a potentially more realistic description of bioaccumulation by zooplankton, although, to date, neither model has been tested independently against field data. 

Table 9.8 provides examples of some of the methods for assessing of the bioconcentration and biomagnification potential of a range of hydrophobic organic chemicals. Included are sample calculations for Koc, the dissolved chemical concentration in water, the BCF in algae, zooplankton, a benthic invertebrate species (i.e., a benthic detritovore, e.g., amphi-pod), and fish (e.g., rainbow trout). 

Ideally, future investigations on the ecology of zooplankton feeding should preferably be conducted in situ, although laboratory experiments are needed to investigate the effects of potentially important signaling substances between Phaeocystis and different predators in presence of realistic alternative prey. 

The predominance of phytoplankton-derived carbon in diets of many fish species, despite its small contribution to floodplain production, can be explained by the selective consumption of algae. However, Bayley (1989) argued that phytoplankton production was too low to contribute significantly to regional fish production. He based his argument on a food chain with three trophic levels (phytoplankton—zooplankton— fish), and assumed a 10% transfer efficiency between trophic levels. This model is inappropriate for detritivorous and herbivorous fish which consume plant materials directly. A model with two trophic levels would be more appropriate and would indicate a higher potential contribution to... 

Alcaraz, M., Saiz, E., and Estrada, M. (1994). Excretion of ammonia hy zooplankton and its potential contribution to nitrogen requirements for primary production in the Catalan Sea (NW Mediterranean). Mar. Biol. 119, 69—76. 

Interestingly, there is evidence that cell death processes in phytoplankton may be driven by allelopathy (e.g., Casotti et al., 2005), and that similar processes may also occur in zooplankton (e.g., Romano et al, 2003). So far, caspase activities have not been detected in these situations, but the potential usefulness of the assays should be assessed. 

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