Zooplankton grazing rate

Dimethyl sulphide (DMS) is produced in sea-water as a consequence of enzymatic action in phytoplankton and zooplankton grazing. Measurements of DMS indicate an average global sea-to-air flux of 40 Tg of sulphur per year (1.3). Instantaneous emission rates to the atmosphere, however, can vary considerably, and depend on the nature and concentration of the phytoplankton, the time of year and the prevailing meteorology. For example, nigh concentrations of Phaeocystis, a prolific producer of DMS, have been found in the North Sea and when it blooms, emissions of DMS may increase by a factor of 40-60 over those in the absence of blooms (4). The sea-to-air flux is also dependent on the depth at which the phytoplankton is found since DMS is slightly soluble in water and can be oxidized in aqueous solution. 

Reports on copepods and other crustaceans dominate available zooplankton grazing data on Phaeocystis spp. These data show a wide range of rates, even for similar predator-prey combinations. We therefore attempt to summarize available quantitative data in relation to some of the mechanisms proposed to control the feeding on Phaeocystis. The shortage of quantitative feeding studies on microzooplankton and Phaeocystis was pointed out already a decade ago (Weisse et al. 1994), and the number of such studies is still limited, especially for protozoan microzooplankton, whether using laboratory cultures (Table 3)... 

Copepods may feed selectively based on nutritional quality of the prey (e.g., Houde and Roman 1987). However, it is not clear how food quality of the different forms of Phaeocystis affects zooplankton grazing. It has been suggested that Phaeocystis spp. are of low nutritional value due to their low content of polyunsaturated fatty acids (PUFA) (Al-Hasan et al. 1990 Claustre et al. 1990 Rogers and Lockwood 1990 Nichols et al. 1991 Cotonnec et al. 2001 Tang et al. 2001 Turner et al. 2002). This contrasts with the high survival and development rates observed in other... 

One example of an inducible defense that is quick (several days) and easy to observe in the lab is an increase in colony size and spine length in the conunon green phytoplankton, Scenedesmus (Chlorococcales, Chlorophyta), when exposed to water in which the common zooplankton grazer, Daphnia (Cladocera, Crustacea), is cultured. Researchers have demonstrated reduced grazing rates of Daphnia on... 

Neufeld et al. (2002a) have shown that this behavior can be explained by the interplay between excitable plankton population dynamics and chaotic flow, similarly to the excitable behavior described in the previous section. In a chaotic flow a steady bloom filament profile can be generated, that does not decay until it invades the whole computational domain as an advectively propagating bloom. The condition for the existence of the steady bloom filament solution in the corresponding one-dimensional filament model is that the rate of convergence, quantified by the Lyapunov exponent, should be slower than the phytoplankton growth rate, but faster than the zooplankton reproduction rate. In this case the phytoplankton does not became diluted by the flow and the zooplankton is thus kept at low concentration unable to graze down the bloom filament. 

Fig. 4.45. The phosphorus residence time, the uptake rates of phytoplankton and zooplankton and POC export fluxes at three sampling stations in the ECS. (a) Residence time for TDP, suspended matter, and net-plankton (b) The phosphorus assimilation rates and carbon assimilation fluxes of phytoplankton (c) The phosphorus grazing rates and carbon grazing fluxes of zooplankton (d) POC export fluxes from the upper 35 m (Zhang et al., 2004) (With permission from Elsevier s Copyright Clearance Center)...

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