Time - series feeding rates of

A flow-through system was used to measure grazing rates of Thysanoessa raschii under conditions simulating encounters with gradients of phytoplankton concentration. Ingestion rates of krill fed Chaetoceros gracilis, measured over 1 O-min intervals, closely tracked changes in phytoplankton concentration. The relationship between ingestion rates and pigment concentration was linear from 0.5-9 fig pigment liter-‘, and zero ingestion was predicted at 0.25 pg pigment liter-’ (intercept significantly different from zero, P < 0.05). Clearance rates were almost constant (15.4+ 3 ml krill-1 h-l) at all concentrations. This indicated that the changes in ingestion rate resulted from changes in food concentration, rather than as an active response of the krill to the food gradient. The results do not support the hypothesis that krill elevate their feeding rates in response to encounters with high concentrations of phytoplankton in patches or layers. The strategy of krill may be to remain within food concentrations encountered in the field, rather than to feed faster. Zooplankters live and feed in a spatially heterogeneous environment, where food sources are often distributed in patches or layers. Many laboratory feeding studies suggest that they must exploit fine-scale concentrations of food to meet their intake requirements (Lasker 1966, 1975; Mullin and Brooks 1976; Stanlaw et al. 198 1; McClatchie 1985). It is difficult to simulate the gradients in food concentration and associated physical variables such as light and temperature in the laboratory. Consequently, virtually all laboratory grazing studies apply to feeding at near-constant food concentrations. Frost (1972), however, allowed Calanus to feed on declining cell concentrations. It is unclear how zooplankton grazers respond to the gradients in food concentration that they experience in the field. Many euphausiids are strong vertical migrators and they are generally opportunistic feeders (Lomakina 1973; Mauchline 1980). They encounter a range of different potential food organisms at varying concentrations during their migrations. Phytoplankton and detritus are uniformly distributed 1 This research was supported by a Natural Sciences and Engineering Research Council (Canada) grant to C. M. Boyd, a Petroleum Research Foundation grant to P. J. Wangersky, and a Dalhousie Graduate Fellowship to S. McClatchie. 2 Current address: Portobello Marine Laboratory, P.O. BOX 8, Portobello, Otago, New Zealand. in the water column only when mixing is intense, as during winter storms (Venrick 1984). Discontinuities in phytoplankton abundance occur in Langmuir cells (Weller et al. 1985), chlorophyll lenses associated with the thermocline (Derenbach et al. 1979), and at oceanic fronts (Pingree et al. 197 5). High-resolution vertical profiling with an in situ fluorometer documented microscale layering of phytoplankton in coastal waters (Derenbach et al. 1979). The average thickness of these layers was 34 cm, with chlorophyll concentrations 8-20 times higher than the average. Chlorophyll layers extended for at least 10 m horizontally and were thought to persist for several days, depending on the degree of turbulence and the sinking rate of the cells comprising the layers (Derenbach et al. 1979). This type of structure may have considerable trophic significance for herbivorous euphausiids capable of locating and remaining within the lenses, particularly if these zooplankters were previously malnourished at ambient food concentrations. Thysanoessa raschii is a neritic species of krill with an arctic-boreal distribution in Atlantic and Pacific Oceans (Lomakina 1973). It occurs in abundance in the Gulf of St. Lawrence (Sameoto 1976) and in north Norwegian fjords (Falk-Petersen and Hopkins 198 l), but there are few studies of its trophic ecology. Biochemical studies describing seasonal changes in its lipid com-