Feeding behaviour and non-linear responses in dilution experiments in a coastal upwelling system

11 páginas, 6 figuras, 4 tablasThe occurrence of non-linear responses in several dilution experiments conducted in the coastal upwelling system of the Ría de Vigo, Spain, was examined in relation to the possible violation of the basic assumptions of the technique. In addition to linear responses, 2 types of non-linear responses (saturated and saturated-increased responses) were obtained. Saturated responses are those showing constant net growth rates at lower dilutions, while saturated-increased responses depict the increase of net growth rates in low-diluted bottles. Evidence relating these 2 non-linear responses to nutrient limitation or changes in the microzooplankton community was not definitive. In contrast, saturated and saturated-increased responses were frequent when the percentage of microzooplankton was relatively low within a very abundant and diverse plankton community. We suggest that saturated feeding responses were related to the achievement of a maximum ingestion rate by microzooplankton and that saturated-increased responses were associated with selective feeding by microzooplankton at times when microzooplankton was feeding at its maximum ingestion rate. Simulated dilution experiments incorporating these 2 assumptions, 2 consumers and 3 prey were able to reproduce the 3 types of responses. The results indicate that non-linear responses in dilution experiments must be expected in regions with high and diverse food abundance, which should allow prey selection by microzooplanktonThis work was funded by the project REM2000- 0880-C02-01 MAR (Ministerio de Educación y Ciencia) and the project PGIDT01MAR4020PN (Xunta de Galicia). I.G.T. was supported by an FCT (Portuguese Foundation for Science and Technology) doctoral fellowship (SFRH/BD/11309/2002)Peer reviewe