Three methods which have been used to determine nutrient uptake hnetic parameters were compared using steady-state NH,-limited cultures of the chrysophyte Pseudopedinella pyriforrnis. The first 2 methods involved a multiple flask incubation where different concentrations of substrate were added to each flask. Method 1 used a variable incubation time, while the incubation time of Method 2 was short and constant. The third method, the perturbation method, involved one large addition of the substrate to one culture and hence the nutritional past history varied throughout the experiment. This method was used also with the diatoms Skeletonema costatum and Chaetoceros debilis in Si-Limited chemostats. Results inhcate that, for nutrient-limited cultures, kinetic parameters are best estimated using multiple additions of the substrate and a short constant incubation time (Method 2). It appears that this method determines membrane transport, which is still not completely free of feedback inhibition even when the incubation time is very short (e.g. 2 min). The short incubation time is necessary because the maximal uptake velocity (V,) decreases with increasing incubation time especially for phosphate and ammonium. Method 3 provides valuable information on a third parameter, V,, the approximate assimilation rate of the limiting nutrient, that is not obtained by the other methods. Multlple sequential additions of the limiting nutrient to N-or Si-limited Skeletonema costaturn and Chaetoceros debilis revealed that if the additions were small (e.g. 2 pM NH,), there was no change in subsequently determined nutrient uptake kinetic parameters. If the sequential additions were larger (e.g. 6 PM) then the maximal uptake rate slowed with time.

The trophodynamic interaction between a heterotrophic zooflagellate, Pseudobodo sp. (2 to 4 pm) and a small (-2 pm) prasinophyte, Micromonas pusilla, was studied in continuous culture. This zooflagellate was capable of phagocytizing Micromonas and causing a rapid decline in cell numbers of the latter. Maximum growth rate of the zooflagellate was about 2 d-I and maximum clearance rate was about 1 X 1 0 -~ 1 ind-I d-l. A video system was used to record its feeding behaviour. We attempted to estimate population parameters by deliberately perturbing the experimental system and using systems identification procedures to fit non-linear dynamic models to the resulting time series. Our results suggest that the response of zooflagellates to fluctuating food densities is intrinsically more complicated than observations of steady-state growth would suggest. These complexities include both time lags in the response of ingestion to increasing food density, and a complex response of cell size, physiological state and depth rate to decreasing food density. Present observations suggest that considerable caution should be exercised in the use of steady-state chemostat results to predict or model zooflagellate populat~ons in the field.

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Ammonium uptake by the marine diatom Thalassiosira pseudonana was measured by trapping chemostat-grown cells on an in-line filter and following changes in the concentration of ammonium pulses pulled across the filter. Over time intervals longer than 1 min, this technique yielded results comparable to those obtained using conventional techniques on cell suspensions. The new technique also allowed changes in uptake kinetics within the first minute of exposure to ammonium to be resolved. Saturated uptake rates were highest (26 d-l) within the first 15 S, and declined to 11 d-I after 1 min. The affinity for ammonium remained constant at 17 d-' PM-'. Implications of these results for models of uptake and assimilation, and possible adaptation of the technique for use in the field, are discussed.

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