Effects of pulsed nutrient enrichment on enclosed phytoplankton:: ecophysiological and successional responses

A microcosm experiment was carried out during early spring 1994 in order to investigate the effect of Si-sufficient but low N:P ratio or high N:P ratio nutrient inputs, added at different frequencies, on phytoplankton succession patterns. Eight Perspex cylinders were filled with 30 1 of coastal water from Masnou (20 km north of Barcelona) and placed in a chamber at 17degreesC under a 12:12 h light:dark photoperiod. Four experimental conditions were randomly assigned to duplicate containers: low N:P ratio (N-deficient) or high N:P ratio (P-deficient) nutrient inputs (including sufficient Si in all cases), in combination with addition intervals of 2 or 6 d. Integrated chlorophyll a (chl a) concentrations in the P-deficient containers were significantly higher than in the N-deficient ones, presumably due to the faster recycling of P with respect to N, but differences between addition intervals were non-significant. All microcosms presented an initial bloom dominated by diatoms. In the post-bloom phase, after depletion of the nutrient in short supply, dinoflagellate abundances were higher and diatom abundances lower in the N-deficient than in the P-deficient microcosms. Within nutrient treatments, the qualitative composition of the phytoplankton assemblages was similar across frequencies and replicates. In contrast, characteristics, such as total phytoplankton biovolume or the abundance of dominant taxa, presented significant variability, presumably due to non-linear interactions, even within replicates. This finding suggests the importance of focussing on assemblages rather than on individual taxa when attempting to derive regularities concerning the response of phytoplankton to environmental factors.