In situ growth rates of marine phytoplankton: approaches to measurement, community and species growth rates

Taxonomic structure and biomass weighting are important determinants of measurable marine phytoplankton community growth potential. Maximal diel-averaged growth rates of communities appear to fall between 3 and 3.6 doublings day−1. Mean net growth rates are considerably lower. Ranges of community growth rates measured in tropical, sub-tropical and (summer) temperate ecosystems are similar. There appears to be a broad dichotomy between the growth potential of diatom species as compared to non-diatoms. Doubling rates of small diatoms frequently exceed community biomass doubling rates by wide margins. Diel-averaged growth rates of large diatoms, microflagellates and non-motile ultraplankton populations are lower and similar in magnitude to community growth estimates. Maximum growth rates of species measured in situ are in good agreement with maximum growth rates measured in laboratory cultures. High specific productivity of sub-dominant or rare diatom species or assemblages will be diluted in the lower specific growth rates of microflagellate and non-motile ultraplankton assemblages. Specific rates of grazing upon species and functional groups remain to be quantified, but stability of community size and taxonomic structure implies close linkage between growth and mortality rates at the species level over time intervals of several generations.