The effects of humic matter additions on nutrient dynamics, primary production, biomass formation and species development were studied in microcosm experiments with different natural marine phytoplankton communities. Humic matter additions were compared with additions of nitrate or phosphate, and the effects of different pulsed additions of humic matter were also evaluated. The maximum chlorophyll a concentration attained in the microcosms was higher when humic substances were added (r2 = 0·95 for the regression line, significance of regression line according to f-test: P < 0·001). Alkaline phosphatase activity became high in the microcosms where humic substances were added, suggesting a shortage of inorganic phosphorus. Increased concentrations of nitrate and ammonia after the addition of humic matter and increased numbers of bacteria (r2 = 0·86 for the regression line, significance of regression line according to f-test: P < 0·001) indicate a bacterial degradation of humic compounds. Among the phytoplankton groups that increased their growth rate in response to addition of humic matter were dinoflagellates, diatoms, monads (2-4 μm) and autotrophic flagellates (5-10 μm). Oligotrich ciliates also showed a higher growth rate in microcosms to which humic substances were added. We conclude that humic matter stimulates bacterial, phytoplankton and ciliate growth in coastal waters and that humic bound nitrogen is to some extent available for phytoplankton biomass formation. The large amount of humic matter reaching coastal waters by land runoff makes organically bound nitrogen a potentially important nitrogen source for coastal phytoplankton. © 1993 Academic Press. All rights reserved.
