Blooms of nuisance phytoplankton species often occur after periods of extensive rainfall and river runoff which introduce large quantities of dissolved humic material into coastal waters. The influence of hydrophobic (humic and fulvic acids) and hydrophilic acids extracted from river runoff on eight nuisance marine phytoplankton species was investigated. Additions of both organic fractions at concentrations from 5 to 50 micrograms per liter induced both quantitative and qualitative responses in all species examined. Organic additions stimulated cell yield and growth rate in the dinoflagellates GYMNODINIUM SANGUINEUM, PROROCENTRUM MICANS AND PRORCENTRUM MINIMUM, the raphidophyte HETEROSIGMA AKASHIWO and the diatoms ASTERIONELLOPSIS GLACIALIS and SKELETONEMA COSTATUM. Cell yield of GYRODINIUM AUREOLUM, however, decreased with additions of both organic fractions, although growth rate was not affected. The similarity in observed responses to both organic fractions in each species suggests that hydrophobic and hydrophilc acids influence phytoplankton growth via similar mechanisms and that the presence of these organic fractions at concentrations typically present in runoff can potentially stimulate bloom formation. Species specific responses to the addition of humic material included: an increase in chain length of the diatom A. GLACIALIS, increases in photosynthetic and respiration rates of the dinoflagellate P. MINIMUM as well as a direct corelation between growth rates of P. MINIMUM and specific molecular weight fractions of organic additions.

Qualitative effects observed with additions of humic material included the induction of meiosis (ALEXANDRIUM TAMARENSE, G. AUREOLUM, P. MICANS and P. MINIMUM), alternate vegetative cells forms (G. AUREOLUM, G. SANGUINEUM and P. MINIMUM) and aberrant cell forms (A. GLACIALIS AND S. COSTATUM). As well as a growth stimulator, dissolved humic material is hypothisized to serve as a chemical "cue" of nearshore environments suitable for the survival of the benthic cyst stage resulting from dinoflagellate meiosis, thus favoring long-term survival of bloom species. The complexities of both quantitative and qualitative responses exhibited by a wide variety of bloom species demonstrate that both hydrophobic and hydrophilic acids serve complex physiological and ecological functions in relation to the formation and development of nuisance phytoplankton blooms.