We examined the relat~ve Importance of phytoplankton and ciliates as prey for metazoan zooplankton and the role of predation in regulating ciliate populat~ons In 2 Long Island (USA) bays Depth-integrated primary production (mg C m h-') was dominated by nannoplankton < 5 pm In d~a meter throughout the year, ranging from > 9 5 " of total production in mid-summer to an average of about 60%, in winter and early spnng Predator exclusion and addition experiments conducted in microcosms showed that the mortality coeff~cient of cil~ates (d.') from zooplankton predation was higher when larger phytoplankton ( > l 0 pm) contributed less to total primary productivity For adult copepods an Increase In the percentage ciliate contribution compdred to phytoplankton contnbution to total carbon Intake also coinc~ded with the higher prrcrntages of small microdlgal production Egg production rates of Acartia spp were positively correlated to the net growth coefficient of cili~ites In contrast, mlcrometazoa routinely obtalned > 7 0 % of t h~~r total carbon ratlon from phytoplankton, and at times d u n n g spnng and summer, removed 23 to 52' 0 of the total depth-~ntegrated prlmary production In a d d i t~o n to protozoa, w e suggest that microinetazoa part~cularly copepod nauplii, may serve as a trophlc llnk between phytoplankton and mesozooplankton In Long Island bays

ABSTRACT. Durlng the summer of 1985, a small (2 to 3 pm) chrysophyte, Aureococcus anophagefferens, bloomed and dominated the phytoplankton assemblage, and recurred each summer for the followlng 12 yr in bays of Long Island, New York, USA. Macronutrlents remained at h~g h levels throughout the years but Secchi d~s c depths were as low as 30 cm in somc arcds, corresponding to a 1 ",, light depth of about 80 cm. In batch culture, A. anophagefferens malnta~ned hlgh growth rates at low irrad i a n c e~. The ln~tial slope of the growth versus irradiance curve, a,, was 0.021 + 0.003 divisions d-' (pm01 quanta m-2 S-')-' which is similar to that for bloom formers such as Skeletonema costaturn. In continuous culture, constant and fluctuating irradiance regimes resulted in equivalent steady state growth rates at 0.46 divisions d.'. Short-term carbon fixation per unit chlorophyll a was about twice as high in the fluctuating irradiance regime compared with constant irradiance. Differences between short-term carbon fixation and cell growth could not be accounted for by dark fixation or exudation and are assumed to be due to photorespiration. The photosynthesis versus irradiance parameter, P,,,,, (light saturated photosynthetic rdte), was similar in both irradiance regimes; however, g, (initial slope) was consistently higher in the fluctuating regime, indicating that photosynthetic efficiency is higher in fluctuating irradiance.