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
Physical factors and invertebrate predators regulate the seasonal abundances of 2 Chesapeake Bay copepods, Scottolana canadensis and Oithona colcarva, which occur during spring and fall, respectively. The absence of population growth of either species during winter can be predicted on the basis of field temperatures, since laboratory experiments revealed that egg production and development were arrested at low temperatures. Temperature and salinity conditions in the field did not contribute to the absence of these species in summer, but were optimal for the growth of both species. A comparative analysis of field and laboratory data revealed that the per capita rate of population change in the field (d-') was consistently lower than that determined in the laboratory for comparable physical conditions. Predation by two invertebrate predators, the planktonic copepod Acartia tonsa and the lobate ctenophore Mnerniopsjs leidy~, had profound detrimental effects on summer populations of S. canadensis and 0 . colcarva. Predation rates frequently exceeded the ability of copepod populations to increase.
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