ZOOPLANKTON GRAZING, PHYTOPLANKTON GROWTH, AND EXPORT FLUX - INFERENCES FROM CHLOROPHYLL TRACER METHODS

Chlorophyll tracer methods were used on six cruises in the Southern Ca lifornia Eight to assess mesozooplankton grazing rates (gut fluorescen ce), fecal pellet export from the euphotic zone (pigment flux to sedim ent traps), the partitioning of grazing impact between micro- and meso zooplankton (pigment budget), and phytoplankton growth. Mesozooplankto n grazing estimates ranged from 16 to 44% of phytoplankton growth rate s during the six cruises, with the mean seasonal average for the sprin g cruises about double (39%) that from the autumn cruises (20%). Only 23 to 32% of the measured mesozooplankton grazing on phytoplankton was lost rapidly enough from the euphotic zone to be recovered as phaeopi gment flux into sediment traps. Since most (68 to 77%) of the grazing activity of mesozooplankton, as inferred from the gut pigment method, does not settle rapidly out of the euphotic zone, it is functionally e quivalent (i.e. remineralized within the euphotic zone) to that of mic rozooplankton as defined by the pigment budget method. The gut fluores cence estimates of mesozooplankton grazing were about equal generally to the sum of micro- and mesozooplankton grazing as inferred from the pigment budget model. Moreover, estimates of phytoplankton growth from the pigment budget were consistently low by a factor of 3 and 4 compa red with rates inferred from C-14 estimates of production and an assum ed C:Chl ratio of 50. Previous studies in systems dominated by large p ellet-producing metazooplankton and/or direct cell sinking of phytopla nkton have demonstrated that the combined pigment budget estimates of phytoplankton losses were consistent with independent estimates of phy toplankton growth. The lack of such agreement in the present study sug gests that the approach may not account adequately for the grazing of protistan microzooplankton.