Standing Stocks and Microzooplankton Grazing Rates in the Equatorial Pacific

Michael R. Landry

Standing Stocks (Transect Cruises):

Samples will be taken from predawn hydrocasts at each station. Population abundances of bacteria, cyanobacteria ( Synechococcus and Prochlorococcus), and chlorophyll-containing nanoplankton will be determined from preserved samples frozen in liquid N and analyzed by flow cytometry (FCM) with internal standards of fluorescent beads. Biomass of nanoplankton will be estimated from calibrated relationships between forward and right-angle light scatter and carbon of cultured phytoplankton. The size structure of pico-plankton will be measured directly (shipboard) on unpreserved samples with a Coulter N4MD submicron particle analyzer. Slides for population and biomass assessment by epifluorescence image analysis will be prepared by us and analyzed by Sieracki et al.; thus constituting an intercomparison of methods. Larger volume samples for rare phytoplankton (e.g., large diatoms and dinoflagellates) and ciliated protozoa will be preserved by us for later microscopical analysis by Sieracki et al.

Microzooplankton Grazing Rates:

Phytoplankton growth and microzooplankton grazing rates will be estimated for three depth strata (mixed layer mid-euphotic zone, and chlorophyll max) using the dilution assay with fluorescently-labelled prey as an internal standard for relative grazing rates. Rate estimates will be derived from total chlorophyll (fluorometry) and various populations of phytoplankton determination by HPLC pigments (R. Bidigare) and FCM. We will run these experiments at each station on the transect cruises; Sieracki et al. will run dilution experiments on the time-series cruises.

Short-term experiments involving uptake of fluorescently-labelled prey, bacteria (FLB) and algae (FLA), will be run at each station to assess grazing rates of different taxa, size-selection by grazers, and the occurrence and rates of algal mixotrophy. FLB-uptake experiments will be run in conjunction with H-thymidine work by D.L. Kirchman and H.W. Ducklow to test the assumption that grazing balances growth rate. Rates will be estimated from the time-course (30--60 min.) of particle uptake by microzooplankton as determined by FCM and epifluorescence microscopy (slides) using formalin-killed samples as controls.

Growth and grazing mortalities of heterotrophic and photosynthetic bacterial populations will be independently determined from differences in the rates of change of populations (determined by FCM analysis) in 24-h incubations with and without the prokaryotic inhibitor ampicillin. FLBs will be used as internal controls. Parallel, long-term incubations will also measure the rate of decline of FLAs and will constitute an independent rate estimate of microzooplankton grazing on algae in the nanoplankton size range.