Phytoplankton growth and microzooplankton grazing rates from CCE LTER Process cruises in the California Current System, 2006 - 2017.

Rates of phytoplankton community growth and microzooplankton grazing on phytoplankton were assessed from chlorophyll a analyses of in situ dilution incubations as described in Landry et al. (2009). For each experiment (Array #), seawater was collected from predawn CTD (~2 a.m. local time) casts at 6-8 depths spanning the upper to lower euphotic zone. For each depth, we prepared a pair of polycarbonate bottles (2.7 L), one with whole seawater (100%), and one with 33% whole seawater (diluted with 0.1-µm filtered seawater) at each depth. Seawater was filtered directly from the Niskin bottles using a peristaltic pump, silicone tubing and an in-line Suporcap filter capsule that had previously been acid washed (10% trace-metal grade HCl followed by Milli-Q and seawater rinses). Dilution treatment bottles received pre-measured volumes of filtered water from the collection depths, and then were gently filled (silicone tubing below the water level) with unscreened water from the Niskin bottles. The filled bottles were tightly capped, placed into net bags and clipped onto attached rings at the depth of collection on a tether line attached to a satellite-tracked surface drifter (WOCE SVP) with a top strobe light, Globalstar telemetry and a 3-m holey-sock drogue centered at 15-m (mixed layer). Incubations were done in situ for 24-h (daily) to get daily averaged rates. In most cases, repeated experiments/deployments were done over the course of 2-5 days, using water collected each morning at the location of the drifter. These back-to-back experiments define experimental “cycles”. The second set of experiments were set up, before recovering the first. Hand recovery of the array, switching of net bags and redeployment was generally completed in 15-20 min. Rate estimates are based on initial and final subsamples (250 ml) taken for fluorometric analyses of Chl a. The samples were immediately filtered onto GF/F filters, and the Chl a extracted with 90% acetone in a dark refrigerator for 24 h. Extracted samples were shaken, centrifuged and quantified on a calibrated Turner Designs model 10 fluorometer. Instantaneous rates of phytoplankton growth (µ) and microzooplankton grazing (m) were estimated as m = (kd – k)/(1-x) and µ = k + m, where k and kd are, respectively, the observed net rates of change of Chl a in the natural and diluted treatments and “x” = the fraction of natural grazer density in the dilute treatment.