Dataset: phyto_growth
Deployment: HX275

Initial samples for size-fractionated chlorophyll (<5, 5 to 20, and >20 um, in quadruplicate), nutrients (nitrate, nitrite, silicic acid, phosphate), and microzooplankton abundance and composition (in duplicate, see below) were taken from the WSW carboy at intervals during experiment set-up. Initial chlorophyll levels in diluted bottles were calculated from these measured WSW values and known dilution factors. Coefficients of variation for quadruplicate initial chlorophyll samples averaged 7.9%, 13.5%, and 8.9% for the <5, 5 to 20, and >20 um size fractions, respectively. During May (all but outer shelf experiments) and July cruises, all diluted bottles and two 100% WSW bottles were enriched with nitrate (4.7 umol L-1 as NaNO3) and phosphate (0.27 umol L-1 as Na2HPO4). The other two 100% WSW bottles were left unenriched. Bottles were screened to collection-depth light levels with neutral density screening and incubated on deck in seawater-cooled incubators for 24 hr. All bottles were then sampled in duplicate for size-fractionated chlorophyll (filtration volumes ranged from 0.15 to 1.08 liter depending on WSW chlorophyll and dilution levels); 100% WSW bottles were additionally sampled for microzooplankton abundance and composition.

Net growth rates (k, d-1) for total chlorophyll and individual chlorophyll size fractions were calculated as (1/t)(ln[Pt/Po]), where Pt = final chlorophyll concentration, Po = initial chlorophyll concentration, and t = incubation time in d. Intrinsic growth rates (µ,d-1) of phytoplankton were estimated from the y-intercept of net growth rates regressed upon fraction WSW. For experiments exhibiting saturated grazing (i.e. a leveling of net growth rate across the least-dilute bottles) (Gallegos 1989), instrinsic growth rate estimates were based on regression of net growth rates in only the most dilute bottles (generally those with <40% WSW). Microzooplankton grazing rates (g, d-1) were estimated from the slope of the regression for experiments with linear relationships between net growth and fraction WSW, and as g = µ_n - k_n (where k_n = net growth rate of phytoplankton in enriched, 100% WSW bottles) for experiments with saturated grazing. In experiments with nutrient enrichment, unenriched phytoplankton growth rates (µ_o) were calculated as µ_o = k_o + g, where k_o = net growth rate of phytoplankton in unenriched, 100% WSW bottles. Estimates of µ_o were used to compare microzooplankton grazing to phytoplankton growth in situ (g : µ_o). These ratios represent the fraction of primary production consumed each day by microzooplankton grazing. Ratios were arctan transformed for estimation of means and standard deviations.

More detailed methods reference in Strom, et al. (2006).