[excerpted from Strom, et al. (2006) Microzooplankton grazing in the coastal Gulf of Alaska: Variations in top-down control of phytoplankton. Limnol Oceanogr in press.]
\n
Water drawn from multiple Niskin bottles closed at a single depth was pooled into two 25-liter polycarbonate carboys. Most often, water was collected from the depth corresponding to 50% of surface irradiance (50% Io, 3 to 10 m). Once during April, and once per station during July, water was collected from the depth of the subsurface chlorophyll maximum (SCM, 12 to 25 m). The contents of one carboy were gravity-filtered (0.2 um) to generate particle-free filtered seawater (FSW, the diluent for the dilution series). The contents of the other were gently pre-screened through 200 um Nitex mesh to exclude macrozooplankton (WSW, the whole seawater for the dilution series). Using gentle siphoning and mixing techniques, FSW and WSW were combined in known proportions in 2.35-liter polycarbonate bottles to generate a dilution series consisting of 9, 16, 24, 41, 61, and 100% WSW (each in duplicate). An additional pair of bottles diluted to 4% was added during the May and July cruises, as well as an additional pair of 100% WSW bottles to control for the effects of nutrient enrichment on phytoplankton growth rate. Clean techniques and inert materials (silicone, polycarbonate) were used throughout.
Phytoplankton Growth Rates in the coastal Gulf of Alaska
\n
Phytoplankton growth rates, the phytoplankton growth rate response to nutrient enrichment, and microzooplankton grazing rates on phytoplankton in three different size classes were measured during three cruises to the coastal Gulf of Alaska in 2001.
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.
\n
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 (\u00b5,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 = \u00b5n - kn (where kn = net growth rate of phytoplankton in enriched, 100% WSW bottles) for experiments with saturated grazing. In experiments with nutrient enrichment, unenriched phytoplankton growth rates (\u00b5o) were calculated as \u00b5o = ko + g, where ko = net growth rate of phytoplankton in unenriched, 100% WSW bottles. Estimates of \u00b5o were used to compare microzooplankton grazing to phytoplankton growth in situ (g : \u00b5o). 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.
\n
More detailed methods reference in\u00a0Strom, et al. (2006).