Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
• Data Files
• Related Publications
• Parameters
• Instruments
• Deployments
• Project Information
• Program Information
• Funding

Phytoplankton Growth Rates in the coastal Gulf of Alaska

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.

[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.]

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.

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).

Program in a Nutshell

Goal: To understand the effects of climate variability and climate change on the distribution, abundance and production of marine animals (including commercially important living marine resources) in the eastern North Pacific. To embody this understanding in diagnostic and prognostic ecosystem models, capable of capturing the ecosystem response to major climatic fluctuations.

Approach: To study the effects of past and present climate variability on the population ecology and population dynamics of marine biota and living marine resources, and to use this information as a proxy for how the ecosystems of the eastern North Pacific may respond to future global climate change. The strong temporal variability in the physical and biological signals of the NEP will be used to examine the biophysical mechanisms through which zooplankton and salmon populations respond to physical forcing and biological interactions in the coastal regions of the two gyres. Annual and interannual variability will be studied directly through long-term observations and detailed process studies; variability at longer time scales will be examined through retrospective analysis of directly measured and proxy data. Coupled biophysical models of the ecosystems of these regions will be developed and tested using the process studies and data collected from the long-term observation programs, then further tested and improved by hindcasting selected retrospective data series.

U.S. GLOBEC (GLOBal ocean ECosystems dynamics) is a research program organized by oceanographers and fisheries scientists to address the question of how global climate change may affect the abundance and production of animals in the sea.

The U.S. GLOBEC Program currently had major research efforts underway in the Georges Bank / Northwest Atlantic Region, and the Northeast Pacific (with components in the California Current and in the Coastal Gulf of Alaska). U.S. GLOBEC was a major contributor to International GLOBEC efforts in the Southern Ocean and Western Antarctic Peninsula (WAP).