Michael R. Landry; University of Hawaii

"Structure and dynamics of plankton communities in the Antarctic Frontal Zone: Interactions of physical forcing, iron limitation and microzooplankton grazing."

The Southern Ocean is a vast and variable environment with a potentially large role in global carbon cycling. The JGOFS Southern Ocean Program seeks to advance the understanding of this region by investigating seasonal and spatial dynamics in three important subsystems: the Ross Sea shelf, the Antarctic Polar Front Zone, and the high nutrient, low chlorophyll waters in between. We propose to contribute to this effort by determining microbial community structure and by assessing rates of phytoplankton growth and microzooplankton grazing on four cruises covering the spring, summer and autumn seasons in the open ocean frontal zone and adjacent high nutrient waters of the Antarctic Circumpolar Current. The overall goal of this research is to understand how physical processes (advection, mixing and light), iron limitation and grazing interact to determine plankton community structure and production in the open oceans.

Population abundance and biomass of the plankton community will be assessed by analytical flow cytometry (bacteria and phytoplankton) and microscopy (nano and microplankton) to determine their temporal and spatial patterns relative to physical features of the polar front. Growth rates of phytoplankton and bacteria and microzooplankton herbivory and bactivory will be measured with a suite of complementary methods including dilution, fluorescently labeled prey, size fractionation, and a lysozyme assay for bacterivory. Results of this project will contribute to the general understanding phytoplankton control mechanisms and trophic interactions in high nutrient, low chlorophyll regions of the world's oceans. They will also help to identify how potential global change effects on water column stability and physical forcing in the Southern Ocean may alter food web structure, carbon storage and export from the euphotic zone.

We propose to investigate microbial community structure, growth rates and grazing interactions in the Antarctic Polar Front Zone (APFZ) on four cruises: the Spring Survey Cruise (October 1997), the Spring Process Study (November December 1997), the Summer Survey Cruise (December 1997 through January 1998), and the Autumn Process Study (March 1998). Microbial community composition will be described using a combination of flow cytometric (FCM) and microscopical methods. Biomass will be determined from abundances, mean biovolumes per cell, and appropriate C:biovolume conversion factors.

METHOD	SAMPLES	FRACTION ANALYZED
Large vol. FCM, ship	5100 ml, live	autotrophic nano & microplankton
Dualbeam FCM, lab	3 ml, presvd./frozen	auto & heterotrophic picoplankton
Epifluor. Microscopy	50100 ml, presvd.	auto & heterotrophic nanoplankton
Inverted Microscopy	1,000 ml, presvd.	auto & heterotrophic microplankton

Specific growth and mortality rates of autotrophic populations will be estimated from dilution experiments, following the refined protocol of Landry et al. (1995, MEPS 120: 5363). This technique uses a series of nutrient (+Fe) enriched dilution treatments to determine rates of grazing mortality and phytoplankton growth with nutrients. The grazing mortality estimate is subsequently used with the observed net rates of growth in unamended, undiluted treatments to estimate the ambient growth rates of phytoplankton. The ratio growth rate estimates with and without added nutrients (+Fe) provides an index of nutrient sufficiency over the 24h incubation. Mean community rate estimates will be determined from shipboard analyses of chlorophyll a. Rates for component populations will be determined by flow cytometry and taxa specific pigments (HPLC). Shipboard FCM will be used to assess the state of pigment photoadaptation by phytoplankton (magnitude of red autofluorescence per cell), which is important for determining incubation light levels (relative to water column mixing) and interpreting pigment based growth rate results from dilution experiments.

Microzooplankton bacterivory will be investigated by combination of seawater dilution, disappearance of fluorescently labeled bacteria, and an acid lysozyme assay. Dilution experiments for bacterivory are analogous to those for herbivory, except that the dilution treatments will be supplemented with glucose and the population responses will be measured by LAL assay (in collaboration with D. Caron) and flow cytometry. FLB will be incorporated into dilution experiments as an alternative assessment of grazing on bacteriasized particles. The acid lysozyme assay, a fluorometric enzymatic technique, will be used on the survey portions of the front cruises to identify relative "hot spots" of grazing activity for detailed experimental study.