Plankton Community Response to Iron-Fertilization in the "Northern Patch" at 56°S

Taxon-specific pigment and microscopical analyses were coupled with seawater dilution experiments to investigate plankton community dynamics during the 2002 Southern Ocean iron-fertilization experiment in the low-silicate waters north of the Antarctic Polar Front Zone (56°S). Preliminary results show an overall response by the phytoplankton community of a nearly 4-fold increase in biomass over a 28-day period. Phytoplankton biomass reached 60 μg C L^-1 approximately 12 days after the initial fertilization and was comprised almost entirely of flagellates, mainly in the 5-20 μm size fraction. Equally high biomass was still apparent two weeks later, but both pigment analyses and microscopy revealed a dramatic shift in phytoplankton community composition from the previous dominance of flagellated cells to pennate diatoms. By this time, cells >20 μm comprised half of the phytoplankton biomass, many of which were diatoms in chains of >100 μm in length. Similarly, the biomass of micro-grazers (2-200 μm) increased 3-fold by day 12, largely attributed to an increase in grazers >20 μm in length. By the time of diatom dominance, grazer biomass appeared to be in decline. The accumulation of accessory pigments over time, specific for different phytoplankton groups, also demonstrated a shift in the composition of the phytoplankton community. Fucoxanthin, a proxy for diatom biomass, continued to increase throughout the sampling period, while pigments representative of flagellate populations (peridinin, 19'-butanoyloxyfucoxanthin, and 19'-hexanoylfucoxanthin) peaked earlier and exhibited a plateau or decrease in concentration by day 28. Both phytoplankton biomass and pigment concentrations were constant over time at stations outside of the iron-fertilized patch. Preliminary rate estimates from dilution experiments point to higher growth rates for the total phytoplankton community inside the patch (mean = 0.45 d^-1) versus outside the patch (mean = 0.29 d^-1) with the largest excess of growth over grazing occurring early in the time series. The highest rate of mortality due to grazing (0.42 d^-1) occurred midway through the sampling period, corresponding to the peak in grazer biomass. Group-specific rate estimates derived from accessory pigments infer differential growth and grazing responses to the iron-fertilization. These group-specific differences in rates will be discussed in relation to standing stocks.