Iron and manganese in the Ross Sea, Antarctica: Seasonal iron limitation in Antarctic shelf waters

Dissolved iron and manganese and total dissolvable iron were measured in wa ter column samples collected From the polynya region of the southern Ross S ea during cruises in November-December 1994 (spring 1994) and December 1995 to January 1996(summer 1995). Iron and manganese addition bottle incubatio n experiments were also performed during these cruises in order to assess t he nutritional sufficiency of ambient iron and manganese concentrations for growth of the phytoplankton community. Generally high dissolved iron conce ntrations (> 0.5 nM) and relatively complex iron and manganese vertical pro files were obtained during the spring 1994 cruise, compared with the summer 1995 data. Dissolved iron concentrations in the upper water column average d 1.0 nM during spring 1994 and 0.23 nM ill summer 1995, excluding two stat ions where concentrations exceeding 1 nM are attributed to inputs from melt ing sea ice. The observed differences in the distribution of iron and manga nese between spring 1994 and summer 1995 are attributed to seasonal decreas es in the upwelling of bottom waters and melting of sea ice, which supply t hese metals into the upper water column, combined with the cumulative remov al of iron and manganese from the water column throughout the spring and su mmer, due to biological uptake, vertical export and scavenging by suspended and sinking particles. Results of the metal addition bottle incubation exp eriments indicate that ambient dissolved iron concentrations are adequate f or phytoplankton growth requirements during the spring and early summer, wh en algal production is highest and Phaeocystis antarctica dominates the alg al community, whereas low dissolved iron concentrations limit algal communi ty growth later in the summer, except in the stratified, iron-enriched wate rs near melting sea ice, where diatoms are able to bloom. Our observations and the inferred seasonal distributions of P. antarctica and diatoms in the se waters suggest that iron availability and vertical mixing (i.e., irradia nce) exert the primary controls on phytoplankton growth and community struc ture in the southern Ross Sea during the spring and summer.