Pn. Sedwick et al., Iron and manganese in the Ross Sea, Antarctica: Seasonal iron limitation in Antarctic shelf waters, J GEO RES-O, 105(C5), 2000, pp. 11321-11336
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.