Nutrient and carbon removal ratios and #uxes in the Ross Sea, Antarctica

Net community production (NCP) and nutrient de"cits (Def(X)) were calculated using decreases in dissolved CO 2 and nutrient concentrations due to biological removal in the upper 200m of the water column during four cruises in the Ross Sea, Antarctica along 76330@S in 1996 and 1997. A comparison to excess dissolved and particulate organic carbon showed close agreement between surplus total organic carbon (TOC) and NCP during bloom initiation and productivity maximum; however, when TOC values had returned to low wintertime values NCP was still signi"cantly above zero. This seasonal NCP, 3.9$1mol Cm~2, must be equivalent to the particle export to depths greater than 200m over the whole productive season. We estimate that the annual export was 55$22% of the seasonal maximum in NCP. The fraction of the seasonal maximum NCP that is exported through 200m is signi"cantly higher than that measured by moored sediment traps at a depth of 206m. The removal of carbon, nitrate and phosphate (based on nutrient disappearance since early spring) and their ratios showed signi"cant di!erences between regions dominated by diatoms and regions dominated by the haptophyte Phaeocystis antarctica. While the *C/*N removal ratio was similar (7.8$0.2 *Corresponding author. E-mail address: [email protected] (C. Sweeney). 0967-0645/00/$ see front matter ( 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 0 6 4 5 ( 0 0 ) 0 0 0 7 3 4 for diatoms and 7.2$0.1 for P. antarctica), the *N/*P and *C/*P removal ratios for diatoms (10.1$0.3 and 80.5$2.3) were signi"cantly smaller than those of P. antarctica (18.6$0.4 and 134.0$4.7). The similarity in *C/*N removal ratios of the two assemblages suggests that preferential uptake of phosphate by diatoms caused the dramatic di!erences in *C/*P and *N/*P removal ratios. In contrast to low *C/*P and *N/*P removal ratio in diatomdominated areas early in the growing season, de"cit N/P and C/P ratios in late autumn indicate that the elemental stochiometry of exported organic matter did not deviate signi"cantly from traditional Red"eld ratios. Changes in biologically utilized nutrient and carbon ratios over the course of the growing season indicated either a substantial remineralization of phosphate or a decrease in phosphate removal relative to carbon and total inorganic nitrogen over the bloom period. The species dependence in C/P ratios, and the relative constancy in the C/N ratios, makes N a better proxy of biological utilization of CO 2 . ( 2000 Elsevier Science Ltd. All rights reserved.