Primary productivity, pulse amplitude modulated fluorometry and nutrient drawdown data from the marginal ice zone: BROKE-WEST survey 30o-80oE, 2006

Pulse Amplitude Modulated Fluorometry Data Fv/Fm values determined using pulse amplitude modulated fluorometry (PAM). Samples were dark-adapted prior to measurement so that non-photochemical quenching was relaxed. Values provide an indication of cell health.

Abstract

Primary productivity was measured in the Indian Sector of the Southern Ocean (30 degrees to 80 degrees E) as part of a multi-disciplinary study during austral summer; Baseline Research on Oceanography, Krill and the Environment, West (BROKE-West Survey, 2006). Gross integrated (0-150 m) productivity rates within the marginal ice zone (MIZ) were significantly higher than within the open ocean, with averages of 2110.2 plus or minus 1347.1 and 595.0 plus or minus 283.0 mg C m-2 d-1, respectively.

In the MIZ, high productivity was associated with shallow mixed layer depths and increased Pmax up to 5.158 mg C (mg chl a)-1 h-1. High Si:N drawdown ratios in the open ocean (4.1 plus or minus 1.5) compared to the MIZ (2.2 plus or minus 0.79) also suggested that iron limitation was important for the control of productivity. This was supported by higher Fv/Fm ratios in the MIZ (0.50 plus or minus 0.11 above 40 m) compared to the open ocean (0.36 plus or minus 0.08).

As well, in the open ocean there were regions of elevated productivity associated with the seasonal pycnocline where iron availability was possibly increased. High silicate drawdown in the north-eastern section of the BROKE-West survey area suggested significant diatom growth and was linked to the presence of the southern Antarctic Circumpolar Current front (sACCF). However, low assimilation numbers (12.8 to 23.2 mg C mg chl a-1 d-1) and Fv/Fm ratios indicated that cells were senescent with initial growth occurring earlier in the season.

In the western section of the survey area within the MIZ, high NO3 drawdown but relatively low silicate drawdown were associated with a Phaeocystis bloom. NO3 concentrations were strongly negatively correlated with column-integrated productivity and chlorophyll biomass which was expected given the requirement for this nutrient by all phytoplankton groups. Regardless, concentrations of both NO3 and silicate were above limiting levels within the entire BROKE-West survey area (N greater than 15.7 micro M, Si greater than 18.3 micro M) supporting the high nutrient low chlorophyll status of the Southern Ocean.