Resource limitation of phytoplankton growth in the Crozet Basin, Subantarctic Southern Ocean

• Published in: Deep Sea Research Part II: Topical Studies in Oceanography
• Main Authors: Sedwick, PN, Blain, S, Queguiner, B, Griffiths, FB, Fiala, M, Bucciarelli, E, Denis, M
• Format: Article in Journal/Newspaper
• Language: English
• Published: Pergamon-Elsevier Science Ltd 2002
• Subjects: Earth Sciences; Oceanography; Biological Oceanography; Indian; Southern Ocean
• Online Access: https://doi.org/10.1016/S0967-0645(02)00086-3; http://ecite.utas.edu.au/26115

In January-February 1999, we performed shipboard iron- and macronutrient-addition experiments in the Crozet Basin, Indian sector of the Subantarctic Southern Ocean, to evaluate the sufficiency of ambient iron and macronutrient concentrations for algal growth. Experiments were conducted with near-surface seawater collected from three locations in a narrow latitudinal band characterized by relatively low algal biomass (<0.7 g l-1 chlorophyll a), low dissolved iron concentrations (<0.33 nM), and strong meridional gradients in temperature, salinity and macronutrient concentrations: (1) the Polar Frontal Zone (PFZ) near 46S, 65E ( 19 M nitrate and 1.2 M silicic acid); (2) the confluence of the Subantarctic and Subtropical Fronts (SAF/STF) near 4412S, 6323E ( 5.4 M nitrate and 0.5 M silicic acid); and (3) the southern Subtropical Zone (STZ) near 4318S, 6231E (<0.1 M nitrate and 1.4 M silicic acid). Our experimental results reveal three distinct regimes of resource limitation of phytoplankton growth. In the PFZ, iron availability exerted the primary limitation on nitrate drawdown and biomass accumulation, thus community growth, with silicic acid availability exerting a secondary limitation on diatom growth and biogenic silica production. Within the SAF/STF, iron deficiency was also the primary limitation on algal community growth; however, here we observed evidence of secondary limitation of nitrate drawdown and biomass accumulation by silicic acid deficiency, via control of algal community structure - such that iron addition preferentially stimulated the growth of non-diatom nanoplankton - suggesting that the algal community was poised close to co-limitation by iron and silicic acid. As expected, our experimental results indicate that macronutrients (nitrate/phosphate) were the primary limitation on community growth in the STZ waters; however, our results also suggest that iron deficiency imposed a significant secondary limitation on community growth, particularly diatom growth, such that the algal community was poised close to co-limitation by macronutrients and iron. We conclude that these same regimes of resource limitation are likely to regulate phytoplankton growth and export production over much of the open-ocean Subantarctic region during the mid to late summer. 2002 Elsevier Science Ltd. All rights reserved.