Control of phytoplankton growth by iron and silicic acid availability in the subantarctic Southern Ocean: Experimental results from the SAZ Project

Abstract

Abstract. Subantarctic Southern Ocean surface waters in the austral summer and autumn are characterized by high concentrations of nitrate and phosphate but low concentrations of dissolved iron (Fe,-0.05 nM) and silicic acid (Si, <1 gM). During the Subantarctic Zone AU9706 cruise in March 1998 we investigated the relative importance of Fe and Si in controlling phytoplankton growth and species composition at a station within the subantarctic water mass (46.8øS, 142øE) using shipboard bottle incubation experiments. Treatments included unamended controls; 1.9 nM added iron (+Fe); 9 [tM added silicic acid (+Si); and 1.9 nM added iron plus 9 [tM added silicic acid (+Fe+Si). We followed a detailed set of biological and biogeochemical parameters over 8 days. Fe added alone clearly increased community growth rates and nitrate drawdown and altered algal community composition relative to control treatments. Surprisingly, small, lightly silicified pennate diatoms grew when Fe was added either with or without Si, despite the extremely low ambient silicic acid concentrations. Pigment analyses suggest that lightly silicified chrysophytes (type 4 haptophytes) may have preferentially responded to Si added either with or without Fe. However, for many of the parameters measured the +Fe+Si treatments showed large increases relative to both the +Fe and +Si treatments. Our results suggest that iron is the proximate limiting nutrient for chlorophyll production, photosynthetic efficiency, nitrate drawdown, and diatom growth, but that Si also exerts considerable control over algal growth and species composition. Both nutrients together are needed to elicit a maximum growth response, suggesting that both Fe and Si play important roles in structuring the subantarctic phytoplankton community.

Importance of deep mixing and silicic acid in regulating phytoplankton biomass and community in the iron-limited Antarctic Polar Front region in summer

“…In waters north of the APF, low mean Si(OH) 4 concentration ð o 2 mmol m À 3 Þ suggests that cells were probably suffering from Si(OH) 4 deficiency (Franck et al, 2000). As a consequence, Si(OH) 4 limitation leads to the dominance of smaller nanophytoplankton mainly haptophytes, which usually prevail over larger cells under Si(OH) 4 limitation condition (Hutchins et al, 2001). In contrast, sufficient supply of Si(OH) 4 ð 45 mmol m À 3 Þ in waters south of the APF lead to high TChl-a concentrations and diatom-dominated phytoplankton community structure.…”

“…In general, the waters north of the APF have typical characteristic of low dissolved iron (DFe), Si(OH) 4 , chlorophyll-a (Chl-a) concentrations, and they are dominated by haptophytes, especially in areas remote from continental influence (e.g. Banse, 1996;Clementson et al, 2001;Hutchins et al, 2001). Surface waters south of the APF are usually rich in macronutrients (NO 3 , PO 4 , Si(OH) 4 ) with a phytoplankton community dominated by diatoms .…”

“…Thus, iron-limited cells are less efficient at coping with an environment with rapid irradiance fluctuations than iron-replete cells (Strzepek and Harrison, 2004;van de Poll et al, 2009;Alderkamp et al, 2012). In addition, under iron-Si(OH) 4 colimitation conditions, growth of non-silicious, iron-efficient phytoplankton species such as eukaryotic picoplankton and cyanobacteria often dominate over larger cells (Hutchins et al, 2001). This study aims to delineate the respective physiological responses of natural phytoplankton communities under varying nutrient and light regimes within a region subjected to CO 2 fluctuation between 45°S and 55°S along a meridional transect at 10°E in the Atlantic sector of the Southern Ocean.…”

Surface chlorophyll concentrations in relation to the Antarctic Polar Front: seasonal and spatial patterns from satellite observations

“…Cross-frontal mixing could be important if different factors were limiting phytoplankton growth north and south of the front. There is often a strong gradient in silicic acid concentrations across the PF, with low concentrations to the north, which may limit diatom growth (Tréguer and Jacques, 1992;Hutchins et al, 2001). Silica may also be limiting south of the PF during times of strong drawdown by the biota (Nelson et al, 2001;Mengelt et al, 2001).…”

“…The Southern Ocean is thus the largest of the High-Nitrate, LowChlorophyll (HNLC) regions (Martin, 1990). In recent years, substantial evidence has accumulated that the limited availability of the micronutrient iron limits growth rates, and in conjunction with loss processes maintains chlorophyll concentrations (and phytoplankton biomass) at relatively low levels within the Southern Ocean (Martin et al, 1990a,b;Helbling et al, 1991;de Baar et al, 1995;Van Leeuwe et al, 1997;Sedwick et al, 1997Sedwick et al, , 1999Takeda, 1998;Boyd et al, 1999Boyd et al, , 2000Boyd et al, , 2001Hutchins et al, 2001). Strong grazing pressure, light limitation, and silicon limitation of diatom growth have also been suggested to limit biomass accumulation in this region (Banse, 1996;Mitchell et al, 1991;Nelson and Smith, 1991;Boyd et al, 1999Boyd et al, , 2001Frank et al, 2000;Brown and Landry, 2001;Nelson et al, 2001;Hutchins et al, 2001).…”