Iron limitation across chlorophyll gradients in the southern Drake Passage: Phytoplankton responses to iron addition and photosynthetic indicators of iron stress

Abstract

Processes influencing phytoplankton bloom development in the southern Drake Passage were studied using shipboard iron-enrichment incubations conducted across a surface chlorophyll gradient near the Antarctic Peninsula, in a region of water mass mixing. Iron incubation assays showed that Antarctic Circumpolar Current (ACC) waters were severely iron limited, while shelf waters with high ambient iron concentrations (1-2 nmol L 21 ) were iron replete, demonstrating that mixing of the two water masses is a plausible mechanism for generation of the high phytoplankton biomass observed downstream of the Antarctic Peninsula. In downstream highchlorophyll mixed waters, phytoplankton growth rates were also iron limited, although responses to iron addition were generally more moderate as compared to ACC waters. Synthesizing results from all experiments, significant correlations were found between the initial measurements of Photosystem II (PSII) parameters (F v : F m , s PSII , and p) and the subsequent responses of these waters to iron addition. These correlations indicate that PSII parameters can be used to assess the degree of iron stress experienced in these waters and likely in other regions where photoinhibition and nitrogen stress are not confounding factors.

Regional impacts of iron-light colimitation in a global biogeochemical model

“…However, observations of natural communities across gradients of iron availability in the Southern Ocean suggest that community-wide responses to long-term iron limitation may diverge from the more clearcut, short-term responses of iron-fertilization experiments on isolated assemblages. In particular, Hopkinson et al (2007) showed that changes in α chl between naturally iron-rich shelf and iron-poor open-ocean waters had no clear relationship to iron availability.…”

“…For example, in bottle incubations of natural samples from the Drake Passage, Hopkinson et al (2007) reported that iron addition increased α chl , θ and P B m (the light-saturated chlorophyll-specific growth rate, equal to P C m /θ ). Marchetti et al (2006a) also reported large increases in these three parameters, within an iron-fertilized patch of the subarctic Pacific during the SERIES experiment.…”

The influence of iron and light on net community production in the Subantarctic and Polar Frontal Zones

“…F v /F m is a good indicator of the overall physiological stress of phototrophs, with low values generally indicating excessive light or limitation by nutrients (including Fe) (Kolber et al, 1994(Kolber et al, , 1998Kolber and Falkowski, 1993;Geider et al, 1993). In the Southern Ocean, where N and P are replete and irradiance is moderate, F v /F m is generally linked to iron limitation stress (Hopkinson et al, 2007). In some regions, F v /F m is confounded by taxonomic differences in biophysical properties (Moore et al, 2005;Suggett et al, 2004Suggett et al, , 2009), but iron limitation tends to overwhelm the taxonomic signal in HNLC regions such as the Southern Ocean (Suggett et al, 2009).…”

Natural iron enrichment around the Antarctic Peninsula in the Southern Ocean

“…There is ample evidence that the low chl-a concentrations in the pelagic waters of the Southern Ocean are the result of biomass-limiting concentrations of Fe (Martin et al, 1990;Boyd and Law, 2001;Oztürk et al, 2004;Hopkinson et al, 2007;Boyd et al, 2007). Conversely, elevated chl-a concentrations in coastal and plateau areas are thought be the result of re-suspension of iron-rich sediments, iron diffusion from sediment and subsequent upwelling (Korb et al, 2008;Blain et al, 2007Blain et al, ,200812 M. V. Ardelan et al: Natural iron enrichment around the Antarctic Peninsula Scotia Ridges as well as the shelf regions surrounding South Georgia and the South Sandwich Islands.…”