Control of Phytoplankton Growth by Iron Supply and Irradiance in the Subantarctic Southern Ocean: Experimental Results From the SAZ Project

The influence of irradiance and iron (Fe) supply on phytoplankton processes was investigated, north (47øS, 142øE) and south (54øS, 142øE) of the Subantarctic Front in austral autumn (March 1998). At both sites, resident cells exhibited nutrient stress (Fv/ F m < 0.3). Shipboard perturbation experiments examined two light (mean in situ and elevated) and two Fe (nominally 0.5 and 3 nM) treatments under silicic acid-replete conditions. Mean in situ light levels (derived from incident irradiances, mixed layer depths (MLDs), wind stress, and a published vertical mixing model) differed at the two sites, 25% of incident irradiance I 0 at 47øS and 9% I 0 at 54øS because of MLDs of 40 (47øS) and 90 rn (54øS), when these stations were occupied. The greater MLD at 54øS is reflected by tenfold higher cellular chlorophyll a levels in the resident phytoplankton. In the 47øS experiment, chlorophyll a levels increased to > 1 /•g L -• only in the high-Fe treatments, regardless of irradiance levels, suggesting Fe limitation. This trend was also noted for cell abundances, silica production, and carbon fixation rates. In contrast, in the 54øS experiment there were increases inchlorophyll a (to >2/•g L-•), cell abundances, silica production, and carbon fixation only in the high-light treatments to which Fe had been added, suggesting that Fe and irradiance limit algal growth rates. Irradiance by altering algal Fe quotas is a key determinant of algal growth rate at 54øS (when silicic acid levels are nonlimiting); however, because of the integral nature of Fe/light colimitation and the restricted nature of the current data set, it was not possible to ascertain the relative contributions of Fe and irradiance to the control of phytoplankton growth. On the basis of a climatology of summer mean MLD for subantarctic (SA) waters south of Australia the 47 ø and 54øS sites appear to represent minimum and maximum MLDs, where Fe and Fe/ irradiance, respectively, may limit/colimit algal growth. The implications for changes in the factors limiting algal growth with season in SA waters are discussed.