The influence of irradiance and iron (Fe) supply on phytoplankton processes was investigated, north (47° S, 142° E) and south (54° S, 142° E) of the Subantarctic Front in austral autumn (March 1998). At both sites, resident cells exhibited nutrient stress (Fv/ Fm < 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 I0 at 47° S and 9% I0 at 54° S because of MLDs of 40 (47° S) and 90 m (54° S), when these stations were occupied. The greater MLD at 54° S is reflected by tenfold higher cellular chlorophyll a levels in the resident phytoplankton. In the 47° S experiment, chlorophyll a levels increased to >1 μg L-1 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 54° S experiment there were increases in chlorophyll a (to >2 μg L-1), 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 54° S (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° and 54° S 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. Copyright 2001 by the American Geophysical Union.

Item Type:	Refereed Article
Research Division:	Biological Sciences
Research Group:	Ecology
Research Field:	Marine and estuarine ecology (incl. marine ichthyology)
Objective Division:	Environmental Management
Objective Group:	Marine systems and management
Objective Field:	Oceanic processes (excl. in the Antarctic and Southern Ocean)
UTAS Author:	Crossley, AC (Dr Clive Crossley)
UTAS Author:	Sedwick, PN (Dr Peter Sedwick)
UTAS Author:	Trull, T (Professor Thomas Trull)
ID Code:	22375
Year Published:	2001
Web of Science® Times Cited:	99
Deposited By:	IASOS
Deposited On:	2001-08-01
Last Modified:	2011-11-07
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