Role of iron, light, and silicate in controlling algal biomass in subantarctic waters SE of New Zealand

Phytoplankton processes in subantarctic (SA) waters southeast of New Zealan d were studied during austral autumn and spring 1997. Chlorophyll a (0.2-0. 3 mu g L-l) and primary production (350-650 mg C m(-2) d(-1)) were dominate d by cells <2 mu m (cyanobacteria) in both seasons. The photochemical effic iency of photosystem II (F-v/F-m) of cells was low (0.3), indicating physio logical stress. Dissolved Fe (DFe) levels in surface waters were subnanomol ar, and the molecular marker flavodoxin indicated that cells were iron stre ssed. In contrast, Subtropical Convergence (STC) and subtropical waters had higher algal biomass/production levels, particularly in spring. In these w aters, DFe levels were >1 nmol kg(-1), there was little evidence of Fe-stre ssed algal populations, and F-v/F-m approached 0.60 at the STC. In addition to these trends, waters of SA origin were occasionally observed within the STC and north of the STC, and thus survey data were interpreted with cauti on. In vitro Fe enrichment incubations in SA waters resulted in a switch fr om flavodoxin expression to that of ferredoxin, indicating the alleviation of Fe stress. In another 6-day experiment, iron-mediated increases in chlor ophyll a (in particular, increases in large diatoms) were of similar magnit ude to those observed in a concurrent Si/Fe enrichment; ambient silicate le vels were 4 mu M. A concurrent in vitro Fe enrichment, at irradiance levels comparable to the calculated mean levels experienced by cells in situ, res ulted in relatively small increases (approximately twofold) in chlorophyll a. Thus, in spring, irradiance and Fe may both control diatom growth. In co ntrast: during summer, as mean irradiance increases and silicate levels dec rease, Fe limitation, Fe/Si colimitation, or silicate limitation may determ ine diatom growth.