Sub-Antarctic zone mooring study of interannual variability in particulate carbon export - Phytoplankton Data

Papers arising from phytoplankton experiments associated with the SAZ (Subantarctic Zone) project. This work was complete as part of ASAC (AAS) project 1156. Taken from the abstracts of the referenced papers: Subantarctic Southern Ocean surface waters in the austral summer and autumn are characterised by high concentrations of nitrate and phosphate but low concentrations of dissolved iron (Fe, ~0.05 nM) and silicic acid (Si, less than 1 micro M). 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 (46.8 degrees S, 142 degrees E) using shipboard bottle incubation experiments. Treatments included unamended controls; 1.9 nM added iron (+Fe); 9 micro M added silicic acid (+Si); and 1.9 nM added iron plus 9 micro M 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 response, suggesting that both Fe and Si play important roles in structuring the subantarctic phytoplankton community. The influence of irradiance and iron (Fe) supply on phytoplankton processes was investigated, north (47 degrees S, 142 degrees E) and south (54 degrees S, 142 degrees E) of the subantarctic Front in austral autumn (March 1998). At both sites, resident cells exhibited nutrient stress. 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 degrees S and 9% I0 at 54 degrees S because of MLDs of 40 (47S) and 90 m (54S), when these stations were occupied. The greater MLD at 54S is reflected by tenfold higher cellular chlorophyll a levels in the resident phytoplankton. In the 47S experiment, chlorophyll a levels increased to greater than 1 micro gram per litre 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 54S experiment there were increases in chlorophyll a (to greater than 2 micro grams per litre), 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 54S (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 54S 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.