Vol. 93: 267-275,1993 MARINE ECOLOGY PROGRESS SERIES Mar. Ecol. Prog. Ser. Published March 11 Phytoplankton biomass in an Antarctic coastal environment during stable water conditions - implications for the iron limitation theory Frederico Pereira Brandini Centro de Estudos do Mar. Universidade Federal do Parana. AV. Beira Mar s/n, Pontal do Sul, Paranagua 83255 (PR). Brazil ABSTRACT: The summer plankton community of Admiralty Bay (King George Is.) was studied in February 1987 under stable water column conditions which developed inside the bay after an un- usually long period of predominant moderate to low wind speeds. The bay contained Fe in non-limiting concentrations, and the hypothesis was tested that Fe enrichment in natural environments would increase phytoplankton biomass. The phytoplankton was numerically dominated by nanoflagellates and small pennate diatoms. The protozoans were dominated by tintinnids and heterotrophic dino- flagellates, mainly Gymnodinium spp. Although the light/nutrient regime was optimum for autotrophic growth, the grazing pressure of microzooplankton precluded phytoplankton biomass accumulation, maintaining low chlorophyll a concentrations throughout the whole bay area during the 7 d of survey. The overall conclusion is that Fe enrichment of natural environments dominated by the microb~al network may not result in h ~ g h algal b~omass. INTRODUCTION In recent years, a potentially stimulating effect of iron on the growth rate of phytoplankton in nutrient- rich/low-biomass areas such as the North Pacific and Southern Ocean has been demonstrated using bottle experiments (Coale 1988, Martin & Fitzwater 1988, De Baar et al. 1990, Martin et al. 1990). Iron addition is also said to favour the dominance of large diatoms (Hudson & More1 1990). Large-scale fertilization with iron in the Southern Ocean has hence been proposed as a possible mechanism for increasing oceanic pri- mary production, transferring more fixed carbon to the deep sea and ultimately reducing atmospheric carbon dioxide levels (Martin et al. 1990). The evidence supporting this so called 'iron hypo- thesis' was critically reviewed at a recent meeting (Ano- nymous 1991) when the role of other biomass-limiting factors, such as grazing, were considered and a series of recommendations formulated concerning the implica- tions of the 'iron hypothesis'. Pilot experiments were proposed in order to test the shrnulation of iron under natural conditions and predict the ecological conse- quences of large scale iron-enrichments in the Southern Ocean. One of the reasons why the iron hypothesis has generated so much controversy is because the regions considered so far are open ocean sites where factors other than iron availability can be invoked to explain low phytoplankton biomass in the presence of high nutrient concentrations. For instance, the low-grazing and highly stable (non-light-limited) regime inside enclosure experiments, typical of highly sheltered areas during the early spring-bloom periods, do not occur in the deeply mixed open ocean waters used to test the effect of iron on phytoplankton development. In this paper I draw attention to a region -the coastal and shelf waters surrounding the Antarctic Continent - which can provide insight into the iron question. Since shelf waters contain iron above limiting levels for phytoplankton growth (Martin et al. 1990),they should support large phytoplankton blooms during stable periods. Indeed, in topographically protected areas along the Antarctic Peninsula, e.g. Deception Island and Gerlache Strait, such blooms have been reported for several years (Burkholder & Sieburth 1961, Holm- Hansen & Mitchell 1991), although they do not occur throughout the shelf regions. Here I present the results of an investigation conducted in Admiralty Bay, I ng George Islands, dur- 0 Inter-Research 1993