Iron supply has a key role in stimulating phytoplankton blooms in high-nitrate low-chlorophyll oceanic waters. However, the fate of the carbon fixed by these blooms, and how efficiently it is exported into the ocean's interior, remains largely unknown. Here we report on the decline and fate of an iron-stimulated diatom bloom in the Gulf of Alaska. The bloom terminated on day 18, following the depletion of iron and then silicic acid, after which mixed-layer particulate organic carbon (POC) concentrations declined over six days. Increased particulate silica export via sinking diatoms was recorded in sediment traps at depths between 50 and 125 m from day 21, yet increased POC export was not evident until day 24. Only a small proportion of the mixed-layer POC was intercepted by the traps, with more than half of the mixed-layer POC deficit attributable to bacterial remineralization and mesozooplankton grazing. The depletion of silicic acid and the inefficient transfer of iron-increased POC below the permanent thermocline have major implications both for the biogeochemical interpretation of times of greater iron supply in the geological past, and also for proposed geo-engineering schemes to increase oceanic carbon sequestration.

The proposed photoprotective role of the UV-A absorbing, extracellular pigment scytonemin was studied in the terrestrial cyanobacterium Chlorogloeopsis sp. strain O-89-Cgs(1). UV-A (315-400 nm) caused growth delay, cell growth restarting only when scytonemin had accumulated in the extracellular envelopes. Cultures with scytonemin were more resistant to photoinhibition of photosynthesis than cultures without scytonemin, the differential resistance being much greater to UV-A-caused photoinhibition than to photoinhibition caused by visible light. The presence of scytonemin in the extracellular envelopes was correlated with the inability of UV-A radiation to induce strong photopigment fluorescence (685 nm emission), regardless of the specific content os photosynthetic pigments. The physical removal of the scytonemin containing extracellular envelopes brought about the loss of UV-A resistance as measured by photobleaching rates of chlorophyll a under conditions of physiological inactivity (desiccation). These observations provide strong evidence for the proposed protective role of scytonemin, as a passive UV-A sunscreen, in cyanobacteria.

We initiated and mapped a diatom bloom in the northeast subarctic Pacific by concurrently adding dissolved iron and the tracer sulfur hexafluoride to a mesoscale patch of high-nitrate, low-chlorophyll waters. The bloom was dominated by pennate diatoms and was monitored for 25 d, which was sufficiently long to observe the evolution and termination of the bloom and most of the decline phase. Fast repetition-rate fluorometry indicated that the diatoms were iron-replete until day 12, followed by a 4-5-d transition to iron limitation. This transition period was characterized by relatively high rates of algal growth and nutrient uptake, which pointed to diatoms using intracellularly stored iron. By days 16-17, the bloom was probably limited simultaneously by both iron and silicic acid AcknowledgementsWe thank the captains, crews, and participating scientists onboard the vessels John P Tully, El Puma, and Kaiyo Maru during this study. We also thank Bill Crawford, Sheila Tows, and Frank Whitney (Institute of Ocean Sciences, Sidney, Canada) for shore-side logistical support. We acknowledge the support of Maurice Levasseur (University of Laval, Quebec, Canada) in providing unpublished data for this manuscript. We thank NASA and Orbimage for the provision of SeaWiFS satellite images presented in Figs.

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In July 1993 we collected hydrographic data and information on chlorophyll distribution on the continental shelf north of Cape Hatteras and across the shelf break at Cape Hatteras. The data show that a warm, transparent mixed layer lies over much colder, euphotic, chlorophyll‐rich bottom water on the shelf. This layer has temperature and salinity properties characteristic of the Middle Atlantic Bight (MAB) cold pool, a distinctive mass of cold bottom water formed when cold water from the Gulf of Maine and Scotian Shelf is isolated from surface water by vernal warming and seasonal stratification [Houghton et al., 1982]. The constant density of this chlorophyll‐rich water (σθ = 25.0–25.6) combined with a strong chlorophyll gradient along the 25 σθ isopycnal at the shelf break indicates that chlorophyll advected off the shelf at Cape Hatteras in July 1993. TS diagrams further indicate that cold pool water, and the chlorophyll it contained, mixed into upper levels of the Gulf Stream. Thus the MAB may contribute to the nutrient budget of Atlantic surface waters through a long loop of circulation that transports deep water from the Labrador Sea to Cape Hatteras.

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