A LINE IN THE SEA

THE ocean has considerable spatial and temporal heterogeneity in bioma ss and productivity owing in part to the effects of ocean circulation and mixing(1,2). Water mass boundaries (fronts) in coastal waters are well-known sites of enhanced biological activity(3,4). Comparatively l ittle is known of open-ocean fronts, and one of the few biological stu dies of an oceanic front showed phytoplankton biomass at only slightly higher densities than in surrounding waters(5). Here we present photo graphs and measurements from satellites, aircraft, ships and the Space Shuttle Atlantis which show dramatic biological responses to circulat ion and mixing processes associated with an open-ocean front. Breaking waves (whitecaps) caused by water turbulence and mixing, and very dar k green water caused by extremely high concentrations (>20 mg of chlor ophyll a per m(3)) of buoyant diatoms (Rhizosolenia sp.) made a distin ct line in the sea visible for hundreds of kilometres. The line traced the northern edge of a westward-progagating (50 km per day) tropical instability wave (1,000-km wavelength) delineating the boundary betwee n cold, upwelled waters and warmer waters to the north. High phytoplan kton biomass and primary production associated with the extensive diat om patches may explain anecdotal observations of high animal abundance along this frontal boundary.