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Answer:
Hydrothermal activity at the Earth's submarine spreading centers may trigger phytoplankton blooms in the overlying open ocean.
Although the largest buoyant plumes from deep sea volcanoes attain neutral buoyancy (through entrainment of ambient, denser fluid) prior to reaching the surface, exceptionally-grand expulsions of buoyant fluid during hitherto unwitnessed large eruptions could transport micronutrients to the euphotic zone. It has been hypothesized that over volcanically-active seafloor in the weakly stratified regions of the world ocean (primarily above 60 degrees latitude), hydrothermal plumes of the magnitude thus far observed may rise all the way to sea level. In such a case, surface waters would be mixed with (highly diluted) hydrothermal fluid, resulting in anomalous levels of mineral precipitates, dissolved compounds, and heat.
Hydrothermal effluent is enriched (relative to sea water) in a variety of essential nutrients, including Fe, S, and H (Lilley et al, 1995). If these elements are transported to a surface ecosystem in which primary production is limited by micronutrient availability, the specific growth rate of the photosynthetic autotrophs will be bolstered. Results from the IronEx II experiment (Behrenfeld et al, 1996) suggest that in surface environments like the Southern Ocean which have high-nitrogen, low chlorophyll character, such a nutritious infusion is a necessary and sufficient condition for a bloom. Blooms from brief and infrequent plume penetration of the thermocline would be as episodic as are sea floor eruptions; however, each resultant patch might persist for weeks because the largest hydrothermal plumes become stabilized as forced baroclinic vortices (Speer, 1989).
Phytoplankton blooms over vent systems could alternatively (or supplementally) be induced by a reduction in predation on phytoplankters. Burd and Thomson (1994) document the migration of surface zooplankton (copepods and their predators) from their typical surface habitats to the hydrothermal plume tops 2 km below. If surface herbivores migrate, the balance between productivity and predation may result in conditions sufficient to cause a bloom not only in the Southern Ocean, but even in a surface ecosystem replete in iron -- like the North Atlantic over the Mid-Atlantic Ridge. There the condition might indeed be necessary and sufficient, whereas it may only supplement a nutrient induced bloom in the Southern Ocean. In both cases, however, it is also possible that only herbivore predators depart for the deep, in which case predation pressure on the herbivores will be reduced, precluding a phytoplankton bloom. This presents an optimal foraging strategy for such predators in which they move between the deep and surface prey populations.
A final effect of plume penetration is that surface waters could become anomalously warm (likely of order 10^-2oC) and enriched in mineral precipitates (perhaps -0.5% light transmissivity). The change in temperature would result in a small increase in specific growth rate, and it might be accentuated by increased absorption of solar energy due to the dark mineral precipitates. This, too, could result in a near-surface bloom, although the deeper phytoplankton populations adapted to the 1% light level would suffer the decrease in transmitted light. Such an occurrence would simultaneously boost iron levels; in the high-nitrogen, low-chlorophyll Southern Pacific, the response could be a thermally enhanced bloom.
References:
Behrenfeld, M.J., A.J. Bale, Z.S. Kolber, J. Aiken, and P.G. Falkowski. 1996. Confirmation of iron limitation of phytoplankton photosynthesis in the equatorial Pacific Ocean. Nature, Vol. 383, 10 October, pp. 508-511.
Burd, B.J. and R.E. Thomson. 1994. Hydrothermal venting at Endeavour Ridge: effect on zooplankton biomass throughout the water column. Deep-Sea Research I, Vol. 41, No. 9, pp. 1407-1423.