Transfer of seep organic matter to the surrounding deepsea ecosystem

The extent to which primary and secondary production in seep sediments is transferred to higher consumers and to the surrounding (non-seep) deep-sea ecosystem has been investigated largely through stable isotopic analyses. A detailed study of mobile consumers at relatively shallow (500-600 m) seeps in the Gulf of Mexico by MacAvoy et al. (2002a) revealed a range of dependence on seep production. Gastropods, sea stars, crabs and selected fishes sampled near seeps exhibited isotopic signatures consistent with 50-100% dependence on seep-derived carbon. In contrast vagrant predators (the isopod Bathynomus giganteus and, to a lesser extent, the hagfishes Eptatretus and Rochina crassa) appeared to rely mainly on photosynthetically derived foods, but were responsible for some export of seep production. These authors conclude that only a limited number of background species are exploiting seep production in the Gulf of Mexico, due either to sufficient food in the ambient deep sea or to biotic defenses by seep prey. However, Kelley et al. (1998) suggested that light C in sediments of the Gulf of Mexico reflect widespread export of organic matter from seeps to background deep-sea bacteria.

Sahling et al. (2003) suggested that export of seep production is related to depth in the Sea of Okhotsk based on stable isotopic analyses. Galatheid crabs at seeps in the Gulf of Alaska (4500 m) (Levin & Michener 2002) and in the Sea of Okhotsk (1450-1600 m) (Sahling et al. 2003) consume prey with tissues derived from chemosynthetically fixed carbon. There is less evidence of this for mobile urchins, crabs, asteroids and cnidarians collected at seeps on the Oregon and California margin; these taxa seem to have photosynthesis-based diets (Levin & Michener 2002). Quantitative energy budgets that examine the transfers of photo- and chemosynthetically derived carbon into and out of seeps have yet to be constructed.

The possibility that organisms with complex life cycles may actively contribute photosynthet-ically derived organic matter to seeps should also be considered. Fatty acid analysis by Pond et al. (2000) has shown that the early life stages of hydrothermal vent shrimp feed on phytoplankon, probably in surface waters, before taking up a benthic existence at vents. No comparable ontogenetic patterns are known for seep taxa, although sablefish commonly seen feeding at 500-m seeps on the northern California margin are known to have pelagic life stages that feed near the surface (Adams et al. 1995).

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