Sediment flushing observations, earthquake slumping, and benthic community changes in Monterey Canyon head

A large area of axis sediment (>500 m(2)) may be annually removed from the head of Monterey Submarine Canyon with the first onshore storm of the fall/winter storm season. In this scenario, flushing events are followed by accumulation of sediment and organic debris-especially macro-algae-in the shallow axis. Net accumulation of this fill material increases during the calmer spring and summer until the next fall-flushing. The benthic community at a canyon axis station was characterized by highly fluctuating populations of opportunistic polychaete worm; and gammarid amphipods, primarily Capitella spp., Armandia brevis, and Orchomene pacifica. The canyon axis community was very different from communities living at two other stations where sudden flushing does not occur-an adjacent sloping-wall station and a sandflat station. Sloping-wall and sandflat stations harbored more and longer-lived species, larger individuals, and a less-variable population structure during a year of sampling. The Loma Prieta earthquake in the fall of 1989 triggered small sediment slumps on the canyon walls, but it did not trigger axis-flushing. The usual seasonal flushing of the axis occurred 2 weeks after the earthquake with the arrival of the first storm. Benthic communities were reduced in abundance inside earthquake-induced slumps; however, the slumped areas were rapidly colonized by Prionospio pygmaea, a polychaete opportunist common to the sandflat. Surprisingly, the physical and biological impacts of the earthquake were much less severe than the seasonal axis-Bushing associated with storms. Observations of sediment-flushing combined with measurements of benthic community changes in Monterey Canyon head represent a step towards an ecological model of mass wasting with implications for the continental shelf and slope and possibly the deep sea. (C) 1997 Elsevier Science Ltd.