Population Biology of Leptostracan CrustaceansLarge accumulations of macrophyte detritus provide food and refuge for a dense (3.5 million per square meter) assemblage of amphipod and leptostracan crustaceans in a Southern California submarine canyon. Laboratory and field experiments demonstrated that the detritus provided an effective refuge for the crustaceans. Fishes became more effective predators as the density of prey populations increased and/or as the thickness of the detritus mat was reduced (increasing number per m3 but not per m2). The density (m2) of the mat crustaceans fluctuated seasonally, being greatest in the winter and spring following storms which eliminated large portions of the habitat. Surprisingly few animals appeared to be directly eliminated by the storm disturbance, rather the important effect of storms was to reduce the quantity and quality of the detritus refuge by concentrating the crustaceans into smaller patches with less detrital cover. During the calm summer months the bacterium Beggiatoa sp. spread out over large portions of the detritus mat. Beneath the bacteria oxygen concentration was much reduced, and infaunal density was two orders of magnitude lower than in unaffected portions of the mat. The summer increase in bacterial cover constituted a biological disturbance that functionally reduced the habitat area available to the mat fauna and left them more vulnerable to predation. Secondary production here is among the highest reported from natural environments. This is possible because the mat crustaceans are unable to graze down their food supply, the detritus and its microbial community. The refuge provided by the detritus for large populations of invertebrates is also crucial; larger refugia translated into increased carrying capacity which allowed both greater population sizes and production, much of which become available to fishes.
Most marine ecosystems are supported by allochthonous material that enters food webs via detritivores. Because the delivery of detritus is governed by physical processes, seafloor features that collect it constitute an important source of patchiness over a wide range of scales. The La Jolla Canyon system, by accumulating organic debris, provides a large food source from shallow to continental slope depths. This resource supports large numbers of fishes and presumably increases local production in higher trophic levels.
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