The effect of migratory seabirds guano on the soft bottom community of a SW Atlantic coastal lagoon

The purpose of this work was to evaluate the contribution of guano by pisci vorous birds and their effect on soft bottom benthic species during the sum mer of 1996 and 1997 in a SW Atlantic coastal lagoon (Mar Chiquita, 37 degr ees 45'S, 57 degrees 26'W). The abundance of piscivorous seabirds (mainly b lack skimmers, South American tern, snowy-crowned tern, common tern) in san dbars near the mouth of the lagoon oscillated between 552 (SE = 373) in 199 6 and 1686 (SE = 935) individuals in 1997. In this area, these birds produc e 0.27 g of guano m(-2) d(-1). Benthic samples showed that bird resting are as have a significantly larger density of macrofauna (e.g., polychaetes and meiofauna nematodes) than areas not used by birds. Experimental addition o f guano in areas without birds [three treatments: (1) no addition, without guano, (2) low addition of guano, 0.23 g m(-2) d(-1), and (3) high addition , 0.57 g m(-2) d(-1)] showed no significant changes in the concentration of nitrogen, phosphorus and organic matter. Nematodes increased in abundance when the addition of guano increased, but ostracods and polychaetes showed no mean effect. The SW Atlantic intertidal burrowing crab Chasmagnathus gra nulata decreased the amount of sediment removed. All observations and exper iments presented here showed that the input of guano by piscivorous seabird s affect abundance and behavior of several benthic species. This evidence a lso supports the hypothesis that the structure of estuarine benthic communi ties in areas used by piscivorous seabirds can be controlled through the in put of nutrients (i.e., a "bottom up" process).