The purpose of this work was to evaluate the contribution of guano by piscivorous birds and their effect on soft bottom benthic species during the summer of 1996 and 1997 in a SW Atlantic coastal lagoon (Mar Chiquita, 37°45′S, 57°26′W). The abundance of piscivorous
seabirds (mainly black skimmers, South American tern, snowy-crowned tern, common tern) in sandbars near the mouth of the lagoon oscillated between 552 (SE = 373) in 1996 and 1686 (SE = 935) individuals in 1997. In this area, these birds produce 0.27 g of guano m−2 d−1.
Benthic samples showed that bird resting areas have a significantly larger density of macrofauna (e.g., polychaetes and meiofauna nematodes) than areas not used by birds. Experimental addition of 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 granulata decreased the amount of sediment removed. All observations and experiments presented here showed that the input of guano by piscivorous seabirds affect abundance and
behavior of several benthic species. This evidence also supports the hypothesis that the structure of estuarine benthic communities in areas used by piscivorous seabirds can be controlled through the input of nutrients (i.e., a "bottom up" process).
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