The effect of migratory shorebirds on the benthic species of three southwestern Atlantic Argentinean estuaries

We experimentally evaluate the effect of migratory shorebirds on the benthi c fauna of three southwestern Atlantic Argentinean stop-over and wintering sites: Bahia Samborombon (35 degrees 30'-36 degrees 22'S,57 degrees 23'W), Mar Chiquita coastal lagoon (37 degrees 40'S, 57 degrees 26'W), and Bahia B lanca (38 degrees 48'-39 degrees 25'S, 50 degrees-62 degrees 25'W). The exp eriments consisted of exclusion ceilings and controls (both 1 m(2)), with 1 0 replicates each and aligned at the same tidal level. During December 1994 - May 1995 these experiments were done, twice in Bahia Samborombon, twice a t two sites (Sotelo and Celpa) 4 km apart in Mar Chiquita coastal lagoon, a nd three times in Bahia Blanca. Three polychaete species (Laconcreis actua, Nephtys fluviatilis, and Heteromastus similis) were found in similar densi ties in both areas of the Mar Chiquita lagoon. L. acuta and N. fluviatilis were affected in Sotelo, but there was no treatment effect in Celpa. In Sot elo the most abundant shorebirds were Hudsonian godwit (Limosa haemastica), American golden plover (Pluvialis dominica), and White-rumped sandpiper (C alidris fuscicollis), and in Celpa the most abundant shorebirds were White- rumped plover (Charadrius falklandicus), and lesser yellowlegs (Tringa flav ipes). The largest polychaete densities were in the upper sediment layer (0 -6 cm), which was also the most affected layer. Three polychaete species we re found in Bahia Samborombon (L. acuta, N. similis, and Neanthes succinea) , but only H. similis showed a treatment effect. The most abundant shorebir ds in Samborombon were white-rumped sandpiper and two-banded plover. No eff ect was detected in Bahia Blanca, where the most abundant shorebirds were w hite-rumped sandpiper and American golden plover. In all cases, the species affected by shorebirds were the most abundant species. From the two sites of Mar Chiquita, there was a treatment effect only in Sotelo, which was als o the area with higher shorebirds counts. However, much lower densities obs erved in Samborombon (similar to the unaffected area of Mar Chiquita) also produced a significant decrease in infaunal abundance. These evidences also suggest that just selection of study sites may produce different views of the interaction between shorebirds and benthic species.