Dry season distribution of hydroids in a small tropical estuary, Pernambuco, Brazil

Hydroid distribution patterns along a horizontal ecocline in the Rio Formoso/Rio Ariquindá/Rio Porto Alegre system, a small and seasonally poikilohaline estuary on the tropical northeast coast of Brazil, were investigated. Collecting was undertaken during the dry season, in November 1993, by diving (with and without SCUBA). Nine stations were sampled on a transect extending from a reef outside the river mouth to a mangrove system at the upper end of the estuary. Four major regions were distinguished along the ecocline based on numerical analyses of hydroid species/station location data: (1) nearshore reef — a sandstone ledge outside the entrance of the estuary; (2) river mouth — a sandy and shelly flood-tidal delta area inside the entrance of the Rio Formoso; (3) mid-estuary — a muddy and rocky, mangal-fringed, poikilohaline region in the middle reaches of the Rio Ariquindá; (4) upper estuary — a muddy, mangal-dominated, strongly poikilohaline region at the upper end of the estuary in the Rio Porto Alegre. Species numbers were highest in mid-estuary and on the nearshore reef, intermediate at the river mouth, and lowest in the upper estuary. Only three eurytopic species, including two euryhaline opportunists (Clytia gracilis, Obelia bidentata) and an estuarine endemic (Garveia franciscana), were found at the two landwardmost stations in the upper estuary. Euryhaline marine species comprised part or all of the fauna at stations along the rest of the system. Changes in hydroid species composition along the estuarine gradient were attributed primarily to differences in substrate types and salinity characteristics from one site to another.