Colonization of Exclosures in a Costa Rican Stream: Effects of Macrobenthos on Meiobenthos and the Nematode Community

Abstract

ABSTRACT Interactions of macro- on meiobenthos were studied in the sediment of a small stream in northwestern Costa Rica. Three different exclosure types (n = 5) were exposed for 21 days: (1) treatment macrofauna (2 mm mesh size) allowed meiobenthic plus macrobenthic colonization; (2) treatment meiofauna (500 μm mesh size) allowed meiobenthic colonization but reduced macrobenthic colonization; and (3) plastic pipe exclosures (35 μm mesh size lid) inhibited access of both meio- and macrobenthos. In each cage, standardized hard substrates were provided for algal colonization. We postulated impacts of macrobenthos on (1) meiofaunal abundance, (2) algal biomass, and (3) nematode community composition. Reduction of macrobenthos did not result in increased population densities for any meiobenthic taxonomic group, but gastropods were more abundant in macro treatments. Ambient population densities were not reached inside the exclosures except for ostracods. Algal biomass significantly increased when macrobenthos was reduced, yet the influence of a caging effect during the experiment cannot be ruled out. Bacterivorous nematodes were dominant in both treatments. Species of the genera Eumonhystera and Plectus (colonizers) were predominantly found in meio treatments, while the genera Rhabdolaimus and Dorylaimus/Mesodorylaimus (persisters) were mainly present in macro treatments.