Fish diversity and species composition in small-scale artificial reefs in Amazonian floodplain lakes: Refugia for rare species?

We designed a controlled experiment to evaluate the effects of introducing small-scale artificial reefs on fish species composition and diversity in four black water lakes located on islands in the Anavilhanas National Park in the Rio Negro, Amazonas, Brazil. We built eight 37.5 m3 artificial reefs consisting of frames enclosing wooden branches to provide habitat complexity, and placed two of them in each lake approximately 60 m from the margins, one situated close to a forest that abuts the lake directly, and one close to a sand/clay beach between the edge of the lake and the forest. Controls consisted of open water areas 100 m from the reefs and the same distance from the lake margins as the reefs. We collected fish with gill nets in 2009 and 2010 in each of these lakes, during the four main phases of the hydrologic cycle (flooding, high water, receding, and low water). Rarefaction curves indicated that we had sampled sufficiently to represent the species diversity at these sites. ANOSIM indicated no difference the two types of lake margin, and only season of the hydrologic cycle was a statistically significant response variable. Shannon's diversity index (H′) and Pielou's evenness (J′) did not allow us to reject the null hypothesis that reefs do not enhance overall diversity, but we did reject this hypothesis on the basis of substantial differences in community composition: of the 107 species collected, 69 were found at both reefs and control sites, but 26 were only found at reefs and 12 only at control sites. The unique species at reef sites were mainly those of low abundance, and which therefore had relatively little effect on the composite diversity indices. These were mainly species that do not migrate out of the lake as part of the life cycle and use the flooded forest as refuge from predators, so the seasonal unavailability of the flooded forest habitat during the dry hydrologic phase probably keeps their populations at low density. Artificial reefs may therefore function to buffer rare and non-migratory species from local extinction in times of drought, and should be considered in efforts to preserve aquatic biodiversity.