Meiofauna in tropical montane streams: community composition and responses to hurricane disturbance

Abstract

Meiofauna is a group of small-size heterotrophic organisms that inhabit sediment accumulations in aquatic ecosystems (e.g., ponds, rivers, wetlands, bromeliads). In streams, these organisms are regulated by discharge, sediment characteristics, and biotic interactions. Hurricanes are a natural disturbance that affects these variables, therefore impacting meiofaunal community dynamics. Hurricanes are expected to increase in frequency and intensity according to climate change models. The current knowledge on tropical freshwater meiofauna and how hurricanes impact them is scarce. Quebrada Prieta, in El Yunque National Forest, Puerto Rico, offers a unique opportunity to understand how hurricanes impact tropical stream meiofauna. The Luquillo LTER program has studied hurricane effects on ecosystems in Quebrada Prieta since 1989. However, studies have been biased towards large body-size biota. This site was affected by two intense hurricanes in September 2017. Irma and María generated heavy rainfall that increased fine sedimentation in the stream, and powerful winds defoliated, debranched, and uprooted vegetation in the riparian zone. The main objective of this dissertation was to characterize the meiofaunal community of a tropical stream and understand how it is affected by hurricanes. The objective was accomplished using three approaches: meiofaunal characterization in a tropical stream, assessment of hurricane impacts on the meiofauna in the same stream, and experimental manipulation of major variables affected by hurricanes under laboratory conditions. A total of 62 morphospecies within twelve major taxonomic groups were observed in Quebrada Prieta. The results also show that the meiofaunal community of Quebrada Prieta is strongly structured by stream discharge, coarse sand, and macroinvertebrates. Also, the results demonstrate that the meiofauna in Quebrada Prieta is adapted to hurricanes. After the hurricanes, the meiofauna exploits available resources in the stream, thus increasing richness and abundance. Moreover, after three months of the impact of the hurricanes, the meiofauna richness and abundance started to diminish, with some meiofaunal phyla returning to pre-hurricane values. Sediments, organic matter, and shrimps were variables regulating the meiofauna after hurricanes in situ and in the experimental setup. Discharge and sediment characteristics are the main variables regulating stream meiofauna globally. However, other variables can regulate stream meiofauna, but they depend on local characteristics of the stream like biotic interactions and disturbance regimes.