Effects of extreme low flows on freshwater shrimps in a perennial tropical stream

1. Long-term data on rainfall suggests that perennial rainforest streams rarely are subject to drying of riffles or pools in the wet, non-seasonal Caribbean climate of Puerto Rico. Unusually low rainfall in 1994 caused some headwater riffles to dry out completely, resulting in isolated pools, reduced pool volumes and loss of access to microhabitats by benthic invertebrates. 2. From 1992 to 1998, shrimp populations were sampled bimonthly using baited traps in six pools along 1200 m (from 305 to 480 m in altitude) of Quebrada Prieta, a second-order headwater stream in the Luquillo Experimental Forest (Caribbean National Forest). 3. Following contraction of the smaller and shallower pools in the most upstream section of the stream, mean densities of the dominant shrimp (Atya lanipes ) increased from 22 to 75 shrimp m(-2) of pool area during the 1994 drought year. A second common species (Xiphocaris elongata ) increased from 5 to 14 shrimp m(-2) . A smaller percentage of adults of both species was gravid during the drought. 4. Following the 1994 drought (1995-1998), densities of both shrimp species and reproductive activity of Atya returned to predrought (1990-1993) levels. However, the reproductive activity of Xiphocaris remained lower than in the predrought period. 5. It is suggested that prolonged droughts, even in tropical rainforest biomes, may significantly alter aquatic communities through localised crowding effects resulting from habitat contraction, and lead to prolonged decreases in reproductive output. Consequently, major alterations in aquatic populations and communities would be predicted by current climate change scenarios of decreased total rainfall and increased variability.