PARENT SESSION
Symposium 7: When the Rivers Run Dry: Cross-Disciplinary Perspectives on Societal Responses to Drought and Ecosystem Impacts
Organized by: A Covich and C Pringle
Tuesday, August 5. 1:30 PM to 5:00 PM, SITCC Oglethorpe Auditorium.

Fragmentation of river networks, freshwater shrimp distributions and Caribbean climate change.

Covich, Alan^*,1, Crowl, Todd², ¹ University of Georgia, Athens, GA, USA² Ecology Center, Logan, UT

ABSTRACT- Long-term data on rainfall suggest that perennial rainforest streams are rarely subject to drying of riffles or pools. However, droughts coupled with increasing urban demands for freshwater are decreasing stream flow and can alter some species ability to migrate upstream from estuaries to headwater pools. Decreased flows and pool depths restrict habitat, diminish water quality and can disrupt chemical communication that influences predator-prey dynamics. This disruption of connections within river drainage networks is likely to occur more often as drought frequency and intensity are predicted to increase over the next several decades while human populations also continue to increase their needs for freshwater. As part of our long-term studies of the effects of variable flow regimes on benthic detritivores, we sampled shrimp populations bimonthly using baited traps in pools along 1200 m (from 305 to 480 m in elevation) of Quebrada Prieta, a second-order headwater stream in the Luquillo Experimental Forest (Caribbean National Forest). 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. Following contraction of high-elevation pools, densities of the numerically dominant shrimp (Atya lanipes) increased from 22 to 75 shrimp per m² of pool area and a second common species (Xiphocaris elongata) increased from 5 to 14 shrimp per m². A smaller percentage of adults of both species was gravid during the drought. Following the 1994 drought (1995-1998), densities of both shrimp species and reproductive activity of Atya returned to pre-drought (1990-1993) levels. Following the drought, Xiphocaris reproductive activity remained lower than in the pre-drought period. Predatory shrimp (Macrobrachium carcinus) moved farther upstream into the headwaters during drought conditions presumably because of decreased habitat availability at lower elevations. These increased concentrations of predator and prey densities along with degradation of habitat quality may increase vulnerability of shrimp populations, especially in rivers where harvest regulations are lacking.

Key words: chemical communication, predator-prey interactions, upstream migration, pool-depth refugia