Effects of a tropical stream poisoning: do they reflect effects of small-scale experiments?

Small-scale experiments in tropical streams have suggested that freshwater shrimps play a critical role in determining the quality and quantity of benthic organic matter and overall nutrient dynamics. We quantified the effects of a whole-reach shrimp poisoning event in the Sonadora, a second-order stream draining the Luquillo Experimental Forest in northeastern Puerto Rico. The illegal poisoning (for shrimp harvest) caused massive mortality of shrimps and aquatic insects. Atyid and xiphocaridid shrimp abundances in pools of the poisoned reach were reduced by ~95%, relative to abundances in an upstream reference reach. A survey of poisoned vs. reference pools, combined with a manipulative experiment (in which atyid and xiphocaridid shrimps were added to 3 poisoned pools), showed that reduced shrimp abundances due to the poisoning had strong impacts on benthic resources. The benthos of poisoned pools, where shrimp abundances were reduced, had 4 times more chlorophyll a, 6 times more algal biovolume, 4 times more fine particulate organic matter, 14 times more fine particulate inorganic matter, 5 times more carbon, and 4 times more nitrogen than did the benthos of pools in the reference reach. These increases in benthic resources were consistent with increases in algae, organic/inorganic matter, and nutrients in previous small-scale shrimp exclusion experiments conducted in the study river and tributaries. Effects of shrimp poisoning on the benthos varied by habitat, with riffles showing fewer significant differences than did pools. Compared to reference riffles, poisoned riffles had higher standing stocks of fine particulate inorganic matter, nitrogen, and biovolume of filamentous algae, and lower epilithic C:N ratios. Overall, previous small-scale exclusion experiments were highly predictive of the direction of effects due to large-scale shrimp removal by poisoning. Our study provides a tropical data set to add to the short list of stream studies that examine the predictive power of small-scale experiments for larger scales.