1. Harvest-related poisoning events are common in tropical streams, yet research on stream recovery has largely been limited to temperate streams and generally does not include any measures of ecosystem function, such as leaf breakdown. 2. We assessed recovery of a second-order, high-gradient stream draining the Luquillo Experimental Forest, Puerto Rico, three months after a chlorine-bleach poisoning event. The illegal poisoning of freshwater shrimps for harvest caused massive mortality of shrimps and dramatic changes in those ecosystem properties influenced by shrimps. We determined recovery potential using an established recovery index and assessed actual recovery by examining whether the poisoned reach returned to conditions resembling an undisturbed upstream reference reach.3. Recovery potential was excellent (score=729 out of a possible 729) and can be attributed to nearby sources of organisms for colonization, the mobility of dominant organisms, unimpaired habitat, rapid flushing and processing of chlorine, and location within a national forest.4. Actual recovery was substantial. Comparison of the reference reach with the formerly poisoned reach indicated: (1) complete recovery of xiphocaridid and palaemonid shrimp population abundances, shrimp size distributions, leaf breakdown rates, and abundances of oligochaetes and mayflies on leaves, and (2) only small differences in atyid shrimp abundance and community and ecosystem properties influenced by atyid shrimps (standing stocks of epilithic fine inorganic and organic matter, chlorophyll a, and abundances of chironomids and copepods on leaves). 5. There was no detectable pattern between any measured variables and distance downstream from the poisoning. However, shrimp size-distributions indicated that the observed recovery may represent a source-sink dynamic, in which the poisoned reach acts as a sink which depletes adult shrimp populations from surrounding undisturbed habitats. Thus, the rapid recovery observed in this study is consistent with results from other field studies of pulse chlorine disturbances, harvest-related fish poisonings, and recovery of freshwater biotic interactions, but it is unlikely to be sustainable if multiple poisonings deplete adult populations to the extent that juvenile recruitment does not offset adult shrimp mortality.
Additional Project roles:
The poisoning occurred 140 m upstream from the 186 bridge crossing Quebrada Sonadora. Seven of our recovery study pools were located in the reach between the 186 bridge crossing and the swinging footbridge where the USGS gage is located. These 7 pools were located at the following distances upstream from the 186 bridge crossing: 357 m (= a reference pool), ~290 m (= a reference pool which was a small pool located adjacent to the "swimming hole"), 240 m (= a reference pool), ~215 m (= a reference pool), 190 m (= a reference pool), 140 m (= the first formerly poisoned pool which was the pool at upstream end of the poisoning), and 100 m (= a formerly poisoned pool). Eight of our recovery study pools were located downstream from the 186 bridge crossing. These 8 pools were all formerly poisoned pools and were located in the left bank (looking downstream) thread of the braided reach below the 186 bridge unless otherwise notes. These 8 pools were located at the following distances downstream from the 186 bridge crossing: 15 m (a pool on the right bank upstream from the "island" of the braided reach below the 186 bridge), 28 m, 47 m, 93 m (a pool of a mid-channel thread of the braided reach), 124 m, 140 m, 164 m.