Luquillo LTER

From 15 June to 28 July, 1999, we assessed recovery of shrimp, fish and leaf pack invertebrate abundances, shrimp size distributions, algal colonization, accrual of fine particulate inorganic and organic matter, and leaf decomposition rates in pool habitats of the upper 315 meters of the 500-m reach affected by the March 1999 poisoning of the Sonadora. We used a 250-meter reach upstream from the poisoning as a reference reach for comparison to the formerly poisoned reach. Five reference pools and ten formerly poisoned pools were chosen for study. In each pool, we haphazardly placed 6 unglazed ceramic tiles (7 x 15 cm) and 6 leaf packs, each tethered to cobble from the channel with cable ties and metal binder clips. Leaf packs were made from air-dried leaves of Cecropia schreberiana Miq. cut into smaller pieces, weighed to approximately 5 g, and held together with a binder clip. In the two most upstream study pools of the formerly poisoned reach, tiles and leaf packs were disturbed by swimmers (i.e. picked up and tossed). Tiles and leaf packs in the third study pool of the formerly poisoned reach suffered high washout during a storm 2 days after placement. The seven other study pools in the formerly poisoned reach were below the Road 186 bridge crossing of the Sonadora but similar to the pools of the upstream reference reach in physical parameters. We randomly sampled, from each study pool, one tile on days 11, 16, 20, 25, and 34 and one leaf pack on days 6, 11, 14, 16 and 20. We also sampled 25 leaf packs on day 0 to obtain an air-dried to oven-dried ratio in order to convert the original air-dried weight of each leaf pack to an estimated oven-dried weight. We sampled tiles and leaf packs by cutting cable ties and raising the tile or leaf pack out of the water within a 363-µm hand net. The tile or leaf pack and its associated hand net contents were placed into a ziplock bag and transported to the laboratory in a cooler. Laboratory processing of tiles consisted of scraping the top surface of the tile with a razor blade and scrubbing the entire tile with a toothbrush. After removing insects from the resulting homogenate, two subsamples of known volumes were filtered onto combusted and pre-weighed glass fiber filters (Whatman GF/F, 0.7 µm). Mass of organic (AFDM) and inorganic matter was determined for one filter, dried at 50 °C for 24 h, weighed to the nearest 0.001 g, burned at 500 °C for 3 h, and re-weighed. The second filter was frozen until analyzed for chlorophyll a using a Turner Designs fluorometer (model 10AU) and standard methods (APHA, 1985). Laboratory processing of leaves consisted of rinsing off insects and sediments, oven-drying at 50 °C for 24 h, weighing to the nearest 0.001 g, burning at 500 °C for 3 h, and re-weighing. Insects and small benthic invertebrates were live-picked from sediments, preserved in 70% ethanol, and identified to family or order. We analyzed the abundances of the four most common invertebrate taxa (Ephemeroptera, Chironomidae, Oligochaeta and Copepoda). Shrimp abundance in each study pool was assessed by trapping and by conducting standardized observations. Minnow traps, each baited with ~200 mL of dry cat food, were set overnight at an approximate density of 1 trap per m2 of pool surface area. On the following morning, trapped shrimps were identified to family, measured for carapace length ( 1 mm), and released. Daytime observations were conducted at each tile and leaf pack on days 8, 13, 20, 25 and 33 and were standardized within an observation date but were not standardized between observation dates. We recorded the number of shrimps in each family. Day 8 observations were conducted for one minute, recording shrimps occurring on the tile or leaf pack. On days 13, 20, and 25, observations were "spot checks," recording shrimps on the tile or leaf pack and in an estimated 5-inch radius around the tile or leaf pack. On day 33, observations were conducted for 10 minutes, again recording shrimps on the tile or leaf pack and in the 5-inch radius. 10-minute observations were not done on day 33 in one upstream reference reach pool. On day 42, density of the green stream goby (S. plumieri) in each pool was determined by snorkeling the entire pool and counting all individuals observed. Visibility was similar across pools.