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

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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.

Date Range: 
1999-03-26 04:00:00 to 1999-04-06 04:00:00

Publication Date: 

2011-06-07 04:00:00



Additional Project roles: 

Name: Eda Melendez-Colom Role: Data Manager
Name: Effie A. Greathouse Role: Associated Researcher
Name: Nina Hemphill Role: Associated Researcher
Name: Ernesto Garcia Role: Associated Researcher
Name: William H. McDowell Role: Associated Researcher
Name: James G. March Role: Associated Researcher
Name: Alonso RamIrez Role: Associated Researcher



From 26--29 March 1999, we assessed effects of the poisoning in 5 pools and 5 riffles (= cascades) in the upper 100 meters of the 500-m reach affected by the poisoning (Fig. 1). We used 5 pools and 5 riffles in a 100-m reach upstream from the poisoning as a reference (Fig. 1).

Three of the pools and 3 of the riffles in each of the reaches were sampled on 26 March for epilithic chlorophyll a, FPOM, fine particulate inorganic matter (FPIM), carbon (C), nitrogen (N), carbon to nitrogen ratio (C:N, mass ratio), and algal biovolume using a quantitative suction sampler modified from Loeb (1981). In each habitat unit, 5 rock surfaces were chosen that satisfied 2 criteria:a surface angle of less than 45 degrees; and a depth of at least 5 cm. Six Loeb samples were taken from each rock surface; 3 were pooled and put on ice for later chlorophyll a analysis and the other 3 were pooled for later analysis of FPOM, FPIM, C, N, C:N and algal biovolume. A single minnow trap baited with ~200 mL of cat food was placed and left overnight in each of 3 pools upstream and 3 pools downstream from the poisoning.

On 27 March, trapped shrimps were retrieved in the morning, identified to genus, measured for total length, and returned to the pool in which they were trapped. The additional two pools and two riffles in each of the reaches were then Loeb sampled as described above. Insects occurring on rock surfaces were then sampled by sweeping a 250-mm hand net over a 100 cm2 area in each of the 20 habitat units (5 pools and 5 riffles in each reach). Trapping was repeated in the 3 pools sampled on 26 March using the procedure described above; traps were re-set on the evening of 27 March and retrieved on the morning of 28 March.


We conducted a manipulative experiment, adding atyid and xiphocaridid shrimps, trapped in the upstream reference reach, to pools of the poisoned reach and then monitored how quickly they modified the benthic depositional environment. Response variables measured were epilithic chlorophyll a, FPOM, FPIM, C, N, and C:N. Three manipulated pools were compared to 3 control pools in which no shrimps were added. All 6 pools were located in the poisoned reach. Trapping to collect shrimps for the experiment occurred on 28 and 29 March, and numbers added were based on measurements of pool surface area and previously observed shrimp densities in pools of the Sonadora (12 individuals/m2; Pringle et al. 1999). All 6 pools were Loeb sampled, using methods described above, 1 day prior to adding shrimps (“day 0”) and 6 days after adding shrimps (“day 6”). For the first 2 days after adding the original batch of atyid and xiphocaridid shrimps, we did not observe any of the added atyid shrimps in one of the manipulated pools; thus, on day 2, we supplemented this pool with additional atyid shrimps again based on previously observed shrimp densities.

Laboratory and data analyses

Chlorophyll a samples were filtered onto pre-ashed glass fiber filters (Whatman GF/F, 0.7 µm) which were then frozen until conducting standard chlorophyll a analysis methods (APHA1985) using a Turner Designs fluorometer (model 10AU). Loeb samples intended for analyses of FPOM, FPIM, C, N, C:N, and algal biovolume were split into 2 sub-samples:10 mL were preserved in 10% formalin for later algal biovolume analyses, and the volume of the remaining sample was measured and filtered onto a pre-ashed, pre-weighed glass fiber filter. This filter was dried at 60°C for 24 hours, cooled in a dessicator, and cut into roughly equal sections which were then weighed to the nearest 0.0001 g. One filter section was used for total C and N analyses, which was determined using a Perkin Elmer CHN analyzer. The second filter section was combusted at 500 °C, cooled in a dessicator, and weighed again to determine mass of organic (AFDM) and inorganic matter.

All algal biovolume sub-samples from a single pool were combined and concentrated to a known volume prior to conducting Palmer cell counts on a 0.1-mL aliquot. The aliquot was scanned at 400x magnification, and 500 algal cells or filaments were identified. Abundances of pennate diatoms too small for identification to species at 400x magnification were determined at 1000x using Naphrax permanent mounts of a separate cleaned aliquot (Carr et al. 1986). To calculate biovolume, average lengths, widths, and depths of 10 cells or filaments of each taxon were used in equations that match the geometric shape of the cell or filament (Gruendling 1971). Aquatic insects were identified to order.



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