Ciencias

The purpose of this study was to compare how aquatic macroinvertebrates are affected by certain management practices and agrochemicals in organic and conventional rice cultivations (treatments) in northwestern of Costa Rica. We sampled macroinvertebrates in both treatments, at the water entrances (irrigation) and exits (drainage) during two cycles (8 months total) of rice field cultivation. We employed a water quality index using macroinvertebrates (BMWP/CR) as bioindicators in both management treatments. Insect family mean (P = 0.0019) and species mean richness (P = 0.0340) were greater in the organic vs. the conventional treatments as well as at the entrances rather than their exits. Both macroinvertebrates mean abundance (P = 0.0281) and insects mean abundances (P = 0.0065) were greater at the organic vs. the conventional treatments. The water quality index (BMWP/CR) was greater in the organic treatment at the entrance (124) comparing with the exit (72), and also at the conventional entrance (92) vs. the exit (66), thus showing that the management practices affected the macroinvertebrate community. The organic treatment showed the settlement of a greater number of families and species of macroinvertebrates both in general and in those considered sensitive to pollution than in the conventional treatment. This sensitive group of macroinvertebrates (Baetis sp., Fallceon sp., Leptohyphes sp., Tricorythodes sp., Farrodes sp., Phyllogomphoides sp., Hydroptila sp., Mayatrichia sp., Neotrichia sp., Oxyethira sp., Nectopsyche sp.1, Nectopsyche sp.2, Oecetis sp.) can be used as a bioindicators of water quality in these agroecosystems. On the contrary, more macroinvertebrates resistant to pollution were found in the conventional compared to the organic treatment, showing that aquatic macroinvertebrates respond to the type of management/products that are applied to the rice field.

The Palo Verde marsh restoration was performed in Costa Rica to reduce invasive Typha domingensis stands. The invasion of this cattail in the 1980s decreased the habitat quality, and thousands of waterbirds that used this seasonal marsh were not returning. In 2002, the Organization for Tropical Studies decided to manage the marsh using a tractor with angle-ironpaddle wheels to crush cattail. We evaluated the land cover changes in response to the prescribed management activities through GIS and vegetation-assessment analysis. Crushing activities started in July 2002. We defined three management plots: A and B) with crushing activities for different lengths of time and C) the control plot with passive management. We evaluated wetland cover changes, richness, and dominance of plant species in the three plots. We found that owing to management activities 1) cattail cover area decreased from 35% to 9% (Plot A) and from 62% to 7% (Plot B), while in Plot C it remained almost constant (always greater than 60%); 2) the area of total live vegetation including cattail decreased, giving way to open water and bare ground; 3) plant species richness was higher in the crushed plots than in the control plot; 4) cattail dominance decreased and more species were sharing the marsh area in managed plots than in the control plot; and 5) cattail cover was negatively related to species richness. We recommend continuing the control of cattail by crushing in order to rehabilitate the marsh and conserve its biodiversity.