Efeitos de forçantes ambientais e do perifíton sobre macroinvertebrados bentônicos em reservatórios do semiárido neotropical

Abstract

The present study had as objective to identify possible abiotic and / or biotic forcings of the community of benthic macroinvertebrates in three neotropical shallow reservoirs of Northeast Brazil. The study was carried out in two periods (dry and rainy) in the reservoirs Saco I, Cachoeira II and Borborema, belonging to the Pajeú-PE River Basin. The following hypotheses were raised: (i) the environmental variables and the periphyton are factors that direct the distribution and structure of the benthic macroinvertebrates in three shallow reservoirs of the Neotropical semiarid; (ii) the community structure of benthic macroinvertebrates is distributed heterogeneously among the different coastal zones of the reservoirs. In the field the physical-chemical variables of the water were measured in situ through a multiparameter probe (Horiba / U52). A sample of 500 mL water was collected at each sampling point for further analysis of chlorophyll a and nutrients. Samples from the periphytic algae community were collected from the sediments in the coastal region in triplicate in each zone of the reservoir (fluvial, transition and lacustrine), using a Petri dish with an area of 13.5 cm², using a spatula and water Distilled. The community of benthic macroinvertebrates was sampled using a Petersen type dredge with an area of 0.034 m². The peripheral algae community was dominated by diatoms (53.41%) and cyanobacteria (43.38%) in the Saco I and Borborema Reservoirs, while the Cachoeira II reservoir was dominated only by the diatoms. The species Cylindrospermopsis raciborskii (1.2 x 106 ind.cm-2) and Chroococcus minutus (6.0 x 10³ ind.cm-2) presented the highest densities in the Saco I reservoir. In the Cachoeira II reservoir, the diatom species Gomphonema sp. (1.0 x 104 ind.cm-2) and Pinnularia major (1.9 x 104 ind.cm-2) presented the highest densities in the rainy season, while in the Borborema reservoir were the species Geitlerinema amphibium (1,7 x 104 ind.cm-2) and Scenedesmus sp. (8.3 x 104 ind.cm-2) in the rainy season. The community of macroinvertebrates was represented by the taxonomic groups Diptera, Gastropoda, Coleoptera, Hemiptera, among other taxa. In total, 8,247 individuals from 32 taxa were quantified. The functional food groups were dominated by scrapers (68.60%) and lower contribution of the collectors (6.13%). The scrapers were represented by the taxa Chironomidae, Melanoides tuberculatus and Biomphalaria straminea. The macroinvertebrates did not present significant variation (F = 12.98, p > 0.05) in the distribution of the different zones of the reservoirs, as well as the density did not have significant seasonal variation (F = 0.028; p = 0.86). The species richness of the macroinvertebrates did not present significant difference between the zones of the reservoirs (F = 0.67, p = 0.09). The highest richness and diversity of species were presented in the lacustrine zone in the Borborema and Cachoeira II reservoirs (S = 10, H'= 2.07 and S = 16, H' = 1.71bits.ind-1), respectively, except in the Saco I reservoir where the transition zone presented (S = 8, H' = 1.75 bits.ind-1).The density of herbivorous macroinvertebrate was significantly influenced by the periphytic algae in the three reservoirs, explaining more than 89,9% of the total variance. The herbivores showed a positive relation with diatoms and negative with the cyanobacteria. We conclude that high densities of herbivorous invertebrates were positively related to higher diatom availability. Habitat heterogeneity and environmental variables did not directly influence the density of benthic macroinvertebrates.