Macroinvertebrates associated with floating macrophytes in urban channels (Balneário Cassino, Rio Grande, RS, Brazil)

Abstract

The Balneário Cassino is located at south coastal plain of Rio Grande do Sul State. It has channels to pluvial running in all of its extension, with dense stands of floating macrophytes. The aim of this study was to characterize macroinvertebrates associated with these macrophytes, which were sampled in five sampling points (P1 Pistia stratiotes; P2 Spirodela intermedia; P3 Eichhornia crassipes; P4 Salvinia minima and P5 Eichhornia crassipes), monthly, with a net (500 ? mesh size) in triplicates, between March 2000 and February 2001. The macrophytes were washed on 500 ? mesh size sieve, the retained material preserved in 80 % alcohol and then separated at stereomicroscope. The plants were dried (60o C) to obtain the dry weight. At each point dissolved oxygen, temperature, pH and electric conductivity, and nutrient (P and N) concentrations were measured. The associated community was assessed with taxa abundance and occurrence, and calculated the Shannon- Wiener diversity index and homogeneity (J’, Pielou). The Dice-Sorensen similarity among plants was estimated. Eichhornia crassipes (P5) had the highest relative density (116,723.61 org. 100g DW-1) and the higher taxa number (46). In all plant species Chironomidae (Diptera), Oligochaeta and Copepoda predominated in abundance and density. The associated community showed high similarity among macrophyte species, with significant difference in H’(p < 0.05), varying between 2.51 (E. crassipes P3 in spring) and 0.98 (S. minima P4 in summer). Low concentration of dissolved oxygen (between 1.4 and 7.6 mg.L-1) and high nutrient concentration (total-P between 0.01 and 4.81 mg.L-1 , and total-N between 0.35 and 14.94 mg.L-1) suggesting an eutrophication process in these channels, mainly during summer months, when increased population and domestic sewage contribution to this systems. This process is reflected by macroinvertebrate community, by the predominant groups, and indicates the deterioration of water quality of these channels.

Key words: macroinvertebrates, aquatic macrophytes, phytofauna, urban channels.