Limnetica 40
Macroinvertebrate communities and macrophyte decomposition could be affected by land use intensification in subtropical lowland streams
In lowland streams with little or no riparian forest, autochthonous organic matter can be an important source of the energy supporting the aquatic trophic webs, but different kind of land use in their catchment areas can directly affect the aquatic community structure and indirectly affect the decomposition rate of coarse particulate organic matter (CPOM), among other processes. We investigated whether intensification in the land use affects the autochthonous (aquatic macrophyte) CPOM decomposition rate (k), by affecting the macroinvertebrate community (e.g. density, biomass and richness of taxa and functional groups). Stems of the emergent macrophyte Schoenoplectus californicus (C. A. Mey) were introduced, for 338 days, into two streams with different land use intensity (extensive pasture vs intensive agriculture-dairy). In our study, the stream draining the more intensified land use pasture (hereafter intensive stream) presented higher water nutrient concentrations and decreased dissolved oxygen percentage, pH and discharge. The stream in extensive pasture (hereafter extensive stream) presented a significantly higher decomposition rate (k = - 0.010 ± 0.00068 days-1) than the intensive stream (k = - 0.006 ± 0.0005 days-1). Ephemeroptera, Amphipoda, Chironomidae and Oligochaeta, accounted for 94 % of the total abundance of macroinvertebrates communities. Moreover, 89 % of total analysed individuals belonged to the collector-gatherers functional group, whereas scrapers dominated the total biomass (85 %). Density and biomass of macroinvertebrates increased in the last dates in both systems, being higher in the extensive stream. An increase in scraper density, biomass and richness in the last sampling dates co-occurred with a rapid loss of S. californicus mass in the extensive stream. Our results show that the stream with higher land use intensification presented more simple macroinvertebrates communities, what could negatively affect a key ecosystem function for these subtropical lowland streams, the decomposition rate of Schoenoplectus californicus.
