Organic Matter Decomposition as an Indicator of Island Stream Functioning Under Riparian Forest Modification

ABSTRACT On remote oceanic islands, changes in land uses frequently involve the clearance of native forest, establishment of exotic commercial plantations and intensification of agricultural and livestock activities. These transformations often result in the alteration or removal of native riparian vegetation, with potentially strong effects on stream ecosystem structure and function. Aquatic decomposers and organic matter decomposition are particularly sensitive to land uses changes due to their dependence on terrestrial litter inputs. Here we assessed organic matter decomposition, and associated aquatic hyphomycetes and macroinvertebrates in Azorean streams under riparian forest modifications (cryptomeria plantations and pastures) in comparison with non‐impacted streams (native laurel vegetation). Ochroma pyramidale wood and Clethra arborea leaves were used as substrates enclosed in fine‐ and coarse‐mesh bags to assess organic matter decomposition driven by microbes alone and by the combined activity of microbes and macroinvertebrates, respectively. We found that organic matter decomposition was faster in streams surrounded by pastures due to higher dissolved nutrient concentrations and water temperature, while streams flowing through native riparian vegetation and cryptomeria plantations showed similar organic matter decomposition. Leaf decomposition was faster in coarse‐ than in fine‐mesh bags only in streams adjacent to cryptomeria plantations, a pattern associated with the higher shredder abundance compared with the other streams. Moreover, leaves decomposed faster than wood due to their lower toughness and greater susceptibility to biological degradation. Aquatic hyphomycete taxa richness was lower in pasture streams, but their reproductive activity did not significantly differ among stream types. Benthic macroinvertebrate taxa richness and abundance did not significantly differ among stream types. These findings highlight the complex interactions between land‐use transformation and organic matter decomposition processes emphasizing the importance of riparian management for maintaining ecosystem functioning. Moreover, our findings underscore the vulnerability of stream ecosystem functioning to riparian land‐use change on remote oceanic islands, where even moderate alterations in riparian vegetation can significantly affect key ecological processes.