Influence of Saprotrophic <i>Phytophthora</i> on the Decomposition Rate of Leaf Litter in Alpine and Mediterranean Streams

ABSTRACT The decomposition of leaf litter from the riparian vegetation is a source of nutrients and organic matter for stream food webs. The decomposition rate is influenced by water temperature, leaf characteristics, and aquatic saprotrophic microbial communities. Although the microbial agents Phytophthora lacustris and Phytophthora gonapodyides are frequently isolated from submerged litter, their ecological role in decomposition and their leaf and temperature preferences are largely unknown. In this study, we surveyed Phytophthora colonisation of naturally occurring leaf litter in 26 streams across an altitudinal gradient in the Pyrenees (NE Spain). At 14 of these sites, we also studied Phytophthora colonisation and decomposition (i.e., area and mass loss) of Alnus, Populus and Quercus leaves for 3 months using baiting bags, mimicking the decomposition of leaves that reach streams during leaf fall in autumn. In both experiments, Phytophthora were isolated and identified by sequencing of the ITS region. Leaf species was the main determinant of decomposition, with Alnus leaves decomposing more rapidly than less labile Quercus leaves. The presence of saprotrophic Phytophthora species also correlated with mass loss, especially in the later stages of decomposition. Altitude and climate had the least impact on decomposition, mainly affecting decomposition at the beginning of the decomposition process. The two most common Phytophthora species in Pyrenean streams were P. lacustris (78% of streams) and P. gonapodyides (75%). Phytophthora lacustris was limited to warmer regions, which could be because it has a higher drought‐tolerance than P. gonapodyides . Compared with P. lacustris , P. gonapodyides preferred leaves with a lower nitrogen (N) concentration and a slower decomposition. Overall, leaf litter decomposition was correlated with the presence of saprotrophic Phytophthora species: P. gonapodyides was found in leaves that were less decomposed (across N‐rich and N‐poor litter species) than those of P. lacustris , indicating that they may play different roles in the decomposition process. These species differed in their tolerance to higher and lower temperatures and in their leaf chemical composition preference and, therefore, their ecological niche also differed. Predicted temperature increases could reduce the potential distribution of P. gonapodyides , but the implications for the decomposition of N‐poor litter at the early stage of the decomposition process would depend on the extent of the contribution of P. gonapodyides to this process.