Moderators of Organic Matter Decomposition in Portuguese Streams: A Field Study and Literature Review

ABSTRACT Organic matter decomposition is a fundamental ecosystem process in streams, which have a large aquatic‐terrestrial interface. The sensitivity of organic matter decomposition to changes in environmental conditions promoted by human activities is relatively well understood at large and local scales. In contrast, less is known about the natural variability of organic matter decomposition along natural gradients, especially at intermediate spatial scales. This gap undermines the use of organic matter decomposition as a tool to assess stream functional integrity in biomonitoring programmes. This study aimed at identifying the main moderators of organic matter decomposition under undisturbed conditions to establish natural baseline variation in organic matter decomposition rates in Portuguese streams. A field study was carried out to assess organic matter decomposition rates in 37 least‐disturbed streams distributed throughout continental Portugal and Madeira Island. This study addressed gaps in empirical research by using poplar organic matter, including wood, and undertaking the incubation in spring, while previous studies were carried out preferably in autumn/winter and used alder and oak leaf litter. A systematic literature review was also conducted to compile organic matter decomposition rates from least‐disturbed streams across continental Portugal and the Azores archipelago. Organic matter type (leaves or wood), shredder access (fine‐mesh or coarse‐mesh bags), season (autumn/winter or spring/summer), water nitrate concentration, the presence of exotic vegetation, distance to the source, and percentage shade emerged as main moderators of organic matter decomposition in the field study. Organic matter type and identity (genus), shredder access, season, stream type (national Water Framework Directive typologies), hydrological regime (permanent or intermittent), water temperature, conductivity, and soluble reactive phosphorus concentrations were also important moderators of organic matter decomposition for the compilation of the field data and literature review data. There was considerable variation in organic matter decomposition across least‐disturbed streams, reflecting differences in organic matter type and identity, shredder access, season, and environmental conditions. Therefore, for organic matter decomposition to be used as a tool to assess stream functional integrity, it is necessary to standardise the protocol by selecting specific organic matter types and species, mesh types, and incubation seasons to minimise variation caused by these factors. Choosing stream typologies relevant for the decomposition process (e.g., as outlined in the Water Framework Directive), along with identifying reference decomposition ranges for each typology, may facilitate the assessment of deviations in stream functioning from reference conditions.