From headwaters into the estuarine zone: changes in processes and invertebrate communities in response to abiotic conditions

Abstract

We analysed the relationship between functional and structural variables (leaf litter decomposition, biofilms growth, primary productivity, invertebrate communities’ taxonomic structure, and biological traits) and environmental conditions from headwaters into the estuary, which remains under-investigated. Six river sections (headwaters, middle reach, and four large river sections with increasing salinity) and 18 sites were selected in the Mondego river (Central Portugal). Hypotheses of the expected variations were drawn, based on theory (e.g. River Continuum Concept—RCC) and previous studies. Abiotic conditions were strongly associated with macroinvertebrate communities’ taxonomic composition, but the relationship with functional parameters was different than the predicted one. Decomposition rates were similar from headwaters to the first large river sections (not expected) but increased in the estuarine sections. The high rates of decomposition were associated with the low salinity and especially with the high densities of the shredder Gammarus sp. Biofilm growth rate peaked in intermediate sections and was lower in the headwaters and in the estuary (not expected). The lower algae productivity in the estuary was not compensated by the heterotrophic component of the biofilm. The use of multiple biological traits indicated an unexpected higher functional similarity among the upper river sections, and differences within large river sections (not evident with taxonomic composition). These results show contradictions to widely accepted river concepts, such as the RCC, and highlight the importance of management practices adjusted to each river section to keep the ecosystem functioning. They also increased the knowledge on the transition zone between fresh and estuarine water which is important in the context of mitigation of climate change effects, considering the expected intrusion of saline water upstream.