Freshwater ecosystems play a key role in the global carbon cycle by collecting, transporting, and processing a significant portion of global organic carbon. These processes can be disrupted in ...non-perennial rivers due to their changing hydrological patterns. We investigated how environmental factors influence organic matter dynamics in the Algars, a Mediterranean non-perennial river basin in the North-East Iberian Peninsula. We conducted seasonal sampling in 16 sites across the river network, collecting samples for (i) storage of benthic organic matter, (ii) transport of dissolved organic carbon and particulate organic matter, and (iii) organic matter processing via aerobic respiration in sediments (Raz–Rru method). We observed pronounced spatial and temporal fluctuations in organic matter processes, especially during distinct periods like summer and autumn. Consistent seasonal patterns of organic matter transport showed a remarkable longitudinal increase downstream, similar to observed aerobic respiration in sediments. Notably, high-flow events doubled observed seasonal transport (mean DOC load: 2344 ± 735 kg/day). Irregular spatial storage patterns between dry and wet channel sections were related to land use and flow intermittency. Notably, storage in dry channel sections was generally ten times higher than wet sections. Our study emphasizes the intricate influence of specific environmental variables on organic matter processes, within different organic matter fractions (for example, coarse and dissolved organic matter). Frequency of non-flow events, seasonal hydrological changes, and land use predominantly govern organic matter dynamics in the Algars basin. Understanding organic carbon dynamics in non-perennial systems will help estimate the impact of hydrological alterations associated with global change on river systems.
Non-perennial rivers and streams are ubiquitous. Nonetheless, our understanding of their hydrological patterns is minimal. Hydrological models are powerful tools to study and characterize ...hydrological patterns, but few can simulate extremes such as non-flow events. We aimed to capture and accurately simulate the flow intermittency spatial and temporal patterns in a Mediterranean river basin with the restructured Soil and Water Assessment Tool (SWAT+). We calibrated the model using a multi-objective optimization algorithm and using data from two gauging stations in the mainstream for the period 2000-2020. Furthermore, we validated the flow intermittency simulations against stage data series at 14 stations. The results indicated that the model accurately simulates flow intermittency and low flows in the study period. We observed a significant variation in flow intermittency in both space and time with a remarkable inter-annual variability. We also observed an increase in flow intermittency over the study period.
Catchment urbanisation results in urban streams being exposed to a multitude of stressors. Notably, stressors originating from diffuse sources have received less attention than stressors originating ...from point sources. Here, advances related to diffuse urban stressors and their consequences for stream benthic communities are summarised by reviewing 92 articles. Based on the search criteria, the number of articles dealing with diffuse urban stressors in streams has been increasing, and most of them focused on North America, Europe, and China. Land use was the most common measure used to characterize diffuse stressor sources in urban streams (70.7 % of the articles characterised land use), and chemical stressors (inorganic nutrients, xenobiotics, metals, and water properties, including pH and conductivity) were more frequently reported than physical or biological stressors. A total of 53.3 % of the articles addressed the impact of urban stressors on macroinvertebrates, while 35.9 % focused on bacteria, 9.8 % on fungi, and 8.7 % on algae. Regarding ecosystem functions, almost half of the articles (43.5 %) addressed changes in community dynamics, 40.3 % addressed organic matter decomposition, and 33.9 % addressed nutrient cycling. When comparing urban and non-urban streams, the reviewed studies suggest that urbanisation negatively impacts the diversity of benthic organisms, leading to shifts in community composition. These changes imply functional degradation of streams. The results of the present review summarise the knowledge gained to date and identify its main gaps to help improve our understanding of urban streams.
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