Origin and dynamics of dissolved and particulate nutrients in a minimally disturbed Mediterranean river with intermittent flow

Hydrological pathways, speciation, and in-stream spatio-temporal variations (including initial flood pulses), along with soil/sediment characteristics, pointed out to the most probable factors and processes driving the origin, levels and dynamics of nutrients in a minimally disturbed Greek temporal river. Nitrogen and phosphorus were predominately found in the organic form and, despite the high suspended sediment transport, in the dissolved phase. Forest soil leaching and erosion fuelled the river with organic nutrients both in the dissolved and particulate phase, especially at the upstream portion of the basin. This part of the basin exhibited “pristine” dissolved nutrient levels (e.g. 30μg/l DIN, 1.04μg/l P-PO₄), despite substantial transport through precipitation and subsurface flow, suggesting that certain biogeochemical processes acting in the watershed and in-stream effectively reduce riverine dissolved nutrient concentrations. Intense erosion caused a significant downstream increase of suspended sediments and associated nutrient concentrations. Flushing processes, that were detected even in a monthly base in particular river stretches, enhanced dissolved and particulate nutrient concentrations. Initial flood pulses created “hot moments” chiefly for suspended sediments (RMF during initial flood events reached up to 7) and associated nutrients (e.g. RMF of POC during initial flood events reached up to 32), although flood event sediments presented lower nutrient portions compared to usual sediment transport. Flush peaks of ammonia (max 19mg/l NH₄) and nitrite (max 42μg/l NO₂), that surpassed aquatic quality standards, were attributed to rapid mineralization of organic matter upon rewetting and subsequent nitrification. River bed sediments were characterized by organic carbon and nitrogen losses, due to leaching and mineralization processes, and phosphorus retention, thus affecting nutrient ratios and hence photosynthesis in receiving waters.