Effects of riparian vegetation removal on nutrient retention in a Mediterranean stream

We examined the effects of riparian vegetation removal on algal dynamics an d stream nutrient retention efficiency by comparing NH4-N and PO4-P uptake lengths from a logged and an unlogged ream in Riera Major, a forested Medit erranean stream in northeastern Spain. From June to September 1995, we exec uted 6 short-term additions of N (as NH4Cl) and P (as Na2HPO4) in a 200-m s ection to measure nutrient uptake lengths. The study site included 2 clearl y differentiated reaches in terms of canopy cover by riparian trees: the fi rst 100 m were completely logged (i.e., the logged reach) and the remaining 100 m were left intact (i.e., the shaded reach). Trees were removed from t he banks of the logged reach in the winter previous to our sampling. In the shaded reach, riparian vegetation was dominated by alders (Alnus glutinosa ). The study was conducted during summer and fall months when differences i n light availability between the 2 reaches were greatest because of forest canopy conditions. Algal biomass and % of stream surface covered by algae w ere higher in the logged than in the shaded reach, indicating that logging had a stimulatory effect on algae in the stream. Overall, nutrient retentio n efficiency was higher (i.e., shorter uptake lengths) in the logged than i n the shaded reach, especially for PO4-P. Despite a greater increase in PO4 -P retention efficiency relative to that of NH4-N following logging, retent ion efficiency for NH4-N was higher than for PO4-P in both study reaches. T he PO4-P mass-transfer coefficient was correlated with primary production i n both study reaches, indicating that algal activity plays an important rol e in controlling PO4-P dynamics in this stream. In contrast, the NH4-N mass -transfer coefficient showed a positive relationship only with % of algal c overage in the logged reach, and was not correlated with any algal-related parameter in the shaded reach. The lack of correlation with algal productio n suggests that mechanisms other than algal activity (i.e., microbial heter otrophic processes or abiotic mechanisms) may also influence NH4-N retentio n in this stream. Overall, this study shows that logging disturbances in sm all shaded streams may alter in-stream ecological features that lead to cha nges in stream nutrient retention efficiency. Moreover, it emphasizes that alteration of the tight linkage between the stream channel and the adjacent riparian zone may directly and indirectly impact biogeochemical processes with implications for stream ecosystem functioning.