Effects of nutrient inputs in a forested Mediterranean stream under moderate light availability

Nutrients were artificially increased in an oligotrophic, forested stream by adding nitrogen (as nitrate and ammonia) and phosphorus (as reactive phosphate) continuously for 44 days. This addition increased the nutrient concentrations by 10 (phosphorus), 6 (ammonia) and 4 times (nitrates) with respect to basal concentrations. The effect of the enrichment on the structure and metabolism in benthic habitats (rocks, sand and leaf litter) was studied during a period of moderate light availability (maximum light at the streambed ranged between 20-120 μE m -2 s -1 ), prior to leaf occurrence in the riparian vegetation. A BACI design was used to determine the significance of the addition by comparing an upstream non-enriched (control) reach with a downstream enriched one (impacted). Nutrient addition caused a 3-fold increase in chlorophyll density on rocks and sand, and a remarkable increase in algal cell density, but did not cause changes in the community composition of the algal community. The increase in algal biomass was not reflected in higher net community metabolism (NCM) when this was measured as oxygen production, but higher carbon incorporation was detected at the end of the experiment. Nutrient addition did not significantly alter bacterial cell density on rocks, sand and leaf litter substrata. Accordingly, no substantial increase in community respiration (CR) was detected. Neither did heterotrophic activity (extracellular enzymes) change significantly because of addition, except for a decrease (although weak) in phosphatase in sand. Enrichment did not cause a general increase in macroinvertebrate density or biomass, but some taxa increased in the enriched reach. Scrapers (Ancylus fluviatilis) and detritivores (Oligochaeta, Lumbriculidae significantly augmented their density or biomass on certain substrata. Overall, enhanced nutrient availability was able to cause a remarkable biomass increase in the autotrophic component of the stream, in spite of the moderate light availability.