Importance of stream temperature to climate change impact on water quality

摘要

The sensitivity of some aspects of water quality to climate change wasassessed in the Seine River (France) with the biogeochemical modelRIVERSTRAHLER, which describes the transformations and fluxes of C, N, P andSi between the main microbiological populations, the water column and thesediment, along the entire river network. Point and diffuse sources areprescribed, stream temperature undergoes a sinusoidal annual cycleconstrained by observations, and runoff is calculated by a physically-basedland surface model. The reference simulation, using meteorological forcingof 1986–1990 and point sources of 1991, compares very well withobservations. The climate change simulated by a general circulation modelunder the SRES emission scenario A2 was used to simulate the related changesin runoff and stream temperature. To this end, a statistical analysis wasundertaken of the relationships between the water and air temperatures inthe Seine watershed over 1993–1999, using 88 points that correctly sampledthe variability of the tributaries. Most of stream temperature variance wasexplained by the lagged moving average of air temperature, with parametersthat depended on Strahler stream order. As an interesting simplification,stream temperature changes could be approximated by air temperature changes.This modelling framework was used to analyse of the relative influence ofthe water warming and discharge reduction induced by climate change onbiogeochemical water quality in Paris and downstream. Discharge reductionincreased phytoplankton growth and oxygen deficits. Water warming decreaseddissolved oxygen, increased phytoplankton biomass during the growth period,and reduced it afterwards, when loss factors dominate. It was also shownthat these impacts were enhanced when point source inputs of nutrient andorganic carbon increased.