Downscaling experiment for the Venice lagoon. II. Effects of changes in precipitation on biogeochemical properties

A downscaling approach is used to assess potential effects of variations in nutrient loadsinduced by possible future climate changes on biogeochemical properties of the Venice lagoon. Theanalysis is based on a hierarchy of dynamic and statistical models linking climatic change to effectson biogeochemical processes. The outputs of regional climate model simulations for present day andfuture (A2 and B2 IPCC emission scenarios) conditions are used to force 2 statistical models that pro-vide forcing and boundary conditions for a 3D coupled transport-biogeochemical model of thelagoon. Results in terms of spatio-temporal dynamics of biogeochemical properties provide evidenceof significant impacts of climate change. Under both the A2 and B2 scenarios, we observe an ampli-fication of the seasonal precipitation patterns, with drier summers and wetter winters, which affectthe timing of nutrient inputs to the lagoon. Nutrient loads are generally higher in the wintertime andlower in the summertime than in present-day conditions. In winter, nutrients are not used by phyto-plankton whose productivity is low, and are mainly exported from the system. Conversely, reducednutrient inputs to the lagoon in summer cause a reduction in planktonic productivity of the ecosys-tem. Between the 2 emission scenarios, the A2, which has higher levels of atmospheric greenhousegas concentrations, shows more intense impact. Analysis of the effects of additional scenarios indi-cates that policy-driven changes in nutrient loads can overbalance changes due to climate drivenmodification of precipitation patterns.