Riverine impact on future projections of marine primary production and carbon uptake

[摘要] Riverine transport of nutrients and carbon from inland waters tothe coastal and finally the open ocean alters marine primary production (PP)and carbon (C) uptake regionally and globally. So far, this process has notbeen fully represented and evaluated in the state-of-the-art Earth systemmodels. Here we assess changes in marine PP and C uptake projected under theRepresentative Concentration Pathway 4.5 climate scenario using theNorwegian Earth system model, with four riverine transport configurationsfor nutrients (nitrogen, phosphorus, silicon, and iron), carbon, and totalalkalinity: deactivated, fixed at a recent-past level, coupled to simulatedfreshwater runoff, and following four plausible future scenarios. Theinclusion of riverine nutrients and carbon at the 1970 level improves thesimulated contemporary spatial distribution of annual mean PP and air–sea CO 2 fluxes relative to observations, especially on the continentalmargins (5.4 % reduction in root mean square error (RMSE) for PP) and inthe North Atlantic region (7.4 % reduction in RMSE for C uptake). Whilethe riverine nutrients and C input is kept constant, its impact on projectedPP and C uptake is expressed differently in the future period from the historicalperiod. Riverine nutrient inputs lessen nutrient limitation under futurewarmer conditions as stratification increases and thus lessen the projecteddecline in PP by up to 0.66  ±  0.02  Pg C yr −1 (29.5 %) globally,when comparing the 1950–1999 with the 2050–2099 period. The riverine impact onprojected C uptake depends on the balance between the net effect of riverine-nutrient-induced C uptake and riverine-C-induced CO 2 outgassing. In thetwo idealized riverine configurations the riverine inputs result in a weaknet C sink of 0.03–0.04  ±  0.01  Pg C yr −1 , while in the moreplausible riverine configurations the riverine inputs cause a net C sourceof 0.11  ±  0.03  Pg C yr −1 . It implies that the effect of increasedriverine C may be larger than the effect of nutrient inputs in the future onthe projections of ocean C uptake, while in the historical period increasednutrient inputs are considered the largest driver. The results aresubject to model limitations related to resolution and processrepresentations that potentially cause underestimation of impacts.High-resolution global or regional models with an adequate representation ofphysical and biogeochemical shelf processes should be used to assess theimpact of future riverine scenarios more accurately.

[学科分类] 大气科学