Climate extremes dominating seasonal and interannual variations in carbon export from the Mississippi River Basin

Abstract

Knowledge about the annual and seasonal patterns of organic and inorganic carbon (C) exports from the major rivers of the world to the coastal ocean is essential for our understanding and potential management of the global C budget so as to limit anthropogenic modification of global climate. Unfortunately our predictive understanding of what controls the timing, magnitude, and quality of C export is still rudimentary. Here we use a process-based coupled hydrologic/ecosystem biogeochemistry model (the Dynamic Land Ecosystem Model) to examine how climate variability and extreme events, changing land use, and atmospheric chemistry have affected the annual and seasonal patterns of C exports from the Mississippi River basin to the Gulf of Mexico. Our process-based simulations estimate that the average annual exports of dissolved organic C (DOC), particulate organic C (POC), and dissolved inorganic C (DIC) in the 2000s were 2.6 ± 0.4 Tg C yr À1 , 3.4 ± 0.3 Tg C yr À1, and 18.8 ± 3.4 Tg C yr À1, respectively. Although land use change was the most important agent of change in C export over the past century, climate variability and extreme events (such as flooding and drought) were primarily responsible for seasonal and interannual variations in C export from the basin. The maximum seasonal export of DIC occurred in summer while for DOC and POC the maximum occurred in winter. Relative to the 10 year average (2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010), our modeling analysis indicates that the years of maximal and minimal C export cooccurred with wet and dry years (2008: 32% above average and 2006: 32% below average). Given Intergovernmental Panel on Climate Change-predicted changes in climate variability and the severity of rain events and droughts of wet and dry years for the remainder of the 21st century, our modeling results suggest major changes in the riverine link between the terrestrial and oceanic realms, which are likely to have a major impact on C delivery to the coastal ocean.

Century‐long increasing trend and variability of dissolved organic carbon export from the Mississippi River basin driven by natural and anthropogenic forcing

“…The model was calibrated against a database collected from published field experiments and metadata analyses (e.g., the amount of DOC leached per unit area in diverse ecosystems such as cropland, grassland, and forest). Details regarding calibration procedures can be found in our previous publications [ Tian et al ., ; Liu et al ., ].…”

Modeling <i>p</i>CO<sub>2</sub> variability in the Gulf of Mexico

“…The results of three model experiments covering the period of 2004-2010 are presented in this study, in which, Experiment 1 (Exp1) was a "control run", with observed riverine inputs from USGS and biological sources and sinks of DIC and alkalinity in the water column; Experiment 2 (Exp2) was a "no-biology run", where all biological sources and sinks of DIC and alkalinity were disabled, similar to the experiment described in Fennel and Wilkin (2009);and Experiment 3 (Exp3) had the same set up as Exp1, but the riverine inputs (water, nutrients, and carbon of the Mississippi-Atchafalaya River) were taken from the Dynamic Land Ecosystem Model (DLEM; Tian et al, 2015) simulation for the period of [1904][1905][1906][1907][1908][1909][1910] (Fig. 4).…”

Climate change-induced increases in precipitation are reducing the potential for solar ultraviolet radiation to inactivate pathogens in surface waters

“…Space-for-time modeling based on data from over 1,000 boreal lakes in Norway indicate that future increases in air temperature and precipitation will increase DOM concentrations by as much as 65% in surface waters 41 . Modeling of carbon flux from the Mississippi River basin to the Gulf of Mexico from 2001–2010 showed that DOM export was approximately 30% higher during a wet year and 30% lower during a dry year compared to the average during the study period 4 . Modeling studies that focus on hydrologic changes and hold other factors such as land use change constant have estimated an increase in annual DOM export into the Gulf of Maine of up to 40–50% or more in some rivers from 1930 to 2013 42 .…”

“…While warming temperatures have been the central focus of studies on climate change from genes to ecosystems 2 , increases in extreme precipitation events (droughts and floods) are also rendering fundamental changes in aquatic ecosystems ranging from inland lakes 3 to the Gulf of Mexico 4 and the Great Barrier Reef 5 . Outbreaks of waterborne infectious diseases are often associated with heavy precipitation events 6 .…”