Abrupt changes in air temperature and precipitation - do they matter for organic matter?
Temnerud, Johan; Weyhenmeyer, Gesa
We analyzed 120 years long time series of air temperature and precipitation from 29 respective 44 sites distributed all over Sweden and determined abrupt changes (regime shifts) by using three different methods. The used methods are the Excel add-in ‘Sequential Regime Shift Detection version 3.2’ (SRSD), the software Change-Point Analyzer version 2.3 and the manually performed CUSUM/Pettit-test. Since all three methods revealed similar results we focused on the first method. The SRSD use yearly mean values and assumes no apriori of when the abrupt change should occur, but the time series must be continuous. The differences in the mean between two time periods (regimes) were tested with Student’s t-test, assuming that the variances for both periods are the same. Serial correlation was modelled by the first order autoregressive model (AR1). The AR1 coefficient in this study was estimated by MPK (Marriott, Pope and Kendall). For air temperature we found shifts in 1930 and 1989 (the largest), for precipitation in 1920, 1979 and 1998 (the largest). Accordingly we analyzed more than 30 year long time series of water discharge and watercourses chemistry (16 variables) of 87 sites (1975-2005), and ice cover for 333 lakes. Except of ice cover we were not able to find any strong response in water chemistry to the abrupt air temperature increase in 1989. First a weak abrupt change in precipitation in 1998 had an impact on water discharge and consequently on watercourses chemistry. Most profound effects were observed on absorbance (420 nm), chemical oxygen demand (both increased) and sulphate (decreased), to a lesser degree on the ratio between absorbance and CODMn, turbidity, organic nitrogen and chloride. Our results clearly show that a sudden strong air temperature increase itself is not sufficient to cause abrupt change in watercourses chemistry in Sweden. However, a sudden increase in precipitation and discharge can lead to abrupt change in water chemistry. The increase in air temperature could liberate more and more easily mobilized organic matter in the system, making the system more sensible to changes in precipitation. Sudden increases in precipitation and discharge are predicted for most parts of Sweden. Such abrupt changes might have far reaching consequences for biological processes in aquatic ecosystems.
Publisher: Joensuu University Press
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