High-frequency metabolism study in a large and shallow temperate lake reveals seasonal switching between net autotrophy and net heterotrophy

Abstract

Respiratory CO 2 release from inland waters is a major process in the global carbon cycle, retaining more than half of the carbon flux from terrestrial sources that otherwise would reach the sea. The strongly lake type-specific balance between primary production and respiration determines whether a lake acts regionally as a net sink or source of CO 2 . This study presents two-year (2009, 2010) results of high-frequency metabolism measurements in the large and shallow polymictic eutrophic Lake Võrtsjärv (area 270 km 2 ; mean depth 2.8 m). We estimated the net ecosystem production (NEP), community respiration (R) and gross primary production (GPP) from continuous measurements of oxygen, irradiance, wind and water temperature. A sinusoidal model fitted to the calculated metabolic rates showed the prevalence of net autotrophy (mean GPP:R [ 1) from early spring until August/September, whereas during the rest of the year heterotrophy (mean GPP:R \ 1) prevailed, characterizing the lake as CO 2 neutral on an annual basis. Community respiration lagged behind GPP by approximately 2 weeks, which could be explained by the bulk of the phytoplankton biomass accounted for by filamentous cyanobacteria that are considered mostly inedible to zooplankton, and the seasonally increasing role of sediment resuspension. In the warmer year 2010, the seasonal peaks of GPP, R and NEP were synchronously shifted nearly 1 month earlier compared with 2009. The strong stimulating effect of temperature on both GPP and R and its negative effect on NEP revealed by the multiple regression analysis suggests increasing metabolic rates and increasing heterotrophy in this lake type in a warmer climate.

Vertical patterns of metabolism in three contrasting stratified lakes

“…Coupling between R and GPP-Coupling between GPP and R was weak in our lakes (maximum correlation was 0.49; Table 3) compared with previous studies in lakes (Sand-Jensen and Staehr 2009;Coloso et al 2011;Laas et al 2012), where correlations were generally . 0.9.…”

Ecosystem respiration: Drivers of daily variability and background respiration in lakes around the globe

“…Although the relationship between average respiration and average GPP across lakes has been well characterized (del Giorgio and Peters 1994; Duarte and Agusti 1998), with the exception of this study there are to date relatively few estimates of the relationship between daily (or similar) respiration and GPP within lakes (Staehr et al 2010;Coloso et al 2011a;Laas et al 2012). Those that are available generally indicate strong coupling of respiration to GPP.…”

Diurnal sampling reveals significant variation in CO2 emission from a tropical productive lake

“…CO 2 undersaturation and/or autotrophy have been recorded in lakes, especially on those with high production rates (e.g. Xing et al, 2005;Gu et al, 2011;Laas et al, 2012). Productive lakes support lower respiration rates than the unproductive ones and then tend to be net CO 2 sinks (Duarte and Agusti, 1998).…”