Inorganic and organic carbon variations in surface water, Konza prairie LTER site, USA, and Maolan karst experimental site, China

Two natural groundwater-fed streams were selected to examine the diurnal trends of water geochemistry, CO2 emissions, and sources of OC on 25 May 2012 at the Konza Prairie Long-Term Ecological Research (LTER) site, USA and 29 August - 30 August 2012 at the Maolan Karst Experimental (Maolan) Site, China. For the stream at the Konza LTER site, little variation in water chemistry was observed among the upstream, midstream and downstream locations, indicating the groundwater and stream water chemistry was mostly stable on a daily basis. 13CDIC was the highest at the downstream site due to the largest CO2 degassing. The autochthonous particulate organic carbon (POC) fraction at the upstream, midstream and downstream sites was 12-35%, 39-65% and 75-88%, respectively. Estimation of the C/N ratio for POC samples at the three locations was 10.4-15.0, 14.0-15.9 and 12.2-13.3. This is comparable to previously measured C/N ratios of suspended POC. For the stream at the Maolan site, there was little or no diel variation in the spring water physical and chemical parameters. However, all parameters show distinct diel changes in the spring-fed midstream pond with flourishing submerged plants. Temperature, pH, DO, SIC, 13CDIC increased during the day and decreased at night, while EC, [HCO3-], [Ca2+], and pCO2 behaved in the opposite sense. Strong aquatic photosynthesis was indicated from maximum DO values (two to three times higher than normal water equilibrated with atmospheric O2). In the downstream pond with fewer submerged plants but larger volume, all parameters displayed similar trends to the midstream pond but with much less change. We attribute this pattern to the lower biomass/water volume ratio. The diel variations in the two ponds resulted from the aquatic photosynthetic effect, demonstrating that natural surface water systems may constitute an important sink of carbon