Diurnal hydrochemical variations in a karst spring and two ponds, Maolan Karst Experimental Site, China: Biological pump effects

Abstract

• There are almost no diurnal hydrochemical variations in the spring waters. • In the surface waters, however, all parameters show distinct diurnal changes. • Hydrochemistry of land surface waters may be strongly modified by submerged photosynthesis. • Land surface water systems may constitute an important sink of carbon. A karst spring and two downstream ponds fed by the spring at the Maolan Karst Experimental Site, Guizhou Province, China, were used to investigate the effect of submerged plants on the CO 2 –H 2 O–CaCO 3 system during a time of spring base flow in summer when underwater photosynthesis was strongest. Temperature, pH, electrical conductivity (EC) and dissolved oxygen (DO) were recorded at 15 min intervals for a period of 30 h (12:00 29 August–18:00 30 August, 2012). [Ca 2+ ], HCO 3 - , CO 2 partial pressure ( p CO 2 ) and saturation index of calcite (SI C ) were estimated from the high-frequency measurements. Water samples were also collected three times a day (early morning, midday and evening) for δ 13 C DIC determination. A floating CO 2 -flux monitoring chamber was used to measure CO 2 flux at the three locations. Results show that there was little or no diurnal variation in the spring water parameters. In the midstream pond with flourishing submerged plants, however, all parameters show distinct diurnal changes: temperature, pH, DO, SIc, δ 13 C DIC increased during the day and decreased at night, while EC, HCO 3 - , [Ca 2+ ], and p CO 2 behaved in the opposite sense. In addition, maximum DO values (16–23 mg/L) in the midstream pond at daytime were two to three times those of water equilibrated with atmospheric O 2 , indicating strong aquatic photosynthesis. The proposed photosynthesis is corroborated by the low calculated p CO 2 of 20–200 ppmv, which is much less than atmospheric p CO 2 . 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, a pattern that we attribute to the lower biomass/water volume ratio. The diurnal hydrobiogeochemical variations in the two ponds depended essentially on illumination, indicating that photosynthesis and respiration by the submerged plants are the dominant controlling processes. The large loss of DIC between the spring and midstream pond, attributed to biological pump effects, demonstrates that natural surface water systems may constitute an important sink of carbon (on the order of a few hundred tons of C km 2 /a) as DIC is transformed to autochthonous organic matter. The rates of sedimentary deposition and preservation of this organic matter in the ponds, however, require quantification in future work to fully assess the karst processes-related carbon sink, especially under global climate and land use changes.