Magnitudes and controls of organic and inorganic carbon flux through a chain of hard‐water lakes on the northern Great Plains

Abstract

Whole‐lake carbon (C) mass‐balance budgets were constructed for a chain of six hard‐water lakes to quantify the relative importance of organic carbon (OC) and inorganic carbon (IC) exchanges between atmosphere, water column, sediments, and rivers. Mean summer C fluxes were calculated for each lake during the ice‐free periods (May to September) of 1995‐2007 by measuring deposition of IC and OC in lake sediments, export of C to outflow rivers, lotic C influxes, and atmospheric exchange of CO2. Unlike soft‐water lakes, IC in rivers accounted for 68.2‐85.6% of total C (TC) influx to these hard‐water lakes, CO2 efflux accounted for 0‐44.5% of total C export (median 2.8%), and sedimentation buried similar amounts of OC and IC. Deposition of C in sediments accounted for 1.8‐61.7% of total export and was correlated to water residence time, while C efflux through rivers accounted for 32.6‐98.2% of total export, mainly as IC (69.6‐85.1% of TC). Unexpectedly, estimates of net ecosystem production based on OC mass balances suggested that all lakes were autotrophic (production > respiration) during summer, despite elevated dissolved organic carbon content (5.6‐16.1 mg C L‐1), pCO2 values, and net CO2 emissions to the atmosphere from three lakes. We conclude that C fluxes within and among these hard‐water lakes are regulated by hydrologic inputs of dissolved IC rather than by lake metabolism, IC and OC pathways are only loosely coupled, and future climate variability will alter C fluxes in similar lakes mainly through regulation of mass transfer from land.