DISTRIBUTION OF NATURAL ISOTOPES OF CARBON IN LINSLEY POND AND OTHER NEW ENGLAND LAKES1

One product of lake metabolism, the CO2 that is produced anaerobically, is naturally labeled by its high C13 content, but in the presence of carbonates, even the modest amounts found in medium‐hard Linsley Pond, such enrichment is not distinctive. Groundwater near the lake is highly charged (175 ml/liter) with CO2; because its source includes ancient carbonate, it is naturally labeled as C14‐poor (Δ=‒87.5‰). Combining these data with estimates of surface inflow, outflow, and other gains and losses of water and carbon, we can account for the C14 deficiency observed in hypolimnetic water (Δ = ca. ‒100‰) by a groundwater contribution of the order of 1 liter/(m2 × day), perhaps less than 1% of the flow expected if the lake were not well insulated by a clay basin seal. Under ice, during the winter of 1961–62, C14 depletion of the deep water developed so rapidly as to suggest direct injection of groundwater and its accumulation by gravitational flow; however, groundwater carbon is ordinarily cycled metabolically, as shown by the fact that organic surface mud has an apparent age of ca. 1,000 years (δC14 = ‒133‰). Inadequate knowledge of the hydrology of Linsley Pond interferes with rigorous description of the carbon balance, as does recent strong but irregular contamination of air by artificial (hydrogen‐bomb) C14. By 1960, submerged pondweeds and surface waters of several lakes were enriched in artificial C14 by less than 5%, while water‐lily leaves, which use atmospheric rather than aquatic sources of carbon, were enriched by ca. 22%. Using the enrichment of Linsley Pond, 1.9% per year, as a tracer, we find the invasion of atmospheric CO2 to be slow, ca. 1.2 moles/(m2 × yr) [1.4 mg/(cm2 × yr)]. Contributions from groundwater and from air being small, and metabolism being an internal cycle that is nearly closed, the carbon balance of a small lake is seen as part of the hydrologic cycle and as dominated by bicarbonate in surface runoff.