Carbon dioxide (CO2) accumulates under lake ice in winter and degasses to the atmosphere after ice melt. This large springtime CO2 pulse is not typically considered in surface‐atmosphere flux estimates, because most field studies have not sampled through ice during late winter. Measured CO2 partial pressure (pCO2) of lake surface water ranged from 8.6 to 4,290 Pa (85–4,230 µatm) in 234 north temperate and boreal lakes prior to ice melt during 1998 and 1999. Only four lakes had surface pCO2 less than or equal to atmospheric pCO2, whereas 75% had pCO2 >5 times atmospheric. The δ13CDIC (DIC = ΣCO2) of 142 of the lakes ranged from –26.28‰ to +0.95‰. Lakes with the greatest pCO2 also had the lightest δ13CDIC, which indicates respiration as their primary CO2 source. Finnish lakes that received large amounts of dissolved organic carbon from surrounding peatlands had the greatest pCO2. Lakes set in noncarbonate till and bedrock in Minnesota and Wisconsin had the smallest pCO2 and the heaviest d13CDIC, which indicates atmospheric and/or mineral sources of C for those lakes. Potential emissions for the period after ice melt were 2.36 ± 1.44 mol CO2 m−2 for lakes with average pCO2 values and were as large as 13.7 ± 8.4 mol CO2 m−2 for lakes with high pCO2 values.

Groundwater discharge is a neglected source of freshwater and dissolved constituents to the ocean. It can occur via diffuse seepage and point source spring discharge. Two naturally occurring trace gases, 222Rn and CH4, are present in groundwater at concentrations that are elevated by several orders of magnitude relative to seawater, and they may be useful in tracing groundwater inputs to surface waters. Water samples collected near a submarine spring in the northeastern Gulf of Mexico displayed radon and methane concentrations inversely related to salinity and considerably greater than those found in surrounding waters. Coastal water 222Rn and CH4 inventories varied directly with groundwater seepage rates. The integrated quantities of 222Rn and CH4 in the nearshore waters overlying a seepage meter transect showed a significant positive relationship (95% C.L.) to direct measurements of seepage. Diffusive fluxes (178±56 dpm m‒2 d‒1), obtained by three different approaches at the same site, showed that these waters receive only a small contribution of 222Rn by diffusion. In contrast, benthic flux chamber measurements revealed a total advective‐diffusive contribution of 5,200±1,800 dpm m‒2 d‒1 (n = 14). Radon inventories in these shallow nearshore marine waters are consistent with the input of radon‐bearing groundwaters.

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