US Long-Term Ecological Research Network

Recent syntheses suggest that globally, about half of the annual net global carbon input from the atmosphere to terrestrial ecosystems is passed on to streams, lakes and rivers.  This flux is of great consequence to the global C cycle, but is currently very poorly constrained.  To address questions about carbon cycling and potential climate change feedbacks we gathered a collaborative team of researchers and constructed a carbon budget for the ~6400 km2 Northern Highlands lake district (NHLD), integrating lake, stream, wetland and forests into the same framework (Buffam et al. in press).  We used a combination of approaches including new field surveys in wetlands (Buffam et al. 2010), tower-based CO2 flux measurements (Desai et al. 2008), modeling using NTL-LTER and other data, and published literature.  This is one of the first ever complete carbon budgets at a regional scale to incorporate aquatic ecosystems.

We found that growing upland forests are the largest contributors to current annual surface-atmosphere carbon exchange (schematic at right), thus in the short-term, management for increased forest productivity would increase the regional C sequestration.  However, >80% of the landscape's carbon storage is within lake sediments and wetland peat (Fig. 4). The disconnect between current rates and pool sizes implies very different turnover times in the different regional C pools.  Lake sediments and peatlands have accumulated over millennia, whereas most of the C in forests accumulated during the past century as secondary succession followed large-scale deforestation for timber.  For long-term C sequestration on the scale of centuries or more, preservation of the peat and lake sediment pools may be more critical than encouraging forest growth.

How important are surface waters for regional carbon cycling?

Because surface waters almost invariably serve to remove/receive C from their terrestrial watersheds, any C budget that does not account for that loss will overestimate terrestrial C accumulation.  In the NHLD, surface waters receive and process on the order of 7% of the annual net terrestrial C inputs.  This percentage is small relative to the global estimate (40-60%) – not because C exports to surface waters are low in the NHLD, but because current forest productivity (and thus net ecosystem exchange, NEE) is so high.

As important as the amount of carbon exported to surface waters, is what happens to it once it gets there?  Most C is partitioned among three possible pathways with contrasting implications for climate feedback: (1) CO2 evasion to the atmosphere (mostly from lakes), (2) organic carbon burial in lake sediments, or (3) riverine runoff from the region.  In the NHLD, all of these are of similar magnitude (Buffam et al. in press), and the factors influencing the partitioning of carbon among these different fates is the subject of recent and ongoing research in the LTER program (Hanson et al. 2004, Cardille et al. 2009).

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