SPATIAL AND TEMPORAL VARIABILITY IN THE ACCUMULATION OF ORGANIC CARBON IN LAKE SEDIMENTS REFLECTS NUTRIENT ENRICHMENT AND CATCHMENT DISTURBANCE: AN EXAMPLE FROM 116 MINNESOTA LAKES

Lakes are recognized as important sites for processing and storing carbon, yet there exists considerable uncertainty regarding the rate of organic carbon (OC) burial in lake sediments. Moreover, little attention has been given to how these rates change over time. Using sediment-core chronologies (based on ²¹⁰Pb, ¹³⁷Cs) and loss-on-ignition data (% organic matter), we estimated OC burial rates for the past 150-200 years in 116 Minnesota lakes. These lakes are fairly small (median area <100 ha) and span multiple ecoregions: Northern Lakes and Forests (NLF), North Central Hardwood Forests (NCHF), Western Corn Belt Plains (WCBP) and Northern Glaciated Plains (NGP). All OC burial estimates were corrected for sediment focusing.

Organic carbon burial rates for individual lakes (across all time periods) range from 4 to 196 g C m^-2 yr^-1. The mean rate for all lakes increases through time, from 16 g C m^-2 yr^-1 prior to European settlement to a modern value of 42 g C m^-2 yr^-1. Mean pre-settlement rates are relatively similar for all ecoregions (13 to 26 g C m^-2 yr^-1), but mean modern rates show substantial separation. Lakes in the WCBP and NGP ecoregions, located in southern areas of the state where agricultural activity is most intense, accumulate OC at a mean rate > 70 g C m^-2 yr^-1. In contrast, lakes in the NLF ecoregion, which covers the northeastern portion of Minnesota and is subject to less anthropogenic disturbance, experience a mean modern OC burial rate of 27 g C m^-2 yr^-1. Thus, increases in OC burial between the pre-settlement and modern periods are most pronounced (approx. fourfold change) in the WCBP and NGP ecoregions, while the smallest increase (approx. 2.4x) occurs in the NLF ecoregion. The more heavily urbanized NCHF ecoregion shows a 3x increase in OC burial since pre-settlement. Analysis of a subset of lakes reveals that modern OC accumulation rates are highly correlated with water-column measurements of total phosphorus (r² = 0.61), chlorophyll a (r² = 0.55), and total nitrogen (r² = 0.53), as well as percent agricultural land in the catchment (r² = 0.55). Total mean annual OC burial by Minnesota lakes is 43.3 x 10¹⁰ g C yr^-1, or approximately 1.6% of total annual CO₂ emissions from fossil fuel combustion in Minnesota.