WHAT IS DRIVING RECENT LAKE ORGANIC CARBON ACCUMULATION RATES: CLIMATE WARMING OR POLLUTION?

Organic content in lake sediments, measured using loss-on-ignition (LOI) methods, is frequently used as a proxy for landscape and aquatic productivity, and, by inference, of climate conditions favorable to productivity. Depending on limiting factors in the ecosystem, these climate conditions might be temperature, precipitation, turbidity (light availability), or combinations of these. Lake sediment paleoclimate reconstructions frequently describe maximum or minimum percent LOI as indicative of warm or cold air temperatures, respectively, such during the Medieval Warm Period (MWP) A.D. 950-1250, or the Little Ice Age (LIA) A.D. 1350 – 1890.

Analyses of eight sediment cores, from multiple lakes in New Hampshire and Maine, reveal trends of rapidly increasing LOI values in the last 100 years, and sharply higher values in the last 30 years. Standard paleoclimate interpretations of these trends would attribute them to warming-enhanced productivity in the watersheds. However, lake monitoring associations, and the environmental consultants they rely on for understanding, almost entirely focus on nutrient enrichment as primary factors, even where eutrophication is occurring without changes in nutrient concentrations in the water column. These associations requested sediment core analyses to help identify the dominant drivers of recent “greening” of their lake waters.

The study lakes range from highly oligotrophic to moderately mesotrophic. All are over 10 m deep at the core site; two are over 50 m deep. Commonalities also include resident complaints of shoreland loss, abundant organic flotsam where waters were previously clear, and macrophytes where stony shoals once existed. ICP-OES records, with Pb210-established chronologies from these lakes, clearly show the arrival and use of combustion engines in the lake area after A.D 1910, along with rising LOI values. The records all also show a sharp increase in LOI values after A.D 1980. Particle sizes of minerogenic material, and magnetic susceptibility of the core sediments, also increase near the top of most of these cores. While nutrient influxes are a popular explanation for changes observed in lake conditions, sediment analyses suggest that climate warming and lake-shore erosion are the strongest influences on many New England lakes right now.