Changing nutrient and productivity regimes in Lake Superior:causes and time course.

Abstract

Understanding nutrient and productivity trends in Lake Superior is necessary to better manage the largest freshwater lake in the world. Investigations of six sediment cores, from the western basin of Lake Superior, were used to characterize the greater lake basin. Sedimentary geochemical trends were measured as indicators of historic change due to effects associated with increasing population and land use changes in Lake Superior’s watershed. Measurements were concentrated within the last two centuries, providing historic context to evidence of observations of the mid to late 1800s and into the present. Bulk and stable isotopic composition of C and N indicate that the lake has undergone productivity changes in response to nutrient availability within the strongly P-limited system. Interpretable response by the δ13C productivity proxy was significant despite the highly oligotrophic condition and low sedimentation rates of the lake. Primary production of the lake has shown a marked increase starting in the early 1900s reaching a maximum in the 1960s, heavier by 1.7‰ on average (δ13CBulk), observed most strongly in proximity to the populated western end of the lake. Subsequent recovery from nutrient enrichment is observed as decreased productivity by 1980, in agreement with P reduction mandated by the United States and Canada. Measurements of n-alkane biomarkers were used as an additional method of determining changes in aquatic production. Carbon isotopic analysis of algal-derived biomarkers support bulk measurement indicating changes in aquatic primary production drive the sedimentary organic geochemical record. N cycling within the lake appears to be driven by different environmental factors in individual regions. δ15N in the most western samples appears to be dominated by aquatic primary productivity chances, but in other regions of the lake productivity has much less influence of δ15N. Sedimentary nitrogen isotopes provide little indication to the source or timeline of the lake wide increase in nitrate concentration.