Challenges to Lake Superior's condition, assessment, and management: A few observations across a generation of change

Selected comparisons of water quality and biological properties in lakewide samplings of the early 1970s and 2005-2006 illustrate a range of ecological changes within Lake Superior over the last three decades. Comparisons depict warmed surface layers, and increased chloride and nitrate concentrations-confirming trends described in recent literature. Our comparisons also depict some spatial dimensions of change, showing vertical and horizontal patterns throughout the lake as a function of depth and from shallow to deepest waters. The selected physico-chemical examples speak to different scales of source drivers for change (from local, to basinwide, and even global) and highlight a lake in which some fundamental properties have been influenced in a short period relative to its long flushing time (similar to 170 years). One legacy of the past 30 years of study seems clear: the notion that Lake Superior, due to its vastness, is resistant to environmental forcing and very slow to change, has been modified. We use two important biological components to evaluate change and also to contrast biological distributions, highlighting that some fundamental aspects of food webs vary with depth. Reflecting on these observations, we offer a perspective on how well we keep track of the condition and functioning of the lake, and how we might improve assessments to more actively inform management. Without more frequent biological sampling across all depth zones of the lake, there will continue to be limited ability to assess the nature and causes of ecological change, even when some changes are detected. Knowing that physico-chemical changes can occur relatively quickly (within decades), that the mechanisms for change can be expressed over different spatial dimensions of the lake, and that biology is distributed heterogeneously over these spatial dimensions, we argue the need to increase the degree (the spatial comprehensiveness, frequency, and integration of components) to which the lake is assessed.