Multi-core investigation of a lotic bay of Lake of the Woods (Minnesota, USA) impacted by cultural development

The freshwater Zippel Bay (Minnesota, USA), from its headwaters to its outlet on Lake of the Woods, a larger lake, was identified as a degraded system because of augmented nutrient and sediment loads, and for low oxygen affecting aquatic life. Anthropogenic impacts were purportedly from stressors including agriculture, urban development and hydrologic modifications, so paleolimnology using multiple indicators was employed in the bay and its two primary tributaries to characterize long-term trends since European settlement of the region. Isotope analyses of the sediment cores indicated notable shifts in sedimentation rates in response to human activities, and the application of a new diatom-based nutrient model to fossil diatom assemblages indicated that phosphorus levels have approximately doubled over the last ∼150 years. While Zippel Bay appears to be naturally eutrophic, increased nutrient loads have resulted in significant algal blooms, particularly during late summer. The multiple-core approach revealed that anthropogenic activities upgradient of the two tributaries varied in their contributions to aquatic degradation in Zippel Bay, and that despite the close proximity of the cores, there has been notable spatial variation in water quality. This research also further validates the application of traditional freshwater paleolimnological methods to lotic systems with sufficient sedimentary regimes. This study has important implications for Zippel Bay as it quantifies the extent of limnological impacts and underlines the need for remediation.