Pollen and phytolith paleoecology in the St. Louis River Estuary, Minnesota, USA, with special consideration of Zizania palustris L.

Abstract

Development of the catchment of the St. Louis River Estuary (SLRE) since Euro-American settlement resulted in quantifiable impacts to the aquatic ecosystem. Zizania palustris L. (northern wild rice) is an important food commodity for the state of Minnesota and the SLRE contains several wild rice stands. Known threats to SLRE wild rice stands include excessive sediment and nutrient loading. This paleoecological study combined pollen and phytolith analyses from five sediment cores from the SLRE to investigate three objectives: (1) enhance identification procedures by distinguishing Z. palustris pollen from other aquatic and upland grasses present in the fossil record; (2) reconstruct the 250-year history of upland and aquatic plants with a particular focus on wild rice; and (3) establish the utility of these indicators for paleolimnology studies of plant communities in a lotic system. Pollen and phytolith analyses of sediment samples yielded paleo-environmental data that confirmed that wild rice microfossils can be conclusively identified from sedimentary records and that Z. palustris was continuously present in the SLRE since the mid-1700s. Although our core sites were not ideally situated for assessment of wild rice history, we documented reorganization of Zizania stands apparently associated with shifts in water level. We recommend that retrospective studies for wild rice be based on sediment collected within or closer to littoral environments that support wild rice stands.