Phosphorus targets and eutrophication objectives in Saginaw Bay: A 35 year assessment |
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| Affiliation: | 1. NOAA Great Lakes Environmental Research Laboratory, Ann Arbor, MI 48108, USA;2. Wayne State University, Department of Biological Sciences, Detroit, MI 48202, USA;3. University of Michigan, Cooperative Institute of Limnology & Ecosystems Research, Ann Arbor, MI 48109, USA;4. Michigan State University, Department of Fisheries & Wildlife, East Lansing, MI 48824, USA;5. Biology Department, Eastern Michigan University, Ypsilanti, MI 48197, USA;6. Department of Environmental Sciences, The University of Toledo, Toledo, OH, USA;1. Department of Biological Sciences, University of Notre Dame, Galvin Life Sciences Center, Notre Dame, IN 46556, USA;2. Institute for Great Lakes Research, CMU Biological Station, Central Michigan University, Brooks 217, Mount Pleasant, MI 48859, USA;3. Department of Biology, Central Michigan University, Brooks 217, Mount Pleasant, MI 48859, USA;1. University of Michigan Water Center, 214 S. State St., Ste. 200, Ann Arbor, MI 48104, USA;2. Michigan Tech Research Institute, Michigan Technological University, 3600 Green Ct. Ste. 100, Ann Arbor, MI 48105, USA;3. School for Environment and Sustainability, University of Michigan, 440 Church St., Ann Arbor, MI 48104, USA;1. Department of Geography, Western Michigan University, Kalamazoo, MI 49008-5424, USA;2. Key Laboratory of West China''s Environmental System, Lanzhou University, Lanzhou, 730000, China;3. Department of Geological Sciences, Case Western Reserve University, Cleveland, OH 44106-7216, USA;4. Great Lakes Environmental Research Laboratory, 4840 South State Road, Ann Arbor, MI 48108, USA |
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| Abstract: | An aggregated view of total phosphorus and chlorophyll a in Saginaw Bay indicates that concentrations of both constituents declined approximately in concert with declining total phosphorus (P) loads stabilizing by the late 1980s. A more spatially focused view reveals that total phosphorus declines outside of the Saginaw River plume, accompanied by more subtle chlorophyll a decreases. In contrast, soluble reactive phosphorus and ammonia have recently declined throughout the bay, while nitrate has remained relatively stable. Concentration data from nearshore transects do not exhibit large differences from open-water sample sites. The 440 tonne P/year target phosphorus load established in the 1978 amendments to the Great Lakes Water Quality Agreement has almost never been met, and total phosphorus concentrations regularly exceed the 15 μg/L concentration objective proposed in documentation supporting the 1978 amendments. Seasonal patterns in both total phosphorus and chlorophyll a are more pronounced in the most recent data, with peaks occurring in September–October. This apparently evolving seasonal pattern may result from seasonal changes in Saginaw River flow inputs, or seasonal variation in dreissenid mussel feeding and filtration rates. The adaptive management framework stipulated in the 2012 Great Lakes Water Quality Protocol should promote better monitoring of Saginaw Bay water quality into the future, with enhanced opportunities to better understand the factors that have maintained ongoing eutrophication symptoms. |
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