Lake Ontario water quality during the 2003 and 2008 intensive field years and comparison with long-term trends K. T. Holeck, 1,* L. G. Rudstam, 1 J. M. Watkins, 1 F. J. Luckey, 2 J. R. Lantry, 3 B. F. Lantry, 4 E. S. Trometer, 5 M. A. Koops, 6 and T. B. Johnson 7 1 Cornell Biological Field Station, Department of Natural Resources, Cornell University, 900 Shackelton Pt. Road, Bridgeport, New York 13030, USA 2 United States Environmental Protection Agency, Region 2, 290 Broadway, New York, New York 10007-1866, USA 3 New York State Department of Environmental Conservation, Lake Ontario Fisheries Research Unit, Cape Vincent Fisheries Research Station, 541 East Broadway, P.O. Box 292, Cape Vincent, New York 13618, USA 4 U.S. Geological Survey, Great Lakes Science Center, Lake Ontario Biological Station, 17 Lake Street, Oswego, New York 13126, USA 5 United States Fish and Wildlife Service, Lower Great Lakes Office, 1101 Casey Road, Basom, New York 14013, USA 6 Great Lakes Laboratory for Fisheries and Aquatic Sciences, Fisheries and Oceans Canada, 867 Lakeshore Road, Burlington, Ontario L7R 4A6, Canada 7 Ontario Ministry of Natural Resources, Glenora Fisheries Station, RR #4, 41 Hatchery Lane, Picton, Ontario K0K 2T0, Canada *Corresponding author: kth1@cornell.edu Phosphorus loading declined between the 1970s and the 1990s, leading to oligotrophication of the offshore waters of Lake Ontario during that time period. Using lake-wide data from the intensive field years of 2003 and 2008 and from available long-term data sets on several trophic state indicators (total phosphorus [TP], soluble reactive silica [SRSi], chlorophyll a and Secchi disc transparency [SDT]), we tested the hypothesis that oligotrophication of the offshore waters of Lake Ontario has continued in the 2000s. Significant differences between 2003 and 2008 include higher spring (April) TP, SRSi, and SDT in 2008, lower summer (July–August) SDT in 2008, higher summer chlorophyll a in 2008, and lower fall (September) TP, SRSi, and chlorophyll a in 2008. The decline in SRSi from spring to summer was greater in 2008 than in 2003. Change point and regression analyses on the long-term data revealed no trend in spring TP since 1996, in summer chlorophyll a since 1994, in spring SDT since 1998, in spring SRSi or SRSi decline from spring to summer since 1999, or in summer SDT since 2001. Neither the comparison of the 2003 and 2008 surveys nor the analysis of the long-term data supported our hypothesis of continued oligotrophication of the offshore of Lake Ontario in the 2000s. Keywords: oligotrophication, phosphorus, silica, chlorophyll a, Secchi disc transparency, trophic state 7 Aquatic Ecosystem Health & Management, 18(1):7–17, 2015. Copyright Ó 2015 AEHMS. ISSN: 1463-4988 print / 1539-4077 online DOI: 10.1080/14634988.2015.1000787 Downloaded by [Fisheries and Oceans Canada] at 09:34 25 March 2015