Notes The effect of a flood pulse on the water column of western Lake Superior, USA Elizabeth C. Minor a, ⁎, Brandy Forsman b,1 , Stephanie J. Guildford c a Large Lakes Observatory and Dept. of Chemistry and Biochemistry, 2205 East 5th St, University of Minnesota Duluth, Duluth, MN 55812, USA b Department of Chemistry and Biochemistry, University of Minnesota Duluth, USA c Large Lakes Observatory and Dept. of Biology, 2205 East 5th St, University of Minnesota Duluth, Duluth, MN 55812, USA abstract article info Article history: Received 23 September 2013 Accepted 14 March 2014 Available online 21 April 2014 Communicated by Robert McKay Keywords: Flood Nutrients Chlorophyll Lake Superior PAR On June 19 and 20, 2012, western Lake Superior was impacted by a “mega-rain event” that raised lake levels by 8 to 10 cm. Within the flood plume on June 21, 2012, total suspended solids, total phosphorus, and soluble reactive phosphorus concentrations were elevated, with measurements of 87 mg/L, N 100 μg/L, and 5.8 μg/L, respectively. Despite the initially high phosphorus loadings, little impact was seen on water column particulate chlorophyll content, which remained in the range 0.7–1.9 μg/L, in the weeks to months following the flood. Both total phos- phorus and soluble reactive phosphorus levels tracked those of total suspended solids, returning to background levels within two weeks. However, the availability of photosynthetically available radiation (PAR) was impacted for a month after the flood event, due mainly to colored dissolved organic matter that remained in the surface layer of the stratified lake water column. It appears that the mismatch in timing of nutrient and light availability acted as a check on phytoplankton biomass production in the flood-impacted portion of the lake. © 2014 International Association for Great Lakes Research. Published by Elsevier B.V. All rights reserved. Introduction Heavy rainfall events in the United States, including the Midwestern region, are exhibiting increases in prevalence and intensity (Angel and Huff, 1997; Karl and Knight, 1998; Villarini et al., 2011). Globally, large-basin 100-year flood events are also increasing in frequency and modeling predicts them to become more common as climate continues to change (Milly et al., 2002). The impacts of heavy rainfall and dis- charge events on receiving basins, however, remain poorly constrained. The 2012 “Solstice Flood” in the western Lake Superior watershed pro- vided an opportunity to investigate the effects of this 100-year flood event on its receiving basin. On June 19–20, 2012, a “mega-rain event” delivered up to 25 cm of rain to northeastern Minnesota and northwest- ern Wisconsin (www.climate.umn.edu/doc/journal/mega_rain_events. htm, accessed June 2013), with recorded instantaneous rainfall of up to 5 to 10 cm per hour (Czuba et al., 2012). This event, occurring after an already wet spring, caused flooding and damage in nine counties in Minnesota to the extent that they were the subject of a Presidential Di- saster Declaration. Thirteen USGS stream gages recorded their highest recorded streamflows during this storm, including the St. Louis River gage at Scanlon, which has been active for over 100 years (Czuba et al., 2012). The rainfall and subsequent river, stream, and overland flow significantly impacted Lake Superior, the world's largest freshwa- ter lake, raising the overall water level by 8 to 10 cm based upon aver- aging data from two USGS water level stations: 9099064 (Duluth) and 9099004 (Point Iroquois), located, respectively, at the western and eastern ends of the lake (Austin, Jay. A., Large Lakes Observatory and Dept of Physics, University of Minnesota Duluth, personal communica- tion, February 12, 2014). The inputs of total suspended solids and col- ored dissolved organic matter had visibly stained nearshore regions in the far western lake (turning them a bright orange color) by mid-day on June 21. By June 26 (Fig. 1) these inputs impacted over half the areal extent of the far western arm (the region to the west of the Apostle Islands, south of Station SU19 in Fig. 1). Lake Superior, the receiving basin for the June 2012 flood event, is responding rapidly to climate change in terms of increasing surface temperatures and duration of summer stratification (Austin and Colman, 2008). However, it has experienced minimal basin disturbance, e.g., few anthropogenic inputs, little impact from invasive species as of yet, and little recent change in land use (Dobiesz et al., 2010). It can therefore give a fairly clear response to climatic stress. Located along the border between the United States and Canada, Lake Superior is the largest freshwater lake on Earth by area (Herndendorf, 1990); its basin contains approximately 10% of our planet's surface freshwater (Cotner et al., 2004). In addition, its position in the Laurentian Great Lakes system, which has a total area of about 242,000 km 2 (Einsele et al., 2001) makes Lake Superior regionally important, both as a remarkably pristine end-member for comparison with the less remote Great Lakes, and as a source of water to the Journal of Great Lakes Research 40 (2014) 455–462 ⁎ Corresponding author. Tel.: +1 218 726 7097. E-mail addresses: eminor@d.umn.edu (E.C. Minor), fors0063@d.umn.edu (B. Forsman), sguildfo@d.umn.edu (S.J. Guildford). 1 Tel.: +1 218 726 7492. http://dx.doi.org/10.1016/j.jglr.2014.03.015 0380-1330/© 2014 International Association for Great Lakes Research. Published by Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Great Lakes Research journal homepage: www.elsevier.com/locate/jglr