The effect of a flood pulse on the water column of western Lake Superior, USA

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, > 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 phosphorus 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.