Macroinvertebrate response and internal phosphorus loading in a Michigan Lake after alum treatment

Alum treatment is a lake restoration technique that is used to address internal phosphorus (P) loading. We evaluated the macroinvertebrate density and P release rates from sediment cores in Spring Lake, Michigan, 5 yr after an alum treatment and compared the findings with conditions before and 1 yr after application. Total macroinvertebrate density recovered to the near pre-alum level after the decline that was measured in 2006. Community structure also shifted, with the dominant group changing from oligochaetes before alum treatment to chaoborids in 2010. Chironomid density in 2010 was similar to pre-alum density, but this represented a decline from an elevated density measured in 2006. Ceratopogonid density increased in 2010 compared with the prior samplings, but absolute densities were very low compared with other macroinvertebrate groups. Maximum P release rates from sediment cores in 2010 averaged from 1.68 to 2.81 mg P m d under anoxic conditions. These rates are an order of magnitude lower than before alum was applied, indicating the alum application was still effectively reducing P release rates from sediments in Spring Lake. However, the release rates have increased since 2006, suggesting that alum efficacy may be declining. The NaOH-extractable soluble reactive P fraction has increased since 2006, suggesting that the aluminum hydroxide floc is successfully binding P in the sediments. Despite the low release rates of P from the sediment, water column P and chlorophyll concentrations remain elevated in Spring Lake. This points to the continued need for reductions in external P loads to Spring Lake.