The internal and watershed controls on hypolimnetic sediment phosphorus release in Lake Auburn, Maine, USA

Doolittle HA, Norton SA, Bacon LC, Ewing HA, Amirbahman A. 2018. The internal and watershed controls on hypolimnetic sediment phosphorus release in Lake Auburn, Maine, USA. Lake Reserv Manage. 00:00–00. Lake Auburn, Maine, USA, is a historically oligotrophic lake serving as a drinking water source. In 2011–2012, epilimnetic total phosphorus (P) concentrations, turbidity, and cyanobacteria blooms increased. This water quality decline was linked to earlier onset, and increased extent, of late-summer hypolimnetic anoxia, causing internal P loading from lake sediment. This study evaluated chemical P speciation in hypolimnetic sediment, using the Psenner method for sequential chemical extractions, and watershed roles in controlling sediment P release. Surficial sediments (0–2 cm) are high in reducible Fe hydroxide (107–1267 µmol/g) and associated P (15–103 µmol/g), and relatively low in Al hydroxide (77–242 µmol/g). Previous research has shown sediment with molar Al:reducible Fe ratios <3 and Al hydroxide:reducible Fe-bound P < 25 to act as a source of P during anoxia. Lake Auburn surface sediment has molar Al hydroxide:reducible Fe hydroxide ratios between 0.2 and 1.7, and molar Al hydroxide:reducible Fe hydroxide-bound P ratios between 2.0 and 14.5, indicating high risk for internal P loading. Al concentration decreases downstream from catchment lakes and along Lake Auburn tributaries, while Fe export from upstream wetlands is high. The decrease in Al and proportionately greater increase in Fe flux to the lake and in-lake deposition of Al and Fe hydroxides are impacting sediment chemistry, increasing the lake's vulnerability to eutrophication following the onset of hypolimnetic anoxia. Understanding the significance of these critical sediment ratios and influences from watershed sources is imperative for lake managers dealing with systems having significant internal recycling of phosphorus.