Assessment of the hypolimnetic withdrawal system at Pine Lake, Alberta

Abstract

Sosiak A. 2021. Assessment of the hypolimnetic withdrawal system at Pine Lake, Alberta. Lake Reserv Manage. 38:47–66. In 1998, the Pine Lake Restoration Society (PLRS) installed a gravity-fed hypolimnetic withdrawal (HW) system in 4.13 km2 Pine Lake, Alberta, which had severe cyanobacterial blooms. Under HW, median euphotic total phosphorus (TP) declined by 27–32%, and chlorophyll a by 37–41%, typical of HW projects elsewhere. Furthermore, cyanobacterial blooms were less severe during HW. Decreasing hypolimnetic TP and internal loads suggest sediment P release declined in at least the middle basin under HW. There was no evidence of increased hypolimnetic temperatures during HW, which could have enhanced sediment P release. Summer hypolimnetic anoxia declined in the middle and south basins, while in winter anoxia declined in the north and middle basins. No change in TP occurred in 9 Alberta reference lakes, and external P loadings were not reduced by management efforts at Pine Lake, which suggests improvements were due to HW. After N declined during HW in Pine Lake, cyanobacteria shifted from a diverse community with species that fix atmospheric N, and others without that capacity, to one dominated by Aphanizomenon flos-aquae. Correlation analysis found Pine Lake blooms were associated with euphotic TP, total nitrogen (TN), earlier spring ice breakup, and higher air temperatures. Due to increased rainfall, the PLRS was able to operate the system more often than predicted. Pine Lake remains productive and the goal of mesotrophic productivity was not achieved. However, the anoxia analysis suggests lake productivity should decline appreciably by 2032 with continued HW operation. HW provides an affordable option for stratified lakes with high internal P loading. Supplemental data for this article is available online at https://doi.org/10.1080/10402381.2021.2001609 .