Dissolved Oxygen Dynamics in Holland Lake, MN

Abstract

The Minnesota Department of Natural Resources, Division of Fisheries, has been considering Holland Lake for stocking with brown trout. Holland Lake, a Twin Cities Metro Area lake, with a surface area of 0.14 km2 and a maximum depth of about 61 ft (18,8 m) is located in Dakota County, The lake has two shallow subbasins with a substantial amount of rooted vegetation and a deep subbasin, which is thermally suitable for brown trout, However, due to a high oxygen depletion rate in summer, the lake becomes anoxic below the surface mixed layer, A combination of high water temperatures in the surface mixed layer and low dissolved oxygen below the surface mixed layer makes it difficult for brown trout to survive in summer, For possible future aeration of the lake, the dissolved oxygen (DO) dynamics of the lake were studied. Historical records show that DO depletes at a rate of 0.47 mg.l"].day-] after spring oveliu11l in the upper stratum of the metalimnion (from 10 to 20 ft depth), such that the lake develops a negative heterograde DO profile by early July. DO depletes at a lower rate (0.22 mg.1"] .day-I) in the lower stratum of the metalinmion (from 20 to 30 ft depth). By mid-August, the entire metalimnion becomes completely anoxic, To better understand and to quantify the processes that contribute to the DO dynamics in the lake, temperature, DO, Secchi depth, photosynthetically active radiation (P AR), total suspended solids (TSS), total organic carbon (TOC), total respiration rate and chl-a concentrations were monitored or measured at three locations during the SUlIDner of 1999. The Secchi depth varied from 5 to 10 ft (1.5 to 3.0 m)in the deep subbasin and from 2.5 to 9 ft (0.75 to 2.7 m) in the easte11l shallow subbasin. The PAR measurements showed ail attenuation coefficient of about 0.25 ft-I in the deep subbasin; it varied with depth by two orders of magnitude (from 0.2 ft-) to 25 fr]) in the shallow subbasins, indicating increasing macrophyte density with depth. In the deep subbasin, the TSS concentration varied from 1 to 3 mgll with a maximum of 5 mgll in the upper stratum of the metalimnion. About 76% of the TSS were volatile, which indicates that most if not all TSS is organic material. Carbonaceous biological oxygen demand (CBOD) was estimated using the TOC profiles. The average CBOD was 18.7 mg/l, with a maximum of 24 mgll in the upper stratum of the metalimnion.