Effectiveness of Best Management Practices to Reduce Phosphorus Loading to a Highly Eutrophic Lake

Abstract

Reducing nonpoint source pollution is an ongoing challenge in watersheds throughout the world. Implementation of best management practices, both structural and nonstructural, is the usual response to this challenge, with the presumption that they are effective. However, monitoring of their efficacy is not a standard practice. In this study, we evaluate the effectiveness of two wetland restoration projects, designed to handle runoff during high flow events and serve as flow-through retention basins before returning flow further downstream. The Macatawa Watershed is located in west Michigan, is heavily agricultural, and drains into Lake Macatawa, a hypereutrophic lake with total phosphorus concentrations usually exceeding 100 µg/L. We measured turbidity, total phosphorus, and soluble reactive phosphorus both upstream and downstream of these wetland complexes during base flow and storm events. While both turbidity and phosphorus increased significantly during storm events compared to baseflow, we found no significant difference in upstream vs. downstream water quality two years following BMP construction. We also measured water quality in Lake Macatawa, and found the lake remained highly impaired. Possible reasons for the lack of improved water quality: (1) The restored wetlands are too young to function optimally in sediment and phosphorus retention; (2) the scale of these BMPs is too small given the overall loads; (3) the locations of these BMPs are not optimal in terms of pollutant reduction; and (4) the years following postconstruction were relatively dry so the wetlands had limited opportunity to retain pollutants. These possibilities are evaluated.

Keywords: Lake Macatawa; best management practices; eutrophication; watershed restoration.

Figure 1

Land use/land cover in the Macatawa watershed, divided into the eight major sub-basins (separated by white lines). Box (A) (lower center): location of Lake Macatawa Watershed within the lower peninsula of Michigan. Box (B) (upper right): Middle Macatawa wetland restoration area (footprint in yellow) with the two upstream and one downstream sampling locations. Box (C): Haworth wetland restoration area (footprint in yellow) with the upstream and downstream sampling locations. Box (D): Lake Macatawa showing the five sampling locations (white dots) for water quality monitoring.

Figure 2

Daily precipitation and turbidity (NTU) during 2017 sampling season at the Middle Macatawa Upstream and Downstream sites. (A) Turbidity data collected every half hour and (B) discrete baseflow and storm turbidity measurements were taken during monthly baseflow sampling. Hourly precipitation data (panels A and B) were retrieved from the National Climatic Data Center website and summed by day.

Figure 3

SRP and TP concentrations measured at Middle Macatawa restoration site in 2017 (A,C) and over total project history (B,D). Colored data lines in (A,C) magnify the 2017 baseflow data shown in (B,D). Symbols represent storm events. Vertical dotted lines represent approximate completion date of wetland restoration construction.

Figure 4

Nitrate (${\mathrm{NO}}_3^{-}$), ammonia (NH₃), and total Kjeldahl nitrogen (TKN) concentrations measured at the Middle Macatawa restoration site in 2017 (A,C,E) and over total project history (B,D,E). See Figure 3 caption for more explanation.

Figure 5

Mean (1 SD) Middle Macatawa pre- and postrestoration nutrient value comparison at baseflow in 2017 sampling year. p-values in top left corner of each panel represent pre- vs. postrestoration statistical analysis within each site. Mac. Up = Macatawa Upstream site; P. Creek = Peters Creek; Mac. Down = Macatawa Downstream site. See Figure 1B for site locations.

Figure 6

Mean (1 SD) Middle Macatawa pre- and postrestoration nutrient comparison at storm flow as of 2017 sampling year. p-values in top left corner of each panel represent pre- vs. postrestoration statistical analysis within each site. Mac. Up = Macatawa Upstream site; P. Creek = Peters Creek; Mac. Down = Macatawa Downstream site. See Figure 1B for site locations.

Figure 7

SRP and TP concentrations measured at Haworth wetland for 2017 (A,C) and total project history (B,D). Colored data lines in (A,C) magnify the 2017 baseflow data shown in (B,D). Symbols represent storm events. Vertical dotted lines represent approximate completion date of wetland restoration construction.

Figure 8

Nitrate (NO₃⁻), ammonia (NH₃), and total Kjeldahl nitrogen (TKN) concentrations measured at the Haworth wetland for 2017 (A,C,E) and total project history (B,D,E). See Figure 7 caption for more explanation.

Figure 9

Haworth pre- and postrestoration water chemistry comparison at baseflow as of 2017 sampling year. Error bars represent 1 SD. p-values in top left corner of each panel represent pre- vs. post-restoration statistical analysis within each site. See Figure 1C for site locations.

Figure 10

Haworth pre- and postrestoration water chemistry comparison at storm flow as of 2017 sampling year. Error bars represent 1 SD. p-values in top left corner of each panel represent pre- vs. postrestoration statistical analysis within each site. See Figure 1C for site locations.

Figure 11

SRP, TP, and Chl a concentrations measured at the five monitoring stations in Lake Macatawa from 2013 through 2017. The red horizontal lines on TP indicate the interim TMDL goal of 50 μg/L; red horizontal lines on Chl a figures indicate the hypereutrophic boundary of 22 μg/L used by MDEQ for assessing chlorophyll in Lake Macatawa (Holden 2014). Summer 2016 site 4 bottom depth SRP sample (B, asterisked) is a likely outlier due to sediment disturbance.