Effects of canopy formation by floating and submergent aquatic macrophytes on the water quality of two shallow Pacific Northwest lakes

Dense mats of aquatic macrophytes partitioned the littoral zone of two shallow lakes in Washington State into different habitats of varying water quality. Horizontal distribution of dissolved oxygen (DO) and pH in the lakes was associated with the patchy distribution of aquatic macrophytes in the lake, and the vertical distribution of DO and pH was associated with the location of the canopy in the water column. Elevated concentrations of DO and pH were observed in the canopies of submergent species, while lower DO concentrations and pH were seen both in the canopies of floating-leaved species and in the sub-canopy water of both growth forms. Open water was mixed to a greater extent than within macrophyte beds, resulting in higher sub-surface DO concentrations than beneath either submergent or floating plant canopies. Diel changes in DO and pH were associated with the growth form of the macrophytes that formed the plant canopy. Diel pH and DO changes were significant within the canopy of the submergent species that formed in the near-surface waters. Diel changes were not seen in either the surface of floating-leaved macrophytes, or in the sub-canopy of either growth form. In Keevies Lake, in Western Washington, dense canopies of overlapping floating leaves of Brasenia schreberi Gmel. reduced summer surface and sub-canopy dissolved oxygen (DO) to <2mg 1 −1 without significantly changing pH. In canopies of Potamogeton natans L., which has sub-surface, as well as floating leaves, the surface DO concentration was higher than in adjacent open water areas, but sub-canopy DO was as low as beneath B. schreberi canopies. In Bull Lake, in Eastern Washington, surface canopies of the submergent species, Ceratophyllum demersum L. and Myriophyllum exalbescens (Fern.) Jeps., regularly had DO concentrations of >30 mg 1 −1 and pH > 10. Beneath these canopies of submergent plants, DO of <1 mg −1 was common at 0.25–0.50 m, and pH was typically 1–2 units lower than at the surface