Gene's Corner: Aquatic Macrophytes – Benefits and Problems

Written July 2024, by Gene Welch, Professor Emeritus, Civil and Environmental Engineering, University of Washington

Aquatic plants or “macrophytes” provide benefits to lakes by supplying habitat and food for fish and invertebrates that fish feed on. Most aquatic macrophytes are rooted if bottom sediments are stable with sufficient organic matter. Also, most are native (indigenous) to the US, like the pond weeds (Potamogetons). But a few are non-native and can be invasive. Moses Lake has two non-native species, Potamogeton crispus (curly leaf pond weed) and Myriophylum spicatum (Eurasian water milfoil). Both are from Europe. Muenscher’s Aquatic Plants of the United States (1944) lists P. crispus but not M. spicatum among the ten species of Myriophylum present in the US at the time. Interestingly, Eurasian water milfoil does not create a nuisance in Europe, but it has in the US in recent decades. It has invaded large expanses of shallow areas of lakes and reservoirs. Why exotic species are so invasive is not well understood.

Milfoil has another bad habit. It, like most rooted aquatic plants, obtains its nutrients from surrounding water through its leaves, but most of its phosphorus is absorbed from bottom sediments through its roots. When the plant dies, often during the summer, its phosphorus is released as part of internal loading. Internal loading was half of the total loading to Moses Lake in 2020 and 2021. However, most of that was due to chemical-physical release of phosphorus from bottom sediments as indicated by past experiments with sediment cores.

A large portion of the Moses Lake is available to colonize milfoil and other aquatic plants. Macrophytes like lots of light, so their growth tends to be limited below the depth of water transparency, which has averaged 1.7 m over the past 7 years. Transparency is measured with a Secchi disc by MLIRD. About 20% of the lake area is above that depth with usually enough light for rooted macrophytes, although some of the shallow area is hard surface and not easily colonized.

Effects of exotic macrophyte on lake quality and use are often mixed. Reduction of milfoil with herbicide in a Tennessee reservoir resulted in less habitat for invertebrate food organisms for fish, not from toxicity of the herbicide. Harvesting as much as 69% of the exotic Brazilian elodea (Egeria densa) in Long Lake, Kitsap County, in the 1990s resulted in increased phosphorus in the lake, not less. Apparently, the dense coverage of the plant minimized the effect of wind mixing in this shallow lake that otherwise enhanced phosphorus release from sediment as internal loading. Also, this plant did not die until fall, unlike milfoil, so phosphorus from its decomposition had little effect on algal production. On the other hand, fishing for crappie by trolling a spinner/fly was effective among native pond weeds in the 1960s (personal experience), but was no longer possible after the exotic and invasive Brazilian elodea expanded in the 1980s. Dumping a fish bowl in the lake was rumored as the source of the invasive plant.

The attitude in Europe toward aquatic macrophytes is positive. Reducing phosphorus input, resulting in less algae, clearer water, and an increase in rooted macrophytes is considered a goal of lake management. The reverse is increased phosphorus (eutrophication) causes increased algae, which decreases light penetration restricting rooted macrophytes. This reversing process occurred in Long Lake-Kitsap: low phosphorus following an alum treatment, decreased algae and increased macrophytes, in this case Brazilian elodea. Long Lake is shallow with an average depth of 2 meters, so most of the lake could be colonized by rooted macrophytes if light penetration increased. However, most lake users did not want elodea or algae, which was difficult to achieve in a shallow lake with internal phosphorus loading.

So rooted macrophytes can provide pluses and minuses. They may provide refuge and food for fish and opportunities for fishermen, but also create a nuisance for lake users if too abundant, especially if they are invasive exotics. Management decisions may vary depending on how much of the lake area is shallow enough for rooted plants to colonize, their density and distribution, and whether they are exotic/invasive or indigenous.

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