Into the weeds: Aquatic macrophytes

By JIM DE RITO
Henry’s Fork Foundation Conservation Director

Aquatic macrophytes, large plants that live in water, are the primary habitat components of the Henry’s Fork in the low gradient areas of the river below Island Park Dam: Last Chance, Harriman State Park, and Pinehaven.

Macrophytes provide cover for trout, habitat and forage for macroinvertebrates, food for trumpeter swans and other waterfowl, and greatly affect the water level and flows patterns at the reach and microhabitat scales in the river. Their importance has led to research to understand how they change seasonally and annually in the Henry’s Fork and what causes these changes.

Growth of macrophytes is greatest during the spring through summer, reaching their greatest biomass in autumn. In early winter, the macrophytes begin to die off and by early spring they are at their lowest abundance and biomass before another growing season.

Young-of-the-year juvenile rainbow trout utilize macrophytes as cover during the spring through autumn, but as macrophytes die off during the winter, the juvenile trout move to find cover in boulders and cobbles or wood in the river. Adult trout likely also utilize macrophytes as cover, as well as habitat from which to feed.

Macroinvertebrates such as caddisflies, mayflies, and midges utilize macrophytes for food, shelter and egg laying. Research on other waterbodies has shown that densities of aquatic insects are greater in those areas with macrophytes than those areas without macrophytes.

Trumpeter swans feed on macrophytes throughout the year, but are most reliant upon them during the autumn and winter. Too many swans eating macrophytes can lead to a shortage later in winter. To avoid overutilization of macrophytes, waterfowl managers have attempted to disperse some swans, either by hazing or translocation, out of Harriman State Park in the winter.

Species of macrophytes in the Henry’s Fork have been placed into two groups; Group-1 species grow tall and erect and flourish in low-velocity areas with silt; and Group- 2 species are shorter, bottom- dwelling plants that are adapted to disturbed, higher velocity areas.

Group-1 species are thought to provide greater benefits for instream habitat for trout, in that they provide greater flow resistance that increases the water level at a given flow, provide more cover for fish, and are more likely to persist throughout the winter.

The seasonal and annual presence, abundance, and biomass of the two groups have varied considerably over the past 40 years in the Henry’s Fork from Last Chance through Pinehaven. Prior to 1972 and changes in water management at Island Park Dam, conditions were likely not beneficial for Group- 1 because of flow termination at the dam in early winter, creating icing in the river, followed by abrupt high releases in later winter or early spring that would scour the river bottom. After 1972, with more consistent winter flows, the macrophytes responded and peak macrophyte abundance occurred in the late 1970’s. A decline in macrophytes followed, and may have been initiated in part by the sediment release from Island Park Reservoir when it was drawn down in 1979 and then high spring flows in the early 1980s.

Through the late 1980s, macrophyte total biomass and relative composition of Group-1 species continued in a depressed state and the winter of 1989-1990 saw a large reduction as a result of an increasing number of swans. Some macrophyte recovery occurred in the early 1990s, but then a large sediment release from Island Park Reservoir occurred in autumn 1992, negatively affecting macrophytes for the next several years. In 1999, during the most recent survey, macrophytes had recovered to pre-sediment release 1992 levels, but were still very low compared to the late 1970s period.

Several recommendations have been made by researchers to benefit macrophytes: manage Island Park Reservoir for more consistent flows throughout the winter, avoid severe drawdowns of the reservoir and subsequent large sediment releases, and continue efforts to disperse waterfowl during the autumn and early winter. Winter flows for the past several years have been relatively consistent, but there is a limited amount of water available and high flows throughout the entire winter are often not possible. High flows during the early part of the winter can discourage swans from foraging and encourage movement to other areas, benefitting macrophytes. Higher flows, however, are critical in the late winter for increasing habitat for juvenile rainbow trout after macrophytes have died.

A large sediment release from Island Park Reservoir has not occurred since 1992. Avoiding severe drawdowns of the reservoir prevents large sediment releases and insures a higher volume of water at the end of the irrigation season. More water in the reservoir at this time provides greater flexibility with the amount of water that will be available for release in the winter.

The Idaho Department of Fish and Game has conducted waterfowl hazing on the Henry’s Fork through the 1990s and translocation of swans last occurred in the winter of 2004 – 2005. A trend of increasing swan numbers in Idaho, Wyoming, and Montana, and likely on the Henry’s Fork, from 1972 – 2007 will necessitate continuing efforts to disperse swans from the Henry’s Fork in the autumn and winter. A repeat survey of macrophytes in the Henry’s Fork is the next step to assessing their current condition and future management options.

This is part of the online edition of Henry's Fork Country.

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