Biomass Assessment

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Aquatic Plant Biomass Assessment - how green is the Assabet?

One of the goals of the state's Total Maximum Daily Load for the Assabet River is a 50% reduction in plant biomass (the mass of aquatic macrophytes and mat-forming algae) in the river as compared with the levels measured in 1999.

However, plant biomass is notoriously difficult to assess accurately and varies with flow and weather conditions. When the summer is hot and dry, there tends to be more plant growth; when the summer is cool and wet, there tends to be less plant growth. Which species dominate also varies over the season and between years. So, it is critical to have a multi-year assessment to be able to measure real changes in the river's condition above natural variation.

In 2005, OARS worked with the MA DEP to develop a biomass monitoring protocol. In August 2005, the pilot year of the assessment, field estimates of floating and rooted biomass were made on four impoundments: Allen Street, Northborough; Gleasondale, Stow; Hudson; the Ben Smith in Maynard/Stow. Throughout the four impoundments, the dominant floating plants were duckweed, watermeal, and filamentous algae; the dominant submerged plants were coontail and waterweed. Limited patches of the invasive species water chestnut were found in the Gleasondale impoundment which could be controlled by hand-pulling.

The large differences in total biomass calculated in 2005 compared with 1999 and 2000 suggest that annual variation in biomass is relatively high, the methodology relatively inaccurate, and that a long-term baseline of measurements would be needed to be able to detect future changes in biomass attributable to changes in watershed management. (2005 Biomass Report). Since 2005, we have adapted the survey protocol and repeated surveys of the Hudson and Ben Smith impoundments each summer in late August.

Remote sensing: OARS worked with USGS and University of Massachusetts staff to survey aquatic plants in the impoundments using satellite imagery ("remote sensing"). The work resulted in a useable methodology for assessing floating plant cover and differentiating types of floating plants.
(Freshwater Biology 2010:55)