Quantifying efficacy of submersed aquatic vegetation management in the Sacramento-San Joaquin Delta
Submersed aquatic vegetation (SAV) is a term used to describe vascular plants that grow completely underwater except for the flowering parts in some aquatic angiosperms. The Sacramento-San Joaquin Delta (hereby referred to as the Delta) is host to many species of SAV, both native and invasive. The most dominant SAV in the Delta is Egeria densa, an invasive macrophyte from Brazil that has been established in the Delta for approximately 30 years. This fast growing weed clogs waterways and restricts boat navigation. Studies have shown that this weed is eco-engineering the Delta, displacing native SAV species, reducing water flow and turbidity, and creating habitat that supports other invasive species (Santos et al. 2011).
Since 2001, the Department of Parks and Recreation Division of Boating and Waterways (DBW) has been designated the state lead agency in controlling invasive aquatic plants in the Delta. Currently, DBW uses a combination of hydroacoustic mapping, field surveys, and an herbicide treatment plan to monitor and control E. densa and other invasive species such as curlyleaf pondweed (Potamogeton crispus), coontail (Ceratophyllum demersum), and watermilfoil (Myriophyllum spicatum).
The hydroacoustic system used by DBW is a combination of Lowrance™ consumer-grade echosounders and a cloud-based algorithm called Biobase (www.cibiobase.com). Biobase derives SAV and GPS data from the Lowrance-collected sonar files. The Lowrance / Biobase combination has a distinct advantage over other sonar systems for mapping aquatic vegetation, having lower hardware and analysis costs as well as quicker processing times (Radomski and Holbrook 2015). In addition, Biobase outputs are adjusted to Mean Lower Low Tide for consistency across all measurements, an important feature when mapping tidal-influenced systems like the Delta. The service provided by Biobase offers vegetation point data, kriged vegetation grids, default maps and tabular data that can be viewed online or downloaded to the subscription holder. However, the default biovolume maps are .png images which are not GIS-friendly and can only be assessed qualitatively. By acquiring the processed data directly, we were able to use ArcGIS to create SAV map products, quantify change in percent biovolume and percent cover at treatment sites, and provide a metric of treatment efficacy for the 2016 season.