Combining quadrat, rake and echosounding to estimate submerged aquatic vegetation biomass at the ecosystem scale

Measuring freshwater submerged aquatic (SAV) biomass at large spatial scales is challenging and no single technique can cost effectively accomplish this while maintaining accuracy. We propose to combine and intercalibrate accurate quadrat-scuba diver technique, fast rake sampling and large scale echosounding. We found that the relationship between quadrat and rake biomass is moderately strong (R2 = 0.62, RMSECV = 2.19 g/m2) and varies with substrate type and SAV growth form. Rake biomass was also successfully estimated from biovolume10 and its error (R2 = 0.53, RMSECV = 5.95 g/m2), a biomass proxy derived from echosounding, at a resolution of 10 m radius from rake sampling point. However, the relationship was affected by SAV growth form, depth, acoustic data quality and wind conditions. Sequential application of calibrations yielded predictions in agreement with quadrat observations, but echosounding predictions underestimated biomass in shallow areas ( 3 m). Whole-system biomass was more accurately estimated by calibrated echosounding than rake point surveys, owing to the large sample size and better representation of spatial heterogeneity of echosounding. We recommend developing as a one-time event a series of quadrat and rake calibration equations for each growth form and substrate type. Because the relationship between biovolume and biomass depends on SAV growth form, rake and echosounding calibration needs to be conducted frequently. With the two calibrations, rake can thus be used as a rapid ground truthing or in shallow areas where echosounding is inadequate.