Guest Blog: Correlations between EcoSound Biovolume and Aquatic Plant Biomass

Andrew W. Howell and Dr. Robert J. Richardson
 
North Carolina State University; Dept. Crop and Soil Sciences
 
Why do we want to sample submersed vegetation biomass using sonar?

Invasive aquatic plants, such as non-native hydrilla (Hydrilla verticillata), negatively impact waterway systems in the southeastern United States and on a global scale. Often, these aquatic weed species impede recreational activities, power generation, and disrupt native ecological systems. Costs associated with aquatic weed management include expenses accompanied with monitoring, mapping, and implementing control measures. Prompt detection and accurate mapping of submersed aquatic vegetation (SAV) are critical components when formulating management decisions and practices. Therefore, SAV management protocols are often reliant upon the perceived extent of invasion. Traditional biomass sampling techniques have been widely utilized, but often require significant labor inputs, which limits repeatability, the scale of sampling, and the rapidness of processing. Advances in consumer available hydroacoustic technology (sonar) and data post-processing offer the opportunity to estimate SAV biomass at scale with reduced labor and economic requirements.

The objectives of this research were to document the use of an off-the-shelf consumer sonar/gps chartplotter to: 1) describe and characterize a relationship between hydroacoustic biovolume signature to measured hydrilla biomass; 2) develop algorithm for on-the-fly assessment of hydrilla biomass from interpolated biovolume records; 3) define seasonal hydrilla growth patterns at two NC piedmont reservoirs; and 4) create a visual representation of SAV development over time. From these objectives, the expected outcome was to describe a protocol for passive data collection while reducing the economic inputs associated with labor efforts involved in biomass sampling and post-processing evaluations. In our research, a Lowrance HDS-7 Gen2 was utilized to correlate biomass from monospecific stands of hydrilla within two different North Carolina piedmont reservoirs using BioBase 5.2 (now marketed as EcoSound – www.biobasemaps.com), cloud-based algorithm to aid in post-processing.

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Legacy applications of commercial sonar

At Contour Innovations we stand on the shoulders of giants who proved commercial depthfinders are precise scientific instruments for the measurement of aquatic plant abundance and distribution in lakes.   As early as 1980, researchers saw the potential for fathometers/chart recorders/depth finders/sonar/echosounders – whatever you want to call them – to substantially reduce time, effort, and cost in assessing aquatic plant communities in lakes (Maceina and Shireman 1980).

The commercial sounders of the 1980’s had only a fraction of the power and resolution of what Lowrance manufactures today (not to mention integration with GPS) and investigators still boasted of the quality and cost-effectiveness of the data acquired.  Here are some excerpts:

Maceina and Shireman (1980): “The principle advantage of utilizing a recording fathometer for vegetation surveys is that savings in time and manpower can be accomplished.  In Lake Baldwin, 14 transects covering a total distance of 11.3 km were completed in three hours.” p 38.

Duarte (1987): “Direct harvesting is an expensive and time-consuming procedure (see Downing and Anderson 1985).  Two SCUBA divers require 20 min on average to harvest the biomass of six replicate quadrats at a single depth.  In contrast, six replicate echosounder transects require only 8-35 min to obtain biomass estimates for all depths, with the actual time required dependent on the littoral slope and the depth to which the plants grow.  Additional advantages of the echosounder method are (1) a continuous record of the vegetation, rather than at discrete depths only, with the latter resulting in inaccuracies when the mean biomass values are estimated, (2) nondestructive sampling, which allows monitoring of the growth of stands over time and (3) simultaneous recording of other variables such as percent cover (Stant and Hanley 1985), volume occupied by the submerged vegetation, and littoral slope (Duarte and Kalff 1986), which influences macrophyte biomass.” p. 734

In fact, Duarte (1987) publishes biomass prediction equation from acoustic estimates of plant height (a ciBioBase output) for 22 aquatic plant species.

Thomas et al. (1990): “Fortunately, shallow range (0-7 m) chart recorders are standard on many low cost (less than $400) commercial echosounders, so the data acquisition equipment costs are relatively low with respect to fisheries acoustic assessments, which makes this procedure relatively nontechnical and very cost effective” p. 810

The concept of using commercial acoustics for mapping lake bottoms is established and proven.  Contour Innovations has refined, streamlined, and automated the methodology with ciBioBase and delivers an intuitive visualization of the complex underwater world we call littoral zones.

A Raytheon DE-719 “fathometer” relic when plant biovolume was measured on paper charts with the use of planimeters.  Photo from www.euronet.nl.
Paper chart from a Raytheon DE-719 displaying dense hydrilla canopies and bottom in a central Florida lake.  Reproduced from Maceina and Shireman 1980; J. Aquat. Plant Manage.

Classic Literature
Duarte, C.M. 1987. Use of echosounder tracings to estimate the aboveground biomass of submerged plants in lakes. Canadian Journal of Fisheries and Aquatic Sciences 44: 732-735

Maceina, M and Shireman, J. 1980. The use of a recording fathometer for determination of distribution and biomass of Hydrilla. Journal of Aquatic Plant Management 18:34-39.

Maceina, M.J., Shireman, J.V., K.A. Langland, and D.E. Canfield Jr. 1984. Prediction of submerged plant biomass by use of a recording fathometer.  Journal of Aquatic PlantManagement 22: 35-38.

Stent, C.J. and Hanley, S. 1985. A recording echosounder for assessing submerged aquatic plant populations in shallow lakes. Aquatic Botany 21: 377-394

Thomas, G.L., Thiesfeld, S.L., Bonar, S.A., Crittenden, R.N., and Pauley, G.B. 1990. Estimation of submergent plant bed biovolume using acoustic range information. Canadian Journal of Fisheries and Aquatic Sciences 47: 805-812.