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.

Continue reading “Guest Blog: Correlations between EcoSound Biovolume and Aquatic Plant Biomass”

Optimal Percent SAV Biovolume? 50% is a Good Start

At Contour Innovations we’ve long argued the importance of objectively assessing submersed aquatic vegetation (SAV) abundance to better inform management decisions.  Our last blog post discussing a recent controversy over the role of herbicides in indirectly affecting fisheries declines in Wisconsin reinforces why this is so important.  When we talk abundance per se, we need a metric that is quantitative, yet is intuitive.   The percent of the water column taken up by vegetation growth (i.e., percent “biovolume”) represents such a metric and is the primary variable that is mapped in ciBioBase.  Zero means no growth (blue).  100% represents growth all the way to the surface (red; Figure 1).

SAV, Aquatic Vegetation map, Lowrance HDS, Surface growing vegetation
Figure 1. SAV Biovolume map (left), boat tracks (red lines), boat location (red dot), and sonar chart of vegetation growing to the lake surface on Orchard Lake, MN.

Zero is undesirable in lake environments where vegetation growth is natural or where an artificial lake is managed for vegetation-dependent fisheries (e.g., largemouth bass or northern pike).  No vegetation growth can also cause and be an effect of water quality impairments as discussed here).  In contrast, 100% is undesirable from an aquatic recreation standpoint because props get tangled up and it’s difficult to navigate your boat through surface mats of vegetation (Figure 2).

Figure 2. Aquatic Vegetation (100% Biovolume) growing all the way to the water surface on Orchard Lake, MN and impediments to motorized recreation. 

If no plant growth is bad (0%), but plant growth all the way to the surface (100%) is bad, then good MUST be somewhere in between.  Indeed!  From a Fisheries standpoint, 40-60% average biovolume is good because there is habitat for vegetation-dependent species like largemouth bass, bluegill, northern pike, and indicator species like blackchin shiners that are sensitive to vegetation loss (Figure 3).

Figure 3.  Probability of sampling blackchin shiners as a function of increasing SAV % biovolume  in Square Lake, MN (Adapted from Valley et al. 2010 Hydrobiologia 644:385-399)

From a water quality standpoint, 40-60% biovolume is sufficient to anchor sediments and will promoting better water clarity than if nothing was growing.  Finally, 40-60% biovolume means that most growth is below the depth of your outboard prop and thus you generally won’t encounter the situation as seen in Figure 1.

A case study in MN, WI, NC, and FL lakes

CI is currently involved in a collaborative research project where acoustic data with Lowrance HDS was passively collected while conducting point-intercept surveys.  Acoustic data (.sl2 files) were uploaded to ciBioBase and the Biovolume value for each species survey point was extracted from the exported raster grid (“Extract Value From Point” in the Spatial Analyst Toolbox in ArcGIS or see our Point-Intercept on Steroids blog).  Figure 4 displays a wealth of information about the status of plant growth and management in the surveyed lakes.  With on-the-fly data entry for the plant species surveys and uploading of the .sl2 file to ciBioBase, a similar graph could be produced within hours of finishing a survey, and thus facilitating informed and rapid decision making.

Figure 4.  Biovolume at invasive species sample points and native sample points free of invasive species.  Non-vegetated sites are not included in the analysis.  Lakes range from intermediate nutrient levels, Mesotrophic (M), to high nutient levels, eutrophic (E).  Berry, Gibbs, Swan, Wingra, and Round are in WI; Gray’s, Gideon’s, and St. Alban’s Bays are bays of Lake Minnetonka, MN; Waccamaw is NC; Tracy, Kissimmee, Istokpoga are FL lakes.  All MN and WI lakes are infested with Eurasian watermilfoil.  All NC and FL lakes are infested with Hydrilla.  Waccamaw is bog stained and the hydrilla is a recent infestation

Specifically this graph tells us the following:

  1. Invasives grow closer to the surface of lakes than natives and growth seems to be highest in lakes of intermediate productivity (meso-eutrophic)
  2. Natives appear to grow at the 40-60% biovolume level regardless of productivity.
  3. Native growth can be an objective benchmark from which to judge the success of invasive management in non-eradication management regimes.
  4. Aquatic Plant management was successful at bringing down invasive growth to the level of natives in Gray’s Bay of Lake Minnetonka, Kissimmee, and Istokpoga
Something as simple as what is displayed in Figure 4 can bring an objective point of reference to the table when discussing the often controversial nature of aquatic plant management.  With data such as these, discussions by various user and management groups can center on the acceptable level of growth to meet Fisheries, Water Quality, and Invasive Species management goals (which we argue can occur at some intermediate level of plant growth).  Without both species AND abundance data, various factions will continue to take up positions with anecdotal evidence that support their prejudices and the discourse will never get to where it needs to be to tackle these important water resource issues.

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.

Patterns of aquatic plant species domination

In an earlier blog post, we informed you of collaborative research in which CI is involved.  We’ve touched on how species presence/absence surveys using methods like point-intercept and full system acoustic surveys of abundance can be combined to fully understand the dynamics of aquatic plant communities and how they are responding to range of “forces.”  These forces may be natural like seasonal or interannual variability, human induced but unintentional like accelerated eutrophication, or the introduction of invasive species, or intentional management interventions to control nuisance aquatic plant growth.  Whatever the case, entire lake ecosystems are likely to be affected these forces including plant species composition, abundance, and spatial patterns of plant growth.

We can generally expect a bell curve-like response of plant growth at differing levels of productivity (Figure 1).  In nutrient-poor oligotrophic lakes, aquatic plants are typically never very abundant because of nutrient limitations or sediment hardness.  At the other side of the spectrum in overly productive or hypereutrophic systems, the lake is often too murky from algae growth or sediment suspension to support much plant growth.  Goldilocks finds her sweet spot in moderately productive meso- or eutrophic lakes (Figure 1).  The cumulative effects of various stressors continually move the ball towards the right of the productivity curve where thresholds are being approached and sometimes breached.  We’ve spoken about this resilience issue also in a previous post.

Figure 1. Conceptual model describing general patterns of aquatic plant abundance  in  shallow to moderately  deep lakes as a function of lake productivity.  O = Oligotrophic or low nutrient levels; M = Mesotrophic or moderate nutrient levels; E = Eutrophic or high nutrient levels; HE = Hypereutrophic or really high nutrient levels.

Likewise, we could replace the Y-axis in Figure 1 with Species Richness and we’d have the same conceptual model and predictions for how lakes should respond to environmental or human stressors.  Maybe this brings back memories of the Intermediate Disturbance Hypothesis a la Connell (1978) for our readers with an academic history in Ecology?

Having understood these patterns, researchers and managers have done much work to assess aquatic plant communities, make prescriptions on their management or conservation, and evaluate outcomes of management efforts.  Still, assessment techniques have generally been focused either on species occurrence patterns or gross plant abundance patterns but rarely both, and especially at the whole-lake scale.
For instance, the point-intercept method has been used to describe species occurrence patterns in many systems throughout the upper Midwestern US (Madsen et al. 2002, Beck et al. 2010, Mikulyuk et al 2010, Valley and Heiskary 2012).  Indeed, this work and many other studies not cited here has contributed great knowledge on factors contributing patterns of what species grow where.  But they can’t tell us “how much.”
In contrast, hydroacoustics assessments of plant abundance has shed light on how various factors affect patterns of plant abundance in lakes (Valley and Drake 2007, Winfield et al. 2007, Zhu et al. 2007, Sabol et al. 2009, Netherland and Jones 2012).  So hydroacoustics can tell us “how much” but generally not what species grow where unless you are dealing with monocultures.
Duh! Combine results from both methods!
Although it seems obvious regarding the proper solution, prior to today, there were many budget and technological difficulties that made combining both species and abundance surveys at the whole lake scale not very feasible.
Most of these barriers were with the acoustic techniques.  Equipment was costly, it required a lot of specialized training to operate and make sense of the data, you needed powerful computers and a lot of data storage capacity.  
Innovations in acoustic and computing technology has smashed these barriers and now valuable high resolution data on aquatic plant abundance can be logged passively to a $650 depth finder while you conduct your species occurrence surveys.  When you return from the field, just add “upload sonar data” to your list of things to tidy up before heading home for dinner.  30-min later all the abundance data will be waiting in the queue to be combined with your frequency of occurrence species data.
Combining point-intercept and acoustic data into meaningful statistics
In our point-intercept on steroids post we described how to append a biovolume column to your point-intercept data file.  In this investigation we have now taken matters to the next step and defined a potentially useful metric (Dominance) and evaluated its utility across several Minnesota and Wisconsin Lakes and one natural North Carolina Lake (Table 1).
Table 1.  Lakes part of a collaborative study demonstrating a technique for quantifying the impact of individual species on plant abundance patterns.  Zmax = max depth in feet; Prod. = productivity as described in Figure 1.  Invasive plants include Eurasian watermilfoil (EWM), Curly-leaf pondweed (CLP), and Hydrilla (HYD).
What we define as Dominance is a metric that ranges from 0 (no plant growth at all) to 1 (surface growth of one species).  The number of species combined with the biovolume at a survey point determines the dominance value.  So at each survey point:
Species1 / Total Species* x Biovolume = Dominance
*Excludes emergent and free-floating species
This means that 10 species sampled at point X with a biovolume of 100% only gets a value of 0.1.  In many natural glacial lakes, surface growth of aquatic plants is common in shallow areas, but typically, many species contribute to the local assemblage.  In a disturbed or invasive dominated lake, surface growth is common but usually only 1-2 species (e.g., D = 1 and 0.5 respectively) contribute to these dense beds.
Figure 2 demonstrates what we find in lakes that range from oligotrophic, uninfested lakes to borderline hypereutrophic infested lakes.
Figure 2. Patterns in aquatic plant growth in lakes that span a range of productivity (ordered from left to right – see Figure 1 for productivity definitions).  BVw is the average total biovolume in the surveyed areas generated from ciBioBase grid reports. Freq. Monocultures is the frequency of species survey points that had only 1 or 2 species and growth was near the water surface.  BV Natives is the biovolume of species survey points where only native submersed or floating leaf plants were growing.  BV Invasives is the biovolume at sites with invasive species present.
First, with the exception of bog stained Waccamaw that naturally depresses plant growth, the overall abundance of plants as expressed as average biovolume (Blue bars- BVw) by in large follows the bell shaped curve in Figure 1.  Second, the biovolume where only native plants grow is pretty stable across all lake types (again excluding Waccamaw) and invasives (in this case Eurasian watermilfoil) push the biovolume higher.  This patterns of biovolume at surveyed points give us another quantitative indicator about the actual impact of invasives and could serve as a benchmark for management objectives.  Third, the red bars tell us how frequent during each survey we saw surface growing beds of one species.  Interestingly, the frequency increases as the lakes become more productive with invasive plants.
Lake Wingra – an extreme example of Eurasian watermilfoil domination

Lake Wingra is a shallow, eutrophic lake near the campus of University of Wisconsin in Madison, Wisconsin.  Wingra resides in an urban watershed and the lake today is a reflection of a long legacy of watershed and in lake impacts from high runoff, sedimentation, and invasive species proliferation such as common carp and Eurasian watermilfoil.  More information on this lake can be found here.
Today, the lake is dominated by Eurasian watermilfoil (there’s that word again: dominated).  What we are doing now is putting numbers behind this descriptive word so the situation can be improved.
So what does “domination” mean in Wingra?  It means that 50% of the sampled points in the lake had only Eurasian watermilfoil or one other species growing to the surface (Figure 2).  It means that 130 acres of the 281 total acres mapped (45%) were essentially surface-growing monocultures of Eurasian watermilfoil (Figure 3).  These represent objective benchmarks that form the foundation of solutions.  It’s probably not a stretch to assume that 129 acres of surface growing Eurasian watermilfoil is not desirable.  With the tools described here local managers and citizens can work out what is desirable and take measures to get there.  But getting there requires objective, repeatable assessment methods that shed light on both species AND abundance patterns.
Figure 3.  Contours (yellow) delineating the extent of surface-growing Eurasian watermilfoil  beds on Lake Wingra (Dane Co. WI).  The background map is a heat map of aquatic plant biovolume collected with a Lowrance HDS-5 and processed with ciBiobBase.  Areas of red is vegetation growth near the surface.  The few red areas outside of the yellow contour lines represent areas where 1 or more native species contributed to the surface growth.

The future: national risk assessment models
Contour Innovations is currently developing data import capabilities to overlay species surveys on ciBioBase maps.  This will have immediate local benefits for our clients, but the real power of such functionality is the building of a powerful national database of species and abundance surveys.  This can lead to independent research efforts to model aquatic plant growth patterns and model risk of certain aquatic systems to domination by an invasive aquatic plant species.  But a critical mass of cooperation by the water and fisheries resource community including academia and public and private institutions is needed to develop robust models.  Contact us if you are interested in being a part of this effort.  We will be taking this concept to the road at the Midwest Aquatic Plant Management Society meeting in Cleveland, Western Aquatic Plant Management Society in Idaho, Minnesota Chapter of the American Fisheries Society in St. Cloud MN and other to be determined venues.
Ray Valley
Chief Aquatic Biologist
Literature Cited
Beck, M. W., L. Hatch, B. Vondracek, and R. D. Valley. 2010. Development of a macrophyte-based index of biotic integrity for Minnesota lakes. Ecological Indicators 10:968-979.
Connell, J. H. 1978. Diversity in tropical rain forests and coral reefs. Science 199:1302–1310.
Madsen, J. D., K. D. Getsinger, R. M. Stewart, and C. O. Owens. 2002. Whole Lake fluridone treatments for selective control of Eurasian watermilfoil: II. impacts on submersed plant communities. Lake and Reservoir Management 18:191–200.
Mikulyuk, A., J. Hauxwell, P. Rasmussen, S. Knight, K. I. Wagner, M. E. Nault, and D. Ridgely. 2010. Testing a methodology for assessing plant communities in temperate inland lakes. Lake and Reservoir Management 26:54–62. doi: 
Sabol, B. M., J. Kannenberg, and J. G. Skogerboe. 2009. Integrating Acoustic Mapping into Operational Aquatic Plant Management : a case study in Wisconsin. Journal of Aquatic Plant Management 47:44–52.
Valley, R. D., and M. T. Drake. 2007. What does resilience of a clear-water state in lakes mean for the spatial heterogeneity of submersed macrophyte biovolume? Aquatic Botany 87:307–319.
Valley, R. D., and S. Heiskary. 2012. Short-term declines in curlyleaf pondweed in Minnesota: potential influences of snowfall. Lake and Reservoir Management 28:338–345.
Winfield, I. J., C. Onoufriou, M. J. O’Connell, M. Godlewska, R. M. Ward, A. F. Brown, and M. L. Yallop. 2007. Assessment in two shallow lakes of a hydroacoustic system for surveying aquatic macrophytes. Hydrobiologia 584:111–119.
Zhu, B., D. G. Fitzgerald, S. B. Hoskins, L. G. Rudstam, C. M. Mayer, and E. L. Mills. 2007. Quantification of historical changes of submerged aquatic vegetation cover in two bays of Lake Ontario with three complementary methods. Journal of Great Lakes Research 33:122–135.

Point-Intercept on Steroids

Who would’ve known that an obscure technical report describing a sampling methodology would become a classic in the world of Aquatic Plant Management and be adopted as a standard by lake service providers and government agencies?  Although it was old hat in the world of terrestrial Botany and Forest Ecology, Dr. John Madsen appeared to be the first to make point-intercept a standard tool for aquatic ecologists and lake managers with his Army Corps of Engineers Technical Note No MI-02 published in 1999 entitled “Point Intercept and Line Intercept Methods for Aquatic Plant Management.”

Briefly summarized, point-intercept methodology entails creating a grid of GPS points on a waterbody and traveling to those points and sampling the aquatic plants in those areas typically by throwing a double-headed rake and pulling up whatever it catches (Figure 1).

Figure 1. Contour Innovations Aquatic Biologists Jesse Amo (back) and Ray Valley (front) conduct a point-intercept vegetation survey while logging acoustic data on Orchard Lake, Dakota Co. MN.

The simplest and most objective application of the method is to simply record the presence of each species on the rake.  This does not lend much insight into how abundant each species is at each point and a mat of surface-growing vegetation gets the same weight as a lonely sprig (Figure 2).  To address this short-coming, several adaptations to the method have been made by various practitioners including ranking the abundance of different species on the rake.  Although some may argue it’s a “better than nothing” measure of relative abundance, I would argue, not much.  There is no straightforward way to objectively rank the abundance of 5 different species in a gob of plant matter on a rake like seen in Figure 1.  As a consequence, results are not repeatable and four different investigators could produce four different results for the same sample.  Further a relative ranking lends little biological information about the architectural structure or canopy height of aquatic plants.

Figure 2. Conceptual figure of a point-intercept sampling point in two contrasting environments.  In the pure application of the method, if the rake intercepts the diminutive sprig in panel B, it would be given the same weight as the thick mat in panel A.
Biological processes, water quality, physical habitat and recreational conditions all hinge on the state of aquatic plant ABUNDANCE in a waterbody.  As I have described above, point-intercept or any subjective adaptation is not well suited to address aquatic plant abundance concerns.  Nevertheless, point-intercept has many strengths and one shouldn’t throw the “baby out with the bath water.”  Rather, ciBioBase offers a powerful and efficient way of getting more out of your point-intercept species sampling.
To add biovolume to your point-intercept surveys all you need is a Lowrance HDS depth finder, a $10 SD card from your favorite electronic retailer, and a subscription to ciBioBase (single lake and unlimited pricing are available).  No additional set up is necessary.  No technical mapping experience needed.  Just hit record, and jump from point to point like you’ve done in the past.  The HDS unit will passively record the GPS signal and acoustics the entire time.
After you return from the field, upload the data to ciBioBase, get a cup of coffee and catch up on some email.  Approximately 30-min to an hour later, one of the new emails in your inbox will be an alert from ciBioBase informing you that your plant abundance and bathymetric map is processed and ready for viewing.
Not only does passively logging sonar data while conducting species surveys require no additional work, but you sample important interim areas between points and get understanding of the TRUE coverage of plants (not just the frequency of plants sampled with your rake).
Unleashing the power of Point-intercept by using ciBioBase
Although ciBioBase comes with many analytical tools, its full potential to inform aquatic plant management is realized when the data is exported out of ciBioBase and into GIS for analysis with other data layers (Figure 3).
Figure 3. ciBioBase users have the option to export processed point data along their GPS track (Point) or  the uniform grid created by kriging interpolation (Grid).  Users can then import these files into GIS for further analysis with their point-intercept data layers.
By converting the ciBioBase grid text file into a Raster grid and using a “point on raster” analysis utility available both in ESRI’s ArcGIS and Quantum GIS (an open source GIS program), users can grab the biovolume value for a point-intercept sampling point (Figure 4).

Figure 4. Example of biovolume data (grid of blues, purples, and reds with increasing density or biovolume getting a “hotter” color) imported into GIS and overlain with point-intercept species data (yellow points are northern watermilfoil – a native stand-in for its unwelcome foreign cousin Eurasian watermilfoil).  The Point Sampling Tool in Quantum will extract grid values from one raster layer and attach them to a different point-layer.
In the hypothetical example in Figure 4, anywhere where milfoil is present we can see how dense the vegetation growth was at the sampled point and around it.  By using the Point Sampling Tool in Quantum that captures the biovolume grid cell value for each surveyed point, we found that for all milfoil points, average biovolume was 65% (with many points at 100% or surface growing).  For all other vegetated points, biovolume was only 45% with many less 100% values.
How can information on species abundance lead to better management decisions than presence alone?  It is generally unrealistic to eradicate most invasive species, and often a more realistic objective is to manage the abundance to an acceptable level.   Perhaps the surface growing tendency of milfoil (i.e., biovolume = 100%) is the primary management concern and that reducing “biovolume” to say, 45% with much less surface growth like other native plant species, would be a desirable result.   Presence/absence data from point-intercept surveys alone will not inform whether plant abundance is being managed within desirable levels.
Case Study: Whole-Lake Treatments of Fluridone with Both PI and Biovolume data
Valley et al. (2006) describe results of whole-lake applications of the herbicide fluridone to a nutrient-rich Minnesota Lake (Schutz Lake, Carver Co. MN).  As part of the evaluation, hydroacoustic surveys of vegetation biovolume were conducted before and after the treatments in addition to point-intercept species surveys.
The treatments reduced Eurasian watermilfoil below detection levels, but also directly or indirectly played a role in reducing the other dominant native species in the lake, coontail.  In fact, almost all submersed vegetation disappeared 1-2 years following the treatment; however, one would never get that indication by solely looking at the point-intercept statistics (Figure 5).  

Figure 5. Mean whole-lake percent vegetation biovolume from hydroacoustic surveys (bars) in Schutz Lake, Carver Co. MN from Valley et al. 2006.  Percent frequency of occurrence of all vegetation from point-intercept surveys conducted at the same time (numbers above bars).
What had occurred was a situation that went from Figure 1A to Figure 1B.  To a rake, these environments are the same, to a lake manager and concerned citizen, they are strikingly different.  Evaluating results with Point-intercept frequency sampling alone can mask unintended harm to water quality and lake resilience.
In the 2000’s, point-intercept methods gave resource managers an objective and rapid species assessment tool.  Now, ciBioBase adds a critical third dimension to these surveys with no additional effort or training. By implementing ciBioBase as a part of standard aquatic plant assessments, resource managers and citizens will be better informed about the true state of vegetation growth in a lake and how it’s changing as a result of environmental change and our management responses.

References
Madsen, J. D. 1999. Point Intercept and Line Intercept Methods for Aquatic Plant Management. APCRP Technical Notes Collection ERDC/TN APCRP-MI-02.
Valley, R. D., W. Crowell, C. H. Welling, and N. Proulx. 2006. Effects of a low-dose fluridone treatment on submersed aquatic vegetation in a eutrophic Minnesota lake dominated by Eurasian watermilfoil and coontail. Journal of Aquatic Plant Management 44:19–25.

Aquatic Plant Abundance Mapping and Resilience!

Merriam-Webster Defines resilience as an ability to recover from or adjust easily to misfortune or change.  Eminent University of Wisconsin-Madison Ecologist Dr. Steve Carpenter further adds that resilience is the ability for a system to withstand a “shock” without losing its basic functions, http://www.youtube.com/watch?v=msiIV5NdLVs

Resilience is a relatively easy concept to understand, but it can be difficult to measure in lakes without monitoring subtle changes over time.  This stresses the importance of long-term monitoring and being on guard for new changes to water quality, aquatic plants, and fish.  Volunteer networks and agencies across the country are making great strides in monitoring water quality by dropping a disk in the water and scooping up some water and sending it to a lab for analysis.  In essence, taking the lake’s “blood” sample.  Indeed, water quality samples can be very telling.  But what is happening to the rest of the lake “body”?  How is it changing in relation to its liquid diet of runoff or medication to treat invasive species?  Unfortunately, until now, natural resource agencies, lake managers, and volunteers have not had the capabilities to objectively and efficiently assess these changes without time-intensive, coarse surveys of vegetation cover.

Your body’s immune system is the engine of resilience.  When your immune system becomes compromised, you become vulnerable to a wide range of ailments that may not be a threat to someone with a healthy immune system.  The same goes for lakes.  In the glaciated region of the Upper Midwestern US and Canada, healthy lakes are those that have intact watersheds where the hydrologic cycle is in balance.  Without going into great depth, keeping water where it falls (or at least slowing it down), goes a long way in keeping the hydrologic cycle in balance.  Healthy glacial lakes also have clear water, a diverse assemblage of native aquatic plants, and balanced fish communities.  When humans or the environment alter any one of these components, the lake must adjust in order to compensate for those alterations and remain in a healthy state.  The ability of the lake to do so is this concept of resilience (Figure 1).

Figure 1.  Conceptual diagram of a resilient system.  The height of the slope and the deepness of the valley are the compensatory mechanisms that bring a lake back to some resilient baseline condition after a short-term “shock” like a flood or a temporary septic failure.  Lakes with forested watersheds, clear water, native aquatic plants, and balanced fish communities are typically in this condition.

Slowly, as more curb and gutter goes in, green lawns replace native grasses, personal swimming beaches replace marshes, fish are overharvested or overstocked, or invasive species are introduced, the lake slowly loses its ability to compensate (Figure 2).  All of a sudden you hear “I’ve never seen that before” become more common when people describe a phenomenon on the lake that well, they’ve never seen before.   You may start to observe more algae blooms, more attached algae on rocks and plants, plants growing where they’ve never grown before, invasive species taking hold and thriving.  This is an example of the lake losing resilience and succumbing to the vagaries of the environment.  Under these circumstances, the lake can’t compensate anymore and you never know what you will see from year to year.  With no baseline, objective assessment of aquatic plant abundance and no monitoring of change in abundance and cover from year to year, it makes it even harder to know how much the lake has actually changed and what you need to try to get back to with implemented best management practices .

Figure 2.  An example of the consequences of the cumulative impacts of environmental and human stressors on lake resilience.  As lakes become more impacted by various watershed and in lake practices and invasive species, resilience is slowly worn away.  The valley becomes more shallow and a new “domain” enters the picture.  Lake conditions slosh around from one state to the next depending on the vagaries of weather and other disturbances.  Not knowing to expect from one year to the next becomes the norm.

A demonstration of the difference between a resilient lake and one that is losing resilience can be found in a paper published by Valley and Drake in Aquatic Botany in 2007 entitled “What does resilience of a clear-water state in lakes mean for the spatial heterogeneity of submersed macrophyte biovolume?”  Valley and Drake found very consistent patterns of vegetation growth from one sampling period to the next over three years in a clear lake (Square Lake, Washington Co. MN USA; Figure 3).  Each survey in Figure 3 took two days to survey and another week to make these plots.  Not including time on the water, ciBioBase produces these same plots in an hour.
 

Figure 3.  Submerged aquatic plant biovolume (% of water column inhabited by plants) as a function of depth in Square Lake, Washington Co., MN USA.  Notice the consistency of the pattern of vegetation growth from one time period to the next (study took place for 3 years from 2002-2004; Valley and Drake 2007).  Water clarity in Square Lake is high with diverse aquatic plants.

In contrast, patterns of vegetation growth were quite variable in a moderately turbid lake with abundant Eurasian watermilfoil; West Auburn Lake, Carver Co. MN USA; Figure 4).  For example, in summer 2003, a bloom of attached algae formed on Eurasian watermilfoil stems and effectively weighed down the stems and prevented them from reaching the surface.  This bloom was unique to 2003 and was not observed at any other time during the study.

Figure 4.  Plant growth as a function of depth in a moderately turbid Minnesota Lake with abundant Eurasian watermilfoil (West Auburn Lake, Carver Co. MN USA; Valley and Drake 2007).  Plants grew shallower and more variable in this more disturbed lake. 

If stressors continue unabated, then the lake can “tip” into a new, highly resilient domain of poor health (Figure 5).  The feedback mechanisms that used to keep the lake in a healthy state have now switched to new feedback mechanisms that are keeping it in an unhealthy state.  Algae begets more algae, carp beget more carp, stunted bluegill beget more stunted bluegill, if invasive plants are lucky enough to grow, they beget more invasive plants.  Getting the lake back to the original state is nearly impossible at this point.  It’s like Sisyphus rolling the rock uphill only to have it roll right back down again!  Although controversial, at some point, citizens, regulators, and lake managers need to start rethinking expectations and adapting management approaches in highly degraded systems.  Rather than trying to restore a lake to a Pre-European settlement condition through expensive, risky, and Draconian measures, it may be more reasonable to ask: “How can we have good enough water quality to support naturally reproducing stocks of game fish?”  “Can we manage invasive plants in a way that maintains fish habitat AND recreational opportunities?”  After the wailing and gnashing of teeth subsides and some agreement is reached on objectives and management strategies, then it becomes essential to determine whether implemented management practices are having their desired effect.  It doesn’t take two weeks and $10’s of thousands of dollars to do a vegetation survey.  Volunteers can do it, lake consultants can do it, state agencies can do it and they’ll all do it the same objective way with ciBioBase and they can all work together!

Figure 5.  Example of a lake that has flipped into a degraded regime regulated by new feedback mechanisms that keep it in the degraded state. 

The Upshot

Resilience is an easy concept to understand on a basic level, but hard to measure in lakes and changes slowly over time.  This stresses the importance of long-term monitoring and being on guard for those things “you’ve never seen before.”  Uploading data to ciBioBase every time you are on the water gives an objective and quantitative snapshot of the current conditions in your lake of interest.  Be watchful for anomalies in monitored areas.  Vegetation growth should follow a relatively predictable pattern from year to year and if it doesn’t, that may be the first indication that the lake is losing resilience and precautionary conservation measures should be taken.  Conservation measures may include better onsite storm water infiltration (e.g., rain gardens, nearshore vegetation buffers), maintaining a modest amount of aquatic plant growth in the lake, maintaining a balanced fish community in terms of species, size, and abundance.  These efforts will go a long way in protecting the long-term integrity of our beloved lakes!

Suggested Readings:

Carpenter, S.R., 2003. Regime shifts in lake ecosystems: pattern and variation. In: Excellence in Ecology, vol. 15, Ecology Institute Oldendorf/Luhe, Germany.

Scheffer, M., 1998. Ecology of Shallow Lakes. Chapman and Hall, London.

Valley, R.D. and M.T. Drake 2007.  What does resilience of a clear-water state in lakes mean for the spatial heterogeneity of submersed macrophyte biovolume? Aquatic Botany 87: 307-319.