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.