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Starry Stonewort (Nitellopsis obtusa) Invasion in Lake Scugog:

Aquatic Plant Survey and Water Quality Assessment

PRELIMINARY REPORT

(2016)

Prepared for:

Scugog Lake Stewards

By:

Andrea Kirkwood, PhD

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INTRODUCTION

Lake Scugog is a large headwater-lake that flows into the federally-managed Trent-Severn

Waterway (TSW) system. Although a regulated waterbody situated in a predominantly agricultural

drainage basin, Lake Scugog’s shallow depth and abundant aquatic weed-beds have historically

supported highly productive ecological communities, including a recreational sport fishery. The federal

Department of Fisheries and Oceans (DFO) in partnership with the Ontario Ministry of Natural

Resources and Forestry (OMNRF) have performed large-scale assessments of angler activity across the

province and country, and reported that Lake Scugog is in the top 10 Ontario lakes for recreational-

angling pressure (as represented by hours spent on the lake) (OMNRF 2009). As such, addressing the

current aquatic ecosystem health of Lake Scugog has important implications for the regional economy as

well as broader environmental implications for the nationally significant waterway it flows in to.

Implications of a Shifting Macrophyte Community on Fish Habitat

A binational Great Lakes nearshore fisheries habitat workshop concluded much remains to be known

about fish associations with particular habitat types, land-use impacts on fish habitat, and current and

potential threats to fish species and communities (Goforth and Carmen 2003). Lake Scugog serves as an

important aquatic ecosystem to assess all of these key knowledge gaps given the emerging ecological

shifts in fish habitat and community structure in the lake. Long-term OMNRF data-trends show

population increases in panfish and largemouth bass, but notable declines in walleye. There are also

rising concerns about the anticipated impacts of a pike invasion on the native, but rare, muskellunge in

Lake Scugog. Kawartha Conservation has long noted the important role that the submerged macrophyte

community has had in supporting dense populations of sportfish in Lake Scugog (Kawartha

Conservation 2010). Even though the non-native Eurasian Watermilfoil (Myriophyllum spicatum) had

dominated the macrophyte community since the 1960s, desirable sportfish such as largemouth bass and

walleye managed to thrive in the lake. However, the aquatic macrophyte community has undergone

significant changes over the last decade in abundance and community composition, including periodic

population collapses of M. spicatum and its hybrid with a native milfoil species. More non-native taxa

have established, and in particular, the macroalga Starry Stonewort (Nitellopsis obtusa) has produced

extensive “pillow-like” stands in various parts of the lake. Coupled with changes in water quality, N.

obtusa poses a significant threat to the long-term health of the sport fishery in Lake Scugog.

Due to the reported impacts of dense meadows of N. obtusa on fish spawning and recruitment in

Michigan inland lakes (Pullman and Crawford 2010), this algal invader has the potential to negatively

impact fish habitat in Lake Scugog. Interestingly, conditions linked to the decline of the walleye fishery

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(i.e., increasing lake temperature and water clarity) may be the same conditions promoting the

establishment and growth of N. obtusa in Lake Scugog. Since recruitment failure, for example, has been

identified as a significant issue for the walleye population in Lake Scugog, and nursery habitat quality is

an important factor in juvenile development, a study on submerged macrophyte stands and water quality

in the lake is warranted. Conversely, there is concern expressed by anglers that N. obtusa is producing

an inhospitable environment for desirable sportfish such as largemouth bass, which normally thrive in

weedy lakes. In order to understand the growth and establishment success of N. obtusa in Lake Scugog,

a broader understanding of macrophyte community composition and dynamics is required, particularly

as it relates to water quality conditions that promote certain macrophyte taxa over others. Knowing how

the macrophyte community varies spatially and temporally in response to changing water quality and

environmental conditions in Lake Scugog will offer valuable information to lake managers interested in

protecting and optimizing fish habitat in the lake.

Background

Lake Scugog is a large (66 km2) marl lake situated north of the Oak-Ridges Moraine in a watershed

(534 km2) shared by the municipalities of Durham Region (95%) and City of Kawartha Lakes (5%) in

Ontario, Canada (Fig. 1). Lake Scugog also serves as a major headwater-lake to the TSW; a nationally

significant watercourse connecting Lake Ontario with Lake Huron. The lake feature known today was

created in the 1800s after gristmill-dam construction on the Scugog River. With the creation of the large,

but shallow lake (mean depth = 1.4 m) came new habitat opportunities for aquatic plants and wildlife,

including birds and an abundant fishery. Both the recreational and natural heritage features of Lake

Scugog have contributed to it being the main economic driver within the region. Kawartha Conservation

(2010) estimated that municipalities in the watershed receive approximately $10-15 million annually

from tourism and recreational activities centred around the lake. In particular, seven annual fishing

tournaments (targeting largemouth bass and walleye) plus winter ice-fishing activities drive substantial

tourism dollars into the local economy (Kawartha Conservation 2004). The successful introduction and

establishment of key sportfish such as largemouth bass and walleye in Lake Scugog was mainly due to

ideal habitat features such as the abundant submerged macrophyte beds throughout the lake. Although

perceived as a “weedy” lake, submerged macrophytes provide refugia, food, and nursery habitat for

many fish species, including largemouth bass, walleye, and the native muskellunge. Submerged

macrophytes also produce oxygen, remove nutrients, and stabilize loose sediments, which cumulatively

contribute to improved water quality in the lake (Kawartha Conservation 2010).

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Lake Scugog: A Changing Aquatic Ecosystem

Recreational Fishery – The OMNRF Kawartha Lakes Fisheries Assessment Unit has conducted

angler surveys on Lake Scugog since the late 1970s. These data show the long-term trends of sportfish,

including increases in panfish (black crappie, bluegill) and largemouth bass, but also declines in walleye

and more recent concerns about the vulnerable muskellunge population. Robillard and Fox (2006)

reported that the walleye fishery has been struggling in Lake Scugog for several decades, in addition to

changes in fish community structure. Multiple stressors have been implicated in shifting fish community

structure, including: water level fluctuations, increasing temperature, centrarchid competition, impacts to

spawning habitat (i.e., siltation, seasonal water drawdowns) and decreasing phosphorus causing

increased water clarity. On January 1st, 2016, the OMNRF closed the walleye fishery. The OMNRF are

reviewing data related to the part(s) of the walleye life cycle that are likely contributing to the decline.

Once vulnerable life stages are identified, this will focus the list of potential mechanisms that are

ultimately responsible for the decline. However, walleye abundance is likely dependent on a complex

web of direct, indirect, and interacting mechanisms.

Submerged Macrophyte Community – The shallow depth and substantial nutrient loads from a

predominantly agricultural drainage basin have historically contributed to a highly productive

submerged macrophyte community in Lake Scugog. A productive and diverse macrophyte bed has also

served as an important habitat and/or food resource for fish in the lake. Even with the invasion of non-

native M. spicatum decades ago, the fishery for the most part thrived. In the past, the major issue with

M. spicatum was its excessive growth, which was aesthetically displeasing and interfered with

recreational activities (e.g., swimming and boating). However, past population collapses (Painter and

McCabe 1988) and more recent declines point to a shift in community dominance by M. spicatum. In

more recent years, Lake Scugog has been inundated with other invasive non-native aquatic macrophytes

including Curly-leaf Pondweed (Potamogeton crispus), a hybrid of M. spicatum with the native

Northern watermilfoil (M. sibiricum), and N. obtusa. Unlike the other non-native macrophytes, which

are aquatic plants, N. obtusa is a charaphyte alga and relatively new to Canadian inland lakes. N. obtusa

has proliferated in inland waters of American states bordering Canada, including Michigan and New

York State, for just over a decade (Pullman and Crawford 2010).

There are few published reports of N. obtusa in Canada, and those that exist document recent

sightings in Lake Simcoe (LSRCA 2015) and a coastal wetland of Lake Ontario (Midwood et al. 2016).

Overall, invasive non-native macrophytes typically change the community composition of macrophyte

beds in lakes. This can have important implications for the local economy, such as impacts to lake

ecosystem-services (e.g., sport fishery) (Schulz and Dibble, 2012). The establishment of N. obtusa in

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particular raises concerns related to both the Lake Scugog fish community and recreational uses of the

lake. In jurisdictions where N. obtusa is considered invasive, the alga forms dense, rootless mats that can

act as a benthic barrier. This impact to the benthic zone can change redox conditions and, coupled with

possible allelopathy, can harm rooted aquatic plants (Pullman and Crawford 2010).

Figure 1. Watershed map of Lake Scugog. Source: Kawartha Conservation.

As such, mats of N. obtusa have been found to correlate with a significant decrease in the biomass of

competing taxa, which is likely due to both light reduction and allelopathy (Mulderij et al. 2007). With

respect to fish habitat, dense mats of N. obtusa can directly impact fish spawning. Bass and sunfish are

known to regularly spawn in dense stands of native Chara species, but these spawning behaviors did not

occur in dense stands of N. obtusa (Pullman and Crawford 2010). This may in part be due to the hard

and rough branches of N. obtusa, which can be painful if grazed or stepped on by swimmers. Overall, N.

obtusa is regarded as the most aggressive invasive species in inland lakes in the US, with reports that it

has replaced other non-native and nuisance species such as M. spicatum and P. crispus (Pullman and

Crawford 2010).

Aquatic Macroinvertebrate Community – Though there are no published studies focusing on the

macroinvertebrate community of Lake Scugog, a few studies have commented on aquatic insect

associations with submerged macrophytes in Lake Scugog, among other Kawartha lakes (Painter and

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McCabe, 1988, Hinch and Collins, 1993, Borrowman et al. 2014). In particular, the milfoil weevil

(Euhrychiopsis lecontei) and aquatic caterpillars have been implicated in some population crashes of M.

spicatum (Painter and McCabe 1988), whereas others have shown that sunfish can effectively suppress

herbivory via predation (Ward and Newman 2006). More recently, Borrowman et al. (2014) reported

that E. lecontei density is not affected by Myriophyllum biotype, but rather environmental conditions

such as patch depth and conductivity. With the recent invasion of N. obtusa in Lake Scugog, there is

serious concern for benthic organisms, including clams and other macroinvertebrates (Pullman and

Crawford 2010). In its European home range, N. obtusa typically hosts chironomids, while other

charaphytes host amphipods and isopods (Hargeby 1990). In a study from Poland, zebra mussels

attached to N. Obtusa at densities of 1000 individuals/m2, whereas densities on other aquatic plants were

much lower (Lewandowski and Ozimek 1997). Therefore, it is not unreasonable to predict that N. obtusa

could promote an increase in the zebra mussel population in Lake Scugog. Up to now, zebra mussels in

the lake have not been considered a serious problem mainly due to the prevalence of soft benthic

substrate throughout the lake (Kawartha Conservation 2010).

Water Quality – As a large, but shallow lake in a mainly agricultural drainage basin (i.e., > 50%

agricultural land-use in the watershed), Lake Scugog has faced water quality challenges since the early

1970s. Primary factors influencing water quality in Lake Scugog include: intensive erosion of lake

shores and stream banks; discharge from Port Perry sewage treatment plant; shoreline development

(septic systems, fertilizers); and stormwater run-off from urbanizing areas (Kawartha Conservation

2010). Agricultural activities are also an important factor in poor water quality, such as: manure storage

and application; excessive fertilizer use; cropland erosion; and access of livestock to watercourses

(Kawartha Conservation 2010). Based on aquatic macrophyte cover and secchi depth, Lake Scugog has

been historically classified as eutrophic. However, water quality monitoring results over the last decade

indicate Lake Scugog is mesotrophic (Kawartha Conservation 2010). This change in trophic status

suggests that “cultural oligotrophication” may be occurring in the lake (i.e., reduction in phosphorus

loadings). The last water quality report card (2013) grade for Lake Scugog was a C (fair). This grade is

in part based on the total phosphorus concentration marginally exceeding the Provincial Water Quality

Objective for this important algal and plant nutrient.

STUDY OBJECTIVES

The long-term goal of this project is to provide evidence-based information to the Scugog Lake

Stewards (SLS) regarding the composition and distribution of the submerged macrophyte community in

Lake Scugog, and the role that dominant non-native taxa have in community structure. To this end, an

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assessment of water quality will shed light on the conditions that promote and/or are impacted by certain

macrophyte community assemblages. This will be the first time a detailed survey and assessment of

aquatic macrophytes and water quality has been concurrently conducted in Lake Scugog. Ultimately,

knowledge-transfer about the ecological status of Lake Scugog will allow the SLS and OMNRF

biologists to be proactive in targeting fish-habitat areas requiring mitigation over the long-term.

Kawartha Conservation will also be able to use this information to justify expansion of their current

monitoring program, as well as have baseline data for future study comparisons. Finally, the SLS can

use this information to inform public education on the state of ecosystem health in Lake Scugog, in

addition to guiding the development and implementation of future restoration projects.

There are four main study objectives that aim to address the knowledge gaps sought by the OMNRF,

as well as the “Monitoring and Scientific Studies Strategy” recommendations outlined in the Lake

Scugog Environmental Management Plan (Kawartha Conservation 2010):

1. Map macrophyte abundance and community composition in Lake Scugog over several, sequential

field seasons.

2. Assess the occurrence and impact of N. obtusa meadows on submerged macrophyte community

structure and diversity, including interactions with M. spicatum and its hybrid.

3. Assess macroinvertebrate abundance and diversity in macrophyte stands and determine possible

interactions with non-native macrophyte taxa as it relates to habitat quality and herbivory.

4. Determine the role of environmental conditions, including water quality, in influencing non-native

macrophytes in Lake Scugog, and conversely, determine how dense stands of non-native

macrophytes influence trophic state conditions.

The first project objective was addressed initiated in 2016, whereby undergraduate students from the

University of Ontario Institute of Technology (UOIT) and the SLS joined forces to conduct a water

quality and macrophyte sampling program from late spring to late summer, 2016.

STUDY APPROACH

UOIT is the closest university to Lake Scugog (36-km round trip), so is ideally situated for field

work involving sample collection and lab storage on the same day. A preliminary macrophyte mapping

survey was performed during the summer of 2016 by UOIT in collaboration with the Scugog Lake

Stewards. Twelve sampling sites in the lake were chosen based on their proximity to walleye spawning

and water quality impact locations (Fig. 2). Similar to the methodology of Ginn (2011), lake-rake

samples of aquatic macrophytes were taken at each site monthly, from May – September. Due to the

large size of the lake and time required for sampling, the first 6 sites (sites 1-6) were typically sampled

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on the Monday of the sampling-week, processed on the Tuesday of the sampling-week, and the

remaining sites (sites 7-12) were sampled on the Wednesday, and processed on the Thursday of the

sampling-week. Macrophyte samples were taken back to the lab for identification and biomass

measurement. During the monthly sampling events, a suite of field measurements were taken at each site

including: secchi and water depth, pH, dissolved oxygen, conductivity and temperature. Three 1-L

replicate water samples were also collected at each sampling location for subsequent lab analyses (total

nitrogen and phosphorus, turbidity, and chlorophyll a).

Figure 2. Lake Scugog sampling sites including the macrophyte sampling-sites (yellow pins) and Kawartha Conservation

water quality monitoring sites (red pins).

RESULTS

Water quality variables measured over the course of the field season from late spring (May) to late

summer (September) showed notable variability across sites and over time. Dissolved oxygen was very

high in the western arm of the lake (sites 1-5) in May (Fig. 3). Values above 12 mg/L reflect

supersaturated concentrations due to high rates of photosynthesis at these locations. Sites in the eastern

arm of the lake (sites 7-12) experienced supersaturated dissolved oxygen concentrations in June. For the

remaining months, dissolved oxygen varied between 7 and 9 mg/L, likely reflecting concurrent

processes of photosynthesis and respiration. Dissolved oxygen always remained above concentrations

deemed critical to fish for the entire field season.

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Figure 3. Dissolved oxygen across 12 sites over the field season. Note missing values in September due to site inaccessibility.

Figure 4. Temperature across 12 sites over the field season. Note missing values in September due to site inaccessibility.

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Figure 5. Secchi depth across 12 sites over the field season. Note missing values in September due to site inaccessibility.

Temperatures were unusually warm in May, 2016 for all sites (Fig. 4). In June, a cold snap

influenced temperatures for the first 6 sites, but 2 days later, warm weather increased water temperatures

significantly in the eastern arm of the lake (sites 7-12). Overall, temperatures were quite warm in the

lake until September. Considering that oxygen levels remained above 7 mg/L when water temperatures

were warm, this infers that photosynthesis was likely maintaining oxygen levels, in addition to wind-

mixing of surface waters. Secchi depth, a measurement of water clarity, varied over time at each site,

and across sites (Fig. 5). It is not entirely accurate to compare Secchi depth across sites since secchi

depth measurement was limited by site depth, but it is certainly useful to compare secchi depths at the

same site over time. The common trend was that water clarity was highest in spring, and became

generally less clear by September. Typically the deepest sites had the most notable water clarity declines

including sites 3, 4, 6, 8, and 12. As lake water evaporates and phytoplankton increase in biomass by the

end of the summer, it is not unexpected for lake water clarity to decline over the summer.

In the western-arm of the lake, lake-water conductivity was relatively stable in most sites over time,

but several sites including sites 2 and 5 showed generally increasing trends in conductivity over time

(Fig. 6). Sites 2 and 5 are situated near tributary inflows to the lake, so this is likely reflective of variable

conductivities in tributary water influencing lake water conductivity. In the eastern-arm of the lake,

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conductivity values doubled in July, but this was due to a heavy rainstorm that happened the day before

sampling. Catching this rain event clearly shows how dramatically conductivity can change in the lake.

Conductivity represents the ion content of water, and can reflect dissolved salts, nutrients and other

contaminants.

Figure 6. Conductivity across 12 sites over the field season. Note missing values in September due to site inaccessibility.

Principal component analysis (PCA) was used to explore spatial patterns in water quality at different

time points over the field season (May, July, September) (Fig. 7). Considering that the sampling sites are

from one lake, there is a surprisingly high degree of variation in several water quality parameters,

particularly phosphorus and conductivity. Interestingly, conductivity and phosphorus inversely co-vary

in May, yet the opposite occurred in September. This suggests that factors controlling conductivity and

phosphorus differed over the sampling period. Increased rain events in late summer likely reflect the

strong covariation of conductivity and phosphorus in September. It was anticipated that sites closest to

each other, such as those in the west-arm and those in the east-arm, would cluster together. However,

this was generally not the case. Site 9 was a standout with respect to water quality profile in May and

July, and was inaccessible in September. Site 2 also stands out as a unique site based on its water quality

profiles over the sampling season, yet the parameters driving its dissimilar profile appear to change

between sampling periods. Overall, the PCA biplots help to visualize how heterogeneous water quality

is in Lake Scugog, over space and time.

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Figure 7. Principal component analyses of water quality variables in Lake Scugog for May, July and September.

May

September

July

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Figure 8: Relative abundance of aquatic plant and macroalgal species across sites in May, 2016.

Figure 9: Relative abundance of aquatic plant and macroalgal species across sites in June, 2016.

May

June

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Figure 10: Relative abundance of aquatic plant and macroalgal species across sites in July, 2016.

Figure 11: Relative abundance of aquatic plant and macroalgal species across sites in August, 2016.

July

August

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Figure 12: Relative abundance of aquatic plant and macroalgal species across sites in September, 2016.

The measurement of aquatic plants and macroalgae from Lake Scugog confirm how productive and

diverse the macrophyte community is (Fig. 8-12). Sites from the shallower western-arm are generally

more biodiverse than the eastern-arm. The eastern arm is dominated by Chara, especially earlier in the

sampling period, which is not surprising considering the deeper water preference typical of Chara

species. The non-native charaphyte Starry Stonewort (N. obtusa) is present throughout the study period,

but increases in relative abundance across sites throughout the sampling period. Eurasian watermilfoil

(M. spicatum) does co-occur with Starry Stonewort at several sites, but whenever it is the dominant

aquatic plant, Starry Stonewort abundance is either very low, or absent from the samples. When the

relative abundance values for Starry Stonewort and Eurasian watermilfoil from all sites are averaged and

compared over time, an interesting pattern emerges (Fig. 13). There appears to be a temporal (i.e.,

seasonal) abundance pattern for each taxon, whereby Eurasian watermilfoil is more abundant early in

the sampling period, and Starry Stonewort increases in abundance towards the end of the sampling

period.

September

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Fig. 13: Mean relative abundance of Starry Stonewort (Nitellopsis obtusa) and Eurasian Watermilfoil (Myriophyllym

spicatum) from all sites over the sampling period.

PRELIMINARY CONCLUSIONS

For the first time in Lake Scugog, a comprehensive macrophyte survey across the entire lake over a

five-month field season has been conducted. Preliminary analyses have confirmed that Starry Stonewort

is a dominant member of the macrophyte community in the lake, occurring throughout the field

sampling season. Since sampling sites were selected based on their proximity to walleye spawning

grounds in Lake Scugog, there is now evidence that Starry Stonewort is present in these locations,

warranting further studies to investigate impact. Currently, there is very little information on Starry

Stonewort distribution and ecology in Canada other than two studies from coastal areas of the Great

Lakes (Schloesser et al. 1986, Midwood et al. 2016). Although Starry Stonewort was first documented

in Lake St. Clair thirty years ago, it is only over the last decade that it has rapidly invaded inland lakes in

the US and most recently, southern Ontario. Surprisingly, no peer-reviewed studies have investigated

this apparent increase in invasiveness, nor the impacts Starry Stonewort blooms may have on other

macrophytes, invertebrates, and fish in North American lakes. Based on the few studies done in the

United States, sites dominated by Starry Stonewort were deemed to negatively impact fish habitat and

food resources.

Another perplexing phenomenon witnessed in other studies (Painter and McCabe 1988, Borrowman

et al. 2014) is the periodic collapse of Eurasian Watermilfoil in the Kawartha Lakes, long before the

invasion of Starry Stonewort. Though Borrowman et al. (2014) noted possible factors such as herbivores

and environmental conditions, it is still unclear what controls the episodic population declines of

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Eurasian Watermilfoil and its hybrid. Pullman and Crawford (2010) have stated that Starry Stonewort

out-competes other non-native taxa such as Eurasian Watermilfoil in Michigan lakes. The Scugog Lake

Stewards had anecdotally observed that Eurasian Watermilfoil and Starry Stonewort do not co-exist in

Lake Scugog. Although the field study has confirmed that these two non-native taxa co-occur, it does

appear that they do not co-dominate together. That is, when Eurasian Watermilfoil is dominant in the

community, Starry Stonewort is not. This phenomenon seems to, in part, be explained by differential

seasonality of the two species. Future studies should determine if antagonistic interactions occur

between Eurasian Watermilfoil and Starry Stonewort, where one species negatively impacts the other.

It is anticipated that continued study of the macrophyte community in Lake Scugog will elucidate

the role of invasive macrophyte taxa and water quality in fish habitat quality. The project focus on

macrophytes and water quality offers new and vital information for fish-habitat assessment in Lake

Scugog, beyond what the in-house expertise that the OMNRF Kawartha Lakes Fisheries Assessment

Unit can normally deliver. Of most concern to OMNRF is the fish nursery habitat provided by

macrophyte beds in Lake Scugog. Even the habitat of fish normally associated with clean, rocky

substrate (e.g., walleye) is of interest, since this habitat type may be displaced by invading macrophyte

stands. Furthermore, OMNR staff biologists can use the spatially explicit macrophyte data from this

study to target areas for future fish habitat assessment and remediation. Both the OMNR and Kawartha

Conservation will find these results invaluable to informing their own monitoring and management

efforts, particularly for the sport fishery. As the original advocates for a lake-wide macrophyte study, the

Scugog Lake Stewards and membership will use the new scientific information on their lake to guide

stewardship projects related to ecosystem health and restoration.

Since there is a paucity of information related to the invasion of Starry Stonewort in Canada,

information regarding impacts to fish habitat and aquatic ecosystem health from this study will be

beneficial to government agencies and stewardship groups across the country. The Lake Simcoe Region

Conservation Authority has already expressed avid interest in the findings from this project based on the

known establishment of Starry Stonewort in Lake Simcoe (B. Ginn pers. comm.). It is also anticipated

that this study and future work will shed light on the underlying mechanisms of macrophyte community

response to Starry Stonewort invasion, including factors that may impede or promote Starry Stonewort

growth. Transfer of this new knowledge to other stakeholders invested in lake health and ecosystem

services will permit proactive management practices, especially prior to Starry Stonewort infestation.

New knowledge from this study will most certainly be included in updates of Canadian invasive species

guides and manuals, as well as spur on future Starry Stonewort invasion and impact studies in Canadian

inland waters.

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