State of the Lake 2016

CONTENTS

Executive Summary                                                                                       2

Introduction                                                                                                     3

Study Purpose                                                                                                4

Study Methodology                                                                                       4

Study Results                                                                                                 5

Summary Findings                                                                                         10

Citations                                                                                                          12

Figures                                                                                                            13

Tables                                                                                                              30

EXECUTIVE SUMMARY

 

Aquatic Ecosystem Research surveyed the plant community of Lake Wononscopomuc on July 9th and July 10th at the request of The Lake Wononscopomuc Association.  The results of that study are summarized below.  A more complete description of the study is presented in the remainder of this document.

 

most species rich (figs.  8,9,13)

 

 

INTRODUCTION

 

Lake Geomorphic Features:

Lake Wononscopomuc is located in Salisbury, Connecticut and has a surface area of between 348 and 363 acres (Frank et al. 1959, Frink and Norvell 1984, Jacobs and O’Donnell 2001, CAES 2004).  This natural lake has a maximum depth of 32m (108ft), a mean depth of 11m (Jacobs and O’Donnell 2001), contains 4,735,353,634 gallons of water, and the water has a retention time of 1700 days (Canavan and Siver 1995).  Two small streams feed the lake; Sucker Brook on the southeast and an unnamed brook that enters on Wononscopomuc’s northern side.  The outlet flows through Factory Brook to Salmon Creek and, ultimately, to the Housatonic River.  Lake Wononscopomuc is situated in the Housatonic Basin and has a sub-watershed of about 1620 acres that is mostly residential development (Jacobs and O’Donnell 2001). Finally, the lake to watershed ratio is 4.65.

 

Water Chemistry:

The water chemistry of Lake Wononscopomuc is quite rare in Connecticut. There are only a few examples of lakes that mirror the water chemistry parameters of Wononscopomuc; a notable example is East Twin Lake, which is also located in Salisbury. The Town of Salisbury is located in the Marble Valley of Connecticut; this type of bedrock weathers easily and contributes the major ions calcium and  magnesium to the lake waters of this region (Frink and Norvell 1984, Canavan and Siver 1995). The resulting water chemistry of lakes situated in this type of bedrock is generally high in pH, conductivity, and alkalinity (Canavan and Siver 1995).

 

Historically Lake Wononscopomuc had exceptional water clarity and recreational quality.  Since 1939 the average water clarity has been 5.2m (17ft).  Furthermore, the lake has been phosphorus-limited since data has been collected (Frink and Norvell 1984, Canavan and Siver 1995), which can be attributed to the low levels of chlorophyll- a throughout the known history of the lake. Additionally, high levels of calcium and magnesium from bedrock sources may also inhibit some genera of algae and be contributing to the high degree of water clarity (Canavan and Siver 1995). Lake Wononscopomuc is deep in comparison to other natural lakes of Connecticut.  The lake waters stratify every summer season with the thermocline forming between 4 and 7m resulting in a distinct thermal separation of the epilimnion and hypolimnion.  This usually occurs by May of each year but oxygen concentrations of the hypolimnion remain high until July.  At that point, internal loading of phosphorus from the anoxic sediments begins and phosphorus builds up in the hypolimnion throughout the remainder of the summer season as anoxia persists (see 2015 Water Quality Report). That source of phosphorus is sequestered until lake mixing begins in September/October.  When the thermal stratification breaks down and the lake mixes water clarity diminishes as the blue-green algae population expands rapidly under the increased phosphorus conditions.  Finally and importantly, the holistic historical data set suggests that infrastructure and regulatory changes have lead to a condition where the internal production of nutrients (internal phosphorus/ammonia loading) has taken the primary role in determining the water chemistry dynamics of the lake throughout the summer season.

 

Plant Community:

Lake Wononscopomuc has been assessed to have a littoral zone of about 105ac or 30.1% of the water body (NEAR 2008).  This critical area is the region where water

 

 

 

clarity permits macrophytes (i.e. vascular plants) to persist.  In general, the littoral zone has been assessed at about 6m for Wononscopomuc (NEAR 2004, 2008) but plants have been reported to be growing at depths of up to 12m (Frank et al. 1959).  Lake Wononscopomuc houses a diverse set of species that can be related to the ambient water chemistry conditions (see June-Wells et al. 2013, 2016).  Since quantitative approaches have been applied to the assessment the Lake Wononscopomuc’s plant community, between 16 and 18 species have been reported (CAES 2004, NEAR 2004,2008).  Furthermore, three state-listed rare species and two non-native species have been reported.  The state listed species that have been reported are Myriophyllum sibiricum, Potamogeton friesii, and Ranunculus longirostris.  The non- native species that have been reported are Myriophyllum spicatum and Potamogeton crispus.  All of the recent surveys and plant management initiatives have focused on the control of M. spicatum because it has been shown to be the dominant constituent of the plant community from the perspectives of how often it is encountered and its total spatial distribution (CAES 2004, NEAR 2004,2008). NEAR assessed the coverage of M. spicatum at 48ac in 2004 and 51ac in 2008, which they suggest represent 45.7 and 48.5% of the littoral zone, respectively.  All of the species detected in Lake Wononscopomuc during the historical surveys show distinct relationships with water chemistry; in particular, the resident species show strong preferences for hard water systems with high relative pH and bicarbonate as the available carbon source (June-Wells et al. 2013, 2016).  Finally, the non-native species also show strong preferences for the aforementioned water chemistry parameters (June-Wells et al.

2013, 2016), which suggest that Lake Wononscopomuc represents the ideal conditions for populations of M. spicatum and P. crispus leading to their persistent residence.

 

 

STUDY PURPOSE

 

Aquatic Ecosystem Research was engaged by The Lake Wononscopomuc Association to examine the current state of the submerged aquatic plant community. The Association has a long history of conservation-minded management and has been using commercial grade harvesters to manage their population of M. spicatum. The purpose of this study is to determine the current extent of M. spicatum, inventory the plant community, identify the locations of state-listed species, and provide a comprehensive statistical assessment pertaining to the structure plant community.

 

 

STUDY METHODOLOGY

 

Qualitative Plant Mapping:

The qualitative plant survey was conducted by slowly motoring throughout the entirety of the littoral zone to a depth of 6m. Plants were identified visually following Crow and Hellquist (2000) after a sample was collected with a grapple tied to a 20m line. This   type of survey was used to develop a species inventory and to determine each species’ general location. Species ranges were logged on blank lake maps using  dead reckoning and were then georeferenced in ARCMAP ®; a qualitative aquatic plant distribution map was created from those  data.

 

Quantitative Plant Community Modeling:

To evaluate the mathematical properties of the plant community and to develop statistical models, 48 geo-referenced points were established at depths ranging from

 

 

 

0.1-6.0m in Lake Wononscopomuc.  These points were organized by depth; six depth classes were created (0-1, 1-2, 2-3, 3-4, 4-5, and 5-6m) and each class contained 8 points.  At each point the boat was anchored, oriented perpendicular to the nearest shoreline, and the depth was measured off the front of the boat using a weighted drop line.  The plant community was sampled using a grapple tied to a 20m line.  At each point, four grapple tosses (two to each side of the boat) were conducted by throwing the grapple parallel to the nearest shore out to the full length of the line.  Plant species were identified on site following Crow and Hellquist (2000) and their abundance was evaluated using a rank abundance technique.  The abundance of each species was evaluated by two independent observers and was given a rating of 1 – rare, 2 – present but not abundant, 3 – abundant but not dominant, 4 – dominant, or 5 – dense monoculture.

 

Those data were compiled in an Excel Spreadsheet ® and various abiotic/plant community parameters were calculated. Light availability at the sediment-water interface was calculated at each point using the following formula: Iz= e(­kz) ∗Io, where Iz is light intensity at depth z, e is Euler’s number, k is the attenuation coefficient of light

in water (1.7) multiplied by the Secchi depth (5.77m), z is the depth, and Io is the solar constant (340 w/m2).  Point diversity was calculated using Shannon’s Diversity Index [– Σ(pi * ln(pi)) – (S-1/2N)] where pi is the fraction of the community each species is representative of, S is the total lake richness, and N is the total abundance at each

point.  Point richness was calculated by counting the number of species encountered at each point.  Total point abundance was calculated by taking the sum of all species’ rank abundance values at each individual point.  Finally, the relative contribution of each species to the community was calculated by taking the sum of each individual species’ rank abundance values across all points, dividing it by the total rank abundance of all species at each point, and multiplying that quotient by 100.

 

Those statistics were then used to develop the plant community models. General Linear Models (GLM) were employed to determine which mathematical models best fit those data. Models were assessed using Akaike Information Criteria (AIC); in each case the model with the lowest AIC was chosen (i.e. best fit). Those analyses were conducted in R for the following variables as they relate to depth and light availability:

1) Total Point Abundance, 2) Diversity, and 3)   Richness.

 

 

STUDY Results

 

Littoral Zone:

We attempted to validate the previous assessments of littoral zone size by collecting on site depth data and calculating the total available light throughout the assumed littoral zone.  We found that 17% of surface light was still available at a depth of 6m, which suggests that the theoretical littoral zone should extend to a depth of 10.5m (fig. 1).  However, plant sampling in depths greater than 6m yielded only rare individuals; therefore, we then used ArcGIS ® to estimate the size of the littoral zone to the previously reported depth of 6m.  The total size of the littoral zone using that technique resulted in littoral zone size of 84ac or 24% of the Lake Wononscopomuc surface area (fig. 2).

 

 

 

Plant Species Inventory and Rare Species:

Our survey resulted in the detection of 19 unique submerged aquatic macrophyte species (Table 1). The most common species from a spatial distribution perspective were the macroalgae Chara spp. and the non-native macrophyte Myriophyllum spicatum (fig. 2). Sixteen of the detected species were native to Connecticut and the New England Region. Two of the native species that were found were state-listed rare species; namely, Myriophyllum sibiricum and Ranunculus longirostris. Both of these species were found in previous assessments and were concentrated in patches near the Sucker Brook and unnamed brook inlets during this initiative. Furthermore, there were small patches of each on the eastern side of the lake and to the north, near the Factory Brook outlet (fig. 3). Our results confirm previous findings regarding these species and suggest that they are persisting in the lake despite distribution/density of

  1. spicatum. However, Potamogeton friesii, which was found in small patches on the eastern side of the lake by NEAR in 2004 and 2008, was not encountered during our survey Given that this species was found in small patches during the surveys that took place nearly a decade ago, it is possible that it is no longer a resident of the lake or that it was missed during this survey. Future surveys will determine   whether this species is truly extinct from Lake Wononscopomuc.

 

Three non-native species were detected during the 2016 plant survey.  The non-native species that were encountered were Myriophyllum spicatum, Najas minor, and Potamogeton crispus.  Myriophyllum spicatum and P. crispus were found during the NEAR surveys and were known to be resident to Lake Wononscopomuc.  However, N. minor was not found during those surveys; during this survey, it was found in small patches along the northeastern shoreline of the lake.  Najas minor is a seed species from Asia and Africa that was introduced to the United States in the early 1900’s.  It is considered a nuisance because in plant communities where there is little productivity and diversity N. minor can dominate the littoral zone by thwarting the colonization of other species.  It was located along the northern shoreline to the west of the unnamed brook (fig. 2).   Potamogeton crispus was found in a few small patches throughout the littoral zone.  Its patches were located in three principal areas: 1) Along the northeastern shoreline, 2) along the southwestern shoreline, and 3) near the unnamed brook on the northern side of the lake (fig. 2).  It should be noted that P. crispus is an early senescing species; it reaches its maximum productivity when water temperatures are cool (May – June).  Therefore, the locations of this species, as mapped during this survey, may not be representative of its true distribution.  To truly understand the distribution of P. crispus, an early season survey/inspection would need to be conducted.  Myriophyllum spicatum was by far the most widely distributed non-native species and inhabited more of the littoral zone than any other species.  Our survey suggests that this species inhabited an area of 40.1ac in a continuous patch encircling the entire lake, which represents 48.2% of the littoral zone (fig. 2).

 

Quantitative Plant Community Analysis:

Lake Wononscopomuc was analyzed quantitatively using 48 individual points that spanned the littoral zone between 0.1 and 6.0m (fig. 4).  At each point the depth was measured and the plant community analyzed in the manner that was described in the methodology section of this report.  The following is broken down in a manner to describe the plant community in a fashion that starts at the largest (holistic) scale and then reviews each individual part of the littoral zone.

 

Holistic Community Composition

 

 

 

As previously indicated, the aquatic macrophyte community of Lake Wononscopomuc contained 19 species that varied in their relative contribution to the holistic community (Table 1).  The four most dominant species were Myriophyllum spicatum, Chara spp., Potamogeton illinoensis, and Vallisneria americana.  These species represented 29.1, 19.4, 10.5, and 8.2% of the plant community, respectively (fig. 5).  There were only two other species that were detected in densities equal to or exceeding 5% of the plant community; those species were Najas flexilis and Potamogeton zosteriformes.  Notably, all of the dominant species except M. spicatum are native; however, M. spicatum did comprise ~30% of the plant community productivity.  The overall assessment of diversity, which couples species richness and individual species’ productivity, indicated that the Shannon Index was 2.29, which suggests that the plant community would be considered diverse by the general ecological literature standards and that Lake Wononscopomuc is very diverse in comparison to other Connecticut lakes (June-Wells et al. 2016).

 

Abundance, Richness, Diversity, and Abiotic Features of the Lake

The productivity of the lake was assessed using a rank-abundance technique that was described in the methodology section of this report and that data collection      initiative indicated that the plant productivity follows a  third-order  polynomial  curve when productivity  is  regressed  against depth  (fig. 6).  Our results  suggest that the most productive areas of the lake lie between 1 and 2m of water depth.  This is not an uncommon finding for lakes because these areas are generally more insulated from wave action and ice-scour compared to shallower areas and have high levels of ambient light.   However, this pattern may be a result of the yearly drawdown       regime that is undertaken by the Lake Wononscopomuc association to thwart the spread of M. spicatum in  to  waters  important to  homeowners  surrounding  the  lake. To confirm these findings, we also regressed the total rank abundance (i.e. plant productivity) against the availability of light (fig. 7). Plant productivity followed a second-order polynomial model when it was compared to light availability. Plant productivity reached its maximum when light was available at ~225watts/m2  (1.e.

1.2m of depth; figs. 1 and 7).  Finally, depth was a better descriptive factor than light in regards to plant productivity because the r-squared value (i.e. how well the model fits the actual data) was marginally higher for the regression of total abundance by depth (figs 6 and 7); therefore, depth should be used in future assessments of the plant  community productivity.

 

Richness, which is the raw number of species encountered at any given point, was also regressed against depth and light.  In this case neither abiotic variable provided a greater amount of fit (i.e. higher r-squared value); both explained the pattern of richness in Lake Wononscopomuc equally well.  The model   developed for the relationship between species richness and depth was a third-order  polynomial model that indicated the most species rich area of the lake is at 1.5m of water depth (fig. 8). However, the curve (i.e. red line) was relatively flat through the 1 to 2.5m-depth zone, which suggests that those areas also house much of the  species richness. The model that was developed to examine the relationship between light availability and species richness was a second-order polynomial  curve that confirmed the findings from our analyses of depth vs. species richness. The examination of light and species richness suggests that the most rich parts of the plant community exist in water depths where there is greater than 160 watts/m2  (i.e. less than 2.5m deep, fig. 9).  The reasons for these findings are

 

 

 

probably much the same as those described for depth and total abundance. Areas of the lake greater than 1m deep are significantly insulated from wave and ice disturbances including high levels of ambient light, which affords more species of varying growth forms to  persist.

 

Community diversity, as assessed by the calculation of Shannon’s Index, is a measure of the richness of a lake balanced by the relative dominance of each species that comprises the aquatic plant community.  It is important because the diversity of a system is related the total number of community interactions that are occurring including all of the potential auxiliary interorganismal (i.e. bacteria with plants and fish with plants etc.) interactions.  In short, diversity is an important metric that is frequently used in ecosystem assessment and research; it can be very valuable in determining lake management requirements.  As was previously mentioned, the diversity of the lake as a whole was high compared to other Connecticut lakes and is considered diverse in the wider ecological literature.  A more precise examination of diversity within the lake system, which included the specific point-diversity vs. abiotic conditions, yielded important results.  When Shannon’s Diversity Index was regressed against depth and light both models resulted in trends of high explanatory value.  The examination of the relationship between diversity and depth was found to be best explained by a third-order polynomial model where the greatest amount of diversity was found at a depth of 1.45m.  However, that model was flat through the entire 1-2m zone, which suggests that the majority of the richness balanced by the relative dominance of each species (i.e. diverse areas) exists in the 1-2m zone.  The reason for this is most likely due to the insulation from wave and ice action or as a result of the drawdown regime.  Our model of the relationship between Shannon’s Diversity Index and light availability confirmed the aforementioned result where the greatest amount of diversity was found where light was available at a rate of 222.5watts/m2 (i.e. 1.46m). Finally, the model was flat between 200 and 250watts/m2 where water depth is between 1 and 2m deep (fig. 10).

 

Community Composition by Depth Class

 

General Trend:

Overall, the proceeding results suggest that total richness increases from the first to second depth class; following, richness decreases throughout to the final depth class. Myriophyllum spicatum becomes increasingly dominant in the third through sixth depth class as light decreases in availability and larger growth forms are favored. The maximum community contribution of M. spicatum was 50% and that occurred in the deepest two zones.

 

0.1 to 1 meter depth class:

The first depth class, which ranged between 0.1 and 1m of water depth, was found to contain 13 species.  The most dominant species were M. spicatum and Chara spp., which each comprised 21.8% of the community of that depth zone.  The tertiary and quaternary species in regards to dominance were N. flexilis and V. americana; both of those species each comprised 10.5% of the plant community in this shallow water zone.  For complete results, see figure 12.

 

 

 

The second depth class ranged from 1 to 2m of water depth and contained 16      aquatic macrophyte species. The most dominant species was Chara spp. and it comprised 26.8% of the plant community.  The second species in regards to    dominance was M. spicatum, which comprised 16.9% of the plant community productivity. The tertiary and quaternary dominant species were N. flexilis and V. americana, which both comprised 9.8% of the plant community. The change from depth class one to depth class two was the addition of 3 species sharing space in this region of the lake.  This is exemplified by the changes in the relative dominance  of each  of the  four dominant species. For complete  results, see  figure  13.

 

The third depth class encompasses depths that range from 2 to 3m; this zone contained 11 species and the most dominant species in this zone was M. spicatum. It accounted 32.8% of the total abundance in this area of the lake, a distinct increase in its dominance compared to the previously reported  zones.

Furthermore, the secondary and tertiary dominant species changed to P. illinoensis and P. zosteriformes, which each comprised 14.2% of the plant community in the 2 to 3m zone.  Chara spp. and V. americana each comprised the 10% of the plant community and shared the quaternary spot in regards to dominance.  The notable change in this zone is the change in the dominance of M. spicatum and the change in composition (i.e. dominant species), which now favor taller growth forms. For complete results, see figure 14.

 

The fourth depth class that was assessed for Lake Wononscopomuc was the area where water depths were between 3 and 4m.  In this area, there was another distinct shift in the plant community; there were 13 species detected in this area with a distinct dominance by M. spicatum, which accounted for 33.3% of the total abundance in this zone of the plant community. The secondary dominant species was again P. illinoensis, which represented 14.3% of the aquatic macrophyte community. However, the tertiary dominant species shifted to Stuckenia pectinata; this species was present in small amounts in the aforementioned zones but comprised 9.5% of the total plant abundance in this zone.  The  quaternary dominant position was shared by two species; namely, Chara spp. and Potamogeton crispus. This zone is an area where P. crispus was encountered, which is probably representative of its mid-summer distribution. For complete  results, see figure 15.

 

The fifth depth class exhibited a pattern where M. spicatum represented wholly 50% of the community abundance and the total number of species present decreased to below 10.  This zone contained 9 species, which represents a notable drop in community richness.  The secondary dominant was Chara spp.; it comprised 15.2% of this zone’s plant community.  The tertiary species was P. illinoensis, which was found to be 10.8% of the plant community in this zone.  The quaternary position was shared by two species: 1) P. crispus and 2) V. americana.  In water this deep, growth form becomes more important compared to shallower water and larger species tend to dominate, as is exemplified by the relative dominance of M. spicatum and P. illinoensis. Chara spp. can also grow in deep

 

 

 

waters but that is related to its photosystem efficiency because it is a macroalgae with very little vertical structure. For complete results, see figure 16.

 

The sixth and final depth class ranges from 5 to 6 meters of water depth. This depth class is limited by light and contained 3 aquatic macrophyte species. Those species were – organized by relative dominance – M. spicatum, Chara spp., and P. illinoensis. Those species represented 50, 42.3, and 7.6% of the community in this zone. In this area of the lake, large growth form like the species M. spicatum and P. illinoensis, are capable of thriving due to their ability to obtain light and due to the lack of interspecific competition. The macroalgae, Chara spp., is capable of surviving low light conditions due to its ability to harvest light when that resource is limited. Therefore, it not uncommon for species like M. spicatum to dominate  these areas with little impact on other species or recreational value. In general,  little management is needed in these areas to enhance ecological or recreational value when other areas of the lake are sufficiently diverse. For full results, see figure 17.

 

 

SUMMARY FINDINGS

 

The following is intended to summarize the results of this study in a concise way; it is bulleted to allow for easy reading and to simplify the results as to allow for application to future management planning.

 

most species rich (figs.  8,9,13)

 

 

 

 

 

CITATIONS

 

Canavan RW, Siver PA. 1995. Connecticut lakes: A study of the chemical and physical properties of fifty-six Connecticut lakes. New London (CT): Connecticut College Arboretum.

 

Connecticut Agricultural Experiment Station: Invasive Aquatic Plant Program. 2004 survey of Lake Wononscopomuc.

 

Crow GE, Hellquist CB. 2000. Aquatic and wetland plants of northeastern North America I/II.  Madison, WI: University of Wisconsin Press

 

Frank R, Purtill JS, Flavell GF, Ward PJ, Stula M. 1959. A fishery survey of the lakes and ponds of Connecticut. State Board of Fisheries and Game Lake and Pond Survey Unit. Report # 1, Project F-4-R

 

Frink CR, Norvell WA. 1984. Chemical and physical properties of Connecticut lakes. New Haven (CT): The Connecticut Agricultural Experiment Station.

 

Jacobs RP, O’Donnell EB. 2002. A fisheries guide to lakes and ponds of Connecticut including the Connecticut River and its coves. Connecticut Department of Environmental Protection: Bulletin  35

 

June-Wells M, Gallagher F, Gibbons J, Bugbee G. 2013.  Water chemistry preferences of five nonnative aquatic macrophyte species in Connecticut: A preliminary risk assessment tool.  Lake and Reservoir Management 29:303-316

 

June-Wells M, Gallagher F, Hart B, Malik V, Bugbee G. 2016.  The relative influences of fine and landscape scale factors on the structure of lentic plant assemblages.  Lake and Reservoir Management 32:2:2016

 

NEAR Report 2004. Aquatic plant mapping in Lake Wononscopomuc, 2004 NEAR Report 2008. Aquatic plant mapping in Lake Wononscopomuc,    2008