Hydrobiologia DOI 10.1007/s10750-016-2739-1
PRIMARY RESEARCH PAPER
An ecosystem management framework to maintain water quality in a macrophyte-dominated, productive, shallow reservoir Kerri Finlay . Richard J. Vogt
Received: 25 November 2015 / Revised: 9 March 2016 / Accepted: 10 March 2016 Ó Springer International Publishing Switzerland 2016
Abstract Loch Leven, SK, is a well-oxygenated, highly productive, clear-water lake dominated by a nuisance species of submergent macrophyte (Elodea canadensis Michaux) whose removal has been suggested to enhance recreational use. Previous empirical and theoretical work, however, has suggested that macrophytes offer an important ecosystem service in such lakes by sequestering nutrients, anchoring sediments, and providing shelter for filter feeding consumers. Macrophyte removal would introduce a risk of shifting the ecosystem to a less desirable turbid state, potentially dominated by toxic planktonic algal species. Here, we present the results of a contemporary (2014) spatio-temporal field survey of Loch Leven, which showed high water quality along several axes of assessment. Results are discussed within the context of historical sedimentary analysis, which indicated increases in algal biomass since 1980. Given the high potential for increases in planktonic biomass should Elodea be harvested, we propose only targeted macrophyte management for Loch Leven, and that
Handling editor: Jasmine Saros K. Finlay (&) Department of Biology, University of Regina, Regina, SK S4S0A2, Canada e-mail:
[email protected] R. J. Vogt Department of Biological Sciences, Trent University, Peterborough, ON K9J7B8, Canada
large-scale Elodea removal programs would have to be paired with dredging of lake sediments to remove the source of internal nutrient loading. We further suggest that a long-term monitoring program be initiated to allow continued assessment of water quality in this shallow, macrophyte-dominated lake. Keywords Elodea canadensis Alternative stable states Cypress Hills Interprovincial Park Adaptive management
Introduction The submerged aquatic macrophyte, Elodea canadensis, is a common nuisance species throughout North America, Europe, Asia, and Australia. It impedes boating and swimming activities, but its spread rarely has negative effects on water quality or other important ecosystem variables (Zehnsdorf et al., 2015). Despite the high water clarity often associated with macrophyte dominance (Richardson, 2008), management solutions are often sought to improve navigation and esthetic appeal in invaded lakes. Common mitigation options include mechanical harvesting, addition of macrophyte-specific toxicants, alteration of lake mixing regimes, or biological control via the introduction of selective herbivores (Zensdorf et al., 2015). At best, these remedies can be successful in temporarily removing biomass, but they do not
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address the root cause of high macrophyte growth (i.e., high nutrient concentrations), and need to be repeated year after year. At worst, these interventions can be costly, ineffective, and because of a lack of mechanistic understanding of physical, chemical, and biological interactions at the ecosystem scale, their implementation can introduce the risk of unexpected shifts to even less desirable ecological states. Shallow lakes have served as a classic example of how ecosystems can shift between resilient states in response to changes in important ecosystem variables such as temperature or the concentration of limiting nutrients (Scheffer et al., 1993; Beisner et al., 2003; Scheffer & Carpenter, 2003; Adrian et al., 2009). Such phenomena are often discussed within the context of alternative stable states, wherein gradual environmental change has little impact until a threshold is reached and an ecosystem state change occurs (Beisner et al., 2003). In the classical shallow lake example, gradual increases in nutrient concentrations eventually precipitate a shift from a macrophyte-dominated, clearwater state to an algae-dominated turbid-water state (Scheffer et al., 1993), though the timing of the change is difficult to predict because of feedback mechanisms that confer resiliency against state change (Blindow et al., 1993; Scheffer et al., 1993; Scheffer & van Nes, 2007). For example, macrophytes stabilize a clearwater state by competing with phytoplankton for nutrients and light, preventing re-suspension of sediment nutrients, and providing refugia from predation for grazers (Scheffer et al., 1993). In fact, fish can exert a strong influence on lake-state in eutrophic ecosystems (Jeppesen et al., 2000), with zooplanktivores reducing grazing pressure on phytoplankton (Zimmer et al., 2001), and benthivores physically modifying habitat by directly uprooting macrophytes (Zambrano et al., 1998) and by re-suspending sediments (Breukelaar et al., 1994), adding to physical turbidity and delivering phosphorus to the water column in support of further algal growth. Once a turbid lake-state is achieved, nutrient concentrations have to be steeply reduced before water clarity will allow re-colonization of macrophytes and re-establishment of a clear-water state (Scheffer et al., 1993). As such, macrophyte abundance has been demonstrated to be an important control over algal abundance and community composition (McGowan et al., 2005; Bayley et al., 2007; Waters et al., 2015), and caution must therefore be exercised with respect to any management
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intervention that would reduce their abundance, which could result in a shift to potentially toxic, algaedominated states. With this in mind, options for macrophyte management can be grouped into three general categories: (1) those that simply kill macrophytes, (2) those that kill macrophytes and remove the biomass, and (3) those that prevent macrophyte growth. Thorough evaluations of mitigation approaches for Elodea canadensis are found in Richardson (2008) and Zensdorf et al. (2015) and are summarized in Table 1. Alternative stable state theory suggests that macrophyte management strategies that would be most effective in preserving long-term water quality would involve biomass removal or a focus on keeping nutrient concentrations low (e.g., Daldorph, 1999). Any management approach that simply aims to kill or reduce the growth of the macrophytes specifically, without removing the nutrients associated with them, could result in shifts to much less desirable turbidwater states. Study Site Loch Leven (surface area = 10 hectares, max depth = 3.1 m) is a spring-fed impoundment that was created in 1932 by the construction of a dam at the end of a drainage area in Cypress Hills Interprovincial Park, located on both sides of the SaskatchewanAlberta provincial border in Canada in the Prairies ecozone. The park is located in the Cypress Upland ecoregion and is characterized by cold winters and warm summers (mean annual temp = 3°C), with higher precipitation (325–400 mm) than in the surrounding mixed grassland ecoregion. Permanent cottages in a lodgepole pine (Pinus contorta Douglas) forest surround Loch Leven, and the land use includes grazing by cattle in pasture areas and minor development for recreational activities. Local residents have anecdotally reported declining navigation and water quality attributable to increases in lake macrophyte coverage over the last several decades, with the first report of extensive Elodea canadensis coverage dating to 1959. Mitigation efforts have been attempted ever since. Chemical herbicides were used in Loch Leven in the 1960s and 1970s, and an aquatic weed harvester was introduced in 1997, though neither approach effectively reduced biomass in the long term. In 1999
Hydrobiologia Table 1 Summary of the advantages and disadvantages of mitigation approaches for reduction of Elodea canadensis and other macrophyte species biomass Category
Methods
Advantages
Disadvantages
Examples
(1) Kill macrophytes and leave biomass in lake
(a) Mowing of weeds without biomass removal
Minimal cost and effort, results in quick improvement to navigation
Need for repetition, no consideration of biological interactions or ecological ramifications of macrophyte removal
(a) discussed in Nichols, (1991)
(b) Water dyes or jute matting block out sunlight and prevent all primary producer growth (c) Macrophyte-specific toxins (diquat, fluridone)
(2) Kill macrophytes and remove biomass
(d) Lake drainage and winter freezing of sediments (a) Weed harvesters combined with biomass removal
(d) Cooke (2007)
Removal of sequestered nutrients will reduce the potential for regime shifts
(b) Introduction of macrophyte-specific grazers (e.g., Ctenopharyngodon idella)
Costlier and more labor intensive than category 1 May shift system to less desirable state if nutrient loading is also not reduced.
(a) Point sources (e.g., septic tanks) (b) Diffuse sources (e.g., fertilizer application)
(a) Di Nino et al. (2005); van Zuidam and Peeters (2012) (b) Ha et al. (2013); Phillips et al. (1999) (c) Curry (2014)
(c) Growth of cattails on floating platforms, land composting of accumulated biomass (3) Macrophyte growth prevention via nutrient load reduction
(b) Hoffman et al. (2013) (c) Glomski et al. (2005); Wagner et al. (2007)
Reduction in nutrient loading should result in a long-term solution, minimize potential for shifting to algal-dominated state
(c) Water column removal (e.g., ferric dosing)
Requires lengthy and expensive sampling to identify nutrient sources, may delay management efforts
(a) Jones & Lee (1979) (c) Daldorph (1999) (d) Philips et al. (1999) and refs therein
(d) Internal loading (e.g., lake dredging) from sediments
and 2000, white amur grass carp (Ctenopharyngodon idella Valenciennes) were introduced but low temperatures and potential winter anoxia under ice reduced their feeding and macrophyte reduction success. In 2008, a Solar Bee lake aerator (Medora Corporation, Dickenson North Dakota) was purchased to maintain high oxygen concentrations and favorable redox gradients to prevent sedimentary nutrients from entering the water column. During the seven years of use, no significant macrophyte biomass reduction was observed, and the occurrence of a cyanobacterial bloom in 2013 suggests that the aerator has not been effective in improving conditions in Loch Leven.
While lake residents have voiced a clear desire to manage Loch Leven’s macrophyte community, we use this study to outline a series of initiatives that should precede direct intervention to remove vegetation. First, because reports of declining quality are currently anecdotal, we have undertaken an evaluation of lake water clarity, fish habitat, potential toxins, and pathogens with a field study in 2014. Paramount to this field study is identification of primary nutrient sources as potential targets for nutrient load reduction. These sources include (A) natural inflows—as a lake on naturally productive prairie soils, any runoff from the catchment likely contains considerable nutrient
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loadings, (B) septic tank leakage—Loch Leven is surrounded by many seasonal cottages, and (C) internal loading of nutrients—Loch Leven is a shallow, polymictic system with accumulated sedimentary nutrients that can become bioavailable during storms and other mixing periods. Finally, we discuss the need for ongoing monitoring of Loch Leven within the context of a historical study of lake productivity from its sedimentary record, so that the potential for shifts in ecosystem state can be identified, and remedial action can be taken in advance of state changes. Ultimately, any adaptive management plan for Loch Leven should incorporate the mechanistic underpinnings of the physical, chemical, and biological interactions that underlie shifts between alternative ecological states. Such a plan would allow lake managers to maximize water quality and improve recreation, while minimizing risks associated with shifts to algal and cyanobacterial dominance.
Materials and methods Current conditions We sampled six sites (5 littoral, 1 pelagic) once per month (June–Sept) in Loch Leven in 2014 to obtain an overview of potential chemical and biological changes over the course of the ice-free season. Littoral sites ranged in depth from 0.5 to 1.0 m and were situated to capture different points of hydrologic interest, including the primary inflow to the south (L2), the primary outflow to the north (L4), and sites either close to (L3, L4, L5) or more distant from cottages (L1). The pelagic site was located at Loch Leven’s deep station site (3 m), which was located very close to the outflow (Fig. 1). We assessed water temperature with a YSI multiparameter meter, taking readings just below the surface (0.1 m) at the littoral stations, while the pelagic station included a profile, measured at 0.1 m and then every 0.5 m to the bottom. We assessed water clarity at the pelagic station using a Secchi disk (depth, m). We measured pH and O2 concentrations using the YSI meter at 0.1 m for each littoral site, and as a profile at the deep site. We collected water samples for nutrient analyses from each site using a van Dorn water sampler (0.1 m depth), filtered through GF/C filters, and stored
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on ice during transport and at 4°C in the lab until analysis for nitrogen (TN, NO3- and NH4?), and phosphorus (soluble reactive P (SRP) and total dissolved P (TDP)) within four months of collection. During this time period of sample storage, there may have been conversion of nitrogen species and nitrate and ammonia concentrations should be interpreted cautiously. We are confident, however, that total nitrogen concentrations will be representative of nutrient levels in the lake. Nutrient analyses were conducted at the Institute of Environmental Change and Society (IECS) at the University of Regina following the procedures of Stainton et al. (1977). We collected bacterial samples from each site as unfiltered surface water using a van Dorn sampler (0.1 m), stored them in acid-washed polycarbonate bottles, and transported them on ice to the University of Regina for analysis within 48 h of collection for bacterial genetic markers, which is an indication of Escherichia coli T. Escherich (E. coli) source (human vs. other animal) (Tambalo et al., 2012) and for pathogens (Campylobacter, Salmonella, hemorrhagic E. coli). We estimated phytoplankton abundance by measuring Chlorophyll a concentration of whole water samples from 0.1 m van Dorn samples from each site and filtering on GF/C filters within 12 h of collection. Samples were frozen during transport and storage, and chlorophyll pigments were extracted in acetone: methanol: water (80:15:5) using standard trichromatic methods and analyzed via spectrophotometry using a Shimadzu UV-1800 UV Spectrophotometer (Wetzel & Likens, 1996). Historical trends We took a 32-cm sediment core from Loch Leven (Aug 26, 2014), near the deep station at 3 m depth using a gravity corer. The core was sectioned on site (5 mm intervals) and stored on ice for transport. Samples were frozen (-10°C) and analyzed within 4 months of collection. Sediment sections were freeze-dried and analyzed for algal pigments using high performance liquid chromatography (HPLC) and stable isotopes using isotope ratio mass spectrometry (IRMS) at the IECS laboratory at the University of Regina. Pigments were extracted using an 80:15 acetone: methanol solvent and analyzed using a Hewlett Packard model 1100
Hydrobiologia Fig. 1 Map of Loch Leven and Sampling sites. L1–L5 represent the locations of the littoral sites (\1 m depth) that were sampled during summer, 2014. ‘‘Deep’’ is the deepest sample site (3.0 m) near the outflow at the north end of the lake
HPLC and reported as nmol pigment g organic matter-1. Total primary producer biomass (both algal and macrophyte) was approximated using betacarotene and pheophytin a, while diatoxanthin was used to represent diatoms, alloxanthin for cryptophytes, canthaxanthin for colonial cyanobacteria, and okenone for purple sulfur bacteria. Additional pigments were analyzed (aphanizophyll (potentially N2fixing cyanobacteria), myxozanthin (filamentous cyanobacteria), lutein-zeaxanthin (chlorophytes and cyanobacteria), echinenone (total cyanobacteria), fucoxanthin (diatoms with chrysophytes and dinoflagellates)), but these results are not presented graphically here. Interpretation of data followed Leavitt & Hodgson (2001). Freeze-dried sediments were packed in tin capsules before combustion and analyzed using a Thermoquest Delta Plus isotope ratio mass spectrometer equipped with a Thermoquest NC2500 Elemental Analyzer. Carbon (d13C) stable isotopes indicate the relative abundance of inorganic vs. organic carbon in the sediments and nitrogen (d15N) can provide insights into cyanobacterial production and nitrogen fertilization in the landscape. Interpretation of stable isotopes
followed Fry (2008). Sediment chronology was determined by evaluating Cs137 activity on 10 sediment samples distributed through the core as described in Gallagher et al. (2001). Loch Leven was formed in 1932, and thus, Pb210 analyses are unlikely to reach background levels, but we analyzed 10 samples surrounding the Cs137 peak for Pb210 to corroborate the dating results. Sediment age and mass accumulation rates (g cm-2 year-1) were calculated assuming a constant deposition rate since the 1964 atmospheric maximum of Cs137 as recorded in the sediments. We used analysis of variance (ANOVA) to detect spatial and temporal differences in chlorophyll a (Chl a; lg l-1), nitrate (NO3-; lg l-1), ammonium (NH4?; lg l-1), total nitrogen (TN; lg l-1), soluble reactive phosphorus (SRP; lg l-1), and total phosphorus (TP; lg l-1) concentrations. All nutrient concentrations, except for TN, were log transformed to conform to assumptions of normality. Sediment pigment and stable isotope data were plotted with a Loess smoother (span 0.75) to more clearly identify points of inflection in the time series. All statistical analyses and graphing were performed using R version 3.2.0 (R Core Development Team, 2008).
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Results Elodea canadensis coverage was estimated at 50% of total lake area throughout summer 2014 and was present in all areas \2 m in depth. The only areas that were consistently free of macrophyte biomass were the deepest areas of the lake (3 m) and a small, maintained sandy beach area near site L3. The water column was very clear, with Secchi depths always reaching the bottom of the lake (max depth, 3 m). Loch Leven was polymictic in 2014 (Fig. 2) and showed no evidence of thermal stratification or changes in pH with depth (Fig. 2a). Dissolved O2 in the water column increased from July–August, with a slight reduction in September (Fig. 2b), and profiles showed slightly decreased concentrations below 2.5 m, but never fell below 6 mg l-1, which is sufficient oxygen for fish survival (CCME, 1999). There were few spatial trends in Loch Leven nutrient concentrations (Table 2). No one site had consistently elevated nutrient levels (all ANOVA F values \1.2, P [ 0.1). These results suggest that there were no strong external point sources of nutrients to Loch Leven. Total water column nitrogen concentrations ranged from a minimum measured value of 416 lg l-1 up to a maximum of 695 lg l-1, while total phosphorus ranged from 5.18 to 20 lg l-1, and Chl a ranged from 0.20 to 2.32 lg l-1. Temporally, nutrient and chlorophyll a concentrations varied over the duration of the summer (Fig. 3), as all nutrients (except NO3-) were significantly higher in August and September than in June or July (ANOVA, TP: P = 0.021; Chl a: P = 0.0006; NH4? P = 0.0048; SRP: P = 0.0055; TN: P = 0.0008), and some showed declines again in September (NH4? and TN). There was no evidence of bacterial contamination of Loch Leven during 2014. All samples for E. coli were below the detection limit and were not further analyzed for the presence of pathogens (data not presented). The lack of nutrient peaks or pathogenic bacteria in the littoral sites suggests that septic tank leakage from surrounding cottages had no measureable impact on Loch Leven. The Cs137 analyses demonstrated a pronounced peak in activity in the 26- to 26.5-cm section (Fig. 4a), resulting in a constant sedimentation rate of 0.52 cm year-1 which was used to estimate the age of each section of the sediment core. The young age of Loch Leven prevents accurate sediment dating using
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Fig. 2 Depth profiles of a temperature, b oxygen concentration and c pH in Loch Leven, 2014
Pb210 analyses, as the activity of Pb210 never reached background levels (Fig. 4b). Despite this, the Pb210 dating analyses demonstrated a log-linear decay rate over time, with a sedimentation rate of 0.451 cm year-1. Furthermore, the sedimentation rates based on Cs137 and Pb210 is on par with what would be expected if the 32 cm core spanned the life of the lake,
Hydrobiologia Table 2 Average and standard deviation (in brackets) of nutrients (total nitrogen (TN) ammonium (NH4?); nitrate (NO3-), total phosphorus (TP), and soluble reactive phosphorus (SRP)), and chlorophyll a values observed in Loch Leven, 2014 Nutrient TN (lg l-1) NH4? NO3-
-1
Deep
L1
L2
L3
L4
L5
F-statistic
596 (118)
551 (64)
517 (68)
556 (84)
570 (114)
557 (106)
0.296
62.5 (54.4)
65 (41.2)
0.435
6.8 (3.25)
8.52 (6.78)
1.144 0.385
(lg l )
37.5 (12.6)
50 (18.2)
37.5 (5)
42.5 (36)
(lg l-1)
5.29 (2.32)
6.70 (9.38)
39.4 (42.4)
3.19 (1.01)
TP (lg l-1)
11.7 (5.7)
8.84 (2.04)
16.3 (11.0)
11.4 (6.1)
11.0 (6.3)
9.3 (0.83)
SRP (lg l-1)
2.39 (2.76)
2.73 (4.61)
7.68 (9.39)
7.36 (9.20)
4.12 (4.21)
1.74 (2.10)
0.355
Chl a (lg l-1)
0.86 (0.61)
0.53 (0.07)
0.95 (0.92)
0.91 (0.96)
0.66 (0.45)
0.62 (0.32)
0.077
All values are in lg l-1. ANOVA F-statistic results are from the comparison mean nutrient concentrations among sites. All ANOVA P values were [0.1 and are not reported here
which was created in 1932 (=0.40 cm year-1) and thus is considered to be an acceptable estimate of the chronology of the sediments. Sediment core analysis shows that total primary producer biomass production in Loch Leven has been increasing since 1980, with similar upward trends detected as well for diatoms, cryptophytes, cyanobacteria (Fig. 5), and N2-fixing cyanobacteria (data not shown). Purple sulfur bacteria also show a dramatic increase beginning in 1980. These purple sulfur bacteria are obligate anaerobes, indicating that Loch Leven began to experience reduced oxygen conditions roughly 35 years ago. The stable isotope data show a similar trend, with all measures except d13C changing near 1980 (Fig. 6). The increasing %N and decreasing trend of C:N are furthermore indicative of increased production in this system. The recent values of 2% N of the sediments and a C:N ratio near 9.0 suggest that almost all sediment organic matter is composed of autochthonous organic biomass and is indicative of increased primary production in the lake.
Discussion Loch Leven exhibited good water quality in 2014 from the perspective of both human recreational use and fish habitat. Dissolved nutrient and chlorophyll a concentrations establish the lake as oligo- to mesotrophic (Wetzel, 2001), in contrast to the more common eutrophic status of other prairie lakes in the nutrient replete prairie ecozone (Hall et al., 1999; Patoine et al., 2006). Oxygen levels never fell below 6 mg l-1 during 2014, even near the sediments, which is sufficient to sustain local fish populations (CCME, 1999), and high water clarity was sufficient to support visually
orienting predators. There was no detectable fecal contamination as evidenced through the E. coli and pathogen analyses, and no toxic cyanobacterial species were observed, suggesting that this system is presently safe for recreational use. While water quality was unquestionably high in 2014, shallow, macrophyte-dominated lakes like Loch Leven still often support high rates of primary production (Brothers et al., 2013; Hobbs et al., 2014). Macrophytes provide a substrate for periphyton growth, and high water clarity allows for the growth of benthic algae, both of which serve to sequester nutrients from the water column and prevent the proliferation of planktonic algae and cyanobacteria in the water column (Eugelink, 1998; Brothers et al., 2013). It is for this reason that claims of high water quality in 2014 remain compatible with results from paleolimnological and stable isotope analysis that indicated increases in primary producer biomass since 1980. While no cyanobacterial blooms were observed in 2014, pigment data suggest increased incidences of cyanobacterial growth since 1980, timing that is consistent with other regional prairie lakes (Hall et al., 1999; Patoine et al., 2006; Maheaux et al., 2015). Specifically, this timing coincides with anthropogenic influences collectively known as the Great Acceleration, which includes factors such as increasing regional fertilizer use in agriculture, increases in CO2 emissions, and subsequent climate change (Maheaux et al., 2015). Furthermore, the increasing abundance of purple sulfur bacteria over this same interval suggests likely reductions in O2 concentrations in winter under ice, as the water column remained well oxygenated in 2014 during the ice-free season. While this study did not include winter water chemistry, under-ice anoxia could have facilitated
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Hydrobiologia Fig. 3 Boxplots of a chlorophyll a concentrations and nutrients: b NO3-, c NH4?, d SRP, e TP, and f TN by month in Loch Leven, 2014. P values represent ANOVA statistics, and letters above each box denote significantly different averages
internal loading of phosphorus from the sediments (Wetzel, 2001), which was subsequently redistributed into the water column during spring turnover. This proposed mechanism accords with anecdotal reports of a bright pink substance appearing at the lake’s outflow after the formation of ice at Loch Leven in 2015 (no data available). The stable isotope data similarly indicates increasing production in Loch Leven, beginning around 1980,
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increasing by *1990, and then leveling off in the early 2000s. While the d13C signature did not demonstrate strong fluctuations, it does suggest that the sediments consist primarily of terrestrial or aquatic plant material ([-28%) or lacustrine algae (\-26%) and that inorganic carbon (*0 %) did not comprise a large proportion of the sedimentary C (Meyers & Teranes, 2001). The increasing %N and declining C:N ratios are indicative of internally produced biomass
Hydrobiologia
(a)
0 5
Depth (cm)
10 15 20 25 30 35 0
2
4
6
8
Cs137 Acvity (dpm g-1)
(b) 0 5
Depth (cm)
10 15 20 25 30 35 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Pb210 Acvity (dpm g-1)
Fig. 4 Dating results of the 32 cm core taken from Loch Leven in 2014. Cs137 activity over the depth of the core a indicates a sedimentation rate of 0.52 cm year-1, while the Pb210 activity vs. depth b indicates a sedimentation rate of 0.45 cm year-1
rather than older, more refractory organic material from the catchment (Meyers & Teranes, 2001). The d15N signature supports the observation of increasing cyanobacterial growth from the pigment data and might also be influenced by atmospheric deposition. N2 gas from the atmosphere has a stable isotope signature of 0 %, so cyanobacterial nitrogen fixation tends to shift N isotopic signatures lower (Fry, 2008). Although industrially fixed N fertilizers themselves have a similar 0 % signature of d15N, denitrification occurs after application on land and lighter nitrogen isotopes are lost to the environment (Anderson & Cabana, 2005). Therefore, nitrogen entering water systems from fertilizers have a higher isotopic signature and be expected to increase the d15N values (Bunting et al., 2007). Atmospheric deposition of low d15N from anthropogenic sources may also explain the trend of declining d15N in the time series (Holtgrieve
et al., 2011), but atmospheric deposition of N is typically considered to account for only a small fraction of prairie lake N inputs (Patoine et al., 2006), and only a full N budget will be able to tease apart the relative contributions of N2 fixation and N deposition to the d15N in the sediments. Attempts to establish the role of nutrients to Loch Leven’s primary production in 2014 identified no major point sources, with no statistical differences in nutrient concentrations among sample sites. The total lack of E. coli throughout the system removes human waste via septic tank leakage as a potential source of water column nutrients as well. This study did not test loading of nutrients that may have occurred during the spring snow melt period, so we can not exclude spring runoff as a source to the annual nutrient loading, but the summer results presented here do not support any significant nutrient loading from the catchment. Instead, summer nutrients are likely to be internally derived in Loch Leven through several mechanisms. Elodea die-off later in the season due to self-shading can release N and P to the water column (van Donk et al., 1993). Indeed, water column nutrient concentrations were all elevated in August (Fig. 2), which could correspond to a period of Elodea die-back. Additionally, the increasing presence of purple sulfur bacteria in the sedimentary record suggests that periods of anoxia do occur in Loch Leven, likely under ice, which would mediate P release and bioavailability when the lake turns over in spring (Wetzel, 2001). Finally, while the dense macrophyte beds should mitigate physical re-suspension of sediments during the summer, some mixing likely occurs during large storms, potentially bringing additional nutrients into the water column. Loch Leven, macrophyte management, and the potential for state change While the 2014 field sampling program points to high water quality in Loch Leven via several limnological indices (water clarity, oxygen concentration, and the lack of pathogenic bacteria or toxic cyanobacteria), historical pigment analysis clearly indicates water quality may, in fact, have been higher in Loch Leven’s past when considering the dramatic increase in primary production over the past several decades (Fig. 5). Knowing that primary production has increased so steeply, any contemporary water quality
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Fig. 5 Pigment concentrations from a sediment core taken at the deep site in Loch Leven, 2014. a diatoxanthin, b alloxanthin, c canthaxanthin, d okenone, e pheophytin a, and f beta-carotene.
All pigments are in nmol pigment g organic matter-1. Lines are drawn using Loess smoother in R (span 0.75), and gray shaded regions indicate 95% confidence interval
management effort that involves large-scale ecosystem manipulation should be exercised with extreme caution. While the highly prolific submerged macrophyte Elodea canadensis might represent a nuisance for local residents, we argue that it is performing an important ecosystem service in Loch Leven by sequestering nutrients that might otherwise be used in planktonic primary production.
We suggest that management intervention in Loch Leven should never be limited to simply culling macrophyte growth; rather, management of aquatic vegetation must include nutrient mitigation and plant biomass removal. If Elodea canadensis is simply killed via mechanical harvesting, shading by chemical dyes, application of macrophyte-specific toxicants, or grazer introduction (Table 1), subsequent increases in
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Fig. 6 Stable isotope analyses from sediments taken at the deep site in Loch Leven, 2014. a nitrogen stable isotope value (dN15 %), b carbon stable isotope values (dC13 %), c percentage of sediment that was present as nitrogen, and d the ratio of carbon
to nitrogen atoms in the sediment. Lines are drawn using Loess smoother in R (span 0.75), and gray shaded regions indicate 95% confidence interval
water column nutrient concentrations are sure to result in increased phytoplankton or cyanobacterial biomass production, decreases in water clarity, and possible increases in plankton-associated toxins. Instead, we suggest that nuisance Elodea harvesting be kept to a minimum and confined to areas of particularly high recreational use (swimming and boating areas). Any
harvested material should be collected and composted on land, and the cleared areas should be monitored for changes in nutrient concentrations and occurrences of cyanobacterial blooms. Any more comprehensive program of macrophyte removal should be accompanied by dredging of lake sediments to pre-1980s levels to remove accumulated
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nutrients and minimize the possibility for state shift to the less desirable turbid-water state via open water eutrophication. This option would be costly (exceeding $1 million) but has been proven to be a successful strategy for managing shallow prairie lake ecosystems (e.g., Wascana lake, Regina, SK (Hughes, 2004), but see Phillips et al., 1999). Regardless of which management intervention approach is selected, we recommend monthly water quality monitoring in summer to inventory both macrophyte and algal biomass, and test for the presence of potentially toxic cyanobacterial blooms and/or toxins. Additionally, a full lake and sediment nutrient budget will allow for a comprehensive evaluation of the potential for internal loading in the lake. If it appears that the lake is experiencing more frequent or more severe algal blooms, then the possibility of draining and dredging the lake should be evaluated more thoroughly.
Conclusions Loch Leven is a polymictic, highly productive, macrophyte-dominated lake in which primary production cannot be attributed to a specific point source of nutrient loading, either from the surrounding catchment or from contamination by septic tank leakage or fertilizer input. Despite evidence of increases in primary productivity over the past 40 years, with shifts to cyanobacterial and anaerobic bacterial prominence at times, the overall water quality of Loch Leven remains very good, with no evidence of pathogen presence or fecal contamination, no toxic cyanobacterial production, and high water clarity and oxygen availability. Any efforts to mitigate the nuisance macrophyte population in this lake must therefore be taken cautiously to avoid tipping the system into the less desirable turbid-water state. Acknowledgments The authors thank Brittany Hesjdal, Jared Wolfe, Kristen Leigh, and Dr. Brian Sterenberg for sample collection in Loch Leven, Cypress Hills. We thank the Institute of Environmental Change and Society for the chemical, pigment, and stable isotope sample analyses. Dr. Bjoern Wissel and Dr. Peter Leavitt provided sampling equipment and Dr. Christopher Yost provided laboratory analyses of bacterial samples. Dr. Rob Wright was instrumental in the initiation of this project and provided direction and guidance throughout, and the authors also thank Brant Seifert and the staff at Cypress Hills Interprovincial Park for logistic support and assistance with data sources and historical context. Matthew
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Bogard provided comments that improved the manuscript. Funding was provided through the Saskatchewan Ministry of Parks, Culture and Sport, and the University of Regina.
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