Submergent macrophytes in a cooling pond in Alberta, Canada

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Aquatic botany

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ELSEVIER

Aquatic Botany 51 (1995) 243-257

Submergent macrophytes in a cooling pond in Alberta, Canada Barry R. Taylor*, Joseph Helwig 1 Golder Associates Limited, 1011-6 Avenue S. W., Calgary, Alta. T2P OWl, Canada

Accepted 15 March 1995

Abstract The structure of the aquatic macrophyte community and factors leading to high plant production were examined in a 450 ha cooling pond on the prairie of southern Alberta, Canada, that receives thermal effluent from a coal-fired electrical generating station. Late-summer standing crops, coverage and species composition were measured at intervals along ten transects perpendicular to the shore around the littoral zone of the pond. Physical and chemical features of the water column and sediments were measured concurrently to elucidate the factors controlling macrophyte growth. The cooling pond supported a simple but extraordinarily productive plant community dominated by species tolerant of high water temperatures and high concentrations of dissolved solids and alkalinity produced by evaporative concentration. Three of the seven dominant species are uncommon or seldom abundant in other lakes of the region but were apparently favoured here by high water temperatures. Standing crops of macrophytes at most points within the littoral zone were moderate ( < 100 g m -2 dry mass), but were extremely high along the sheltered north and east shores (400-900 g m-2). The spatial distribution of species within the pond was evidently governed by ( 1) presence of suitable substratum, (2) light penetration, and (3) exposure to wave action. The situation in Sheerness cooling pond is comparable with that in other lakes receiving thermal effluent. Keywords: Thermal effluent; Production; Temperature; Plant community; Potamogeton; Ranunculus

1. Introduction Increased production by aquatic plants is a frequent occurrence in cooling ponds and lakes receiving heated industrial effluents. Aquatic macrophytes can become a nuisance in heated lakes if excess production leads to clogging of water intake pipes, deterioration of * Corresponding author. Present address: Terrestrial and Aquatic Environmental Managers, Suite 302, 2255 Hanselman Court, Saskatoon, Sask. S7L 6A8, Canada. 0304-3770/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI0304-3770(95)00475-0

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fish habitat or a reduction in aesthetic appeal or recreational potential of the lake. Nevertheless, there have been relatively few studies examining the effects of thermal effluents on aquatic macrophyte communities (Anderson, 1967; Grace and Tilly, 1976; Haag and Gorham, 1977; Svensson and Wigren-Svensson, 1992). The work of Haag and co-workers on Lake Wabamun, a eutrophic lake in the aspen parkland of central Alberta, Canada, that receives cooling water from a coal-fired electrical generating station, has remained the most comprehensive study of thermal effluent effects on aquatic macrophytes (Haag and Gorham, 1977; Haag, 1979, 1983). Plant growth began 2-3 months earlier at the heated site in Lake Wabamun than elsewhere in the lake because of warmer water and better light penetration unhindered by ice cover. Plants at the warm site also reached peak biomass, flowered and senesced earlier than at the cool site and produced on average five times more seeds (Haag, 1983). In addition to increased production and accelerated life cycles, the species composition and community structure of the macrophyte community is often different at sites receiving thermal effluents because different species are more or less favoured by the higher water temperatures (Grace and Tilly, 1976; Haag and Gorham, 1977). Local environmental factors such as water chemistry, water currents and sediment texture modify the response of the plant community to heat additions. Effects of thermal effluents might be expected to be more pronounced in the cool temperate climate of central Canada than in more southerly regions of the continent. Given the increasing number of water bodies receiving heated water from power stations and industrial discharges, it is important to understand the effects of heat additions on aquatic macrophyte communities. We report here on a preliminary survey of the aquatic macrophyte community and associated environmental factors at a cooling pond for an electrical generating station in south--central Alberta, in the same region as Lake Wabamun. Our objective was to determine the species composition, distribution and standing biomass of macrophytes in the pond, and to measure physical-chemical features of the pond to study how heat additions, interacting with other environmental factors, had influenced the aquatic plant community. We compared our findings against those for other lakes of the region and in particular Lake Wabamun, to test the generality of heat effects in that more thoroughly studied lake to other temperatezone water bodies receiving thermal effluent.

2. Materials and methods 2.1. Site description

The Sheerness Electrical Generating Station, operated by Alberta Power, is located in an area of rolling, treeless prairie in southern Alberta, Canada, about 170 km northeast of the city of Calgary (51°29'N, 11 l°40'W). The station discharges heated water from condenser cooling to a 450 ha cooling pond constructed in 1983 by erecting a rock and earth berm along the south and west sides of a natural hillside (Fig. 1). Make-up water to replace evaporative losses from the pond is pumped from the Red Deer River through a 40 km pipeline, and overflow water is fed into an irrigation canal (Fig. 1). The pond receives no natural drainage, however, and is normally operated as a closed-loop system.

B.R. Taylor, J. Helwig /Aquatic Botany 51 (1995)243-257

tNJ

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1

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245

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Inlet Conal

BE

Pump House

Power Plant

PIPELINE FROM RED DEER RIVER

Discharge Canal

~333 OUTFLOW TO CAROLSIDE RESERVOIR

Fig. I. Bathymetric map of Sheerness Cooling Pond in southern Alberta, Canada.

The cooling pond has a calculated mean depth of 6.3 m; greatest depths, exceeding 14 m, occur along the constructed south and west banks. About two-thirds of the pond is over 9 m deep and 11% of its area (50 ha) is less than 2.5 m deep. The generating plant withdraws water from the north end of the pond through a 150 m canal, and discharges heated water, up to 1 I°C warmer than intake water, to the southeast corner of the pond (Fig. 1 ). The climate of the region is dry continental with short, cool summers and long, cold winters. Annual precipitation totals only 400 mm. Natural lakes of the region are frozen

246

B.R. Taylor, J. Helwig/ Aquatic Botany 51 (1995) 243-257 25'

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Fig. 2. Monthlymeanwatertemperaturesin SheernessCoolingPond (pumphouseinletcanal) fromJanuary 1988 to December1989. from late November to mid-April, but Sheerness pond remains open throughout the winter. Mean monthly temperatures of intake water exceed 20°C in July and sometimes August, and are maintained above 2-3°C in mid-winter (Fig. 2); daily temperatures in midsummer may exceed 27°C. By contrast, mean daily air temperature in January is - 15.6°C at Hanna, the nearest meteorological station, and temperatures can remain below - 20°C for weeks in winter. Mean daily July air temperature is 17.8°C (Environment Canada, 1982). Water chemistry in Sheerness cooling pond has been monitored since 1985 by Alberta Power. Conductivity and pH were measured with electronic meters, major ions by atomic absorption spectrometry and alkalinity, total phosphorus and total nitrogen by standard methods of the American Public Health Association (APHA, 1985). Water in the pond is very alkaline and rich in total dissolved solids (Table 1) because of the evaporative concentration of Red Deer River water, which itself carries relatively high ion concentrations. As would be expected from the high calcium concentration, the water is very hard ( 170270 mg 1- ~as C a C O 3 ) . Sheerness Pond water is well supplied with phosphorus and nitrogen (Table 1) although most N is organic and nitrate concentrations tend to be low, usually less than 0.01 mg 1- ~. Aquatic macrophytes began to colonize the pond soon after it was constructed; macrophytes are confined largely to the north and east banks where the natural slope creates a littoral zone (less than 3 m depth) for up to 100 m from shore. Macrophyte growth is also conspicuous in the inlet canal to the power station, located at the north end of the east bank (Fig. 1). Prolific macrophyte growth creates a nuisance for recreational use of the pond as well as more serious operational problems for the generating station. Coarse steel screens (trash racks) prevent floating debris from entering the generators with cooling water; fragments of aquatic plants have accumulated on these screens in midsummer in such volumes that the capacity of the cooling water system has been significantly reduced. Approximately 30 m 3 of plant material was removed from the trash racks in 1991 (Alberta Power data). 2.2. Methods

Sheerness cooling pond was surveyed on three occasions in 1991, during 21-23 May, 29-31 July and 26-29 August. Only water quality was measured on the first sampling trip.

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247

Table 1 Summary of water chemistry in the Sheerness cooling pond, 1985-1991. Units are all mg 1- ~ unless indicated otherwise Variable

Mean

Standard deviation

Max.

Min.

N

pH (units) Conductivity (/zS c m - ~) TDS a'b Alkalinity Total P Total N Calcium Magnesium Sodium Sulphate Chloride Carbonate Bicarbonate d

8.2 492 301 223 0.12 0.56 45.0 21.4 43.0 81.7 5.4 2.8 241

0.4 20.5 64.5 71.7 0.14 0.23 6.3 3.0 7.2 12.6 2.9 5.8 42.0

8.8 523 470 396 0.40 0.88 59.5 25.6 57.7 101 12.1 18.0 351

7.7 413 256 166 0.02 0.20 38.2 17.2 34.0 60.5 0.50 0 191

11 10 11 10 9 8 11 10 10 11 11 11 11

"Total dissolved solids. OCalculated.

Surface (1 m) water temperature and specific conductance were measured at 11 points around the pond (Fig. 3) on each sampling trip with a YSI Model 3000 temperatureconductivity meter (Yellow Springs Instruments, Yellow Springs, OH). In addition, temperature and specific conductance were measured at 1-m intervals from surface to sediments at the north and south ends of the pond on each trip. Specific conductance values, corrected to 25°C, were used to estimate total dissolved solids (TDS) by multiplying by 0.552. This conversion factor was determined empirically from simultaneous TDS and conductivity data (n = 30) collected in 1983-1984 from two sites on the Red Deer River before installation of the pipeline to the pond (data provided by Alberta Power). In most natural fresh waters, this conversion factor ranges from 0.55 to 0.70 (APHA, 1985). Transparency readings in the deepest part of the pond were obtained on each trip using a 15 cm diameter Secchi disc. Macrophytes were sampled along ten transects about Sheerness pond, as shown in Fig. 3; most sites were sampled only in August, except for Sites T1, T2 and T3 which were sampled in both July and August. Locations were chosen to cover all sides and shoreline types in the pond, but were deliberately weighted toward the north and east sides where plant growth was most abundant. At each transect site, a marker was anchored about 100 m offshore for use as a guide and to measure the distance from shore with an optical rangefinder. At the 100 m sampling point for each transect, transparency was determined with a Secchi disc and in July a sediment sample was collected with a Ponar dredge. Bad weather prevented collection of a sediment sample at Transect 10 and only large rocks were encountered at Transect 6. Sediment samples were analysed by the University of Alberta, Edmonton, for particle size distribution, and contents of water, organic matter and extractable nitrogen and phosphorus by standard methods. Particle sizes used were gravel ( > 2 mm diameter), coarse sand ( > 250/xm), fine sand ( > 63/zm) and silt and clay ( < 63/~m). Water content was

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LEGEND - - -- "-I MACROPHYTE TRANSECTS • SURFACE TEMPERATURE LOCATIONS Fig. 3. Locations of sampling stations for water quality and transects for macrophyte sampling in Sheerness Cooling Pond.

determined as the change in sample mass after drying to constant mass (48 h, 65 °C ); organic matter content was taken as loss on ignition at 550°C. Extractable N and P, the best estimates of the nutrient concentrations available for biological uptake, were measured by extracting sediments in sulphuric acid, followed by N and P determination on the water phase according to methods in APHA (1985). Four or five sample points were determined at approximately equal intervals between the 100 m point and shore. At each point, a diver swam in a 5 m diameter circle around the boat and estimated the amount of plant cover and identified species present along the circumference. At one or more of the sample points, aboveground parts of all vegetation within a 0.25 × 0.25 m z frame was collected, spun to remove surface water with a portable lettuce spinner (a hand-held device that removes water by centrifugal force), sorted by species, and weighed immediately on a calibrated electronic scale. Plant wet mass was converted to dry mass assuming macrophytes contain 85% water (Wetzel, 1975); three confirmatory samples (wet mass 30-335 g) air-dried under forced air produced a comparable conversion factor (range: 78-86% water). Samples of all macrophyte species were preserved for later confirmation of identification. Plant samples were also collected from the power plant intake screens and identified to ascertain problem species.

B.R. Taylor, J. Helwig/Aquatic Botany 51 (1995)243-257

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3. Results

3.1. Physico-chemical conditions Mean surface temperature in the cooling pond already approached 20°C in May 1991 (range: 17.3-20.5°C, n = 11 ) ; surface temperatures peaked near 25°C in July (range 22.527.5°C) and were only slightly cooler in August (mean 23.0°C, range 22.0-26.0°C). Surface temperatures varied by up to 5°C at different points around the pond, with highest temperatures along the south and east sides (Fig. 4), closest to the cooling water discharge canal. In July, recorded surface temperatures at Sites 7, 8 and 9 (Fig. 3) averaged 26.6°C; temperatures at Sites 1, 2 and 11, at the far end of the pond, averaged 22.7°C. Surface-to-bottom temperature profiles at the deepest part of the pond ranged from 20.3 to 16.7°C in May, 26.3 to 23.2°C in July, and 22.5 to 21.0°C in August (Fig. 4). Some T e m p e f a t u r e Profiles South End •

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250

B.R. Taylor, J. Helwig / Aquatic Botany 51 (1995) 243-257

Table2 Summaryof sedimentchemistryin the Sheernesscoolingpond.N= 8 (no samplestakenat Sites6 and 10) Variable

Mean

Standard deviation

Maximum

Minimum

Organic matter (%) ExchangeableP (/xgg- l ) ExchangeableN (/~gg- l ) Water content (%)

6.0 135.7 8.9 43.0

3.9 74.6 3.9 16.1

13.2 288.8 14.4 69.7

1.1 42.2 4.1 20.9

evidence of thermal stratification was indicated in the south end in July, where the top 2 m were about 2°C warmer than the rest of the water column. This temperature gradient probably reflects heated discharge water floating on top of cooler pond water, as was observed in Lake Wabamun ( Haag and Gorham, 1977 ) and in a Finnish cooling pond (Eloranta, 1983 ). No thermal depth gradient was observed at the north end of the pond, farther away from the discharge canal (Fig. 4). Electrical conductivity remained approximately constant throughout the summer, averaging 531/xS c m - ~in May (range 521-534/~S c m - 1) and 502/xS c m - l in August (range 478-531/zS cm -~) at 25°C. Estimated average total dissolved solids (TDS) levels were 277 mg 1-1 and 292 mg 1- l, respectively. Similar or higher TDS concentrations have been recorded previously in routine monitoring since the pond was constructed (Table 1 ). Values of most chemical variables measured in a grab sample taken at 8 m depth in July 1991 (analyzed by the same methods used by Alberta Power) were at the low end of the range measured previously, except for pH (8.3). Light penetration in the pond was severely limited by turbidity and phytoplankton production that turned the water murky green all summer. Secchi depths at the deepest point in the lake were 1.2, 1.2 and 1.0 m in May, July and August, respectively. At the lakeward end of the macrophyte sampling transects, Secchi depth ranged from 1.0 to 1.4 m in July and from 0.5 to 1.2 m in August. Gravel, coarse sand and fine sand collectively composed 79% of sediment samples from eight sites around the pond (range: 62.6-97.3%), with the remainder consisting of silt and clay. Gravel alone constituted less than 10% of sediments, except at Transects 5 (27.1%) and 7 (14.3%) which were near the edge of the berm (Figs. 1, 3). Except along the berms on the south and west shores, there were no larger cobbles or rocks encountered anywhere. The sediments were relatively rich in organic matter (Table 2) but, except at one site, concentrations were less than levels that may inhibit plant growth (Barko et al., 1991). Water content, a measure of the porosity of the sediments, was high, though variable (Table 2), indicating a limited resistance to root penetration and a high proportion of interior spaces in which nutrient-rich interstitial water may accumulate. Although there was substantial variation around the pond, the sediments were richly supplied with phosphorus, sufficient to support luxuriant growth of macrophytes even at the poorest sites (Table 2). Concentrations of nitrogen were much lower, and strongly correlated with the percentage of organic matter in the sediments (r = 0.91, n = 8); hence, much of this N is probably organic, residing in decaying plant detritus. The relationship between organic matter and P concentration was much weaker, apparently due to two

B.R. Taylor,J. Helwig/Aquatic Botany51 (1995)243-257

251

aberrant points (Sites 5 and 9); at the remaining sites, P concentration is strongly correlated with organic matter ( r = 0.96, n --- 6).

3.2. The macrophyte community The macrophyte community of Sheerness cooling pond was simple, consisting of only nine species, of which seven were common or abundant. The common species were: Potamogeton pectinatus L., Potamogeton praelongus Wulfen, Myriophyllum exalbescens Fern., Ceratophyllum demersum L., Ranunculus circinatus Sibth. var. subrigidus Drew, Elodea canadensis Michx., and the macroalga Chara sp. The other two species were rare: Subularia aquatica L. is small and appeared to be restricted to a few points on the east side of the pond, and Isoetes sp. was collected only once. These two species are not discussed further. The remaining seven species form an interesting plant community composed of an unusual mix of common and rare species. The most abundant plants throughout the pond were P. pectinatus, M. exalbescens, and C. demersum. These species are extremely common and widespread in Alberta and elsewhere in North America, especially in alkaline or nutrientenriched waters (Pip, 1979; Mitchell and Prepas, 1990). The remainder of the community in the cooling pond, however, is unusual, and is composed of species apparently favoured by the unique environment o f the pond. Only a small area of the total littoral zone o f the cooling pond was responsible for almost all the macrophyte production (Table 3). No plants at all were found along the south or west shores o f the pond; water depths were too great (4-11 m) even near shore and the substratum o f large rocks (riprap) provided no habitat for plant roots. No windrows of Table 3 Percentagecover and aboveground dry mass (g m- 2) of aquatic macrophytesalong transects in Sheernesscooling pond. See Fig. 3 for locations of transects. Samples collected 29-31 July or 26--29 August 1991 Transect (Month)

Distance from shore (m) 100 Cover

1 (July) (Aug) 2 (July) (Aug) 3 (July) (Aug) 4 (Aug) 5 (Aug) 6 (Aug) 7 (Aug) 8 (Aug) 9 (Aug) 10 (Aug)

10 30 50 100 0 0 0 0 0 0 0 0 0

aSampletaken near shore.

75 Mass 1

50

25

Cover

Mass

Cover

Mass

Cover

Mass

60 80 75 100 90 60 0 0 0 50 0 10 0

46 158

60 85 90 70 85 5 0 0 0 75 15 80 50

39 529 74

100 100 100 70 70 100 0 0 0 95 t00 90 95

264 840 925a

73 137

42

19

84 27

252

B.R. Taylor,J. HelwiglAquatic Botany51 (1995)243-257

plant material were observed on shore along the south or west side, nor were any floating mats of plants ever seen there. Dense plant mats did develop along the east and north shores, especially in the shallow areas near the intake canal, and in the northeast corner of the pond (Table 3). Plants colonized to a depth of at least 3 m, but did not reach the surface in water deeper than 2 m. Deepest sites tended to support mostly C. demersum. A more mixed community of M. exalbescens, E. canadensis, and the two Potamogeton species was found in shallower water, Ranunculus circinatus was mostly found in water less than 2 m deep, and the densest beds were always in shallow water near shore. Chara sp. appeared to be even more restricted to shallow water and was rarely found in water deeper than 1 m. Macrophyte communities on the east shore (Transects 1-3) were strongly dominated by M. exalbescens, which had 50-80% coverage at most points; P. pectinatus and Chara sp. were the only other common species. Neither R. circinatus nor C. demersum was found there, probably because of the detrimental effects of heavy waves. The densest plant beds in the pond, often consisting of pure stands of M. exalbescens, grew near shore and on shallow sand bars right around the mouth of the inlet canal. The north shore supported a more mixed community in which all seven common species were represented. R. circinatus, P. pectinatus and P. praelongus were the usual dominants there, but proportions varied unpredictably from site to site, and even E. canadensis reached 30% coverage at one point. Total plant coverage was high near shore, often approaching 100%, but dropped off quickly because of the steep depth gradient (Table 3). Standing biomass at most sites was moderate, usually less than 100 g m-2; at nearshore sites on Transects 1 and 2 (near the mouth of the inlet canal) however, standing crops reached 530 g m -2 to more than 900 g m -2 (Table 3). Judged by the conspicuous density of plant growth, similar or greater standing crops were present from the sample points at 25 m right to the shore: the shallow water was clogged with nearly impenetrable mats of plants. Again, M. exalbescens was responsible for the heaviest standing crops. Although no quantitative samples were taken, prolific macrophyte growth was observed along the banks at the mouth of the intake canal and within the canal itself. The canal has a steep cross-section that limits the area of plant growth, but any fragments from those beds were immediately carried by the current toward the intake. Dense floating mats, consisting of R. circinatus, C. demersum, and other species, were observed along both shores at the mouth of the canal. Although uprooted, the plants appeared healthy, and R. circinatus was in flower. Accumulations of plant debris were also observed on the safety buoys at the mouth of the canal and in windrows at the waterline. On two occasions, samples were taken from the pile of plant debris accumulated by backwashing the intake screens at the generating station. Significantly, the dominant plants in the pond were not represented proportionately in the screen samples: at least 70-80% of all plant debris was R. circinatus, with the remainder being small fragments of P. pectinatus and C. demersum. None of the other species contributed significantly to the debris samples. 4. Discussion

The unusual species composition and high standing crops of aquatic macrophytes in Sheerness cooling pond illustrate the effects of heat additions to the pond. Standing crops

B.R. Taylor, J. Helwig/Aquatic Botany51 (1995)243-257

253

at most sites in the littoral zone were unremarkable ( < 100 g m-2), except in the extremely dense plant beds along the north and east shores, where biomasses of 500 to over 900 g m-2 were observed. Productive lakes in North America seldom support standing crops of macrophytes in excess of 400 g m - 2, and maxima near 200 g m - 2 are more usual (Chambers and Kalff, 1985; Mitchell and Prepas, 1990). By comparison, standing crops in Lake Wabamun ranged from 260 to over 400 g m - 2 in the thermally influenced area, and averaged about 200 g m - 2 in the rest of the lake (Haag and Gorham, 1977). In a soft-water reservoir used for reactor cooling, maximum plant biomass was 90-150 g m-2 in the heated area and 40-90 g m - 2 in the rest of the lake (Grace and Tilly, 1976). Thus, while mean plant biomass in Sheerness cooling pond was more or less typical for eutrophic lakes of the region, maximum biomass was extreme even for lakes receiving thermal effluent. Growing conditions in Sheerness cooling pond were nearly optimal in most respects and were amenable to high macrophyte production. Essential nutrients were in abundant supply in both the sediments (Table 2) and the water column (Table 1 ). Heat additions from the generating plant removed any impediment from low temperatures or short growing seasons, and had indirectly increased TDS content and water-column nutrient concentrations by evaporative concentration when the pond was being operated as a closed loop. Sediment texture was excellent, at least on the north and east shores, consisting of fine sand and gravel where most macrophyte species make their best growth (Pip, 1979). The major limitation on growth of macrophytes in Sheerness cooling pond was evidently light penetration, which was severely limited by phytoplankton blooms, themselves a response to the warm water and elevated nutrient concentrations. Rapid attenuation of surface light limited the development of large standing crops of macrophytes to shallow water ( < 2 m) on the north and east shores of the pond. This restriction of plant growth agrees with the model of Chambers and Kalff (1985) relating maximum depth of angiosperm colonization to Secchi depth. Thus, while the cooling pond was highly eutrophic, the nutrient richness of the water column served to indirectly limit rooted plant growth as well as directly stimulating it. On the other hand, weak light tends to strongly stimulate shoot elongation, especially if water temperatures are near optimal for growth (Barko et al., 1991). Long stems may exacerbate the problem of intake clogging with plant debris; in particular, the prevalence of R. circinatus as a problem species evidently reflects its habit of growing in shallow water and producing long stems that fragment easily (Dawson, 1976). Plant material on the intake screens indicates that most of the aquatic weed problem in the cooling pond could be relieved by controlling growth of R. circinatus, without necessarily decimating the other species. Within the zone where rooted plant growth was observed (i.e. near-shore areas with suitable substrata), the distribution of individual species appeared to be influenced by light requirements and exposure to wave action. Deepest sites were always dominated by C. demersum; although other species were usually present at the off-shore sites, they were generally less abundant and always less vigorous than C. demersum. The tolerance of C. demersum for low light levels has been noted in other studies (Davis and Brinson, 1980; Engel, 1985; Dale, 1986). Water of intermediate depths ( 1-2 m) supported virtually all the species found in the pond, but were dominated by M. exalbescens, E. canadensis, P. pectinatus and P. praelon-

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gus; C. demersum occurred only sporadically in these communities. Similarly, R. circinatus was common in the mixed-species community, but produced its best growth in very shallow water where light penetration would have been greatest. This species often produced dense, pure stands right along the water's edge. The charophyte Chara sp. (probably Chara globularis Thuill.; Haag, 1983) was also restricted to very shallow water, as has been observed elsewhere (Engel, 1985). Effects of exposure to waves were evidently superimposed on the depth gradient of plant distributions. Wave action strongly favoured M. exalbescens, but completely eliminated R. circinatus and C. demersum from the three east-shore transects. However, R. circinatus persisted along the intake canal, whose size and orientation provided protection from waves. In contrast to the exposed east shore, R. circinatus was abundant and often dominant along the north shore (Transects 7-10) that suffered less severe exposure to waves. The north shore appeared to provide a nearly ideal habitat for most macrophyte species in the pond, in that all seven common species were abundant and productive there. Thermal effluent discharged to standing water bodies can cause shifts in relative abundances of aquatic plants, suppressing normal dominants and favouring ordinarily rare species (Haag, 1983). Such a pattern was observed in Sheerness cooling pond. The simple plant community consists of several exceedingly common species (P. pectinatus, M. exalbescens and C. demersum) associated with others that are regionally uncommon or seldom abundant in most habitats. P. pectinatus is one of the most commonly encountered species of macrophyte in hardwater lakes (Sculthorpe, 1967; Pip, 1984, 1988). In Lake Wabamun, P. pectinatus formed a dense stand right near the mouth of the hot water discharge (Haag, 1983). Similarly, M. exalbescens is one of the most common and widespread species of aquatic plants in temperate climates (Sculthorpe, 1967) and it is abundant in the thermally altered portion of Lake Wabamun (Haag, 1979). The third common species, C. demersum, is cosmopolitan in hard, eutrophic water (Seddon, 1972; Best, 1986). In contrast to these widespread species, the presence of dense growths of R. circinatus (white water buttercup) in the pond was unexpected. While white water buttercup is very common in the Canadian prairies, the usual species is R. aquatilis, not the closely related R. circinatus found in Sheerness cooling pond. In her survey of over 400 central Canadian water bodies, Pip (1979) found R. circinatus in only 0.9% of all sampled habitats, too few even to establish the ecological preferences of the species, though she found R. aquatilis in a wide variety of lakes, ponds and slow-moving rivers. Presumably the resurgence of the less common species at Sheerness is a response to the unnaturally warm water in the pond; R. circinatus is also the species found in Lake Wabamun (Haag, 1983). Similarly, none of the ten or so common species of Potamogeton which normally cooccur with P. pectinatus was present at all in Sheerness cooling pond, but there was significant coverage by the relatively uncommon P. praelongus. In a compilation of surveys of plant communities in 100 south and central Alberta lakes, P. praelongus was found in only 14 and is listed among the dominants in only one (Mitchell and Prepas, 1990). Temperature preferenda for R. circinatus and P. praelongus have not been determined, but the observation of Haag (1983) that R. circinatus in Lake Wabamun grew near the thermal outlet suggests a high thermal tolerance. Similarly, Pip (1989), in a broad survey of 345 aquatic habitats, found P. praelongus associated with the warmest mean July temperature of the 47 plant species compared. Hough (1977) found anatomical evidence in P.

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praelongus, alone out of over 40 species of aquatic plant examined, of the C 4 photosynthetic

pathway, which in terrestrial plants is associated with temperature optima in excess of 30°C (Best, 1986). Hence, there is ample evidence that the presence ofP. praelongus in Sheerness pond is a consequence of the altered thermal regime. Elodea canadensis is not widespread in Alberta, and thus its presence at Sheerness is also surprising. E. canadensis is found throughout Canada and is a major pest species in other regions and in Europe. But in their review of the biology of the species, Spicer and Catling (1988) report that it is rare on the Canadian prairies, and has been reported from only one other location in Alberta: in the thermally altered portion of Lake Wabamun (Haag and Gorham, 1977; Haag, 1983). It is worth pursuing why E. canadensis has not become a problem at Sheerness despite its abundance in Lake Wabamun. Several factors may be responsible. First, E. canadensis is a plant of clear water, and it appears to have limited ability to grow and compete in low light (Barko et al., 1982; Spicer and Catling, 1988). Second, while E. canadensis is a plant of neutral to alkaline waters, its salinity tolerance range may have been exceeded at Sheerness. Pip (1979) found a negative association between the presence of E. canadensis and high TDS and alkalinity, apparently linked to the species' inability to photosynthesize at pH higher than 8 (Simpson and Eaton, 1986). Mean pH in Sheerness cooling pond was 8.2 and values up to 8.6 are sometimes observed (Table 1). Haag and Gorham (1977) found that E. canadensis in Lake Wabamun only grew in iron-rich sediments, but we have no data to confirm this observation. It is most likely that the combination of low light and high levels of dissolved solids and alkalinity in Sheerness pond is keeping E. canadensis in check. The apparent absence of Potamogeton richardsonii (Bennett) Hulten from the cooling pond is unexpected. This species is ubiquitous in standing waters in this region, and is excluded only from very saline lakes. Surveys of 100 lakes and reservoirs in southern Alberta record P. richardsonii in all of them, including Lake Wabamun (Mitchell and Prepas, 1990). In her survey of 430 water bodies in central Canada, Pip (1987) reports that P. richardsonii had the highest frequency of occurrence of any among the 19 species of Potamogeton encountered. Again, the absence of this species from what would otherwise appear to be ideal habitat is presumptively attributable to the altered temperature regime. In summary, thermal effluent in Sheerness cooling pond has produced an extremely productive plant community composed of a few species tolerant of high temperatures as well as high alkalinity and dissolved solids concentrations. Low light penetration from high algal densities restricts rooted plant growth to a narrow band within the littoral zone, but within that zone standing crops can be extremely high. Fine sediments and high nutrient concentrations in both sediments and water contribute to the extraordinary plant growth. The macrophyte species composition is unusual and includes several species that are rare or seldom abundant in other lakes of the region. Within the pond, distribution of individual species is apparently determined by light requirements and tolerance of wave action. Most of the plant material clogging intake pipes to the generating station was produced by one species (R. circinatus) which was not the most abundant species in the pond. The situation in Sheerness cooling pond is comparable with that in the thermally altered portion of Lake Wabamun.

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Acknowledgements This report is part of a larger study undertaken for Alberta Power under the direction of W. Peel, and co-ordinated for Golder Associates (then Environmental Management Associates) by Dr. B.A. Barton. We are grateful to D. Patan for able assistance in the field, to K. Wilkinson for confirmation of plant identifications, and to G. Hutchinson, Dept. of Zoology, University of Alberta, for sediment analyses. M. Kopchia and J. Tingley of Sheerness Generating Station provided essential information on operations of the power plant. We are indebted also to K. Tait and E. Neilson of Monenco Consultants Limited, for data on the bathymetry and thermal regime of Sheerness Cooling Pond.

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