Hydrodynamics and water quality in western Lake Ontario

Journal of Great Lakes Research 38 (2012) 91–98

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Hydrodynamics and water quality in western Lake Ontario Yerubandi R. Rao ⁎, J.E. Milne 1, C.H. Marvin 2 Environment Canada, National Water Research Institute, Aquatic Ecosystem Management Research Branch, 867 Lakeshore Rd, P.O. Box 5050, Burlington, ON L7R 4A6

a r t i c l e

i n f o

Article history: Received 11 April 2011 Accepted 14 March 2012 Available online 17 May 2012 Communicated by Todd Howell Keywords: Nearshore Lake Ontario Physical Processes Resuspension

a b s t r a c t The hydrodynamics and water quality parameters in the nearshore waters of western Lake Ontario (WLO) were examined using time series data from late-spring to early-summer of 2006. In general, the observed water quality parameters were within the expected limits of Lake Ontario. The observations suggest that in this region, the nearshore-offshore gradients of nutrients were influenced by the formation of a coastal boundary layer. Comparison of time series of physical and nutrient measurements reveal that upwelling due to winds from the west and downwelling due to winds from the east affect the water quality during the summer. These observations further confirm that moderate to significant taste and odour events in drinking water were due to strong downwelling episodes of surface water to intake depths in late summer. Resuspension of bottom material by surface wind waves was another mechanism that affects the water quality in WLO. Crown Copyright © 2012 Published by Elsevier B.V. on behalf of International Association for Great Lakes Research. All rights reserved.

Introduction The coastal zones of large lakes are the areas of most immediate concern to the general public. In the last two decades, there has been increasing concern about ecological changes in the nearshore region of the Great Lakes. Effects are directly related to changes in nutrient loading and biogeochemical interactions that involve invasive dreissenid mussels, water quality, and benthic macroalgal growth (SOLEC, 2009). Mackerwicz and Howell (2007) noted that despite significant water quality improvement in the open waters of Lake Ontario, nearshore waters are still suffering from many impairments that severely limit their recreational use and ultimately affect the economic development of the region. Most pollutants that reach nearshore waters enter from the land via tributaries, shoreline discharges, or as surface runoff. Pollutant concentrations can be higher near tributary mouths and discharge sites until they are diluted with the nearshore waters and ultimately with the larger volumes of cleaner offshore water. The Great Lakes hold approximately 20% of global fresh surface water which provides drinking water to over 15 million Canadian and US consumers. Drinking water intakes and discharges alike are typically installed in a narrow band of the lake extending, at most, a few kilometres from the shoreline in water depths less than 20 m. Outbreaks of intense earthy/musty taste and odour (T&O) in drinking and source water have been occurring in the Great Lakes basin

⁎ Corresponding author. Tel.: + 1 905 336 4785. E-mail addresses: [email protected] (Y.R. Rao), [email protected] (JE. Milne), [email protected] (CH. Marvin). 1 Tel.: + 1 905 336 6432. 2 Tel.: + 1 905 336 6919.

(Watson et al., 2007). These are largely caused by geosmin produced by some cyanobacteria in the offshore surface waters and 2methylisoborneol (MIB), produced by both cyanobacteria and actinomycetes on the submerged surfaces and macrophyte biofilms. Recently extensive field studies were undertaken at several water treatment plants to document the lake physics, biology and chemistry before and during the taste and odour events to understand the processes controlling the delivery of geosmin to the water treatment plant intakes. Another important parameter affecting the drinking water treatment systems is episodic elevations of total suspended solids (TSS) concentrations in lake waters. Excessive turbidity may interfere with disinfection and reduce filter performance during the water treatment. The three primary particles that contribute to nearshore turbidity are inorganic materials (suspended solids), algae and other organic matter. Several physical factors combine to make the coastal systems complex and unique in their hydrodynamics, and the associated physical transport and dispersal processes of the coastal flow field are equally complex (Rao and Schwab, 2007). Physical processes such as the lake's thermal cycle and circulation can have a pronounced influence on water quality conditions in the coastal waters of the Great Lakes (Edsall and Charlton, 1997). Therefore, understanding the circulation and mixing in the nearshore region is very important for the loading, pathways and fate of pollutants in lakes and for locating water intakes and waste water treatment plants. The nearshore area of lakes can be characterized from the shore to approximately 10 km offshore as a boundary layer within which the mid-lake motions adjust to the presence of the shores (Csanady, 1972; Rao and Murthy, 2001a). Many field experiments conducted in the Great Lakes have shown a variety of circulation features associated with meteorological forcing, stratification and

0380-1330/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. on behalf of International Association for Great Lakes Research. All rights reserved. doi:10.1016/j.jglr.2012.04.001

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topography (Boyce et al., 1989). Recently, Rao and Schwab (2007) summarized coastal physical processes that affect the water quality in the Great Lakes. Among the general factors that influence the transport and mixing processes are the coastal bathymetry, meteorologically forced events, stratification, inflows and influence of the earth's rotation. Transient wind-driven upwelling or downwelling of the thermocline in coastal waters is one of these physical processes that affect the mixing and transport of contaminated waters in the nearshore zone and brings nutrient rich sub-surface waters to surface levels (Rao and Schwab, 2007). During changeovers from upwelling to downwelling, appreciable mass of inshore water exchanges with offshore water. An example of the consequences of these episodes seen in Lake Ontario is abrupt increase in geosmin levels coincident with increased temperatures due to downwelling forced by winds from the east (Rao et al., 2003b). Another process that can significantly affect the flow and thermal characteristics during early spring and can influence ecological functioning is the thermal bar in the Great Lakes. During fall and winter seasons, the higher wind speeds and the absence of the thermocline allow the effects of wind action to penetrate deeper into the water column. During this period higher turbidity levels associated with resuspension of sediments will have more impact on water quality than other times. The western Lake Ontario (WLO) shore is a continuous urban community that heavily depends on the lake for drinking water and discharge of waste water. In this study, WLO is defined as a region consisting of the city of Burlington and Hamilton shorelines at the western end of the lake. The water quality in the vicinity of city of Hamilton drinking water intakes is influenced by the exchange flow between Lake Ontario and Hamilton Harbour (HH), circulation patterns and dispersal properties of the water movements in this region. Historical current and temperature data show periods of current stagnation, high variability of currents, and frequent episodes of upwelling and downwelling during the summer (Miners et al., 2002). The area is exposed to easterly winds, usually associated with storms, and often quite strong, which have the potential to generate strong alongshore currents of the order of 20 cm/s at a depth of 10–20 m, but also drive surface waters onshore. Easterly winds travelling over a fetch of about 200 km generate large waves capable of causing substantial resuspension of sediments which include sand in the nearshore, changing to silt-sand, then silt-clay with increasing distance offshore (Thomas et al., 1972). Under the influence of weak currents during periods of calm weather lasting several days, contaminants which adsorb onto fine sediments, or settling organic material, could accumulate only to be reintroduced into the water column at higher concentration during wind events. The coastal communities of Burlington and Hamilton draw raw water from water intakes that extend to about 1 km from the shoreline in WLO. The source water at these water intakes are occasionally influenced by adverse water quality (Miners et al., 2002). A thorough knowledge of hydrodynamics in WLO is critical to understanding a range of causal factors affecting source water quality, and formulation of effective strategies to eliminate or mitigate these factors. Because of these concerns and the renewed issue of water quality in Hamilton Harbour, a collaborative study between Environment Canada and City of Hamilton was initiated in 2006. The city of Hamilton currently draws its drinking water from an intake located at 900 m from the shoreline of WLO at a water depth of 9–10 m. In this paper, we use time series measurements of hydrodynamic parameters, bi-weekly water quality measurements and conceptual models to investigate the influence of physical processes on water quality in WLO. We first show that the spatial distribution of mean concentrations of nutrients are somewhat controlled by physical processes in the coastal boundary layer. We next examine the influence of upwelling and downwelling and wave-driven resuspension events on spatial and temporal variability of water quality in western Lake Ontario.

Data and Methods The WLO field study covered a seven-month period from April to late October 2006. The main objective was to monitor the water quality and hydrodynamic conditions at the city of Hamilton intake and the area around the intake (Fig. 1). Bi-weekly sampling at Ww1 to Ww6 and monthly sampling at Wm1 to Wm3 was carried out during this field campaign. In addition weekly samples were also obtained at a few stations during August and September for both nutrients and taste and odour compounds (geosmin and MIB). Discrete water samples were collected using a 6 liter Van Dorn water sampler at 1 m below the surface and 2 m above bottom (Bot-2 m). Water samples were then prepared for total phosphorus unfiltered and filtered (TP and TFP), soluble reactive phosphorus (SRP), nitrate+nitrite (NO3+NO2) and ammonia+ ammonium(NH3+NH4) analyses by filtering through a 0.45 μm cellulose acetate filter for nutrients and Whatman GF/C filters for chlorophyll a and seston. Chlorophyll a (Chla) samples were prepared by filtering 1 liter of sample water through a 42.5 mm glass fiber filter. The prepared samples were then analyzed by the National Laboratory for Environmental Testing (NLET) in Burlington, Ontario. Geosmin/MIB samples were collected in 1 liter amber glass bottles (Brownlee et al., 2004). On several occasions, following Environment Canada sampling protocols, blank and replicate samples were collected. The blank results were set at b0.5 μg/L (below detection) and coefficient of variation for replicates were ~5%. At all stations depth profiles of temperature, pH, conductivity, dissolved oxygen, Chlorophyll a, and turbidity were collected using a YSI 6600 sonde. Statistical analyses were conducted to

Fig. 1. Map of western Lake Ontario with experimental setup and local bathymetry.

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Table 1 Description of temperature, current meters, water quality and meteorological moorings in western Lake Ontario. Station & Parameters

Depth (m) (Inst. depth/ water depth)

Cm1(Currents, waves with Sontek Hydra, Temp mooring 11.5/12 & YSI 6600EDS) Cm2 (ADCP, temp mooring, YSI 6600 EDS) 1 m-bins/18 Cm3 (ADCP, temp mooring)

1 m bins/20.5

Cm4 (ADCP, temp mooring)

1 m bins/31

Position

Deployment period & sample interval

43° 15’ 37”N 18”W 43° 16’ 43”N 56”W 43° 15’ 59”N 03”W 43° 23’ 49”N 18”W

79° 45’ 79° 43’ 79° 40’ 79° 40’

Apr 27 - Oct 20 (hourly for currents, temp, water quality and 3 h for waves) Apr 12 - Oct 30 (hourly) Currents Apr 12 - Jul 24 (hourly), Temp Apr 12 - Oct 30 (hourly) Apr 12 - Oct 30 (hourly)

T5 (Temp) 43° 17’ 25”N 79° 41’ 39”W Burlington Pier (Wind)

+ 10 m

Apr 1 - Nov 1(10 min)

assess temporal and spatial variability of water quality in WLO. Data were analyzed by one-way ANOVA with non-parametric tests for significance. In order to provide continuous records of water temperature and currents within the nearshore zone, 4 moorings at Cm1, Cm2, Cm3 and Cm4 were deployed (Table 1). Except at Cm1, RDI acoustic doppler current profilers (1200-kHz ADCP) were deployed to measure water current profiles. The general accuracy of velocities obtained by ADCPs is within ± 2.5 mm/s. At Cm1 time series of currents, wave height, period and direction were obtained using a Sontek Hydra current meter. The Hydra current meter provided bursts of 2048 scans sampled at 4 Hz at three hour intervals, which were subsequently analyzed to yield summary wave data. The wave characteristics were obtained using the program (SonWave-Pro) provided by Sontek. It uses high precision burst measurements of hydrostatic pressure (P) and horizontal water velocity (U, V) to compute wave parameters using the parametric spectral method (Longuet-Higgins et al., 1963). The wave bed shear stresses is calculated using the Mian and Yanful (2004) formula

τwave

2 
 0:5 3 2π 3 6ρ ν T 7
 5 ¼ H4 2 sinh 2πd L

Apr 27 - Nov 1 (hourly)

ð1Þ

where H is wave height; d is water depth; T is wave period; L is wave length; ν is kinematic viscosity of water; and ρ is water density. The 2 current bed shear stress (τcurrent) is calculated by τcurrent = ρCdUbottom , where drag coefficient of bottom water (Cd) is given as 2.0 × 10 - 3; Ubottom is current speed obtained from the current meter or bottom bin velocity of an ADCP. Water temperature data were obtained from five moorings with thermistors deployed at 1–2 m intervals in the water column. Water temperature was also obtained from the current meter and from the ADCP deployed at the lakebed. The hourly time series of turbidity, dissolved oxygen, conductivity, pH were measured by moored YSI 6600 EDS at 1 m above bottom at Cm1 and Cm2. The performance of turbidity sensor was found to be comparable with values from occasional spot measurements and bi-weekly YSI profiles. The wind data from Burlington Pier which is around 4 km from the water intake

has been used for providing meteorological forcing. The wind stress was obtained from the quadratic law given asτ = ρaCd|W|W, where ρa = 1.2 kg m - 3 is the air density, W is wind velocity at 10 m. In general, drag coefficient Cd increases with the wind speed and estimated as Cd = (0.8 + 0.065 W)x10 -3 for W >1 m s - 1 (Wu, 1980). Results and discussion Mean circulation and concentration of nutrients in the coastal boundary layer The mean currents in Lake Ontario show a lake-wide cyclonic (counterclockwise) circulation with stronger flow along the southern coast than along the northern coast (Beletsky et al., 1999; Huang et al., 2010a). Further it is observed that both cross-shore and alongshore velocities increase with distance offshore and peaks at 3–5 km from shore (Rao and Murthy, 2001a). In WLO the primary driving force of circulation is the wind (Simons, 1980). However, both the shape of the shoreline and gentle offshore slope of the lake bottom in the study area impede any strong currents (Miners et al., 2002). Water movements and chemistry will also be influenced by short period water level oscillations, for example, surface seiches in Lake Ontario. However, because the magnitude of these oscillations are relatively small (b 2 cm) during the study period, their impact is not significant in this study. The measurements during 2006 further describe the characteristics of nearshore circulation in the western end of the Lake Ontario (Table 2). The circulation is consistent with the shoreline configuration and a large counterclockwise circulation is observed. The mean currents are small near the shore (Cm1) with periods of stagnant conditions (b3 cm/s) occurring half the time. Although at intermediate depths (Cm2) the currents are uniform over depth, they are weaker close to the bottom. Further, the current vector rotation is not in the same direction at all places indicating the complex nature of the flow. The bi-weekly water quality measurements obtained at 1 m from the surface and 2 m above the bottom at all stations are averaged to obtain the seasonal mean concentrations in the lake. Fig. 2a shows an example of the mean concentrations of TP and SRP along the transect

Table 2 Summary of currents from April 12 to October 20, 2006 at the surface and bottom depths. Current meter/depth (m)

No of observations

Maximum speed (cm/s)

Mean Speed (cm/s)

Mean Direction (deg)

Standard Deviation (mean speed)

% of stagnation period (b 3 cm/s)

Cm1/10 Cm2/2 Cm2/13 Cm3/3 Cm3/18 Cm4/4 Cm4/27

4456 4988 4988 2463 2463 4804 4804

29.5 30.1 25.2 31 21.2 57.2 31.2

2.7 4.58 4.02 7.41 4.08 11.72 5.33

175 190 181 156 140 138 166

2.6 3.64 3.11 5.36 3.18 8.82 4.12

48 25 29 19 24 11 31

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a)

c)

d) b)

Fig. 2. a) mean concentrations of TP (error bars represent SD) and SRP from stations Ww1 to Ww6 (offshore from Cm1), b) mean concentrations of NH3 + NH4, NO2 + NO3 (mg/L) and chlrophyll a (μg/L), here error bars represent SD, c) simulated TP concentrations from stations Ww1 to Ww6 (offshore from Cm1), and d) model (ELCOM) simulated mean current magnitude from stations Ww1 to Ww6 (offshore from Cm1).

from Ww1 to Ww6 at 1 m below surface and 2 m above the bottom. In general TP concentrations are slightly higher at the bottom compared to the surface (P b 0.05). Also there is significant difference between the shoreline and 3 km offshore (Pb 0.05), but no difference between the station at 3 km and further offshore stations (P = 0.11). Nearshore TP concentrations at the bottom are marginally higher than offshore values. The vertical gradient of TP concentrations also gradually decreases as we move offshore. In contrast SRP concentrations show no clear evidence of nearshore/ offshore variability (P = 0.14). The mean concentrations of chlorophyll a, ammonia + ammonium and nitrate + nitrite at the surface are shown in Fig. 2b. Both ammonia + ammonium and nitrate + nitrite concentrations do not show statistically significant differences between nearshore and offshore areas. Chlorophyll a values increased in the offshore direction. Hecky et al. (2004) hypothesized that the mussel colonization has implications for the removal and fate of materials in the nearshore zone. Smith et al. (2007) observed nearshore to offshore variability of nutrients and chlorophyll a concentrations in Lake Erie and attributed it to the presence of zebra and quagga mussels. However, in the study area in WLO, the presence of these mussels is not significant (Wilson et al., 2006; personal observation of Environment Canada Divers in 2006). Gregor and Rast (1982) had also pointed out that nearshore waters generally contained lower chlorophyll concentrations for a given TP level than offshore waters even before mussel invasions and attributed it to the light conditions among other factors in this zone. In WLO the mean Sechi depth varied from 6.35 m in the nearshore to 7.72 m in the offshore during the study period. The water quality of the broader study area is further affected by episodic Hamilton Harbour plumes through Burlington Ship Canal (BSC). However, the effect of harbour plumes is confined to the vicinity of the BSC and rarely extends to the area of intake (Poulton et al., 1986). Enhanced nutrient concentrations in the nearshore are possible because of the formation of weak currents associated with the frictional boundary layer, resulting in low dispersion of nutrients near the coastal boundaries (Rao and Schwab, 2007). In order to test this hypothesis, we need high resolution coastal currents in the

study region. However, current measurements are only available at a few stations in WLO during the study period. Alternatively, the coastal currents can be obtained from numerical models. Huang et al. (2010b) applied several numerical models to Lake Ontario and found that the Estuary Lake Computer Model (ELCOM) is capable of predicting circulation and temperature reasonably well. In another study, water level fluctuations in Hamilton Harbour were also simulated well with the ELCOM model (Rao et al., 2009). Therefore, in this study the mean depth-averaged currents were obtained from high resolution ELCOM simulations (Rao and Zhao, 2010). In the numerical experiments the modelled area extends over a region of 7.5 km in the x-direction (east–west) and 11.4 km in ydirection (north–south) with a horizontal resolution of 200 × 200 m and vertical resolution of 1 m. The open boundary conditions for the WLO model, such as water levels and vertical temperature profiles were obtained from the lake-wide model runs during the same period (Huang et al., 2010b). When aggregated over time and space scales relevant to the coastal retention period (typically 3–5 months in WLO; Rao and Zhao, 2010), mean flow is very small in the nearshore areas. The model predicted mean currents are qualitatively in agreement with observations at Cm1 and Cm2 in Table 2. For illustrative purposes, we show the depth-averaged mean currents averaged from 1 May to 16 September along a cross-section from Ww1 to Ww6 (Fig. 2d). The flow and structure of the coastal boundary layer was characterized by a frictional boundary layer of a width of 1–1.5 km. Currents in this zone were typically slow, while alongshore flows were slowed by the drag of a shallow bottom and the roughness of the coastline, and cross-shore flows were inhibited by the proximity of the solid coastal boundary. Cross-shore dispersal near the shore is also due to diffusive motions. In Lake Ontario currents at fixed-locations (Eulerian measurements) were obtained at several stations. These measurements were used to calculate the horizontal exchange coefficients in terms of Eulerian statistics as 2 K x ¼ βu′ τ, K y ¼ βv0 2 τ where τ ¼ ∫∞0 Rðτ Þdτ is the integral time scale, and R(τ) is the auto-correlation coefficient and β =1.4 (Rao and Murthy, 2001b). Here the fluctuations u (t) and v (t) are obtained by subtracting the low-pass filtered (>8 h) currents from hourly values.

Y.R. Rao et al. / Journal of Great Lakes Research 38 (2012) 91–98

Further Rao and Murthy (2001a) show that cross-shore exchange coefficients increases with distance from the shoreline, and are relatively small in the nearshore zones. Using current meter measurements in the western end of Lake Ontario similar average values were obtained at Cm1 (Kx =0.98×104 cm2/s) and Cm2 (Kx =1.57×104 cm2/s). The mean alongshore exchange coefficients obtained at Cm1 (Ky = 1.15×104 cm2/s ) and Cm2 (Ky =2.68×104 cm2/s) are also comparable to previous studies (Rao and Murthy, 2001b). In order to provide a basis for comparison between observed TP concentrations and effects of water transport within the coastal boundary layer, we first consider a simple balance of advective (mean currents) and diffusive fluxes. An essential element characterizing the water quality is the transport of particles, both biologically and chemically active, from the lake boundaries into lake interior (Rao et al., 2003a). The concentration gradients could also be because of local sources or sinks through biological processes. Consequently, the local changes in the average concentration can be expressed as     ∂c ∂c ∂c ∂ ∂c ∂ ∂c þu þv ¼ Kx þ Ky −kc þ Sc ∂t ∂x ∂y ∂x ∂x ∂y ∂y

ð2Þ

where c is TP concentration at position x and time t, u and v are vertically averaged velocity components, Kx and Ky are eddy diffusivities, Sc is the source and k is a decay constant. By imposing a no-flux condition at the solid boundary; and assuming diffusive flux as zero at open boundaries, Eq. (2) can be solved numerically using a finite difference scheme. At the shoreline, the input concentrations were taken from occasional measurements made at the entrance of the ship canal. The decay coefficient, k is taken as zero in the simulations mainly because the concentration is assumed conservative. This is a valid assumption because our main interest is qualitative depiction of the concentration and not simulating the phosphorous cycle. The choice of horizontal diffusion coefficients is very important to the prediction of model concentrations. Although eddy diffusivity values may vary in space and time, we use the average observed values at Cm1 and Cm2 to obtain qualitative information. The vertically averaged daily currents obtained from ELCOM simulations were used in this transport model. The mean concentrations after 5 months of simulation are shown in Fig. 2c. The modeled cross-shore distribution of TP concentration closely resembles the observed values at this cross-section. The concentrations are high near the shoreline due to weak cross-shore currents and dispersal. The small discrepancies could be due to the biological processes that were not included in the model or due to errors in model currents. This supports the interpretation that the near-shore/offshore gradients in concentrations to an extent are governed by the flow and structure of the coastal boundary layer in WLO. The mean concentrations would also be higher near the shore because of higher energy at the bottom, which would cause episodic resuspension of sediments and entrainment of particle-bound TP into the water column. Temporal variability of nearshore circulation and nutrient concentrations The variability of nearshore coastal dynamics is determined by prevailing winds over the lake (Boyce et al., 1989). Fig. 3a shows the east (+ve)-west (−ve) and north (+vn)-south (−vn) wind stress components obtained from hourly wind measurements at Burlington Pier. The mean wind speed at this station is 3.5 m/s and the mean direction is towards the south-east, however, strong winds in a range of 13 m/s were observed blowing towards the west on a few occasions. The wind impulse along the axis of Lake Ontario from west to east causes transient upwelling of the thermocline along the northshore and downwelling along the southshore with the transition between the upwelling and downwelling zones taking place at the ends of the basin (Boyce et al., 1989). In the western end of the lake, this kind of variability has been clearly identified before (Rao and Murthy, 2001a).

95

Fig. 3. a) The time series of wind stress components, east (+ve) and north (+ ve) at Burlington Pier, and, b) the thermocline depth (m) at T5.

In previous studies the position of 10 °C isotherm was used to define the thermocline to identify upwelling and downwelling events in Lake Ontario (Rao and Murthy, 2001b; Simons and Schertzer, 1989). Although, this simple definition provides crude estimation for locating the thermocline, because of the shallow nature of WLO, the temperatures were significantly warmer than 10 °C in the water column during most of the summer in a large portion of the basin. Therefore, in this study we define the thermocline as that depth z at which the temperature difference ∂ T/∂ z is maximum (Hutchinson, 1957). Temperature recordings at five thermistor moorings were analysed to study the seasonal warming in WLO (Yerubandi et al., 2007). Because the thermistors were placed at irregular intervals (1–2 m), linear interpolation was used to obtain temperatures at 1 m interval. A general picture of the seasonal cycle of water temperature variations was constructed from these observations (figures not shown here, see Yerubandi et al., 2007 for details). Close to the shoreline at Cm1 in a water depth of 7 m, the water column warmed quickly and remained isothermal for the entire period. However, cooling and warming due to wind induced upwelling and downwelling was clearly observed. At other stations also, the thermal stratification cycle and its variability is closely related to these upwelling and downwelling events caused by prevailing winds. Fig. 3b shows a representative example of the position of the thermocline at station T5. During the early spring, isothermal conditions were observed except in calm wind periods. The thermal stratification was established by day 148. During the thermally stratified period, upwelling and downwelling events can be identified with the movement of the thermocline and favorable wind direction. Time series of depth-averaged low-pass filtered (>24 h) east–west and north–south currents are rotated to provide alongshore and cross-shore currents after aligning to the local shoreline. We have not shown results at Cm3, because the ADCP at this location provided only partial data. The alongshore currents show the flow reversals due to prevailing winds (Fig. 4a to c). In summer cross-shore currents are comparatively weaker than alongshore currents at all stations except at Cm1. However, cross-shore transport appears to be important at both Cm1 and Cm2 during strong wind events from both east and west directions. Previous studies showed that currents have a primary energy peak at a period of 10–12 days corresponding to the large scale response to meteorological forcing, and a secondary peak located at the near-inertial frequency band (Rao and Murthy, 2001a). The primary peak of 10–12 days can be seen at Cm4 (Fig. 4c). A series of eastward wind events starting from day 167 (June 16) raised the thermocline in the western end of the lake (Fig. 3). The strongest upwelling (shallow thermocline) was observed on day 191 (July 10) during this period. The surface temperatures reduced to 13 °C with isothermal conditions below 4 m at T5. During this period surface currents were predominantly eastward. The net

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Fig. 4. The time series of depth-averaged and low-pass filtered (> 24 h) alongshore (dashed line) and cross-shore (solid line) currents at three stations a) Cm1, b) Cm2, and, c) Cm4.

alongshore currents were relatively small coinciding with weak stratification. The mean depth-averaged cross-shore currents do not show any net transport. However, vertical current profiles during this period show an onshore flow from deeper parts of the lake (figures not shown). Beginning at about noon on day 240 (August 28), winds turned to the west and reached a peak (wind stress of −0.15 N/m 2) on day 243 (August 31) (Fig. 3). During this period the thermocline intersected the bottom and remained there till the end of this episode. The surface temperatures increased considerably to 20 °C because of the arrival of warm surface waters from the offshore. Strong currents to the south-west were observed all through the depths at Cm4. However, close to the shoreline cross-shore currents flowed in both directions, except close to the bottom, where they flowed offshore. This particular downwelling event coincided with significant taste and odour in raw water at the Hamilton drinking water intake. Fig. 5a and b shows surface and

bottom geosmin concentrations obtained at selected stations in WLO. Geosmin concentrations clearly show a sharp increase during the week of August 28 and remained elevated till September 5 (day 248). Although geosmin levels were low in 2006 compared to severe episodes in 1998 and 1999 (Watson et al., 2007), the patterns of occurrence were similar. During this episode geosmin produced in the warm surface waters of the offshore are transported to the shoreline by downwelling currents (Rao et al., 2003b). At relatively shorter time scales (hours to days), water movements due to upwelling and downwelling events and associated coastal jets will influence the transport of dissolved or suspended substances in the coastal region (Rao and Schwab, 2007). The temporal and spatial variation of TP and SRP concentrations at the surface and near the lakebed (2 m above bottom) are shown in Fig. 6 (a-h) at selected stations. In general both TP and SRP show significant variability over the observational period. The TP concentrations are slightly higher at the bottom compared to surface values except at the offshore station (WW6). The SRP values varied from spring to summer and episodic increases occurred during strong wind events in late summer and fall. The SRP concentrations have also shown vertical gradients during the spring and fall with occasional higher values at the surface. During the upwelling event of July 10–11, TP concentrations increased at the bottom in shallow waters (Ww1), however these increases were relatively small and no vertical gradients are observed at the rest of the stations. The concentrations in the offshore waters increased with the relaxation of the thermocline after the event. The downwelling event from 28 August coincided with higher concentrations of both TP and SRP at all the stations. In general, during this period the stations located at depths less than 20 m experienced less vertical variability in TP concentrations mainly because the thermocline was below this depth and conditions were well-mixed. At Ww6, bottom TP concentrations were slightly higher than surface values. Soluble

a) Ww1

e) Ww1

b) Ww2

f) Ww2

c) Ww5

g) Ww5

d) Ww6

h) Ww6

a) Surface - 1 m

b) Bottom - various

Fig. 5. The time series of a) surface (1 m), and, b) bottom (2 mab) geosmin concentrations at selected stations in western Lake Ontario.

Fig. 6. Bi-weekly measurements of surface and bottom concentrations of TP and SRP at four stations a) Ww1, b) Ww2, c)Ww5, and, d)Ww6.

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Reactive Phosphorous concentrations showed higher values in the surface in the shallow depths coinciding with the transport from the offshore. As observed with TP concentrations, SRP also showed increased values close to the bottom at Ww6. Rao and Murthy (2001a) noted that when the thermocline is close to the bottom near-inertial oscillations associated with internal waves increased and could be an important mechanism in generating kinetic energy close to the bottom. This may contribute to the bottom stress generation and resuspension of sediments. However, this mechanism may be only important further offshore where the thermocline would be a few meters above the bottom. In WLO, the wind induced surface waves frequently disturb the sediment-water interface and cause sediment resuspension, which may increase the internal release of nutrients from sediments. Significant wave events occurred at times because of high winds over Lake Ontario (Fig. 7a). Wave heights reached about 2.5 m during the strong easterly wind event on day 245, but were typically less than 1 m. Wave periods were typically less than 7 s. Waves at this location are mostly from the east and northeast (Fig. 7b). The dashed line in Fig. 7c and d shows the critical bottom shear stress of 0.08 Pa, obtained for a sediment size of 35 μm, reflective of the area near the Hamilton drinking water intake (Thomas et al., 1972). As shown in other Great Lakes, the shear stresses induced by waves are higher than those driven by currents (Hawley et al., 2004). However, shear stress due to currents shows several episodes where it is close to or above the critical shear stress indicating that currents have to be considered in this region. During the upwelling episode of July 10, wave-induced bed shear stresses did not exceed the critical threshold; therefore it is unlikely that any local resuspension had occurred in this region. The wave shear stresses exceeded the critical shear stress during days 142–146 and during the downwelling episode in late August and early September. Fig. 8a show the hourly observations of turbidity during the study period at Cm2. The turbidity increased to high values on many occasions during the spring period. The high values were also observed during the strong wind events and peaks in wave height due to resuspension of bottom material. However, the peaks in turbidity did not always coincide with peaks in wave height indicating that other sources such as presence of phytoplankton or

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Fig. 8. Time series of a) turbidity at 1 m above bottom, and, b) acoustic backscatter intensity obtained from ADCP located at Cm2.

bioturbation contributed to turbidity. Further, since turbidity was measured at 1 m from the bottom, it is difficult to assess the vertical distribution of turbidity during upwelling and downwelling episodes. On the other hand, acoustic sensors such as ADCP detect acoustic reflections from particles in suspension and the signal strength can provide qualitative information about the distribution of particulate matter in the water column. Fig. 8b shows the hourly time series of average backscatter intensity at Cm2. During the upwelling event, turbidity on day 190 increased to 25 NTU from 2 NTU on the previous day. Signal strength also showed marginal increase in the bottom layer suggesting that the increased TP concentrations close to the bottom (Fig. 6) could be because of upwelling transport from offshore or other biological processes. However, signal strength has not extended to the surface because of the presence of a thermocline at 4 m below the surface (Fig. 3) and weak mixing due to milder westerly winds. During the downwelling event from day 240, strong downwelling flows accompanied by high waves induced significantly stronger resuspension of bottom sediments. This resulted in higher values of turbidity on day 245 (160 NTU) at the bottom. Signal strength also increased through the water column. During this episode complete vertical mixing occurred and was reflected in similar TP concentrations at the surface and the bottom. Conclusions

Fig. 7. Time series of a) significant wave height, b) wave direction, c) bottom shear stress due to currents, and, d) bottom shear stress due to waves at Cm1. The dashed line shows the critical shear stress at this location.

The flow and structure within the coastal boundary layer play a significant role in the nearshore/offshore distribution of nutrients in WLO. The observed circulation in 2006 is complex, but consistent with previous observations. The mean currents are small near the shore due to the formation of a frictional boundary layer and with periods of stagnant waters that contribute to the creation of nearshore to offshore nutrient gradients. The currents and temperature measurements indicate that the wind-induced circulation, in particular upwelling and downwelling of the thermocline due to westerly and easterly winds, respectively, play a major role in the variability of water quality in WLO. This study further confirms that during strong downwelling events geosmin concentrations are delivered to drinking water intakes by currents in WLO. Our measurements also suggest that the wind induced surface waves disturb the sediment-water interface and cause sediment resuspension in WLO. The intense wave-induced sediment resuspension accompanied by downwelling flows due to

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easterly winds likely increases the internal release of nutrients from sediments. From these observations it may be pointed out that mass transport and mixing is significant in the nearshore zones. Therefore, the models simulating the response of nearshore zones should properly accommodate coastal physical processes and associated resuspension due to surface waves and its interactions with the open lake. Acknowledgements This project is partially funded by the City of Hamilton's Plant Capital & Planning Section in 2006 and subsequently by source water protection group. The authors extend sincere thanks to Gordia McInnis for processing geosmin data. We thank Environment Canada's Research Support Branch for their help in field work, and NLET for conducting the chemical analysis. We also thank Dr. Todd Howell and the reviewers for providing critical comments on the manuscript. References Beletsky, D., Saylor, J.H., Schwab, D.J., 1999. Mean circulation in the Great Lakes. J. Great Lakes Res. 25, 78–93. Boyce, F.M., Donelan, M.A., Hamblin, P.F., Murthy, C.R., Simons, T.J., 1989. Thermal structure and circulation in the Great Lakes. Atmos. Ocean 27 (4), 607–642. Brownlee, B., MacInnis, G., Charlton, M., Watson, S., Hamilton-Browne, S., Milne, J., 2004. An analytical method for the shipboard extraction of the odour compounds, 2-methylisoborneol and geosmin. Water Sci. Technol. 49 (9), 121–127. Csanady, G.T., 1972. The coastal boundary layer in Lake Ontario, 2, The summer-fall regime. J. Phys. Oceanogr. 2, 168–176. Edsall, T.A., Charlton, M.N., 1997. Nearshore waters of the Great Lakes. State of the Lakes Ecosystem Conference 1996 Background Paper. ISBN 0-662-26031 U.S. EPA Report #EPA905R-97015a. Gregor, D.J., Rast, W., 1982. Simple trophic state classification of the Canadian nearshore waters of the Great Lakes. J. Am. Water Resour. Assoc. 18 (4), 565–573. Hawley, N., Lesht, B.M., Schwab, D.J., 2004. A comparison of observed and modeled surface waves in southern Lake Michigan and the implications for models of sediment resuspension. J. Geophys. Res. 109, C10S03, http://dx.doi.org/10.1029/ 2002JC001592. Hecky, R.E., Smith, R.E.H., Barton, D.R., Guildford, S.J., Taylor, W.D., Charlton, M.N., Howell, T., 2004. The nearshore phosphorous shunt: a consequence of ecosystem engineering by dreissenids in the Laurentian Great Lakes. Can. J. Fish. Aquat. Sci. 61, 1285–1293. Huang, A., Rao, Y.R., Lu, Y., 2010a. Evaluation of a 3-D hydrodynamic model and atmospheric forecast forcing using observations in Lake Ontario. J. Geophys. Res. 115, C02004, http://dx.doi.org/10.1029/2009JC005601. Huang, A., Rao, Y.R., Lu, Y., Zhao, J., 2010b. Hydrodynamic Modeling of Lake Ontario: An Intercomparison of Three Models. J. Geophys. Res. 115, C12076, http://dx.doi.org/ 10.1029/2010JC006269. Hutchinson, G.E., 1957. A treatise on limnology. Geography, physics and chemistry, vol. 1. John Wiley and Sons. 1015 pp.

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