Frequency of episodic stratification in the near surface of Lake Opeongo and other small lakes

Wind-driven mixing in the epilimnion of a deep lake can be suppressed when there is a weak near surface stratification, which occurs frequently during periods of strong solar heating and weak winds. Using data from a vertical chain of fast response thermistors, we analyze the frequency of near surface stratification in the top 2 meters of the epilimnion in Lake Opeongo, Ontario for the periods between May and August in 2009 and 2010. Near surface thermoclines (as defined by dT/dz> 0.2 WC m 1 between 1 and 2 m) occur for 24% of the sampling period in 2009, 37% of the sampling period in 2010 and correspond to periods of high values of gradient Richardson number. During daytime the epilimnion is stratified up to 45% of the time. At night, cooling generally leads to a more isothermal profile, but near surface thermoclines still form at least 20% of the time. Extended periods of near surface stratification (>1 h), account for more than 80% of the stratified period. We compare these findings with previous observations from the Experimental Lakes Area in Northern Ontario, and discuss the biological implications of episodic stratification. doi: 10.2166/wqrjc.2012.001 s://iwaponline.com/wqrj/article-pdf/47/3-4/227/163525/227.pdf Patricia Pernica (corresponding author) Department of Physics, University of Toronto, 60 St George St, Toronto, Ontario, Canada M5S 1A7 E-mail: ppernica@physics.utoronto.ca Mathew Wells University of Toronto, 1265 Military Trail, Toronto, Ontario, Canada M1C 1A4


INTRODUCTION
The typical thermal structure of a dimictic mid-latitude lake in the summer is usually depicted as a three-layer system: a warm isothermal epilimnion separated from a cool hypolimnion by the seasonal thermocline, a region of high thermal stratification. However, weak temperature gradients can form in the epilimnion from daytime solar heating, so that this layer is not continuously isothermal. These weak temperature gradients are referred to as 'diurnal thermoclines' (Imberger ; Monismith & MacIntyre ) or 'nearsurface thermoclines' (Xenopoulos & Schindler ). () also showed that the flux of carbon dioxide out of a small shallow lake in Finland was strongly controlled by the stability of the lake's temperature stratification. They observed that when the depth averaged buoyancy frequency was greater than 0.09 s À1 , the measured flux of carbon dioxide was almost zero. The presence of stratification in the epilimnion can also change the depth to which active mixing occurs, resulting in fluctuations in irradiance which used a definition of T 0m À T 1m ! 0.2 W C to identify the presence of diurnal thermoclines. The stratification for an average value of velocity shear producing Ri g > 0.25 can also be used to define a near surface thermocline. Approximating the current velocity shear and using Ri g crit ¼ 0.25 generates a critical buoyancy frequency and hence a minimum temperature stratification necessary to inhibit mixing.  Values of Ri g were calculated using Equation (1). The RBR temperature data were averaged over 10 min and were depth interpolated to every 0.6 m prior to computing density. Velocity shear was calculated with Δz ¼ 0.6 m, the size of the velocity depth bins using 10 min averaged velocity data. This produced 10 min averaged values of Ri g .
Since we did not have a temperature sensor at the surface of the lake, we calculated a near surface thermocline as occurring for dT/dz > 0.2 W C m À1 between depths of 1 to 2 m to compare with near surface thermocline data For the 2009 data, this was done by calculat-

RESULTS
The meteorological, temperature, and velocity data for both years are compared in the following section. Further analysis of these data, including calculating buoyancy frequency, gradient Richardson number (Ri g ), and frequency and duration of near surface thermoclines, is also included.   For both years the percentage of near surface thermoclines was slightly higher during the day than at night due to daytime heating (Table 1)    Our definition of near surface thermocline of dT/dz > 0.2 W C m À1 was chosen in order to compare our data with     The south arm of Lake Opeongo has an area of 22 km 2 , so this formula predicts that the mixed layer depth is 7 m, very close to the 5-6 m depth of the epilimnion shown in In both cases one of the common features of these continuously polymictic lakes is that they have shallow depths, less than 5 m, i.e., similar to the depth of the epilimnion in Lake Opeongo. Thus rather than thinking of the epilimnion as an essentially isothermal layer, a better analogy might be to the shallow continuously polymictic lakes where the stratification is weak but constantly changing.

CONCLUSION
The presence of weak thermal stratification in the epilimnetic waters of lakes has strong implications for chemistry gradients, algal physiology, and species diversity as the phytoplankton respond to changing mixing dynamics in the lake. We find that the epilimnion of Lake Opeongo during the summer season is frequently (∼30%) stratified, particularly in daylight periods early in the season (∼40%).
Extended periods of stratification were observed with 80% of the stratified period consisting of near surface thermoclines that persisted for at least 1 h. These events lasted for a significant fraction of the daylight hours, so during these periods the water in the epilimnion was not well mixed, and turbulence as parameterized by Ri g was of a highly intermittent nature.
The reduction of active vertical turbulence due to weak temperature gradients is known to have implications for the distribution of plankton and other aquatic organisms We hope that the observations of persistent temperature gradients in the epilimnion of Lake Opeongo will spur aquatic biologists to make routine measurements of stratification in surface waters. Many temperature probes that are routinely used only record temperature to an accuracy of ±0.1 W C. Hence, to accurately determine the presence of a 0.1 or 0.2 W C m À1 temperature gradient requires a higher accuracy of at least ±0.01 W C. This accuracy is routinely attained by many instruments (such as a CTD (Conductivity, Temperature, Depth)) that are used by oceanographers to measure weak thermal gradients at sea. We suggest that aquatic biologists consider using these more accurate temperature loggers in future studies looking at the spatial distribution of plankton in the euphotic zones of a lake, that typically lie within a lake's epilimnion. By accurately measuring these weak near surface stratifications, aquatic biologists will be able to predict whether the surface waters of the lake are homogeneously mixed, and so determine sensible strategies for the sampling of chemistry and planktonic organisms.