What is it?
Temperature [T, units of degrees Celsius (ºC)] is a measure of molecular vibrational energy.
How is it measured?
Many different methods can be used to measure temperature. Commonly liquid-in-glass thermometers are used to determine temperature. Though this method can be very reliable and accurate, a thermometer is not as useful when making measurements where the thermometer is not easily viewed (such as below the water surface). Methods that are more applicable include thermocouples and thermistors. The thermocouple measures the current generated by two dissimilar (different) metals at different temperatures. The buoy’s probe package utilizes a thermistor. A thermistor consists of a ceramic-like semiconducting material which has the property of decreasing resistance with increasing temperature. By measuring the voltage change across the thermistor, the temperature of the surrounding water may be obtained
Why is it important?
The temperature of water has extremely important ecological consequences. Temperature exerts a major influence on aquatic organisms with respect to selection/occurrence and level of activity of the organisms. In general, increasing water temperature results in greater biological activity and more rapid growth. All aquatic organisms have preferred temperature in which they can survive and reproduce optimally. For example, trout typically need cold water which may not be available in shallow waters during the summer.
Temperature is also an important influence on water chemistry. Rates of chemical reactions also generally increase with increasing temperature. Temperature is a regulator of the solubility of gases and minerals (solids) – or how much of these materials can be dissolved in water. The solubility of important gases, such as oxygen and carbon dioxide increases as temperature decreases. For example, warm water contains less dissolved oxygen (DO) than cold water. Inversely the solubility of most minerals increases with increasing temperature.
Thermal stratification refers to the layering that occurs, particularly in the warm months, during which a warmer, less dense layer (the epilimnion) overlies a colder denser layer (the hypolimnion). Thermal stratification is a common occurance in deep northern temperate lakes such as Onondaga Lake. Between these two layers is a third layer (the metalimnion) where strong vertical differences (gradients) in temperature, and therefore, density prevail. The vertical temperature profile shown below depicts this layering for a hypothetical summer profile.
Note that the set-up of this graph may seem somewhat atypical when compared to formats used in other scientific disciplines – the surface is at the top of the Y-axis (Depth axis), with increasing depths moving down this axis. This convention is commonly used by lake scientists (limnologists) for all vertical profile data, as its configuration is consistent with the character of the vertical measurements of the profile.
Thermal stratification is widely considered to be an important regulator of the overall metabolism of a lake. The epilimnion is usually relatively well mixed, as it is subject to mixing induced from the wind. In contrast, mixing is much more limited in the hypolimnion because these deeper lake layers are isolated from energy inputs imparted to the lake’s surface. Exchange of dissolved substances between the epilimnion and hypolimnion (across the metalimnion) is quite limited because of the low level of turbulence/mixing. Generally the greater the temperature/density gradient of the metalimnion, the less exchange across this layer. This limited mixing has important implications for the cycling of critical constituents such as nutrients and dissolved oxygen.
]Lakes in this climate experience major seasonal changes in the thermal stratification regime that are coupled to the seasonality of meteorological conditions. In early spring, after the loss of ice-cover, vertically uniform low temperatures are observed top to bottom. The absence of vertical temperature gradients allows mixing to occur throughout the water column with only modest energy (wind) input. This interval of uniform temperatures is referred to as spring turnover. Thermal stratification develops when surface waters are heated more rapidly (from increasing air temperature and solar radiation) than the heat can be distributed by vertical mixing. Progressive increases in the temperature of the epilimnion occur during summer, accompanied by increases in the temperature/density gradient in the metalimnion. Cooling of the epilimnion starts in late summer or early fall as air temperatures and solar radiation inputs decrease. The dimensions of the epilimnion deepen progressively until vertically isothermal conditions develop – the onset of fall turnover.
The features of thermal stratification of a particular system, such as the timing of turnover and the onset of stratification, the vertical dimensions of the layers, and the temperature of the layers, are manifestations of a number of system-specific characteristics and the influence of various environmental (forcing) conditions. In particular, these features are regulated by basin morphometry (size, shape, depth), and setting, attendant meteorological conditions, hydrology, and the extent of light penetration. Substantial year-to-year variations in the features of stratification can occur as result of natural variations in meteorological conditions.
Tributary Temperature Cycle
Tributaries (e.g., creeks and rivers) tend to be more dynamic in their temperature variation then are deeper lakes. Just as air temperature varies throughout the day, typically shallower water body’s temperature varies with diurnal solar cycle. Like air temperature, the water temperature typically is at coolest just before sunrise and at it warmest in late afternoon. Variation in cloud cover and air temperature will also effect the temperature of tributaries. Additionally, the sounding topography (hills) and vegetation (tree canopy) effect tributary temperature. Like other aquatic system in our region, tributaries experience a seasonal temperature cycle, with cool temperature in the winter and warmer in the summer. Typically tributaries warm faster in the spring and cool faster in the fall than lakes. This has consequence on a tributaries behavior upon enter a lake (e.g., density currents)
What to look for in our systems?
Seasonal heating and cooling will be clearly manifested for all robotic deployments. Additionally, Onondaga Creek will experience diurnal variation in temperature. The overall seasonal stratification regime described above will be observed in all the lakes that are part of the Network.