The influence of submersed aquatic plants on temperature gradients in shallow water bodies

Abstract

In Lake Opinicon, Frontenac County, Ontario [Canada] the temperature gradient at midday from 10 cm below the water surface to soil, in clear weather in Aug., was found to be 10.degree. C/m in shallow water where there was a large macrophyte biomass and only 0.2.degree. C/m in deeper water with fewer plants. There was a direct correlation between the log of the biomass per cubic meter and the steepness of the gradient. Reference is made to temperature gradients developed in calm weather during phytoplankton blooms in Scottish lakes. The biomass of aquatic macrophytes [Elodea canadensis and Vallisneria americana] growing in a heated greenhouse in tubs with a capacity of 1 m3 increased from early Jan. to the end of Feb. With increased biomass, the temperature gradient became larger. The size of the gradient correlated with the incoming radiation. Temperature gradients within large populations of the pondweed Potamogeton richardsonii were enhanced by low temperatures near the bottom. Diurnally produced temperature gradients with an algae bloom in the same tubs in the open in late June showed strong correlation with solar radiation, 8.degree. C/m with cumulative radiation of 22 MJ/m2 and 4.6.degree. C/m with 122 MJ/m2. The gradients produced by a population of P. richardsonii with tips 17 cm from surface were from 5.2-3.4.degree. C/m but showed less correlation with radiation when the wind velocity was above a threshold value. No gradient was produced in tubs of water without plants.