Recurrent internal waves in a small lake: Potential ecological consequences for metalimnetic phytoplankton populations

Abstract

Lay AbstractWind mixing is one of the main factors controlling the ecology of lakes. Large, deep lakes, as well as small, shallow ones, can develop a seasonal thermal stratification. Because both light and heat come from above, a warm and illuminated surface layer develops above a colder, darker, and heavier bottom water layer under the influence of gravity. Gravity also controls the tendency for the microscopic plants that make up the floating plankton—the phytoplankton, which are a bit heavier than water—to fall from the top layer to the bottom layer, where they die and decompose, releasing the nutrients contained in their bodies. For this reason, the warmer surface layer, or epilimnion, becomes depleted in plant nutrients and the deep layer becomes enriched with nutrients. Specialized phytoplankton can grow at the interface of these two layers, leading to the development of a deep‐chlorophyll maximum in the intermediate layer known as the metalimnion. Strong winds can stir the waters of the upper layer and increase resource availability to the plankton in the upper layer in two ways. First, the mixing can bring nutrients across the metalimnion and up into the epilimnion. Second, the wind increases turbulence in the epilimnion, thereby keeping the phytoplankton suspended in the lighted layer and resuspending cells from the metalimnion. In this study, a particular consequence of wind mixing known as internal waves, which are waves that occur in the metalimnion, was observed in a small lake. These waves are caused by the wind physically pushing the warm surface layer to the downwind side of the lake, where the warm water accumulates and pushes down on the metalimnion, inducing a lakewide tilt in the metalimnion. When the wind relaxes, the waves on the surface of the lake diminish, but larger waves can be induced in the metalimnion as it tries to reestablish its equilibrium with gravity. In much the same way as plucking a guitar string can produce a primary tone as well as several overtones, the oscillation in the metalimnion has more than one frequency. This study links lake physics and ecology by characterizing the different modes of internal waves and examining the potential ecological consequences of each for phytoplankton productivity. Until now, the exploitation of internal waves by specialized phytoplankton has been thought to be important mainly in large, deep lakes. This study highlights the occurrence and ecological importance of both vertically complex internal waves and the dominance of a deep chlorophyll maximum over the entire summer stratification period in a small shallow lake.