Remote sensing evidence for the episodic transport of phosphorus from the littoral zone of a thermally stratified lake

Abstract

Summary 1 Esthwaite Water in Cumbria is a small, thermally stratified lake fringed with beds of reed in the shallow littoral. In this study, we used a combination of in situ measurements and airborne remote sensing to investigate some of the physical processes influencing the transport of water and nutrients from the littoral zone. 2 The analysis of water samples collected from the reed beds, the littoral zone and the open water showed that significantly higher concentrations of dissolved reactive phosphorus (DRP) were frequently recorded in the reed beds in early summer. 3 Experiments with surface and near-surface free-running drogues demonstrated that the movement of water from the littoral zone was strongly influenced by the development of a secondary thermocline. When there was no secondary stratification, the surface currents generated by light winds seldom exceeded 2 cm s-1. When a secondary thermocline was present, surface current speeds of 5–10 cm s-1 were recorded even when the wind speed was less than 200 cm s-1. 4 A series of thermal surveys using a Daedalus Airborne Thematic Mapper (ATM) demonstrated that plumes of warm water frequently developed in the littoral zone when the weather was calm. Some of these plumes covered several hundred square metres and persisted for several hours. Others were only a few metres in width and acted as episodic ‘pumps’ that appeared and dispersed in less than an hour.