Riparian shading controls instream spring phytoplankton and benthic algal growth.

S J Halliday,M Loewenthal,H P Jarvie,D S Read,M J Bowes,R A Skeffington,A J Wade

doi:10.1039/c6em00179c

Abstract

Dissolved oxygen (DO) concentrations showed a striking pattern in a multi-year study of the River Enborne, a small river in SE England. In each of three years (2010-2012), maximum DO concentrations were attained in mid-April, preceded by a period of steadily increasing diurnal amplitudes, followed by a steady reduction in both amplitude and concentration. Flow events during the reduction period reduce DO to low concentrations until the following spring. Evidence is presented that this pattern is mainly due to benthic algal growth which is eventually suppressed by the growth of the riparian tree canopy. Nitrate and silicate concentrations are too high to inhibit the growth of either benthic algae or phytoplankton, but phosphate concentrations might have started to reduce growth if the tree canopy development had been delayed. This interpretation is supported by evidence from weekly flow cytometry measurements and analysis of the diurnal, seasonal and annual patterns of nutrient concentrations. As the tree canopy develops, the river switches from an autotrophic to a heterotrophic state. The results support the use of riparian shading to help control algal growth, and highlight the risks of reducing riparian shade.

Highlights

It is generally accepted that nuisance algae and a shi in plant community composition can be a consequence of nutrient enrichment by nitrogen (N) and phosphorus (P) compounds, which is in turn due to increasing human effluent inputs and runoff from intensive agriculture
Reducing photosynthetic rates and water temperatures, and though modelling studies tend to show this would be highly effective in reducing algal growth especially under scenarios of increased water temperature resulting from climate change,[6,8] observational evidence of its effectiveness is more limited
This paper explores the controls on algal growth in a small river in SE England, the River Enborne, where riparian shading, by deciduous trees, is heavy but seasonal

Summary

Introduction

It is generally accepted that nuisance algae and a shi in plant community composition can be a consequence of nutrient enrichment (eutrophication) by nitrogen (N) and phosphorus (P) compounds, which is in turn due to increasing human effluent inputs and runoff from intensive agriculture (e.g. ref. 1 and 2). It is generally accepted that nuisance algae and a shi in plant community composition can be a consequence of nutrient enrichment (eutrophication) by nitrogen (N) and phosphorus (P) compounds, which is in turn due to increasing human effluent inputs and runoff from intensive agriculture A number of papers have suggested recently that promoting riparian shading would be a more effective, and certainly more cost-effective, management tool for the control of nuisance algae in rivers.[6,7,8] Riparian shading is expected to work by reducing photosynthetic rates and water temperatures, and though modelling studies tend to show this would be highly effective in reducing algal growth especially under scenarios of increased water temperature resulting from climate change,[6,8] observational evidence of its effectiveness is more limited. Using high-frequency hydrochemical data coupled with weekly grab sampling of a wider range of chemicals and the river's phytoplankton community, we can test the hypothesis that riparian shading controls algal growth for at least part of the year

Methods

Results

Conclusion