Algal growth control by terrestrial leaf litter: A realistic tool?

Abstract

When barley straw and deciduous leaf litter decompose aerobically in water, inhibitors are released that suppress the growth of nuisance algae. Barley straw has been widely used for algal control in small, shallow lakes and we review the advantages and disadvantages of the method. It is particularly effective at promoting the switch from algal to macrophyte domination. Despite its cheapness and apparent safety in the short term, however, the use of barley straw requires considerable management effort and the long-term ecological safety of such un-natural litter inputs is unknown. We therefore recommend it to lake managers primarily as a short-term measure. Deciduous leaf litter from a range of woody species can suppress the growth of Chlorella and Microcystis very effectively in laboratory bioassays and, in field trials with medium-sized ponds, the addition of leaf litter produced significant inhibition of the filamentous alga Cladophora glomerata. We followed the development of algal inhibitory activity over 2.5 years with freshly fallen oak leaves placed in a large tank of aerated water and using Chlorella as the test species. Two periods of inhibitor release were identified: 4-90 days (early phase) when soluble, relatively stable inhibitors were present in tank liquor, probably generated from oxidized tannins; and 120-900+ days (late phase) when inhibitors were relatively unstable in solution and were associated primarily with fine particulate organic matter (FPOM). Late phase inhibitors may, as suggested for barley straw, be generated during the oxidative breakdown of lignin. The prolonged and powerful anti-algal properties of these natural litter inputs offer possibilities for low-effort, sustainable management of lakes and catchments so as to reduce the problem excessive algal growth.