Overgrazing of edible algae as a mechanism behind red tides and harmful algal blooms

Abstract

It has recently been suggested that there is a positive synergistic relationship between herbivorous zooplankton and their algal food, with herbivores stimulating the production of their preferred species while negatively affecting competing algal species. A prerequisite for this positive interaction hypothesis is that herbivores and phytoplankton coexist in balance in which recycled nutrients from herbivores are immediately utilised by phytoplankton, which, in turn, are grazed by the herbivores. Empirical evidence suggests that this balance is more stable during increasing primary productivity than during its decline. During longer periods of highly stratified water in summer and early autumn, primary productivity may decrease because the export of nutrients from the euphotic zone outweighs import. If there is a lag between the decline in primary production and the decline in herbivore biomass, the herbivores may overgraze their preferred algal species, which, in turn, cannot fully utilise the recycled nutrients. Hence, nutrients may become available for non-grazed phytoplankton species that cause red tides and harmful algal blooms (HABs). In other words, it is hypothesised that HABs and red tides are caused by the collapse of the balanced positive (synergistic) relationship between herbivores and their preferred algal food.