The effect of aquatic vegetation on turbidity; how important are the filter feeders?

Abstract

A review of the literature suggests that aquatic macrophytes can enhance water clarity and reduce phytoplankton biomass through shading, reduction of nutrient availability, excretion of allelopathic substances and reduction of resuspension. In addition, vegetation fields are reported to enhance grazing on phytoplankton by providing a day-time refuge against fish predation for planktonic filter feeders such as Daphniaand by providing a suitable habitat for macrophyte associated filter feeders such as Sida crystallina, Eurycercus lamellatusand Simocephalus velutus. I use a graphical and a simple mathematical model to explore how top-down control by these grazers may interact with the effect of reduced phytoplankton production due to the other factors mentioned. The analysis suggests that grazing tends to be an all-or-none effect, driving phytoplankton to a very low biomass once a certain threshold level of grazing pressure is exceeded. This threshold level is predicted to increase with the productivity of the phytoplankton. Thus, the model suggests that, in plant beds, productivity reducing factors such as shading and reduced nutrient concentrations can pave the way for top-down control of phytoplankton even by a relatively moderate population of filter-feeders, and that phytoplankton biomass will decrease sharply beyond a critical macrophyte (or grazer) density. Indeed such a discontinuous response is observed in field experiments. Also, the idea that filter feeding cladocerans such as Daphniaplay a key role is in line with the observation that brackish lakes where Daphniadoes not thrive tend to be turbid despite the often dense weed beds.