Does the fish-invertebrate-periphyton cascade precipitate plant loss in shallow lakes?

Alternative equilibria dominated by either submerged plants or phytoplankton have been described for communities in shallow lakes. A nutrient-mediated increase in periphyton (algae attached to plant surfaces) is often described as being responsible for the loss of plants from shallow lakes, yet this violates the stochastic assumptions of alternative equilibria. To determine if periphyton is capable of forcing a switch between these communities, the factors governing the success of aquatic plants were surveyed in 17 plant-dominated shallow lakes in Norfolk (United Kingdom) that varied in nutrient concentration and fish biomass. In these lakes, plant biomass was negatively related to the density of periphyton. However, the density of periphyton on the plants was correlated with the density of grazing invertebrates, not nutrient concentration. In turn, the biomass of fish determined the density of invertebrates. This cascade from fish to periphyton via invertebrates appeared to be evident even though plant-dominated lakes are heterogeneous and complex. Under conditions of plant dominance, periphyton appeared to have a stronger influence on plant growth than phytoplankton. Our data support a model where, within the range of nutrients where alternative equilibria are possible, fish are the prime determinants of community structure in shallow lakes, through a cascading effect of predation on grazing invertebrates influencing the biomass of periphyton and hence, plants. We suggest that the stochasticity required for alternative equilibria in shallow lake communities is often derived from the vagaries of fish colonization and reproduction.