The impact of bird herbivory on macrophytes and the resilience of the clear-water state in shallow lakes: a model study

Wolf M. Mooij,Jan J. Kuiper,Cassandra van Altena,Elisabeth S. Bakker

doi:10.1007/s10750-016-2779-6

Wolf M. Mooij, Jan J. Kuiper + Show 2 more

Open Access

https://doi.org/10.1007/s10750-016-2779-6

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Abstract

Shallow lakes have the potential to switch between two alternative stable states: a clear macrophyte-dominated and a turbid phytoplankton-dominated state. Observational and experimental studies show that in some lakes herbivory by birds may severely decrease macrophyte biomass, while in other lakes, the removed biomass by herbivory is compensated by regrowth. These contradictory outcomes might arise because of interplay between top-down control by bird herbivory and bottom-up effects by nutrient loading on macrophytes. Here, we use the ecosystem model PCLake to study top-down and bottom-up control of macrophytes by coots and nutrient loading. Our model predicted that (1) herbivory by birds lowers the critical nutrient loading at which the regime shift occurs; (2) bird impact on macrophyte biomass through herbivory increases with nutrient loading; and (3) improved food quality enhances the impact of birds on macrophytes, thus decreasing the resilience of the clear-water state even further. The fact that bird herbivory can have a large impact on macrophyte biomass and can facilitate a regime shift implies that the presence of waterfowl should be taken into account in the estimation of critical nutrient loadings to be used in water quality management.

Highlights

Shallow lakes have the potential to switch between two alternative stable states: a clear macrophytedominated and a turbid phytoplankton-dominated state (Jeppesen et al, 1990; Scheffer, 1990; Carpenter, 2003)
We used an established ecosystem model, PCLake, to test in a standardized way for different nutrient loadings what the impact of birds is on macrophyte biomass, and whether birds could induce a regime shift from a clear- to a turbid-water state
Our model predicted that: (1) bird herbivory lowers the critical nutrient loading at which the regime shift occurs, implying that at intermediate nutrient loadings birds can trigger a regime shift, (2) bird impact on macrophyte biomass through herbivory increases with nutrient status, and (3) improved food quality can enhance the impact of birds on macrophytes and thereby decrease the resilience of the clearwater state even further

Summary

Introduction

Shallow lakes have the potential to switch between two alternative stable states: a clear macrophytedominated and a turbid phytoplankton-dominated state (Jeppesen et al, 1990; Scheffer, 1990; Carpenter, 2003). Macrophytes play an important role in the resilience of the clear-water state (Carpenter & Lodge, 1986; Scheffer et al, 1993), because they stabilize this state in multiple ways. They reduce resedimentation of particles from the sediment of the lake Because zooplankton are key grazers of phytoplankton and zooplankton abundance can be severely depleted by fish in the absence of shelter, macrophyte shelter for zooplankton supports the clearwater state in lakes

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