Alternative Stable States

Abstract

The chapters in this book show how profoundly the presence of abundant submerged macrophytes may change the conditions in a lake. Macrophytes and associated epiphytes compete with phytoplankton by taking up nutrients from the water, by shading, and perhaps also by secreting allelopathic substances (Sønder-gaard and Moss, this volume, Chapter 6). They reduce wind-induced water movements which results in increased sedimentation and reduced resuspension (Barko and James, this volume, Chapter 10; Van den Berg et al., this volume, Chapter 25). Macrophytes also represent a large surface area and therefore enhance the density of surface-associated organisms (Diehl and Kornijow, this volume, Chapter 2; Jeppesen et al., this volume, Chapter 5). Such a local concentration of organisms together with the reduced water movements often leads to marked gradients in oxygen conditions, which have a significant impact on phosphorus and nitrogen dynamics and pH (Barko and James, this volume, Chapter 10; Søndergaard and Moss, this volume, Chapter 6). The plant-associated organisms comprise animals such as snails that graze periphyton from plant surfaces (Brönmark and Vermaat, this volume, Chapter 3; Crowder et al., this volume, Chapter 14; Diehl and Kornijöw, this volume, Chapter 2; Jones et al., this volume, Chapter 4), but plant beds also host macrofiltrators such as mussels (e.g., Anodonta) and microfiltrators (e.g., Sida), which may occur in such high densities that they help reducing turbidity of the water within the plant beds (Jeppesen et al, this volume, Chapter 5).