Synergy between shading and herbivory triggers macrophyte loss and regime shifts in aquatic systems

Abstract

Macrophytes play a keystone role in shallow aquatic ecosystems. In lakes, macrophytes stabilize clear‐water conditions with high biodiversity and their decline can cause a shift to a turbid state with lower biodiversity. Various mechanisms have been suggested as triggers of macrophyte collapse. Herbivory by waterfowl and fish seems to be one of the obvious factors, but the response of macrophytes to herbivory is ambiguous. We hypothesized that herbivory alone does not typically cause macrophyte collapse, but that shading from periphyton can enhance the effect of herbivores. Shading of macrophytes is supposed to increase with eutrophication due to changes in the top–down control cascading from fish via macroinvertebrates to periphyton. We elaborated on this idea by fitting a macrophyte growth model with different herbivore grazing and periphyton shading scenarios. In addition, we performed a meta‐analysis on existing experimental herbivore exclosure studies with respect to periphyton growth. The model supported our proposed hypothesis and the reviewed field studies appeared to point in the same direction. We suggest that a significant herbivore impact may indicate a reduced resilience of vegetation to eutrophication, making it an early warning signal for an imminent macrophyte collapse leading to a sudden shift of the system to turbid conditions.

Interactive Effects of Rising Temperature and Nutrient Enrichment on Aquatic Plant Growth, Stoichiometry, and Palatability

“…As a result, V. spiralis profited from nutrient enrichment in the sediment, but suffered from competition by algae, in particular periphyton, under external nutrient loading (Figure 3), as also illustrated in the SEM (Figure 7). The periphyton biomass densities observed in our study correspond to a reduction in light availability of approximately 20% at the high end of the observed periphyton densities (up until approximately 300 µg cm −2 ), if periphyton directly grows on the leaves of the plants (Hidding et al, 2016). Therefore, in our study, V. spiralis suffered from competition by periphyton, which may have to a limited extent resulted from shading, but it is not possible to pinpoint whether light competition or nutrient competition was driving the observed effect.…”

Periphyton density is similar on native and non‐native plant species

“…Under eutrophic conditions, submerged plants compete strongly with algae for light and nutrients (Scheffer, Hosper, Meijer, Moss, & Jeppesen, 1993). Especially epiphytic algae, which grow attached to plants, are a major cause of shade and contribute to the decline of native submerged vegetation under increasing nutrient loading (Hidding, Bakker, Hootsmans & Hilt, 2016;Phillips, Eminson, & Moss, 1978;Phillips, Willby, & Moss, 2016). Although native vegetation declines under these conditions, non-native plants typically grow excessively in eutrophic conditions and can dominate the vegetation (Hussner, 2012;Van Kleunen et al, 2015).…”

The “Forgotten” Ecology Behind Ecological Status Evaluation: Re-Assessing the Roles of Aquatic Plants and Benthic Algae in Ecosystem Functioning

“…In contrast to earlier assumptions, periphyton density is often not controlled by nutrient loading but top-down by a fish-grazer-periphyton cascade (a high number of fish feeding on grazing macroinvertebrates results in high periphyton biomass, whilst a low number of fish results in greater grazing activity by macroinvertebrates, leading to a lower periphyton biomass; Jones and Sayer 2003). In addition, herbivory by birds and fish might play a significant role in preventing macrophyte reestablishment (Bakker et al 2013), particularly when combined with periphyton shading (Hidding et al 2016). All these interactions blur a simple correlation between water chemistry and assemblages of aquatic plants and benthic algae.…”