Interactive effects of light and snail herbivory rather than nutrient
loading determine early establishment of submerged macrophytes
Abstract
Submerged macrophytes play a key role in maintaining a clear-water phase
and promoting biodiversity in shallow aquatic ecosystems. Since their
abundance has declined globally due to anthropogenic activities, it is
important to include them in aquatic ecosystem restoration programs.
That macrophytes establish in early spring is crucial for maintaining
the macrophyte communities for the remainder of the year. However,
factors affecting this early establishment of submerged macrophytes have
not been fully explored yet. Here, we conducted an outdoor experiment
from winter to early spring using the submerged macrophytes Potamogeton
crispus and Vallisneria spinulosa to study the effects of shading,
nutrient loading, snail herbivory (Radix swinhoei) and their
interactions on the early growth and stoichiometric characteristics of
macrophytes. The results show that the effects strongly depend on
macrophyte species. Biomass and number of shoots of P. crispus
decreased, and internode length increased during low light conditions,
but were not affected by nutrient loading. P. crispus shoot biomass and
number showed hump-shaped responses to increased snail biomass under
full light. In contrast, the biomass of the plant linearly decreased
with snail biomass under low light. This indicates an interaction of
light with snail herbivory. Since snails prefer grazing on periphyton
over macrophytes, a low density of snails promoted growth of P. crispus
by removing periphyton competition, while herbivory on the macrophyte
increased during a high density of snails. The growth of V. spinulosa
was not affected by any of the factors, probably because of growth
limitation by low temperature. Our study demonstrates that the
interaction of light with snail herbivory may affect establishment and
growth of submerged macrophytes in early spring. Macrophyte restoration
projects may thus benefit from lowering water levels to increase light
availability and making smart use of cold-adapted herbivores to reduce
light competition with periphyton.