Interactions between stoneworts (Charales) and waterbirds.

Stoneworts (Charales) are green algae that represent an important food resource for many waterbird species in Europe and elsewhere. Browsing avian herbivores (e.g. swan, goose, duck and coot species) consume Charales plant vegetative parts, by head-dipping, up-ending or diving. A lower fibre content and longer growing season may make Charales as attractive to such herbivores as sympatric submerged higher plant species in some circumstances. Charales respond to environmental stress (e.g. drought) by producing abundant diaspores, in the form of oospores (sexual) and bulbils (asexual), both rich in starch, generating abundant food for waterbirds at critical stages in their annual migratory cycles. Waterbirds feed on these by diving (e.g. common pochard Aythya ferina and red-crested pochard Netta rufina) or by filtering from the water column (e.g. dabbling duck species), ensuring dispersal of sexually produced and vegetative diaspores locally (because of predator swamping) and remotely (through endo- and ectozoochorous dispersal by long-distance migratory waterbirds). Greater invertebrate density and diversity associated with Charales canopies enhances their attractiveness over other submerged macrophyte beds to diving predators [e.g. tufted duck Aythya fuligula, common pochard and Eurasian coot Fulica atra (hereafter coot)]. Fish fry preying on these invertebrates use such vegetation as predator cover, in turn providing prey for avian piscivores such as grebes and cormorants. Abundant Charales contribute to maintaining a transparent water column due to canopy density, nutrient effects, dampening of sedimentation/remobilisation of suspended matter and nutrients and allelopathic effects on other plants (especially phytoplankton). Shallow, relatively eutrophic waters can flip between clear-water high-biodiversity (where Charales thrive) and turbid species-poor depauperate stable states (lacking Charales). Shifts between turbid conditions and rich submerged Charales beds have profound elevating effects on aquatic diversity, to which waterbirds show rapid aggregative responses, making them ideal indicator species of ecological change; in the case of Charales specialists (such as red-crested and common pochard), indicators of the presence and abundance of these plants. Large-bodied colonial nesting birds (e.g. cormorants, gulls, heron and egrets) aggregating along lake shores contribute high N and P loadings to water bodies sensitive to such external and internal inputs and can cause local eutrophication and potential loss of Charales. Despite variation from complete seasonal removal of Charales biomass to undetectable grazing effects by herbivorous birds, evidence suggests little effect of avian grazing on biomass accumulation or the stability of community composition (under otherwise stable conditions), but we urge more research on this under-researched topic. We also lack investigations of the relative foraging profitability of different Charales organs to waterbirds and the degree of viability of gyrogonites (fertilised and calcified oospores), vegetative bulbils and plant fragments after passage through the guts of waterbirds. We especially need to understand better how much the carbonate armour of these organs affects their viability/dispersal via waterbirds and urge more research on these neglected plants and their relationships and interactions with other organisms in the aquatic biota.