The allelopathic capacity of submerged macrophytes shapes the microalgal assemblages from a recently restored coastal wetland

We have tested the efficiency of isolated and combined submerged macrophyte cultures to inhibit, through allelopathy, the natural phytoplankton growth. Both plants and microalgae come from the same wetland, a recently restored area in Albufera de València Natural Park (Spain). The need to replant the area under restoration with submerged macrophytes makes this information essential for wetland management. The selection and culture of the submerged macrophytes used in that restoration (four charophytes: Chara hispida, Chara vulgaris, Chara baltica, Nitella hyalina, and one angiosperm: Myriophyllum spicatum) provided a good opportunity to test in the laboratory the allelopathic effect of macrophyte assemblages on environmental phytoplankton communities. Three experiments were carried out using spring communities: in Experiment 1, a diverse phytoplankton assemblage (29% of biomass Chlorophyceae, 26% Cryptophyceae, 19% diatoms and 9% cyanobacteria) was cultivated with exudates from monocultures of C. hispida, N. hyalina and M. spicatum. Experiment 2 proved the allelopathic effect of a macrophyte assemblage exudate (C. hispida, C. baltica, C. vulgaris, N. hyalina and M. spicatum in a mixed culture) on two different phytoplankton communities (one diverse: 53% biomass of diatoms, 27% cyanobacteria, 18% chlorophytes and another dominated by small chlorophytes). The response variables were Chl a concentration, phytoplankton biovolume and main algal groups' biovolumes. When phytoplankton grew in water with exudates from monocultures, microalgal biomass was from 4 to 6 times lower than in the control after 5 days and Chl a concentration was up to 4 times lower. The inhibitory effect of C. hispida was greater than that of M. spicatum. Mixed macrophyte assemblages resulted in even stronger allelopathic effects than monocultured macrophytes; the biomass was reduced by 7 fold after 5 days using the mixed exudates and Chl a concentration was between 3 and 5 times lower. The experiments demonstrate that macrophytes are particularly effective in inhibiting the growth of both small diatoms and the least desirable phytoplankton component in these wetlands (filamentous cyanobacteria), but not chlorophytes (reduction by 37-69, 7-14 and 1-7 fold for diatoms, cyanobacteria and chlorophytes, respectively). The predictions are that spring macrophytes might enhance microalgae that are suitable for grazing (mostly small chlorophytes) and will decrease non-edible filamentous taxa. Thus, restoration managers should replant with the mixture of submerged native macrophytes that provide the most harmful allelopathic effects to promote benefits on aquatic communities in two synergistic ways: by directly reducing microalgal biomass and by indirectly enhancing grazing, which in turn, would promote a clear-water phase. We present replanted macrophytes as ecosystem engineers, i.e. organisms that directly or indirectly modulate the availability of resources for the wetland food webs, by causing changes in primary producer assemblages.