Nutrients recycled by a small omnivorous fish facilitate the growth and change the stoichiometric contents of submerged macrophytes

When restoring shallow lakes, the recovery of submerged macrophytes is of vital importance to obtain clear water conditions. However, post-restoration recovery of small omnivorous fish can increase water nutrient concentrations through excretion. The impact of these recycled nutrients on submerged macrophytes is not well-studied, and it may depend on fish food of varying nutrient contents. We studied the effects of Acheilognathus macropterus on the growth and nutrient stoichiometry of Vallisneria denseserrulata and Hydrilla verticillata . The experiment had six treatments: two controls without recycled nutrients and two nutrient treatments for each of the plant species, respectively. We found that fish released more phosphorus after consuming low N:P food and more nitrogen after high N:P food. The recycled nutrients significantly promoted the growth of both macrophyte species. Interestingly, macrophytes showed higher growth rates in treatments with nutrients from the low N:P food source. Our study, therefore, suggests that nutrient subsidy from fish excretion can significantly stimulate the growth of submerged macrophytes, the magnitude of the effect being determined by the N:P ratio of the fish food. The realization of this growth potential, however, also depends on the phytoplankton response to these nutrients, that expectedly will be weaker at high macrophyte coverage.