Algal coexistence created jointly by neutral competitor and asymmetrical competitors in shallow aquatic ecosystems

• There is competitive and compensatory trade-off between primary producers. • Asymmetrical competitors can coexist as niche differentiation in other dimensions. • Neutral competitor might facilitate coexistence by joining resource competition. • Niche-based and neutral forces jointly shape the species coexistence. • Water quality management can help maintain clear-water state. How to maintain a clear-water state in shallow aquatic ecosystems is a pressing issue, especially in the context of frequent occurrences of eutrophication. The alternative equilibria between clear-water and turbid states depend on the competitive dominance between pelagic algae and epiphytic algae. Therefore, understanding algal interactions under changing nutrient loads is crucial for managing shallow lake ecosystems. According to niche-based theories, such as resource competition theory, trade-offs in the competitive abilities of species for limiting resources lead to algal coexistence. However, neutral theory proposes that species diversity is determined by ecological drift rather than differences in competitive abilities. To clarify which mechanism primarily determines algal coexistence in shallow aquatic ecosystems, we set up a controlled pairwise competition experiment to investigate the response of multiple algal assemblages to changes in initial algal life form under different nutrient loading scenarios. Our results supported the theory of a competitive and compensatory trade-off between primary producers. Our findings showed that the inferior competitor (periphyton) might coexist with the superior competitor (phytoplankton) in the asymmetrical competition for limiting resources (e.g. nutrients or light), owing to their niche differentiation along other dimensions (water qualities). Besides, the neutral competitor (microphytobenthos) may help facilitate equilibrium coexistence by participating in resource competition. Thus, we emphasized that niche-based and neutral forces jointly shaped the algal coexistence. Based on our findings, water quality management can regulate algal competition and mediate the shift between clear-water and turbid states in shallow aquatic ecosystems.