Flow velocity and light level drive non-linear response of seagrass Zostera noltei to ammonium enrichment

We investigated the interactive effects of light (low and high light doses) and flow velocity (low, medium and high levels) under NH4+ enriched conditions on dynamic and morphological variables of Zostera noltei plants in a 5 wk flume experiment. Our results showed a nonlinear response of Z. noltei in this factorial design, with the strongest negative effect of NH4+ enrichment recorded at intermediate flow velocities for almost all the dynamic response variables (i.e. survival, net production, shoot appearance rate). This negative effect of NH4+ enrichment was intensified under low light conditions, where net production was only positive in plants growing at high flow velocity. This positive effect of flow velocity was ascribed to the more horizontal position of the leaves, which allows for higher levels of light capture than under lower flow velocities. However, enhancing current velocity may increase NH4+ uptake rates until they reach adverse levels, which can potentially trigger NH4+ toxicity. This negative effect of flow velocity seems to be neutralized by the higher light capture at high flow, resulting in intermediate current velocities being more harmful for plants. In summary, our results highlight the importance of studying the complexity of interactions among multiple stressors that frequently co-occur in nature to improve our ability to forecast the response of seagrass populations to possible interaction effects in future global change scenarios.