Context dependence of interactions among global change stressors on submerged plants: roles of null models, experimental conditions, and plant traits

Submerged plants play key roles in aquatic ecosystems and are affected by multiple concurrent global-change stressors. These interactions are context-dependent, but the sources of this context dependence have not been systematically evaluated. We synthesized 57 studies and classified interactions between paired stressors as additive or non-additive by comparing observed combined responses with additive, multiplicative and dominance null-model predictions. Non-additive interactions were frequent among individual observations. However, the overall interactions were additive under the additive and dominance null models, and all stressor-pair mean interactions were additive under all three null models. The multiplicative null model identified an overall non-additive interaction. About one-third of observations changed classification or direction among null models. Negative non-additive interactions were detected under high background nutrient conditions and in experiments without fish and in experiments without snails under the multiplicative null model. Species classified as Eutrophic sensitive showed a negative non-additive interaction under the additive null model, whereas species classified as Eutrophic tolerant, C3 or Not warm tolerant did so under the multiplicative null model. Overall, whether interactions were classified as additive or non-additive depended on null-model choice, experimental conditions and plant traits, and no general interaction pattern emerged from current evidence. Therefore, we advocate more experiments under more realistic ecological conditions and more comprehensive reporting of experimental contexts.