Differential response of forest-forming seaweeds to elevated turbidity may facilitate ecosystem shifts on temperate reefs

Underwater light is essential for fuelling coastal productivity. However, elevated turbidity, resulting from land-based activities and climate change, is often overlooked as a threat to coastal ecosystems. Understanding how low light, and specifically the temporal delivery of light, affects the productivity of forest-forming species is necessary to predict how ecosystems and species will respond to future increases in turbidity. Outdoor mesocosm experiments were used to compare the low-light tolerance of 2 forest-forming macrophytes that vary in their distribution in relation to turbidity, and investigate how the temporal delivery of light, i.e. press vs. pulse low-light disturbance, affects net primary productivity (NPP). We showed that the kelp Ecklonia radiata, which dominates reefs with low turbidity, is more productive per unit biomass under high-light conditions than the fucoid Carpophyllum flexuosum, which typifies more turbid waters. Under low light, E. radiata suffered greater tissue loss and had lower NPP than C. flexuosum. Under both press and pulse treatments, E. radiata showed significant losses of lamina biomass and reduced NPP, while C. flexuosum showed net growth under press disturbance, and only lost tissue and had reduced NPP under pulse disturbance. The greater tolerance of C. flexuosum to decreased light, and differential responses of E. radiata and C. flexuosum to press and pulse low-light conditions, provide mechanistic support for C. flexuosum being better suited to turbid low-light environments than E. radiata. These results suggest future increases in turbidity may facilitate a shift from kelp-dominated forests to alternate states, resulting in reduced primary productivity.