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

Abstract

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 kelpEcklonia radiata, which dominates reefs with low turbidity, is more productive per unit biomass under high-light conditions than the fucoidCarpophyllum flexuosum, which typifies more turbid waters. Under low light,E. radiatasuffered greater tissue loss and had lower NPP thanC. flexuosum. Under both press and pulse treatments,E. radiatashowed significant losses of lamina biomass and reduced NPP, whileC. flexuosumshowed net growth under press disturbance, and only lost tissue and had reduced NPP under pulse disturbance. The greater tolerance ofC. flexuosumto decreased light, and differential responses ofE. radiataandC. flexuosumto press and pulse low-light conditions, provide mechanistic support forC. flexuosumbeing better suited to turbid low-light environments thanE. radiata. These results suggest future increases in turbidity may facilitate a shift from kelp-dominated forests to alternate states, resulting in reduced primary productivity.