Coastal darkening substantially limits the contribution of kelp to coastal carbon cycles.

Macroalgal-dominated habitats are rapidly gaining recognition as important contributors to marine carbon cycles and sequestration. Despite this recognition, relatively little is known about the production and fate of carbon originating from these highly productive ecosystems, or how anthropogenic- and climate-related stressors affect the role of macroalgae in marine carbon cycles. Here, we examine the impact of increasing turbidity on carbon storage, fixation and loss in southern hemisphere kelp forests. We quantified net primary production (NPP) and biomass accumulation (BA), and estimated carbon release via detritus and dissolved organic carbon (DOC) across a large-scale turbidity gradient. We show that increased turbidity, resulting in a 63% reduction in light, can result in a 95% reduction in kelp productivity. When averaged annually, estimates of NPP and BA per plant at high-light sites were nearly six and two times greater than those at low-light sites, respectively. Furthermore, the quantity of carbon fixed annually by kelp forests was up to 4.7 times greater than that stored as average annual standing stock. At low-light sites, the majority of C goes directly into tissue growth and is subsequently eroded. In contrast, excess production at high-light sites accounts for up to 39% of the total carbon fixed and is likely released as DOC. Turbidity is expected to increase in response to climate change and our results suggest this will have significant impacts on the capacity of kelp forests to contribute to carbon sequestration pathways. In addition to demonstrating that turbidity significantly reduces the quantity of carbon fixed by kelp forests, and subsequently released as detritus, our results highlight the negative impacts of turbidity on a large source of previously unaccounted for carbon.