Assessing the Impact of Coastal Darkening on Primary Production of Habitat Forming Macrophytes

The transport of dissolved organic carbon (DOC) into aquatic ecosystems is causing a global darkening of coastal seas, negatively impacting water quality, light penetration, nutrient cycling, and overall ecosystem productivity. Climate change is expected to exacerbate brownification by altering catchment hydrology and increasing run-off, raising DOC levels and accelerating coastal darkening. Habitat-forming benthic macrophytes (e.g. seagrasses, macroalgae, aquatic plants) host high biodiversity and support a wealth of ecosystem service provision, including fishery support, water quality mitigation, coastal protection and blue carbon storage. However, their reliance on sunlight for photosynthesis makes them extremely vulnerable to coastal darkening and brownification. The decline of benthic macrophytes due to brownification would induce a cascade of significant ecological and socio-economic repercussions, so it is essential to understand how these ecosystems may respond. Using the Baltic Sea as a model system for future trends in brownification, we take a multidisciplinary approach to elucidate how the primary production of coastal habitats will change throughout this century. Working along a natural gradient of brownification spanning 10° of latitude, we have characterized underwater light fields, and quantified organismal photobiology and community habitat formation to model primary production under current and future brownification scenarios. By reconciling the organismal-community level effects of brownification on these valuable coastal ecosystems, we provide evidence-based support for coastal conservation and management strategies for both mitigating climate change and land-use change stressors and for maintaining coastal ecosystem services.