Macroalgae re- and afforestation are frequently proposed carbon dioxide removal (CDR) strategies, but the carbon storage is often uncertain. In nature, kelp forests grow on hard or sandy substrate leading to as little as ∼0.4% of their primary production being buried in the kelp’s habitat (Krause-Jensen and Duarte, 2016) with the remainder exported. Export of dissolved and particulate carbon from kelp forests is strongly affected by the exchange of water between the kelp forest and the surrounding water. The permanence of storage of the exported carbon is determined by its destination, which is affected by the flow in and around the forest. Additionally, the potential for forests to grow can be limited by nutrients dissolved in the water, the availability of which is determined by the same water exchange rates.

Here, we use OceanBioME to build a numerical model of coupled flow/kelp interactions to study how tidal currents interact with a giant kelp (Macrocystis pyrifera) forest. We first validate our model using observations of currents within and surrounding a kelp forest in Southern California. By varying the kelp density within our model and tracking dissolved tracer released from the kelp forest, we analyse the timescales of water exchange to better understand how the flow influences carbon export and nutrient uptake. We find a density that maximizes the export of tracers which coincides with the density typical of natural kelp forests. Additionally, the drag results in a mean circulation through the forest and a mean displacement of the individuals suggesting that the physical dynamics of the kelp should be an important consideration in future studies and when planning re/aforestation projects.