Lagrangian Pathways and Connectivity in a Reef Canopy

Abstract

Changing global ocean conditions threaten many tropical reef ecosystems. Affected reefs in turn change the local ocean conditions. Local changes in bottom friction, nutrient availability, oxygen consumption and spreading of spawn and diseases determine the transient response of these ecosystems to changing global conditions. To study the mechanisms causing a decrease in coral cover in many reefs including those in the Caribbean, more detailed models of the complex turbulent flow over the reef seafloor are starting to be developed. This study aims to develop Lagrangian tools to analyze the small-scale transport processes and the network of sedentary organisms, the immobile organisms that depend entirely on the flow. This enables studying the local mechanisms driving past and future reef ecosystem changes. Virtual particles are released and tracked in model output of the unsteady flow over a reef canopy. Using a novel scheme for the particle boundary conditions, continuous pathways are traced near the organism surfaces to study the influx and exchange of particles and solutes to and from the sedentary organisms. The decrease of coral organisms is shown to increase entrainment of water from higher up in the water column by eddies that become larger as the spacing between large coral organisms increases. This also promotes the exchange of particles and solutes between smaller organisms in the wake of larger organisms. Waves interact with the larger spacing between organisms and further increase the eddy size and strength. Waves also increase the length scale of influence for individual organisms.