Channel‐Island Connectivity Affects Water Exposure Time Distributions in a Coastal River Delta

The exposure time is a water transport time scale defined as the cumulative amount of time a water parcel spends in the domain of interest regardless of the number of excursions from the domain. Transport time scales are often used to characterize the nutrient removal potential of aquatic systems, but exposure time distribution estimates are scarce for deltaic systems. Here we analyze the controls on exposure time distributions using a hydrodynamic model in two domains: the Wax Lake delta in Louisiana, USA, and an idealized channel‐island complex. In particular, we study the effects of river discharge, vegetation, network geometry, and tides and use a simple model for the fractional removal of nitrate. In both domains, we find that channel‐island hydrological connectivity significantly affects exposure time distributions and nitrate removal. The relative contributions of the island and channel portions of the delta to the overall exposure time distribution are controlled by island vegetation roughness and network geometry. Tides have a limited effect on the system's exposure time distribution but can introduce significant spatial variability in local exposure times. The median exposure time for the WLD model is 10 h under the conditions tested and water transport within the islands contributes to 37–50% of the network‐scale exposure time distribution and 52–73% of the modeled nitrate removal, indicating that islands may account for the majority of nitrate removal in river deltas. Plain Language Summary The transport of nutrients to coastal waters can cause a number of environmental, economic, and human health issues. For example, the Gulf of Mexico's “Dead Zone” off the coast of Louisiana is one of the largest hypoxic (low oxygen) zones in the world and its increasing size has been attributed to increased nutrient delivery by the Mississippi River. There is evidence that river deltas may be able to naturally reduce the nutrient load to coastal waters by removing nutrients. One important factor for predicting nutrient removal in a coastal system is the water transport time scale, or how long water stays in the system. In systems like river deltas, which comprise a complex network of channels and inundated islands that behave as vegetated wetlands, the water transport time scale can vary significantly in time and space. Our research aims to identify how vegetation, river discharge, tides, and channel network geometry influence the water transport time scale