Twenty More Years of Marsh and Estuarine Flux Studies: Revisiting Nixon (1980)

Abstract

In 1980, Scott Nixon reviewed the role of salt marshes in estuarine and coastal productivity. His review was effectively a progress report on the testing of "The Outwelling Hypothesis" (Odum, 1980). Nixon (1980) signaled a crucial turning point in the direction of estuarine flux studies conducted since then. In this review we revisit Nixon (1980), focusing on research and thinking that has been guided by The Outwelling Concept in the last two decades. Since 1980, estuarine flux studies have been conducted at 41 different sites and presented in over 42 publications. More than a third of these were conducted in Europe, Africa, Australia, or Mexico. Our review of these studies highlighted several important advances. The first was evolution of a conceptual approach that decomposes the estuary-coastal ocean landscape into interacting subsystems (i.e., the coastal ocean, estuarine basins, and marsh). Most post-1980 flux studies have addressed interactions between these individual subsystems, often in an hierarchical sense. Over half of these quantified exchanges between marsh-dominated basins and the greater estuary-generally through a single, well-defined tidal channel. From these data, we found that tidal range, subsystem area, and distance to the ocean together explained 87% of the variability in total organic carbon (TOC) exchanges and 92% of the variability in total suspended solids (TSS) fluxes, with exports occurring at lower tidal ranges, areas, and distances. Tidal range explained 40% of the variability in nitrate + nitrite (NN) exchange (with uptake at ranges below about 1.2 m and export at greater tidal ranges) and 39% of available phosphorus (SRP) flux variation (with export at 391