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

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 ranges below about 1.6 m). We were unable to extract similar relationships from whole-estuary exchange studies because so few exist. The geomorphological setting and degree of ecological maturity (analogous to geologic age) of a marsh or basin within an estuary are important controllers of ecological function, thus flux behavior. We applied concepts of community succession and ecosystem development to data from marsh-water column flux studies, and found that slope of flux vs. tidal height relationships was greater for younger marshes compared to all marshes, and much greater for younger marshes compared to older marshes. This change in slope often caused a shift in the inflection point that indicated the tidal range at which export shifted to import, or vice versa. These studies quantified surficial fluxes, though, and a number of post-1980 studies demonstrated the importance of other processes, including subsurface flow, subtidal advection, and the movement of nutrients and organic matter by animals (other chapters in this volume address these processes). Finally, a number of studies showed strong controls on fluxes by exogenous environmental forcing, and we reviewed several studies that used innovative budgeting and modeling of flux dynamics and ecological processes to incorporate these sources of variability. Since 1980 we have learned a great deal more about how estuarine wetlands interact with their estuaries, and of the value of establishing a conceptual framework and system boundaries. Estuarine ecologists have learned a great deal about outwelling as a concept although few flux studies have directly addressed the original Outwelling Hypothesis. We suggest that the question should not be “Is The Outwelling Hypothesis true?” but rather: 1) how are materials being exchanged between different subsystems in estuary-coastal ocean landscapes? 2) what are the mechanisms of this exchange? and 3) how do exogenous forcings control these patterns of exchange? Estuarine scientists are encouraged to view The Outwelling Hypothesis as a conceptual stimulus of ideas and not as a strict statistical hypothesis that must be proven or disproven.