Coastal wetland stability maintained through counterbalancing accretionary responses to chronic nutrient enrichment

The link between anthropogenically modified nutrient loading and coastal wetland stability is not well understood due to limited data from long-term experiments and inconsistent findings from investigations thus far. In this study, we present results from a 13-year oligohaline marsh fertilization experiment aimed at determining whether eutrophic conditions compromise ecosystem stability, defined here as the capacity to keep pace with sea level rise and resist the erosive forces of high-energy meteorologic events. To accomplish this objective, we measured soil surface elevation change and soil shear strength, along with a suite of regulatory processes that included belowground standing crop, belowground decomposition, organic and mineral matter accumulation, soil accretion, and shallow subsidence. Our results identified an apparent compensatory effect of nutrient enrichment on accretionary processes whereby shallow subsidence, attributed to reduced live root standing crop, was balanced by enhanced accretion resulting from greater organic matter accumulation at the soil surface. Consequently, the rate of marsh elevation change measured over a five-year period was unaffected after more than a decade of experimental nutrient enrichment. Furthermore, the structural integrity of the soil matrix did not deteriorate under elevated nutrient conditions; decomposition rates were similar to control plots, and although live root standing crop was reduced, the root system was evidently stronger, as soil strength tended to increase rather than decrease after nutrients were provided for 13 years. Our data suggest that enhanced nutrient loading is an unlikely destabilizing mechanism in this coastal marsh and possibly others. However, additional long-term research will be required in a diverse range of habitats and environmental settings before broad-based, general conclusions concerning the effects of nutrient enrichment on coastal wetland stability can be made with a high degree of certainty.