The Mississippi River: The ecological forcer in the marsh communities of its bird-foot delta, Louisiana, as illustrated over this 25+ year study by its impacts from both warming, likely from climate change, and also from its highly dynamic character

Background/Question/Methods

Wetland ecosystems are systems that are highly dependent upon flooding regimes.  And riverine wetlands, as opposed to those along ocean fronts, are especially impacted by the quantity and quality of river flow.   I have been monitoring 3 relatively recently formed (early 1980s) freshwater wetlands within the bird-foot delta of the Mississippi River for 25+ years by sampling biomass at permanent transects at the end of each growing season. Such lands are rare within a coastal region having the highest rate of wetland loss within the United States.  This study answers the questions: (1) What impact does the River have on new wetlands?; (2) Does the River impact all wetlands in a similar manner?; (3)  Do the dominant species in the wetlands change over time?

Results/Conclusions

The ‘lower marsh’ wetlands of the delta when mature generally are dominated by nearly monospecific stands of Schoenoplectus deltarum and Sagittaria latifolia/platyphilla.  The impact of ecological forcing by the River in these marshes is similar at all 3 sites as shown by remarkably parallel annual change in species abundance and total biomass (standing crop average ~525 g/m²; n = 780 ¼ m² plots; inter-site CC comparisons ~0.40 to ~0.65 over the first 19 yrs of study) and the changes are negatively correlated to annual change in water quality, particularly to Spring flow rates and associated sediment load (volume of flow CC = -0.70; sediment load CC = -0.65).  Also, over the past 20+ yrs, end-of-growing season plant biomass has been increasing ~2%/yr.   The only reasonable explanation to date for this upward productivity trend is the discovered significant warming of the Mississippi River of just under 1⁰ C/decade over the same time period.  With a warmer river comes an earlier start to each growing season, and studies have shown that longer growing seasons often translate into more end-of-the year biomass.  This warming of the River is likely because of climate change.  All 3 sites have recently transitioned to different (but in explainable directions by site) wetland community-states leading to the end-of-the-pattern of similar change in wetland states found at the sites since study inception 25+ years ago.  These new divergences are likely in small part because of the continued River forcings but more from invasion by exotic haplotypes of Phragmites and Typha.  These haplotypes are outcompeting the native species which are extremely important as wildlife food for the thousands of bird migrants to this delta.