Multiple effects of the Mississippi River's water on annual wetland plant biomass within it's bird-foot delta over a 2.5 decade study period - still mainly a climate impact.

Background/Question/Methods

Climate is a major factor that impacts water quality the world over.  The wetlands at the mouth of the Mississippi River are in direct contact with its water through sheet flow.  Potential impacts of water quality on riverine wetlands could come from at least 3 of its parameters: discharge, temperature, and nutrients levels.  These parameters often vary seasonally.  This analysis addresses the question: What impact does the Mississippi River’s water have on the wetlands beyond the reported temperature rise of 1⁰ C/decade suggested as the cause of the ~1.6%/yr over a quarter century increase in wetland plant growth?  Aboveground biomass plots (n = 938; mean ~520 g/m²) were collected once annually at the end of the growing season from two bird-foot Mississippi delta splays at three sites from 1984 to 2010.  Water quality data (discharge, temperature, nutrient loads) on the Mississippi River were obtained.  The Julian day the river exceeded 15°C was determined and subtracted from September 1 to estimate the length of the growing season.  Nutrient loading was determined by multiplying the average monthly concentration by the average discharge.  The growing season nutrient load was calculated as the sum of the average monthly loads from April through August.  We applied quantile regression using total oven dried biomass, as well as the dried biomass of the two most abundant genera as the dependent variables to the independent variables of average annual discharge, spring discharge, average annual nitrate concentration, nitrate load, TKN load, length of growing season, and average summer water temperature.

Results/Conclusions

Quantile regression showed a significant decrease in total biomass with increasing growing season length.  In years with longer seasons, Sagittaria increased and Schoenoplectus decreased.  Sagittaria tended to have lower end-of-season biomass than the Schoenoplectus it replaces during those years.  The temporal trend of increased biomass over the quarter century may be related to a combination of environmental changes.  Quantile regression identified that river discharge has a curvilinear effect with highest total biomass occurring in years with spring discharge averaging 700,000 cfs.  Nitrogen concentrations in the river where not found to control aboveground biomass, but growing season (April - August) nitrate loading followed a similar pattern to discharge with highest biomass at intermediate loading rates.  This study shows how inter-annual variation in growing season length and river discharge adds to explaining the observed trends in biomass in several ways.  Thus, future modeling of wetland dynamics will be more accurate.