Plant community changes within an aridland constructed wastewater treatment wetland

Background/Question/Methods

Constructed wetland systems (CWS) are increasingly used for wastewater treatment. These systems rely on the ability of emergent macrophytes to take up nutrients from wastewater to improve water quality.  However, due to species specific variation in plant nutrient requirements, community composition changes may influence spatial and temporal patterns of nutrient uptake.  In this study, our goal was to examine how community composition varies over seasons in a CWS.  We examined and quantified intra-annual plant community composition changes within a 21 ha CWS at the largest wastewater treatment facility in Phoenix, AZ. Multiple regression allometric biomass models were developed to relate various plant measurements to dry weight for each of the nine species present. Ten 50m transects were established across of the vegetated areas of the CWS.  Every 2 months, beginning in July 2011, we sampled 5 randomly placed 0.25 m² quadrats along each transect.  Plants within each quadrat were measured for predetermined physical dimensions that are used in the allometric biomass models. Aerial imagery was used to calculate vegetated area within the wetland, which we used to scale up aboveground biomass data from the quadrat to the wetland scale

Results/Conclusions

We observed a sharp decrease in total aboveground biomass from July (607,500 kg) to September (199,700 kg).  During this time, average biomass declined dramatically from 2890 gdw m^-2 to 950 gdw m^-2. We attributed this to a ‘thatching’ phenomenon where large stands of Typha latifolia and Typha domingensis grew excessively and toppled over areas of the wetland.  The fraction of total aboveground biomass represented by T. latifolia and T. domingensis decreased by 22% between July 2011 and January 2012.  Over this same period, the fraction of total aboveground biomass represented by Schoenoplectus californicus increased 9% and that of Schoenoplectus acutus and Schoenoplectus tabernaemontani increased 12%.  These changes highlight a shift in community composition that could impact the nutrient uptake ecosystem service expected of the system. Our sampling will continue into Summer 2012 to generate an annual picture of species composition changes and aboveground biomass. We will use plant tissue nutrient analysis, water chemistry data, and a whole system nutrient budget to examine the impact of community composition changes on nutrient uptake from Summer 2011 to Summer 2012.