A Constructed Freshwater Wetland Shows Signs of Declining Net Ecosystem Exchange

Abstract:

The USGS constructed a freshwater-wetland complex on Twitchell Island in the Sacramento–San Joaquin Delta (Delta), California, USA, in 1997 and maintained it until 2012 to investigate strategies for biomass accretion and reduction of oxidative soil loss. We studied an area of the wetland complex covered mainly by dense patches of hardstem bulrush (Schoenoplectus acutus) and cattails (Typha spp.), with smaller areas of floating and submerged vegetation, that was maintained at an average depth of 55 cm.

Using eddy covariance measurements of carbon and energy fluxes, we found that the combination of water management and the region’s Mediterranean climate created conditions where peak growing season daily means of net ecosystem exchange (NEE) reached -45 gCO₂ m^-2 d^-1 and averaged around -30 gCO₂ m^-2 d^-1 between 2002 through 2004. However, when measurements resumed in 2010, NEE rates were a fraction of the rates previously measured, approximately -6 gCO₂ m^-2 d^-1. Interestingly, NEE rates in 2011 doubled compared to 2010 (-13 gCO₂ m^-2 d^-1). Methane fluxes, collected in 2010 to assess a complete atmospheric carbon budget, were positive throughout the year, with daily mean flux values ranging from 50 to 300 mg CH₄ m^-2 d^-1. As a result, methane flux reduced NEE values by approximately one-third, and when the global warming potential was considered, the wetland became a net global warming potential source.

We found that carbon cycling in a constructed wetland is complex and can change over annual and decadal timescales. We investigated possible reasons for differences between flux measurements from 2002 to 2004 and those from 2010 and 2011: (1) changes in methodology, (2) differences in weather conditions, (3) differences in gross primary productivity relative to respiration rates, and (4) the amount of living plant tissue relative to brown accumulations of senesced plant litter. We hypothesize that large mats of senesced material within the flux footprint could have contributed to the difference in flux rates by reducing the productivity of the emergent marsh species. We conclude that for restored or constructed wetlands, it is important to implement long-term monitoring strategies to assess and manage environmental processes associated with carbon storage.