Abstract
How aquatic primary productivity influences the carbon (C) sequestering capacity of wetlands is uncertain. We evaluated the magnitude and variability in aquatic C dynamics and compared them to net ecosystem CO2 exchange (NEE) and ecosystem respiration (Reco) rates within calcareous freshwater wetlands in Everglades National Park. We continuously recorded 30-min measurements of dissolved oxygen (DO), water level, water temperature (Twater), and photosynthetically active radiation (PAR). These measurements were coupled with ecosystem CO2 fluxes over 5 years (2012–2016) in a long-hydroperiod peat-rich, freshwater marsh and a short-hydroperiod, freshwater marl prairie. Daily net aquatic primary productivity (NAPP) rates indicated both wetlands were generally net heterotrophic. Gross aquatic primary productivity (GAPP) ranged from 0 to − 6.3 g C m−2 day−1 and aquatic respiration (RAq) from 0 to 6.13 g C m−2 day−1. Nonlinear interactions between water level, Twater, and GAPP and RAq resulted in high variability in NAPP that contributed to NEE. Net aquatic primary productivity accounted for 4–5% of the deviance explained in NEE rates. With respect to the flux magnitude, daily NAPP was a greater proportion of daily NEE at the long-hydroperiod site (mean = 95%) compared to the short-hydroperiod site (mean = 64%). Although we have confirmed the significant contribution of NAPP to NEE in both long- and short-hydroperiod freshwater wetlands, the decoupling of the aquatic and ecosystem fluxes could largely depend on emergent vegetation, the carbonate cycle, and the lateral C flux.
Highlights
Wetland ecosystems worldwide are recognized for their disproportionately large and complex organic carbon (C) storage potential (Mitsch and Gosselink 2007; Nahlik and Fennessy 2016; Lu and others 2017)
We evaluated the effect of water level and Twater on daily Net aquatic primary productivity (NAPP), gross aquatic primary productivity (GAPP), and respiration of aquatic vegetation (RAq) using generalized additive models (GAM) with the gam function in the ‘‘mgcv’’ package (Wood 2011) in R (R Core Team 2014)
Seasonal fluctuations in water levels differed between the two sites, strong annual patterns in Tair and photosynthetically active radiation (PAR) were consistent between the longhydroperiod marsh and the short-hydroperiod marl prairie over the study period (Figure 2)
Summary
Wetland ecosystems worldwide are recognized for their disproportionately large and complex organic carbon (C) storage potential (Mitsch and Gosselink 2007; Nahlik and Fennessy 2016; Lu and others 2017) They are believed to be highly productive ecosystems with low rates of decomposition due to anaerobic conditions (Chmura and others 2003), wetlands vary in their capacity to sequester C based on hydrology (Kayranli and others 2010; Bernal and Mitsch 2012; Malone and others 2014; Lu and others 2017). In a global synthesis of wetlands, Lu and others (2017) showed that the leaf area index of emergent vegetation was correlated with GEE and Reco, but not with NEE This suggests that the GEE and Reco of wetlands is heavily influenced by emergent vegetation (Lu and others 2017), the hydrological control on the balance between the two flux components may be the dominant driver of the C sequestering capacity of wetland ecosystems (Bernal and Mitsch 2012). There have been few studies that have shown how aquatic primary productivity influences net fluxes of CO2 from wetlands at the ecosystem scale (but see Hagerthey and others 2010)
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