Science Serving Maryland's Coasts

Research Publications: UM-SG-RS-2003-16


Hyperspectral reflectance of emergent macrophytes as an indicator of water column ammonia in an oligohaline, subtropical marsh.




Tilley, DR; Ahmed, M; Son, JH; Badrinarayanan, H


Ecological Engineering 21(23):153-163




Narrow spectral band reflectance indices of two emergent macrophytes (Typha latifolia and Borrichia frutescens) were found to correspond to changes in water column total ammonia (TA = NH3 + NH4+) of a constructed oligohaline (2-5 ppt) subtropical marsh (8 ha) designed as a shrimp farm treatment wetland in south Texas, USA, near the western Gulf of Mexico. A handheld hyperspectral radiometer measured plant leaf reflectance across the visible (VIS, 300-700 nm) and near-infrared (NIR 700-1100 nm) spectrum during the mid-growing season (July-August). Reflectance of the two species and water quality (TA: total phosphorus (TP); salinity, S) were measured along a dynamic nutrient gradient from wetland inlet to outlet. Mean TA concentration was 1.22 mg N l-1 ranging between 0.92 and 1.71 mg N l-1. Salinity fluctuated between 2.5 and 4.5 ppt, while total phosphorus was between 0.01 and 0.86 mg P l-1. T. latifolia, the more dominant wetland cover: (a) reflected 1-5% less in the VIS compared to B. frutescens, (b) generally reflected 7-8% more in the NIR, (c) exhibited an average red-edge (RE) (718 nm) 15 nm greater than B. frutescens, and (d) had an average leaf normalized difference vegetation index [NDVI = (R-774 - R-681)/(R-774 + R-681), where Rlambda is reflectance at wavelength lambda] of 0.82, versus 0.65 for B. frutescens. A multivariate linear regression equation containing photochemical reflectance index (PRI) and RE for T latifolia explained 93% of TA variation (P < 0.001) when 0.10 < TP < 0.20 mg P l-1. For the entire range of water quality conditions, an equation containing PRI, RE and R-493/R-678 for T. latifolia explained 72% of TA variation (P < 0.001). These results indicate that hyperspectral radiometry can quantify wetland ammonia concentrations in a treatment wetland, which could lead to development of remote techniques for continuously monitoring treatment effectiveness.

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