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

Abstract

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=NH 3+NH 4 +) 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 Rλ is reflectance at wavelength λ] 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.