Hyperspectral reflectance response of freshwater macrophytes to salinity in a brackish subtropical marsh

Coastal freshwater wetlands are threatened by increased salinity due to relative sea level rise and reduced freshwater inputs. Remote radiometric measurement of freshwater marsh canopies to detect small shifts in water column salinity would be useful for assessing salinity encroachment. We measured leaf hyperspectral (300-1100 nm) reflectance of freshwater macrophytes (cattail, Typha latifolia and sea oxeye, Borrichia frutescens) in a field study in a subtropical brackish (2.5-4.5 parts per thousand salinity, parts per thousand.) marsh to determine salinity effects on visible and near-infirared spectral band reflectance and to identify reflectance indices sensitive to small (1 parts per thousand) changes in wetland salinity. For sea oxeye, floating-position water band index [fWBI = R-900/minimurn (R-930 - R-980), where R gimel = reflectance at band gimel], normalized difference vegetation index [NDVI = (R-774 - R-681)/(R-774 + R-681)], and a proposed wetland salinity reflectance ratio (WSRR = R-990/R-933) were sensitive to salinity with R-2 of 40, 35, and 65%, respectively (p < 0.01). For cattail, NDVI and photochemical reflectance index [PRI = (R-531/R-570)/(R-570 + R-531)] were sensitive to salinity with R-2 of 29 and 33%, respectively (p ! 0.01). Higher salinity significantly reduced mean reflectance of sea oxeye in 328- to 527-nm and 600- to 700-nm wavebands (p < 0.05), which corresponded to chlorophyll bands. Reflectance of cattail was not significantly affected by the highest salinity, although the spectral band most affected was 670 mn (p < 0.10), which is a chlorophyl a band. Our findings indicate that hyperspectrall radiometry can detect the response of emergent freshwater plants to changes in wetland salinity, which would help with monitoring salinity effects on coastal wetlands.