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TECHNICAL REPORTS

Wetlands and Aquatic Processes

Hyperspectral Reflectance Response of Freshwater Macrophytes to Salinity in a Brackish Subtropical Marsh

^a 1449 Agricultural Engineering Bldg. #142, Biological Resources Engineering Dep., Univ. of Maryland, College Park, MD 20742
^b Dep. of Environmental Engineering, Texas A&M Univ.-Kingsville, Kingsville, TX 78363
^c Division of Environmental Systems Engineering, Pukyong National Univ., Pusan 608-737, Korea

^* Corresponding author (dtilley{at}umd.edu)

Received for publication August 25, 2005. 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, ) marsh to determine salinity effects on visible and near-infrared spectral band reflectance and to identify reflectance indices sensitive to small (1) changes in wetland salinity. For sea oxeye, floating-position water band index [fWBI = R₉₀₀/minimum(R₉₃₀ – R₉₈₀), where R = reflectance at band ], normalized difference vegetation index [NDVI = (R₇₇₄ – R₆₈₁)/(R₇₇₄ + R₆₈₁)], and a proposed wetland salinity reflectance ratio (WSRR = R₉₉₀/R₉₃₃) were sensitive to salinity with R² of 40, 35, and 65%, respectively (p < 0.01). For cattail, NDVI and photochemical reflectance index [PRI = (R₅₃₁ – R₅₇₀)/(R₅₇₀ + R₅₃₁)] were sensitive to salinity with R² 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 nm (p < 0.10), which is a chlorophyll a band. Our findings indicate that hyperspectral radiometry can detect the response of emergent freshwater plants to changes in wetland salinity, which would help with monitoring salinity effects on coastal wetlands.

Abbreviations: fWBI, floating-position water band index • NDVI, normalized difference vegetation index • NIR, near-infrared • PRI, photochemical reflectance index • SRVI, simple reflectance vegetation index • WSRR, wetland salinity reflectance ratio