An In Situ Multispectral Imaging System for Planar Optodes in Sediments: Examples of High-Resolution Seasonal Patterns of pH

A new sediment profile imaging (SPI) instrument, CHEM-SPI, was developed for in situ two-dimensional measurements of biogeochemical solutes using fluorosensor foils in sediments and overlying waters. The CHEM-SPI system was used to simultaneously measure vertical sections of pH, O₂, and pCO₂ distributions in subtidal, surface deposits of Long Island Sound, NY. Images are readily obtained in 5-15 min with inexpensive LED excitation and commercial grade digital cameras having typical pixel resolution of ~50-100 μm over areas <150 cm² sediment. Seasonal examples of in situ deployments of the instrument revealed extensive horizontal and vertical heterogeneity of pH distributions. pH dynamics were associated with complex biogenic structures in the upper few centimeters of marine sediment and the pulsed input of organic matter during the spring bloom period. The pH beneath the sediment-water interface was dramatically depressed by the bloom input of organic matter but macrofaunal activity otherwise dominated pH variations in the bioturbated zone. The CHEM-SPI system allows direct quantitative confirmation of biogeochemical patterns previously inferred qualitatively from color patterns in visible SPI images. The instrument is sufficiently adaptable in design to accommodate new optical sensor foils for other chemical variables.