Evaluation of sediment profile imagery as a tool for assessing water quality in Greenwich Bay, Rhode Island, USA

The Benthic Habitat Quality (BHQ) index was used to assess habitat visible in sediment profile images (SPI) following hypoxia disturbance in a shallow (<10 m) estuarine embayment in Rhode Island, USA. We tested for associations between the BHQ, SPI features and water quality over several assessment windows (1, 3, 7, 14, and 28 days prior to imaging) and at multiple dissolved oxygen (DO) thresholds (2.0 mg l−1, 2.9 mg l−1, and 4.8 mg l−1). Using categorical data analysis, we established empirical relationships between hypoxia prevalence and presence/absence of biogenic features visible in SPI. Fecal pellets, tubes, feeding pits, voids, mounds, and BHQ score were good affirmative features, meaning that their presence (or score greater than 5) indicated a high probability of good water quality. However, low sensitivity to hypoxia precluded their usefulness as indicators, and was attributed to rarity in images and to factors acting on time intervals longer than those examined, e.g. long-term organic enrichment or hypoxia. Burrow structures and the apparent redox potential discontinuity (aRPD), or oxidized layer of surface sediment, were good discriminatory features, with high sensitivity and specificity for both hypoxia and normoxia. Both were strong surrogates for water quality over multiple assessment windows and DO thresholds, and had the highest overall predictive values. We conclude that SPI images can be used to widen the spatial extent of water quality monitoring efforts by utilizing the relationships between aRPD, burrows and hypoxia prevalence.