A Quantitative Assessment of Organic Carbon Content as a Regional Sediment-Condition Indicator

Organic carbon content of sediments, whether directly or indirectly assessed, has often been used as an indicator of marine benthic community condition both in site-specific and regional scale condition assessment studies. The conceptual framework underlying use of this indicator was developed based primarily on site-specific studies. A quantitative analysis of literature data on sediment organic matter impacts in marine systems was conducted to determine whether biotic metrics respond to abiotic indicators of sediment organic content, as predicted by conceptual models, at larger spatial scales. The ability to detect predicted decreases in community metrics (abundance, species richness, species diversity index H', biomass) varied among metrics, with best performance by species richness and H'. There was significant added variation both between and within analytical approaches (loss on ignition, total organic carbon methods), emphasizing the need for careful cross calibration and quality control in studies with multiple laboratory partners. High levels of variability for biotic metrics versus organic carbon metrics appear typical for large scale studies, and organic matter source, site depth, and individual estuarine system differences were important sources of variation. Covariation of organic matter content with percent fine sediments is another known source of variation, but various normalization methods may be inadequate due to inherent sources of variation at estuary level. While likely still useful for point-source studies, multiple major sources of variation appear to limit the usefulness of sediment organic content as a benthic condition indicator at larger spatial scales.