San Francisco Estuary and Watershed Science

Long-term monitoring programs may face protocol or equipment changes for various reasons, and they must determine how to implement change while maintaining the historical consistency and significance of their data. A case study on the US Fish and Wildlife Service’s (USFWS’s) Enhanced Delta Smelt Monitoring program (EDSM) demonstrates what factors a long-term monitoring program might consider when substituting new equipment to collect environmental data, such as turbidity. Turbidity can influence the health, survival, and habitat quality of fish and other organisms. Thus, reliable collection of turbidity data is critical. Turbidity data are commonly presented in two units: nephelometric turbidity units (NTU) and formazin nephelometric units (FNU). The EDSM historically collected turbidity data using NTU units, like other monitoring programs in the San Francisco Estuary. Changes in reported turbidity values by other agencies, and proposed criteria relevant to the protection of the federally threatened Delta Smelt, have shifted focus to FNU values. Before transitioning to only collecting FNU values, the EDSM performed side-by-side collections of both FNU and NTU values for 2 years. Multiple linear regression supported a linear relationship between FNU and NTU values when the covariates of temperature, dissolved oxygen, specific conductance, stratum, and month were included. The discrepancies in FNU and NTU values could be explained by varying light wavelengths, differences in sample collection method, or sensor fouling. The model also demonstrated predictive capabilities for future unit conversion. These findings support the EDSM collecting turbidity in a single unit while maintaining the ability to correlate with historical environmental data, if needed. Other long-term monitoring programs could make similar considerations when faced with programmatic or equipment changes to ensure minimal effect on their historical datasets.