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Improving estimates of ecosystem metabolism by reducing effects of tidal advection on dissolved oxygen time series-Abstract
Citation:
Hagy, Jim, M. Beck, AND M. Murrell. Improving estimates of ecosystem metabolism by reducing effects of tidal advection on dissolved oxygen time series-Abstract. To be Presented at CERF 2015, Portland, OR, November 08 - 12, 2015.
Impact/Purpose:
Present subject of presentation to conference organizers for placement of presentation in conference schedule
Description:
Continuous time series of dissolved oxygen (DO) have been used to compute estimates of metabolism in aquatic ecosystems. Central to this open water or "Odum" method is the assumption that the DO time is not strongly affected by advection and that effects due to advection or mixing can be neglected. In reality, most DO time series are collected at fixed locations and many are strongly affected by tidal advection. In such cases, unrealistic estimates of metabolism can occur, often with unlikely correlations with tides. In our study, we explored the impact of physical artifacts on metabolism estimates at a variety of time scales. We developed a statistical model using locally weighted regression to separate variability in DO due to metabolism from variations due to tides, allowing a partial relaxation of this assumption and improving metabolism estimates. The model requires a DO and water level time series, which are often collected together by the same monitoring instrument. The model does not require a priori knowledge of horizontal DO gradients and does not assume that tidal DO effects are invariant. We tested the method using a simulated DO time series, then applied it to one year of continuous monitoring data from four water quality stations within the National Estuarine Research Reserve System. The model effectively reduced artifacts caused by advection when the magnitude of tidal influence was high and the correlation between tidal changes and the solar cycling are low. Conversely, the model failed when the pattern of tide height was closely associated with the solar cycle. By reducing the effects of physical transport on metabolism estimates, this method increases the potential to empirically relate metabolic rates to causal factors on times scales of several days to several weeks. Estimates of variability associated with physical advection may also be more interpretable, since convolution of physical and biological effects can be reduced.