Seasonal Oxygen Dynamics in a Warm Temperate Estuary: Effects of Hydrologic Variability on Measurements of Primary Production, Respiration, and Net Metabolism
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Seasonal responses in estuarine metabolism (primary production, respiration, and net metabolism) were examined using two complementary approaches. Total ecosystem metabolism rates were calculated from dissolved oxygen time series using Odum’s open water method. Water column rates were calculated from oxygen-based bottle experiments. The study was conducted over a spring-summer season in the Pensacola Bay estuary at a shallow seagrass-dominated site and a deeper bare-bottomed site. Water column integrated gross production rates more than doubled (58.7 to 130.9 mmol O2 m−2 day−1) from spring to summer, coinciding with a sharp increase in water column chlorophyll-a, and a decrease in surface salinity. As expected, ecosystem gross production rates were consistently higher than water column rates but showed a different spring-summer pattern, decreasing at the shoal site from 197 to 168 mmol O2 m−2 day−1 and sharply increasing at the channel site from 93.4 to 197.4 mmol O2 m−2 day−1. The consistency among approaches was evaluated by calculating residual metabolism rates (ecosystem − water column). At the shoal site, residual gross production rates decreased from spring to summer from 176.8 to 99.1 mmol O2 m−2 day−1 but were generally consistent with expectations for seagrass environments, indicating that the open water method captured both water column and benthic processes. However, at the channel site, where benthic production was strongly light-limited, residual gross production varied from 15.7 mmol O2 m−2 day−1 in spring to 86.7 mmol O2 m−2 day−1 in summer. The summer rates were much higher than could be realistically attributed to benthic processes and likely reflected a violation of the open water method due to water column stratification. While the use of sensors for estimating complex ecosystem processes holds promise for coastal monitoring programs, careful attention to the sampling design, and to the underlying assumptions of the methods, is critical for correctly interpreting the results. This study demonstrated how using a combination of approaches yielded a fuller understanding of the ecosystem response to hydrologic and seasonal variability.
KeywordsPrimary production Plankton community respiration Net ecosystem metabolism Dissolved oxygen Estuary
This project was made possible by an excellent support team: Jessica Aukamp, David Beddick, George Craven, Ally Duffy, and Diane Yates. Stephanie Friedman, Susan Yee, Kim Cressman, and Yongshan Yan provided comments that improved the manuscript. We also thank four anonymous reviewers and the associate editor for their detailed and insightful comments and suggestions. This study was funded by the US Environmental Protection Agency (US EPA) and has been reviewed and approved for publication. The views expressed here are solely those of the authors, thus do not necessarily reflect the views or policies of the US EPA. Mention of trade names or commercial products does not constitute endorsement by the authors or the US EPA.
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