Date of Award
MS in Biological Sciences
Using an instrumented water quality network in Morro Bay Estuary, California from 2007 to 2010 (15 min sampling frequency), this study addressed the two objectives of constructing a nitrate budget and assessing the influence of sampling frequency on water quality parameters. These two objectives led to the submission of an original report of research (Appendix A) and a note (Appendix B) to peer-reviewed journals.
The first objective was to characterize the high spatial and temporal variation in physical parameters and nitrate concentrations and to construct a nitrate budget quantifying sources and sinks of nitrate from the ocean, streams, and groundwater, as well as biological processes in the Estuary. Morro Bay Estuary was found to be a non-eutrophic system and a mean net exporter of nitrate, 327.15 t yr-1. Fifty-four percent of the nitrate export was attributed to nitrate sources and internal biological processing. Nitrate loading from streams contributed 37 % to the export of nitrate (124.01 t yr-1), while groundwater nitrate loading supplied a conservative estimate of 46 % of the exported nitrate (153.92 t yr-1), with a neap tide enhancement of the discharge. Denitrification, Zostera marina, and benthic macroalgae assimilation of nitrate were the dominant internal biological processes for removal and retention, but were only 35% of the total nitrate budget.
The second objective was to investigate the impact of sampling frequency and sampling location on understanding dynamics in water quality by degrading a year time series of seven parameters from three water quality monitoring stations to sampling frequencies ranging from 15 minutes to 28 days. In Morro Bay Estuary, the semi-diurnal tidal cycle was the maximum component frequency driving the variability of temperature, turbidity, and dissolved oxygen concentrations. For these parameters, asymptotes were reached and sampling frequencies greater than six hours did not explain the additional variation in the parameters sampled. Whereas, salinity, turbidity, and nitrate concentrations lacked an asymptote, and decreased sampling frequencies led to increased estimated error. Sampling water quality parameters every 28 days can lead to mean annual difference of 30 – 140 % from 15 minute sample annual mean. We recommend sampling frequencies should be selected to oversample the tidal signal to at least hourly frequencies to capture diel cycles and episodic events that contribute significantly to understanding the variability in the estuarine physical and biological dynamics.