Detection and Classification of Phytoplankton Deposits Along an Estuarine Gradient
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Phytoplankton deposition onto sediments affects trophic structures, sedimentary nutrient fluxes, and dissolved oxygen concentrations in coastal ecosystems. Deposition can occur as distinct events that are highly variable over space and time, necessitating detection methods that have similarly high resolution. We present an assessment of a novel rapid detection method that combines water-column and benthic fluorometry with surficial sediment sampling to identify phytoplankton deposition, as implemented in a 2-year study of a Florida estuary (24 monthly collections at 14 locations). Maximum water-column chlorophyll concentration, average benthic chlorophyll fluorescence, and the proportion of centric vs. pennate diatoms at the sediment–water interface were each fitted to sine functions to represent phytoplankton bloom cycles. The phase offsets among the three fitted sine functions were varied to maximize fit to the 336 observations. The fitted cycles were divided into four classes that separate dominance by benthic microalgae from early, late, and post-phytoplankton depositional states. Best-fitting cycles for the proportion of centric diatoms were consistently offset from water-column chlorophyll cycles, indicating peak deposition occurred after peak phytoplankton blooms. Phytoplankton deposition dominated the upstream region of the studied estuary and was associated with reduced dissolved oxygen concentrations. Benthic algae dominated in downstream regions, particularly during low freshwater flow conditions when light absorption by colored dissolved organic matter was low. This approach produced repeatable and consistent patterns that agreed with expected relationships and was practical for sampling with high spatial and temporal resolution.
KeywordsPhytoplankton sedimentation Bentho-pelagic coupling Phytodetritus deposition PAM fluorometry Caloosahatchee River estuary Basal resource
We thank Dr. Gregory Ellis, Dr. Scott Burghart, Ralph Kitzmiller, and Dr. Elon Malkin of the University of South Florida (USF) for their help in data collection and logistics. We thank Dr. Chuanmin Hu and David English of USF for assistance with the calculation of light attenuation. Data were gathered in collaboration with Florida Gulf Coast University (FGCU) scientists Drs. Gregory Tolley, David Fugate, and Michael Parsons. We thank Megan Andresen and Brooke Denkert along with many other FGCU students for their work in the field efforts. We also thank Dr. Peter Doering of the South Florida Water Management District (SFWMD) and anonymous reviewers for their constructive comments. Funding for this project was provided by SFWMD grants 4500020141 and 4500035194. This work was performed to partially fulfill the requirements of the first author’s doctoral degree.
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