MEPS prepress abstract  -  DOI: https://doi.org/10.3354/meps13012

Phytoplankton community structure response to groundwater-borne nutrients in the Inland Bays, Delaware

Daniel M. Torre*, Kathryn J. Coyne, Kevin D. Kroeger, Joanna K. York

*Email: danmtorre@gmail.com

ABSTRACT: To determine the impacts of groundwater-borne nutrients on phytoplankton biomass and community structure, we conducted a series of mesocosm experiments in the Inland Bays of Delaware. Four treatments were tested, including mesocosms coupled directly to submarine groundwater seepage, mesocosms with the addition of pumped submarine groundwater, mesocosms with the addition of phosphate, and control mesocosms with no nutrient addition. We measured chlorophyll a concentrations as a proxy for overall biomass and used genetic sequencing techniques to characterize the phytoplankton community structure. Groundwater carried a high N load to the estuary with NO3- up to 295 µmol L-1 and NH4+ up to 55 µmol L-1. As a result, treatment mesocosms had elevated NO3- and NH4+, while control mesocosms were relatively low in nutrients. In June the highest chlorophyll a concentrations occurred in mesocosms attached to seepage meters after 3.5 days, with significant differences across all treatments. In August, groundwater-amended mesocosms reached the highest biomass concentrations, which peaked after 3 days. There were significant differences across all treatments, except control and phosphate-amended mesocosms which remained unchanged. Community sequence data showed that species assemblage was also impacted by availability of nutrients, with significant differences in community structure for mesocosms receiving nutrients vs control mesocosms in both June and August experiments. Harmful algal species proliferated in high nutrient treatments, including Cylindrotheca closterium, Karlodinium veneficum, Nitzschia sp., and Heterocapsa sp. While the general relationship between nutrient supply and biomass production is well known, we demonstrate the role groundwater-borne nutrients and sediment processes plays in shaping community structure in estuarine primary producers and in promoting harmful algal blooms.