Effects of drift macroalgae on sediment nitrogen cycling in Thalassia testudinum beds of St. Joseph Bay, FL
Anthropogenic nitrogen (N) loading and eutrophication can affect valuable ecosystem services and seagrass habitats by modifying structural and functional aspects of estuarine communities including increasing and prolonging macroalgae blooms. In some contexts, macroalgae may play a key role in N cycling pathways because they can alter sediment chemistry. Previous research has associated drift macroalgae blooms with elevated dissolved inorganic N concentrations in sediments as a result of increased remineralization of organic matter, but drift macroalgae effects on microbial N transformation pathways are not well understood. This study quantified the effects of macroalgae on estuarine N cycling in Thalassia testudinum seagrass beds of St. Joseph Bay, FL, on June 4, 18, and 26, 2017. Sediment physical characteristics, seagrass and macroalgae biomass measurements, and porewater chemistry were analyzed to compare sediments in areas with and without macroalgae cover. Seagrass aboveground and belowground biomass was significantly greater in areas with no macroalgae on June 18, and 26. Sediments at incubation sites with macroalgae had higher porewater sulfide and % organic matter, though this was not significant across all sites and incubation dates. Porewater sulfide concentrations were positively correlated to seagrass shoot density in areas with and without drift macroalgae. A continuous-flow incubation system for intact sediment cores amended with stable isotope additions (15NH4+ and 15NO3- ) was used to measure N transformation rates across the sediment-water interface. Results shows that DNRA (dissimilatory nitrate reduction to ammonium) rates were similar in magnitude to denitrification rates in areas with and without macroalgae, showing similar rates of N removal and recycling within sediments. Anammox rates were a minor source of N2 flux from sediments in macroalgae and no macroalgae areas, though anammox was stimulated under drift macroalgae blooms. N fixation was observed during all incubations, even in sediments with porewater NH4+ above 300 µM. Previous research in St. Joseph Bay during summer and fall had higher denitrification rates, and lower N fixation rates than this study. Presence of drift macroalgae appears to possibly alter organic matter supply during blooms that may affect seasonal sediment N cycling rates in seagrass beds. Though there were not many significant differences in N cycling rates between areas with and without drift macroalgae, data presented in this study provides baseline N transformation rates in a relatively unimpacted bay, St. Joseph Bay, FL, and is an important step in understanding how nutrient loading may alter N cycling rates in other similar systems.
nitrogen, seagrass, coastal ecosystems, estuaries, biogeochemistry, sediment chemistry, Biology, Integrative Biology, Marine Biology
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