Effects of riverine nutrient inputs on the sinking fluxes of microbial particles in the St. Lawrence Estuary

Discharges of nitrogenous nutrients associated with agriculture and wastewater treatment promote eutrophication in coastal waters around the world. Since the Lower St. Lawrence Estuary shows early signs of eutrophication, with the presence of hypoxic and acidified deep waters, a mesocosm experiment was conducted to determine the impact of variations of nitrogenous nutrients from riverine inputs to this estuarine environment during the spring–summer phytoplankton growth season. Four treatments were applied to recreate conditions mimicking 1) upwelled waters at the head of the Lower Estuary (“control”), 2) a mix of upwelled and riverine water (”+N”), 3) the same mix of upwelled and riverine waters with a greater proportion of urea in the total nitrogenous pool (”+Urea”), and 4) waters with a 50% decrease in nitrogenous riverine inputs (”–N”). The composition and vertical fluxes of microbial communities were characterized under these four scenarios. Nitrogenous nutrients were consumed within three days in all treatments. A higher concentration of nitrogenous nutrients led to increases in phytoplankton carbon biomass. The centric diatom Skeletonema costatum dominated in all treatments. The microbial community primarily consumed nitrate in all treatments, but equally preferred nitrate and urea in the +Urea treatment. Mean particle sinking velocities were not statistically different between the two most nitrogen-enriched treatments and the control, but the higher dissolved silicon:nitrate uptake ratio in the –N treatment could have favored silicification of the diatoms, leading to increased sinking speeds. Indeed, the total biogenic carbon sinking flux in the –N was on average 51% higher than in the other treatments (∼391 mg C m−2 d−1 vs ∼253–262 mg C m−2 d−1). These results indicate that efficient control measures concerning the St. Lawrence watershed should consider concentrations and ratios of nutrients that potentially promote phytoplankton growth in the Lower Estuary and consequently increase the amount of carbon exported to and decomposed in hypoxic and acidified deep layers.