Cyanate dynamics under algal blooms and sediment resuspension events in a shallow micro-tidal estuary in the lower Chesapeake Bay

Abstract

Although an emerging component in the marine nitrogen (N) cycle, cyanate concentrations and cycling have not been examined in estuarine systems to date. To better understand controls on cyanate concentrations in estuaries, time series data of cyanate and nutrient concentrations in the Lafayette River, a micro-tidal, sub-tributary of the lower Chesapeake Bay, were examined between June and September 2018, and May to September 2019. Cyanate concentrations ranged from near the detection limit (0.4 nmol L−1) to 82.9 nmol L−1 in 2018, and 6.8–207.4 nmol L−1 in 2019. Variations in cyanate concentrations were highly correlated with chlorophyll biomass in the summer, biomass degradation in early fall, and sediment resuspension that occurred in response to meteorological forcing. Cyanate concentrations increased after Chl a concentrations decreased suggesting algal decomposition as a source of cyanate. High cyanate concentrations in bottom waters, corresponded to wind-induced sediment resuspension events in the Lafayette River, again suggesting organic matter decomposition as a source of cyanate. Cyanate concentrations in sediment pore water varied between years; in summer 2018, cyanate concentrations were up to 150 nmol L−1, while in 2019, they were three times lower. To confirm an algal source for cyanate, a degradation experiment was conducted using Lafayette River water collected during a bloom of Margalefidinium polykrikoides in 2018. In dark incubation bottles, cyanate was one of the first labile organic nitrogen products produced, suggesting the contention that high concentrations of cyanate in late summer and fall were the result of organic matter decomposition. Neither cyanate nor ammonium accumulated in light bottles suggesting production and uptake are tightly coupled and microbes have a high affinity for cyanate in the light. In dark bottles, cyanate production rates were 6.8 nmol L−1 d−1, while microbial removal rates during the late phase of degradation were 1.5 nmol L−1 d−1, suggesting that cyanate may not be a preferred nitrogen substrate for microbes (including nitrifiers) in dark bottles or that microbes have a lower affinity for cyanate in the dark, allowing cyanate to reach steady state at concentrations greater than the detection limit.