Cyanate distribution and uptake in North Atlantic coastal waters

Abstract

Cyanate (OCN−) is a reduced nitrogen compound that may be a source of nitrogen and carbon for marine phytoplankton. Cyanate is produced photochemically and through organic matter degradation but its distribution in marine systems is poorly defined because, until recently, there was no method to measure its concentration in marine environments. Here, we report results from the first regional oceanographic survey of cyanate distributions and uptake. Cyanate concentrations ranged from below detection (0.4 nM) to 11 nM in the coastal North Atlantic Ocean along the North American continental shelf and were slightly higher in nearshore surface waters than offshore surface waters. Subsurface cyanate maxima were observed at many stations suggesting a nonconservative distribution. Subsurface cyanate peak concentrations were tightly correlated with chlorophyll a concentrations integrated throughout the water column (R2 = 0.79). Cyanate N and C uptake were measurable in all regions and seasons sampled. Cyanate N uptake was significantly higher than cyanate C uptake for all cruises (1.3 ± 1.9 nmol L−1 h−1 and 0.4 ± 0.6 nmol L−1 h−1 for N and C, respectively). However, cyanate N uptake was significantly lower in November than in June and August, and cyanate C uptake was significantly higher in November than during June and August. Spatial trends in cyanate distribution and uptake, along with the correlation between depth‐integrated Chl a concentrations and cyanate maxima, suggest that cyanate is taken up in the euphotic zone and is likely produced by degradation of phytoplankton‐derived organic matter in subsurface waters.

Cyanate distribution and uptake above and within the Eastern Tropical South Pacific oxygen deficient zone

“…The concentration of a dissolved compound in seawater reflects the balance of its production and consumption, and in the ETSP, the more uniform, higher OCN − concentrations in the euphotic zone likely reflect higher rates of OCN − production in the ETSP than in the North Atlantic. OCN − uptake rates in the upper oxic layer of the ETSP (0.1 ± 0.1 nmol N L −1 h −1 ) were an order of magnitude lower than uptake rates in the North Atlantic in August (1.1 ± 1.0 nmol N L −1 h −1 ; Widner and Mulholland ), and diatoms, which are dominant in the ETSP (Pennington et al ; Franz et al ) but not in the North Atlantic in August (Pan et al ), have been shown to produce cyanate in culture (Widner et al ). The regional difference in OCN − profile shape may also reflect very different stratification and mixing regimes between the two locations.…”

“…OCN − concentrations in late summer (August) in the coastal North Atlantic between Cape Hatteras and the Gulf of Maine ranged from 0 nM to 25 nM (Widner et al ; Widner and Mulholland ); however, in the ETSP, OCN − concentrations were at or near the limit of analytical detection at most stations and depths measured. The North American Atlantic coastal region is an N‐limited, temperate system influenced by western boundary currents with a relatively wide continental shelf and a mixed, seasonally dynamic phytoplankton community (Townsend et al ; Pan et al ).…”

“…In contrast, the ETSP lies in a tropical Eastern boundary current and has a narrow continental shelf, experiences regular coastal upwelling, diatom dominance, and offshore iron limitation (Hutchins et al ; Pennington et al ; Montecino and Lange ; Franz et al ). At stations on the continental shelf slope in the coastal North Atlantic, the OCN − concentration was less than 2 nM in surface waters and there was an OCN − maximum below the Chl a maximum followed by a gradual decline in OCN − concentration with depth (Widner et al ; Widner and Mulholland ). In contrast, we observed relatively high OCN − concentrations in ETSP surface waters (10–45 nM) that rapidly declined below the pycnocline, and there were no subsurface maxima associated with the Chl a maximum.…”

“…Uptake rates were calculated using a mixing model (Montoya et al 1996;Mulholland et al 2006;Mulholland and Lee 2009). Limits of detection for uptake rates were calculated as in Widner and Mulholland (2017).…”

“…Urea [CO(N 2 H 2 ) 2 ], another simple organic compound, accounts for approximately one third of nitrogen uptake across all aquatic systems and is released to marine systems by bacteria, fecal pellet leaching, sloppy feeding, zooplankton excretion, atmospheric deposition, and anthropogenic sources such as wastewater and fertilizer (Bronk and Steinberg ; Mulholland and Lomas ; Sipler and Bronk ). Because urea and OCN − are both taken up and produced by marine microbes, their distributions are biologically controlled, similar to those of ammonium ( ) and nitrite ( ) (Gruber ; Widner et al ; Widner and Mulholland ). In the oligotrophic North Atlantic Ocean, we found that urea uptake accounted for up to 50% of the total measured N uptake while OCN − uptake accounted for up to 10% of total N uptake (Widner et al ).…”