Isotopic evidence for a marine ammonium source in rainwater at Bermuda

Abstract

Emissions of anthropogenic nitrogen (N) to the atmosphere have increased tenfold since preindustrial times, resulting in increased N deposition to terrestrial and coastal ecosystems. The sources of N deposition to the ocean, however, are poorly understood. Two years of event-based rainwater samples were collected on the island of Bermuda in the western North Atlantic, which experiences both continent-and ocean-influenced air masses. The rainwater ammonium concentration ranged from 0.36 to 24.6 μM, and the ammonium δ 15 N from À12.5 to 0.7‰; and neither has a strong relationship with air mass history (6.0 ± 4.2 μM, À4.1 ± 2.6‰ in marine air masses and 5.9 ± 3.2 μM, À5.8 ± 2.5‰ in continental air masses; numerical average ± standard deviation). A simple box model suggests that the ocean can account for the concentration and isotopic composition of ammonium in marine rainwater, consistent with the lack of correlation between ammonium δ 15 N and air mass history. If so, ammonium deposition reflects the cycling of N between the ocean and the atmosphere, rather than representing a net input to the ocean. The δ 15 N data appear to require that most of the ammonium/a flux to the ocean is by dissolution in surface waters rather than atmospheric deposition. This suggests that the atmosphere and surface ocean are near equilibrium with respect to air/sea gas exchange, implying that anthropogenic ammonia will equilibrate near the coast and not reach the open marine atmosphere. Whereas~90% of the ammonium deposition to the global ocean has previously been attributed to anthropogenic sources, the evidence at Bermuda suggests that the anthropogenic contribution could be much smaller.

Marine biogenic source of atmospheric organic nitrogen in the subtropical North Atlantic

“…The lack of anthropogenic inorganic N precursors and the stagnant air mass back trajectories (AMBTs) are consistent with group 2 aerosols having a marine-sourced NH 4 + as the inorganic N precursor. Moreover, measurements of δ (29), the large contribution of amine salts and amino acids in marine aerosols (24), and the evidence that the surface ocean is the dominant source of ammonia to the marine atmosphere at Bermuda (7,23). The prevalence of reduced-N compounds is also consistent with previous work showing that 29-75% of rain and aerosol WSON is bioavailable (30,31).…”

“…All samples were analyzed for the concentration of ammonium (NH 4 + ), nitrate (NO 3 − ), total N, and non-sea-salt sulfate (nss-SO 4 2− ). These samples were previously analyzed for the concentration and N isotopic composition of NH ) in rain and aerosol, which are discussed below but detailed in other publications (5)(6)(7). A subset of event-based rain (n = 12) and weekly aerosol (n = 20) samples were analyzed for the N isotopic composition of total N (δ 15 N−TN; allowing for the calculation of δ 15 N−WSON by mass balance) and for chemical composition by ultra-high-resolution Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS).…”

New insights into carbon acquisition and exchanges within the coral–dinoflagellate symbiosis under NH 4 + and NO 3 − supply

“…Although the detrimental effect of nitrate on photosynthesis has to be confirmed with different coral species and nitrate concentrations, excess nitrate in reef waters, which is mainly brought by industry and agriculture runoff, will probably severely affect the entire coral primary productivity. On the contrary, ammonium is the principal form of recycled nitrogen from fish excretion [60,62], although rain can also be a significant source in some reef environments [27]. Enhancing fish populations above reefs will probably be beneficial for corals, as already observed with resident fish schools [63].…”

“…Consequently, at certain periods of the year, inorganic nitrogen and phosphorus concentrations can be much higher [16][17][18][19] than the usual levels measured on reefs (less than 0.5 mM for nitrogen [20][21][22], less than 0.1 mM for phosphate [23][24][25]). Furthermore, the geomorphology and the different sources and types of pollution, will also change the abundance of nitrogen sources (nitrate versus ammonium) and the N : P ratios of coastal ecosystems [26,27].…”

Global oceanic emission of ammonia: Constraints from seawater and atmospheric observations

“…The underestimate of [NH x ] may thus be partly driven by uncertainties in the magnitude of continental sources of NH 3 and in the long-range transport of NH x . Isotopic measurements of [NH + 4 ] in precipitation [Altieri et al, 2014] may provide better constraints on the contribution of continental sources to marine atmospheric NH x .…”