The Distribution of Nickel in the West-Atlantic Ocean, Its Relationship With Phosphate and a Comparison to Cadmium and Zinc

Abstract

Nickel (Ni) is a bio-essential element required for the growth of phytoplankton. It is the least studied bio-essential element, mainly because surface ocean Ni concentrations are never fully depleted and Ni is not generally considered to be a limiting factor. However, stimulation of growth after Ni addition has been observed in past experiments when seemingly ample ambient dissolved Ni was present, suggesting not all dissolved Ni is bio-available. This study details the distribution of Ni along the GEOTRACES GA02 Atlantic Meridional section. Concentrations of Ni were lowest in the surface ocean and the lowest observed concentration of 1.7 nmol kg −1 was found in the northern hemisphere (NH). The generally lower surface concentrations in the NH subtropical gyre compared to the southern hemisphere (SH), might be related to a greater Ni uptake by nitrogen fixers that are stimulated by iron (Fe) deposition. The distribution of Ni resembles the distribution of cadmium (Cd) and also features a so called kink (change in the steepness of slope) in the Ni-PO 4 relationship. Like for Cd, this is caused by the mixing of Nordic and Antarctic origin water masses. The overall distribution of Ni is driven by mixing with an influence of regional remineralization. This influence of remineralization is, with a maximum remineralization contribution of 13% of the highest observed concentration, smaller than for Cd (30%), but larger than for zinc (Zn; 6%). The uptake pattern in the formation regions of Antarctic origin water masses is suggested to be more similar to Zn than to Cd, however, the surface concentrations of Ni are never fully depleted. This results in a North Atlantic concentration distribution of Ni where the trends of increasing and decreasing concentrations between water masses are similar to those observed for Cd, but the actual concentrations as well as the uptake and remineralization patterns are different between these elements.

Dissolved Trace Metals in the Ross Sea

“…In addition to being limited by light, primary production in the Ross Sea is limited by DFe and possibly DMn availability (Wu et al, 2019). Most bio-active metals are not fully depleted in the Ross Sea, but notably, Ni is relatively high in AASW (average 6.71 nM vs. 7.19 in HSSW), consistent with other observations in the global oceans (Morel et al, 2014;Middag et al, 2020). However, macro-nutrients and other trace elements such as Zn, Co, Cd and Cu can play a role in influencing phytoplankton growth or determining phytoplankton community composition (Sunda and Huntsman, 2000;Bertrand et al, 2007;Hoppema et al, 2007;Alderkamp et al, 2019).…”

“…All metals except DPb, had their lowest concentrations in AASW and concentrations increased with depth. Remineralization of organic material was probably an important source responsible for the increase (Orians et al, 1990;Bau et al, 1995;Hendry et al, 2008;Bown et al, 2011;Croot et al, 2011;Middag et al, 2013Middag et al, , 2020Bruland et al, 2014). The highest concentrations for all nutrients in the AASW and ML, both macro and micro, were measured at the stations sampled earlier in the season and those close to the Ross Ice shelf (St. 44, 45) where primary productivity was low and indeed was just starting.…”

“…When nutrient concentrations are influenced by phytoplankton growth, we expect the changes in concentrations to reflect phytoplankton elemental stoichiometry (Redfield et al, 1963;Sunda and Huntsman, 2000;Saito et al, 2010;Twining and Baines, 2013). However, mixing is also known to lead to relationships between nutrients that can be linear (Middag et al, 2018(Middag et al, , 2019(Middag et al, , 2020. As observed in other ocean basins, the macro-nutrients NO 3 − and PO 4 3− in the Ross Sea correlate better with each other than with Si(OH) 4 ( Supplementary Table S1, Figure 7).…”

“…The stoichiometric relationships between dissolved (D) metals [e.g., zinc (Zn), cadmium (Cd), cobalt (Co), copper (Cu) and nickel (Ni)] and macro-nutrients {e.g., nitrate (NO 3 − ), silicate (Si(OH) 4 ), and phosphate (PO 4 3− )} indicate that their concentrations are regulated by phytoplankton uptake (de Baar, 1994;Löscher et al, 1998;Cullen and Sherrell, 2005;Cullen, 2006;Lai et al, 2008;Bruland et al, 2014;Dulaquais et al, 2014a;Wyatt et al, 2014;Saito et al, 2017;Till et al, 2017;Middag et al, 2018Middag et al, , 2019Middag et al, , 2020. For example, Zn is used for protein repair, in the enzyme alkaline phosphatase, and for the uptake of CO 2 via the enzyme carbonic anhydrase (Morel et al, 1994;Buitenhuis et al, 1999;Hu et al, 2003;Shaked et al, 2006;Montsant et al, 2007).…”

“…Thus, it is possible that low DCo limits the production of vitamin B12 (Gordon and Sullivan, 2008). The micro-nutrient Ni is usually not depleted in the surface ocean (Butler et al, 2013;Middag et al, 2020) and Morel et al (2014) suggested this could be caused by slow uptake mechanisms.…”