Micronutrient export from glacier to fjord, southwest Greenland: potential impacts on open ocean primary productivity

The accelerated melting of the Greenland Ice Sheet could potentially enhance fluxes of key nutrients, to the surrounding oceans, impacting marine biogeochemical processes and ecosystems. Iron (Fe) is one key micronutrient for marine phytoplankton that may be affected by this increase in meltwater flux, with high export of dissolved and particulate Fe from glacial meltwaters into fjords and a potentially significant increase in the supply of labile and potentially bioavailable Fe to the Greenlandic shelf. However, biogeochemical processing within estuarine-like fjord systems may result in depletion of nutrients, acting as a sink of micronutrients before they can reach the coastal ocean. The extent to which glacially derived micronutrients, specifically Fe, reach coastal waters remains an unanswered question.

Here, we address this question by assessing the concentration of dissolved (<0.45 µm) and labile particulate (determined using the Berger leach) bio-essential trace metals (Fe, Cd, Mn, Ni, Cu, Zn) in two contrasting glaciated fjords in southwest Greenland; one fed predominantly by marine terminating glaciers and the other by a land terminating glacier. We investigate the difference in size fractionated concentrations between fjords and the transport of these metals from stations close to glacial termini down to the fjord mouths. Our findings reveal that each micronutrient exhibits a distinctive behaviour, with some metals enhanced in meltwaters (e.g. dissolved Fe and Mn) and some depleted (e.g. dissolved Cd), relative to marine waters. The spatial variability in our dataset highlights that concentration of Fe and other trace metals (Cd, Mn, Ni, Cu, Zn) enriched in meltwaters become depleted towards the mouth of the fjords, with non-conservative loss from surface waters. Despite this depletion, the concentrations of these metals in waters that reach the coastal zone are significantly higher than typical surface ocean values, both in dissolved and labile particulate form. These data can ultimately be used in combination with a physical understanding of the fjord systems to constrain the capacity of fjords to enhance productivity downstream and deliver micronutrients into coastal and open ocean systems. Furthermore, the direct comparison of land- and marine-terminating glacial fjords could provide valuable information on the potential future impact of retreating glacial systems with enhanced melting.