Glacier depth controls downstream productivity; non-linearity in all Ice Sheet-to-Ocean nutrient fluxes

It is widely hypothesized that climate change will enhance dissolved macronutrient (NO3, PO4 and Si) andmicronutrient (e.g. Fe) fluxes from glaciers to the ocean and thus potentially enhance future marine primaryproduction. Here we combine a subglacial plume model for a tidewater glacier and datasets of estuarine nutrientdistribution from Greenland and Svalbard to investigate how meltwater affects the availability of nutrients inthe marine environment. Combining these data with insight from satellite derived fluorescence quantum yieldsto assess the balance between NO3 and Fe limitation in the North Atlantic, we also evaluate the likely effect ofincreased discharge from the Greenland Ice Sheet on future marine primary production.We show that delivery of macro/micro nutrients from both tidewater and land terminating glaciers is non-linearwith respect to increasing freshwater volume. In the case of tidewater glaciers, macronutrient fluxes to the marinephotic zone are strongly dependent upon glacier grounding line depth. For a simplified model tidewater glacier, anoptimum 'Goldilocks' zone for nutrient delivery into the photic zone for a discharge of 500 m3 s-1 occurs whenthe glacier sits at around 200-600 m depth. The resulting plume entrains seawater and produces an upwelled NO3flux at least two orders of magnitude greater than that from glacial runoff. Runoff itself provides a negligible NO3flux in terms of the likely effect on marine productivity. Shoaling of tidewater glaciers decreases downstreamfluxes of all macronutrients due to the reduced upwelling efficiency. For example, even with a 10-fold increase inmeltwater discharge, the macronutrient flux from our model system is still reduced with a 200 m grounding linerelative to a 600 m grounding line. Further non-linearity in all Ice Sheet-to-Ocean nutrient fluxes arises becauseof non-conservative estuarine mixing behavior for Fe (>80% removal), Si (+12-13% increase) and PO4 (>35%removal) in turbid fjord waters.Our results emphasize the importance of 3-dimensional processes on Ice Sheet-to-Ocean nutrient fluxes. Nutrientfluxes derived simply from meltwater volume and meltwater nutrient concentration are always positive eventhough the enhanced stratification which accompanies meltwater delivery into the ocean leads to, in the absenceof sub-glacial discharge driven upwelling, reduced NO3 and PO4 availability for downstream marine primaryproduction.