Nitrogen Cycling In Shelf Seas: A Stable Isotope Approach

There is some uncertainty regarding the source of nutrients that sustains enhanced shelf sea productivity and, in temperate regions, carbon export. Nutrients are supplied via a combination of cross shelf physical transport from the nutrient-rich deep ocean, and local regeneration of organic matter coupled with local nitrification. Using the spatial distribution and time-series of nutrients, the δ¹⁵N and δ¹⁸O isotope composition of nitrate and the δ¹⁵N of particulate matter, we unravel the relative magnitude and importance of physical transport processes and local regeneration in sustaining an on-shelf nitrate pool. We find cross-shelf and shelf edge gradients in nutrients, δ¹⁵N and δ¹⁸O and apparent oxygen utilization across the Celtic Sea on the NW European shelf, covering a seasonal cycle from 2014 to 2015. Bottom water wintertime and summertime off-shelf nitrate concentrations were ≈18μM, decreasing on-shelf to 10μM in the winter and less than 7μM during the spring phytoplankton bloom as nutrients were assimilated. Bottom water N:P gradients were fairly constant at 15:1 during the winter across the shelf, only decreasing close to the coast. Spring saw decreases from 16:1 at the shelf break to 12:1 on shelf. Towards the surface over the outer shelf, δ¹⁵N-NO₃ increased with nitrate consumption due to phytoplankton growth during the spring as nitrate was depleted, also δ¹⁸O-NO₃ showed a decreasing shoreward trend indicating nitrification on-shelf. Knowledge of the nutrient sources is important as a new supply of nutrients from the deep ocean will be accompanied by a new source of dissolved inorganic carbon. Results imply that some on-shelf regions are likely to be supported by locally regenerated nitrate, which has implications for the carbon budget of the NW European shelf.