Nitrogen Cycling In Shelf Seas: A Stable Isotope Approach

Calum Preece1, Claire Mahaffey1, Jonathan Sharples2, Malcolm S Woodward3 and Keith Weston4, (1)University of Liverpool, Earth, Ocean and Ecological Sciences, Liverpool, United Kingdom, (2)University of Liverpool, Earth, Ocean and Ecological Sciences, Liverpool, L69, United Kingdom, (3)Plymouth Marine Laboratory, Plymouth, PL1, United Kingdom, (4)Centre for Environment Fisheries and Aquaculture Science (Cefas), Lowestoft Laboratory, Lowestoft, United Kingdom
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.