Iron Uptake in a Shelf Sea: Seasonality and Stoichiometry

Primary production by phytoplankton in shelf seas represents a significant contribution to global carbon cycling. Trace metals, such as Iron (Fe), are essential micronutrients for phytoplankton growth, and may ultimately limit primary production. The uptake of iron within natural phytoplankton communities is poorly understood. Using carrier free ⁵⁵Fe, we are able to obtain novel estimates of biological uptake of Fe using trace level Fe additions that do not perturb the system. Here we present results from a study measuring the uptake of carrier free Iron (⁵⁵Fe), in parallel with measurements of Carbon (¹⁴C) and Phosphorus (³³P) uptake; samples were collected from the Celtic sea in spring (April 2015) and summer (July 2015), on-shelf and off-shelf, and in 2 size fractions (Total, >2µm), thus producing a novel dataset of the seasonality of macronutrient and micronutrient uptake by phytoplankton within a shelf sea. Primary production ranged from 1.0 – 2.2 mmol C m^-3 d^-1 in April and 0.2 – 1.5 mmol C m^-3 d^-1 in July, with a corresponding higher biomass in April (0.7 – 3.24 mg Chl m^-3) than July (0.3 – 1.6 mg Chl m^-3). Significant rates of Fe uptake were measured in all samples (e.g. 21 nmol m^-3 d^-1, on-shelf, April), with variable stoichiometry (e.g. Fe/C of 16 µmol/mol, and Fe/P of 0.4 mmol/mol, on-shelf, April). The results will be presented and examined in the context of the available nutrient pools, the phytoplankton community structure and the impact on biogeochemical cycling.