Going Organic In The Shelf Sea: Where Has All The Carbon Gone?

It has been hypothesised that variability in the stoichiometry of organic matter (OM) plays a key role in sustaining the shelf sea carbon pump by delivering carbon rich material below the thermocline, where it can be remineralised and exported to the deep sea. Here, we test this hypothesis by quantifying the magnitude, stoichiometry, source and lability of particulate and dissolved OM (POM and DOM) over a seasonal cycle in the temperate Celtic shelf sea.

Above the thermocline, suspended particulate organic carbon (POC) and particulate organic nitrogen (PON) concentrations were 2 to 3-fold higher in spring and summer compared to winter. Below the thermocline, POC and PON concentrations varied by less than 50%. However, in both layers POM was C-rich and N-poor in summer (C:N > 7) but C-poor and N-rich in winter (C = 4). Segregation of the suspended and sinking particles indicated that in winter suspended particles were C-poor while sinking particles were C-rich (C:N > 7), implying that sinking C-rich material is critical to getting carbon below the thermocline where it can be remineralised and potentially exported in the dissolved phase.

In a similar manner, DOM concentrations varied seasonally but the patterns for C and N differed. DOC concentrations decreased above and below the thermocline from spring through summer and into winter, but the decrease was significantly larger below the thermocline. In contrast, there were no significant seasonal changes in DON throughout the water column. Thus, changes in DOC drove a seasonal decline in the C:N ratio in DOM from spring (18) to summer (16) and finally winter (14). Overall, we find that the seasonal change in DOC concentration is larger than POC implying that the DOM plays an important role in the C dynamics within the shelf sea and is a key player in the carbon export pathway.