Seasonal Patterns of Dark Carbon Incorporation by Natural Phytoplankton Assemblages in the Central Cantabrian Sea (Bay of Biscay)

Abstract

Abstract. Phytoplankton dark carbon incorporation rates measured in the Central Cantabrian Sea (Bay of Biscay) exhibited a clear seasonal pattern. The highest rates were measured in May and September, whereas during winter very low rates were observed. Step‐wise regression analyses were performed with temperature, chlorophyll and the production/chlorophyll ratio as independent variables. The production/chlorophyll ratio was the most significant contributor (29.02%) on a yearly basis. Chlorophyll also explained a relevant percentage of dark carbon incorporation variability (15.36%), whereas temperature accounted for less than 0.5% of variance. Very similar results were obtained when mixing and stratification periods were analysed separately. The ratio between dark and light carbon incorporation showed high values (22–30 %) in January and March at all stations. During late spring and summer this ratio was low, ranging from 4 to 10 %. These results indicate a linkage between dark and light carbon incorporation and, therefore, that the subtraction of dark bottle counts could lead to underestimations of actual primary production rates, especially during low‐production periods.SummaryCarbon incorporation rates integrated over the water column varied from less than 4 to more than 7 mg C · m‐2· h‐1, showing a marked seasonal pattern in close phase with the production/chlorophyll ratio. The highest rates were recorded in May and September at all stations. Verly low rates were measured in winter. Step‐wise regression analysis performed on the data revealed that the production/chlorophyll ratio explained the highest percentage of dark carbon incorporation variability (29.02%) on a yearly basis. The chlorophyll a concentration was also a relevant contributor (15.36%), whereas temperature explained less than 0.5 % of the variance. The percentage of dark to light carbon incorporation exhibited a clear seasonal variation. The highest percentages were measured in January and March. This ratio remained between 4 and 10.3% during late spring and summer.