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Shelf seas play an important role in the global carbon cycle, absorbing atmospheric carbon dioxide (CO2) and exporting carbon (C) to the open ocean and sediments. The magnitude of these processes is poorly constrained, because observations are typically interpolated over multiple years. Here, we used 298500 observations of CO2 fugacity (fCO2) from a single year (2015), to estimate the net influx of atmospheric CO2 as 26.2 ± 4.7 Tg C yr−1 over the open NW European shelf. CO2 influx from the atmosphere was dominated by influx during winter as a consequence of high winds, despite a smaller, thermally-driven, air-sea fCO2 gradient compared to the larger, biologically-driven summer gradient. In order to understand this climate regulation service, we constructed a carbon-budget supplemented by data from the literature, where the NW European shelf is treated as a box with carbon entering and leaving the box. This budget showed that net C-burial was a small sink of 1.3 ± 3.1 Tg C yr−1, while CO2 efflux from estuaries to the atmosphere, removed the majority of river C-inputs. In contrast, the input from the Baltic Sea likely contributes to net export via the continental shelf pump and advection (34.4 ± 6.0 Tg C yr−1). [ABSTRACT FROM AUTHOR]

Understanding the dynamics of upwelling systems, especially the interactions between nutrients and light, has benefited from the application of models of varying complexity. Validation of such models using unialgal cultures or field observations has often proven difficult, but short-term incubations of contained natural assemblages and use of instantaneous physiological indicators offer an alternative approach. In May and June 1996, phytoplankton communities deep in the euphotic zone were sampled from nearly identical physical environments. Replicate samples (20 l volume) were incubated on deck at 50% surface irradiance with either no nutrient additions (Controls) or additions of 20 μM nitrate (Enrichments). Over 24 h, variable fluorescence (Fv:Fm), nitrate reductase activity (NR), nutrients, chlorophyll a and particulate C and N were monitored. Initial chlorophyll a (∼3 μg l-1), phosphate (∼0.2 μM), nitrate (∼1.5 μM) and silicate (∼3 μM) were similar in both months. Changes in NR and Fv:Fm indicated clear physiological responses to changes in irradiance and added nitrate that differed between months. In May, Controls and Enrichments responded in the same way. Fv:Fm stayed constant (0.5), chlorophyll a increased slightly, and NR activity increased markedly in all samples. In contrast, in June, treatments responded quite differently. Fv:Fm was near the theoretical maximum (0.7–0.8) initially and remained constant in Enrichments, but fell sharply in Controls. Declines in controls were also seen for chlorophyll a, and NR activity. Thus, the addition of 20 μM nitrate had a significant effect even though ambient levels of nitrate (>1 μM) should not have been limiting. Small (<20 μm) flagellates predominated in the May samples, but in June large and chain-forming centric diatoms constituted a significant proportion of the phytoplankton community. We conclude that the response of a phytoplankton community to environmental changes can depend on factors that are poorly represented by bulk measurements of chlorophyll, nutrients and particulate elements. [ABSTRACT FROM AUTHOR]