Coupling nitrate sensor technology with automomous gliders to investigate seasonal controls on the biogoechemical cycling of nitrate in a temperate shelf sea
The oceans take up ~30 % of increasing atmospheric carbon dioxide (CO2) concentrations. Shelf seas, though small (7 - 8 % of the total ocean area), play a significant role in the uptake and removal of this CO2 by contributing 15 – 30 % of total oceanic primary production. Shelf seas are highly dynamic systems with seasonal cycles in nutrients, light, stratification and primary production. Two key stages in the seasonal cycles are the spring bloom and the summer formation of a sub-surface chlorophyll maximum (SCM). There is a growing need to increase spatial and temporal resolution of in situ measurements of nutrients and phytoplankton growth alongside the physical factors which drive their seasonal cycles as these measurements are needed to ground-truth and provide new data for biogeochemical models used to predict both contemporary and future changes in climate.
In this study, a novel wet-chemical microfluidic Lab-on-Chip nutrient sensor was deployed for the first time within an autonomous underwater glider where nitrate + nitrite (∑NOx) measurements were comparable to traditional ship-based methods (r2=>0.98; n = 60). The Lab-on-Chip nutrient sensor was able to capture the large drawdown of ∑NOx - within the surface mixed layer due to the onset of the spring bloom in the central Celtic Sea, where surface ∑NOx concentrations decreased from 5.74 µM to 1.42 µM, whilst bottom layer ∑NOx concentrations remained constant (6.86 ±0.16 µM).
During the summer, concurrent measurements of the dissipation of turbulent kinetic energy and ∑NOx at the pycnocline resulted in a mean flux (f∑NOx of~4.2 mmol m-2 d-1) that is double that of previously reported estimates (~2 mmol m-2 d-1) in the Celtic Sea. The mean f∑NOx across the pycnocline was dominated by short mixing events that could potentially supply larger (> 15 mmol m-2 d-1) intermittent fluxes of ∑NOx into the SCM. Using our spring and neap tide f∑NOx estimates to represent the upper and lower limits, the contribution of new production estimated in this study, supported by the f∑NOx into the SCM (58 (21-80) g C m-2), could support all of the estimated annual new production (81.8 g C m-2) in the Celtic Sea in the SCM alone during the summer period (120 days). If this under-estimation of the contribution of the summer SCM to the annual new production shown here in the Celtic Shelf Sea is indicative of all other continental shelf seas of the Northern Hemisphere, then their previously estimated ability to be net sinks of CO2 (0.24 Pg C yr−1; Laruelle et al., 2010)could be underestimated.