Coupling nitrate sensor technology with autonomous gliders to investigate seasonal controls on the biogeochemical cycle of nitrate in a temperate shelf sea

Abstract

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 (NO3-) 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 NO3- within the surface mixed layer due to the onset of the spring bloom in the central Celtic Sea, where surface NO3- concentrations decreased from 5.74 µM to 1.42 µM, whilst bottom layer NO3- concentrations remained constant (6.86 ±0.16 µM). Concurrent measurements of the dissipation of turbulent kinetic energy at the pycnocline resulted in a mean nitrate 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. Using bio-optical sensors (fluorescence and backscatter) deployed on a glider, bio-optical relationships were established between Chl-a and particulate backscatter that were used to derive values of particulate organic carbon (POC) and phytoplankton carbon. During the spring bloom this study showed a strong positive correlation between Chl-a, particulate backscatter and POC, which was decoupled during the summer SCM due to changing phytoplankton biomass and resuspended sedimentary material. This work has shown that using established empirical relationships the potential for bio-optical sensors on gliders to predict POC, and thus Cphyto, in a dynamic shelf seas environment. This thesis demonstrated that autonomous platforms and novel in situ biogeochemical sensors are an invaluable tool for the observation of the changing marine environment and are well placed to provide new insight into biogeochemical, physical and bio-optical fields as well as to augment, with complimentary high-resolution datasets, moorings, ships and satellites methodologies.

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ORCID for Maeve Lohan: orcid.org/0000-0002-5340-3108

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