Abstract
Microfluidics, or lab-on-a-chip (LOC) is a promising technology that allows the development of miniaturized chemical sensors. In contrast to the surging interest in biomedical sciences, the utilization of LOC sensors in aquatic sciences is still in infancy but a wider use of such sensors could mitigate the undersampling problem of ocean biogeochemical processes. Here we describe the first underwater test of a novel LOC sensor to obtain in situ calibrated time-series (up to 40 h) of nitrate+nitrite (ΣNOx) and nitrite on the seafloor of the Mauritanian oxygen minimum zone, offshore Western Africa. Initial tests showed that the sensor successfully reproduced water column (160 m) nutrient profiles. Lander deployments at 50, 100 and 170 m depth indicated that the biogeochemical variability was high over the Mauritanian shelf: The 50 m site had the lowest ΣNOx concentration, with 15.2 to 23.4 μM (median=18.3 μM); while at the 100 site ΣNOx varied between 21.0 and 30.1 μM over 40 hours (median = 25.1μM). The 170 m site had the highest median ΣNOx level (25.8 μM) with less variability (22.8 to 27.7 μM). At the 50 m site, nitrite concentration decreased fivefold from 1 to 0.2 μM in just 30 hours accompanied by decreasing oxygen and increasing nitrate concentrations. Taken together with the time series of oxygen, temperature, pressure and current velocities, we propose that the episodic intrusion of deeper waters via cross-shelf transport leads to intrusion of nitrate-rich, but oxygen-poor waters to shallower locations, with consequences for benthic nitrogen cycling. This first validation of an LOC sensor at elevated water depths revealed that when deployed for longer periods and as a part of a sensor network, LOC technology has the potential to contribute to the understanding of the benthic biogeochemical dynamics.
Highlights
In situ, high frequency observations are crucial to uncover the temporal and spatial complexity in the aquatic environments [1]
Before the Lander deployments, we assessed the quality of the LOC sensor to reproduce vertical nutrient gradients by comparing it to concentrations obtained from water samples
The overall trend was towards increasing temperatures, decreasing NO2- and O2 while increasing NO3- (Fig 4 and Fig 6a). What drives these non-tidal variations in the chemical and physical parameters? To attempt an explanation we examined the velocity profiles from an acoustic Doppler current profiler (ADCP) that was deployed on the sea floor at the depth of 50 m, close to the Lander deployment site (Table 1)
Summary
High frequency observations are crucial to uncover the temporal and spatial complexity in the aquatic environments [1]. The temporal variability can be even higher in estuaries, where current can be fast and freshwater inputs are significant [14]. Despite these improvements, the capacity of many nutrient sensors to perform long term, stable measurements at elevated water depths may be limited due to shallow depth range, large size, large power consumption and/or high detection limits
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