Spatially Complex Distribution of Dissolved Manganese in a Fjord as Revealed by High-Resolution in Situ Sensing Using the Autonomous Underwater Vehicle Autosub

Abstract

Loch Etive is a fjordic system on the west coast of Scotland. The deep waters of the upper basin are periodically isolated, and during these periods oxygen is lost through benthic respiration and concentrations of dissolved manganese increase. In April 2000 the autonomous underwater vehicle (AUV) Autosub was fitted with an in situ dissolved manganese analyzer and was used to study the spatial variability of this element together with oxygen, salinity, and temperature throughout the basin. Six along-loch transects were completed at either constant height above the seafloor or at constant depth below the surface. The ca. 4000 in situ 10-s-average dissolved Mn (Mnd) data points obtained provide a new quasi-synoptic and highly detailed view of the distribution of manganese in this fjordic environment not possible using conventional (water bottle) sampling. There is substantial variability in concentrations (<25 to >600 nM) and distributions of Mnd. Surface waters are characteristically low in Mnd reflecting mixing of riverine and marine end-member waters, both of which are low in Mnd. The deeper waters are enriched in Mnd, and as the water column always contains some oxygen, this must reflect primarily benthic inputs of reduced dissolved Mn. However, this enrichment of Mnd is spatially very variable, presumably as a result of variability in release of Mn coupled with mixing of water in the loch and removal processes. This work demonstrates how AUVs coupled with chemical sensors can reveal substantial small-scale variability of distributions of chemical species in coastal environments that would not be resolved by conventional sampling approaches. Such information is essential if we are to improve our understanding of the nature and significance of the underlying processes leading to this variability.

Trends in microfluidic systems for in situ chemical analysis of natural waters

“…[48][49][50] Of these, colorimetry -in which the sample is mixed with an analyte-specific reagent to produce a measurable colour -has proved to be by far the most popular method and has been used for in situ analysis of a range of chemical parameters including nitrate and nitrite, [21,22,24,29,31,34,35,38] phosphate, [27,38] iron, [23,26,28,36,39,[42][43][44] manganese, [25,26,28,37,40] sulfide [32,33,35,39,41] silicate [30,32,33,38] and pH. [51][52][53][54][55] Colorimetry lends itself well to microfluidic in situ analysers as it is chemically robust, offers excellent analytical performance (limits of detection typically in the order of 10 nM [21,25,36]) and requires relatively small, cheap and easily-sourced components.…”

“…A good example is the Mn analyser reported by Statham et al [37,40] described schematically in Fig 1a and shown in Fig 1b. Built in-house using commercially available components, the system operates by continuously pulling water from the environment using a peristaltic pump. The water is then propelled into 800 µm diameter polytetrafluoroethylene (PTFE) tubing along with a flow of reagent consisting of a solution of 1-(2-pyridylazo-)-2-napthol (PAN) mixed with an iron-specific chelating agent to remove any iron interference.…”

“…[40] To demonstrate the utility of the sensor, it was deployed in-the-field to ascertain the distribution of Mn within Loch Etive -a fjord-like deep water basin on the west coast of Scotland (Fig 1c). [37] The sensor was fitted to the autonomous underwater vehicle (AUV) "Autosub" which systematically transected the loch at either fixed depth or fixed height above the seafloor. In doing so, the sensor was able to map the spatial distribution of Mn across depth and location.…”

“…Small sized peristaltic pumps are easily commercially sourced, relatively cheap and easy to M a n u s c r i p t use and consequently have proved consistently popular. [25,26,[28][29][30][32][33][34][35]37,[39][40][41][42][43] Nonetheless, peristaltic pumps can suffer from drifting flow rates due to variation in the elasticity and plasticity of the pump tubing with changes in temperatures [35] and, crucially, are relatively power hungry. [56] Consequently, reported deployments of peristaltic pumped sensors have been limited to a day or less, except where power could be externally supplied via cabling.…”

“…[67] Any widespread oceanic deployment would likely need to include deployment on low-power underwater profiling vehicles such as oceanic gliders or Argo floats. There have been several reports of microfluidic in situ chemical sensors being deployed on underwater vehicles, however, they have so far been limited to deployments on Remotely Operated Vehicles (ROVs) [35,39,50] and AUVs [37,40,48] where power is less critical. The advent of low-power precision pumping, as previously discussed, makes deployment on low-power mobile A c c e p t e d M a n u s c r i p t platforms feasible and we anticipate that this will be reported in the near future.…”