Sensors used in FerryBox Systems
The FerryBox-technology itself is already very mature in the direction of reliability, long-term stability or operability. Therefore, the critical paths are suitable sensors or analysers that run unattended over long time periods; especially sensors for chemical/biological parameters within the goals of the Marine Strategy.
Fortunately, sensors in FerryBoxes have different advantages in comparison to in situ sensors in other observation systems such as buoys, piles or gliders:
- The sensors are under controlled conditions (flow, temperature, shock)
- The energy consumption and mechanical dimensions are not critical.
- The maintenance is cheaper and easier than on offshore installations.
In addition, FerryBoxes, installed inside the ships, provide an ideal environment for more complex sensors, eventually with higher power consumption and vulnerability to extreme conditions. Here, complex (mechanical, biochemical, optical) systems will function more reliable than under in situ conditions. Since several FerryBox systems already provide full-automated cleaning and antifouling devices, the problem of bio-fouling, that often prevents the reliable, quality-assured measurements over longer time periods, can be neglected.
Despite of these advantages the number of sensors used today operational on FerryBox systems is limited and often limited to physical parameters. Some of the reasons are:
- The sensor/instrument works but sometimes needs human interaction and therefore fails under completely unattended operation.
- The sensor/instrument needs too much personal for calibration, check and maintenance prior and after its deployment
- The sensor/instrument’s long-term stability is poor.
Therefore, there is a strong need for initiatives to prompt science and industry for developing “FerryBox instruments” in order to achieve the goals from the Marine Strategy. Once these developed instruments work and are accepted by the community this could be as well a starting point for the industry in the future to improve these sensors for in situ application and therefore expand their markets.
The following summary lists contains sensors that are already in (operational) use, sensors that function in the lab and new sensors on the horizon. The comments are results from the EU-FP5 Project “FerryBox”, from discussions within the “FerryBox Community” and from investigations in the Helmholtz-Zentrum Geesthacht.
Reliable standard sensors. No problems, if placed near the water inlet
Reliable conductivity sensors with long-term stability available. However, only few flow-through systems for small FerryBox systems.
Sensors exist, either optical (“optodes”) or with membranes (“Clark-type”) that are robust and stable over some months.
Standard sensors with and without mechanical wipers exist. However, only few flow-through systems for small FerryBox systems.
For this task there exist fluorometers. However, there is a high dependence on physiological parameters of the phytoplankton and prior illumination that influence the signal (uncertainty for chlorophyll-a concentration : up to 100 %!). New experimental systems that measure the optical absorption (PSICAM) do not have these restrictions, but are more complex to handle and need to be cleaned very thoroughly. High potential to measure simultaneously other algal pigments.
Even if relative stable pH electrodes exist their long-term stability for seawater is not enough. Optical systems have the potential to reach the accuracy and long-term stability needed for measurements within the carbon cycle (=0,01pH). The complex handling and necessary temperature stabilisation limited the application to labs on research ships. High demand for pCO2-budget estimations.
There exist instruments that measure the optical absorption of seawater and from this calculate the nitrate concentration. The algorithms should be improved. However, due to high absorption of bromide in seawater the sensitivity for nitrate is very limited.
All instruments use standardised or non-standardised wet chemistry and different more or less complex hydraulic/mechanical process control. Due to mechanical or hydraulic failures the reliability is not very high and frequent maintenance is required (Remark: Different approaches with electrochemical devices (ISFET etc.) can principally not work due to the very low nutrient concentrations in seawater). There is a very high demand for small, reliable and accurate nutrient analyser!
The differentiation of algal groups, e.g., diatoms, green algae, dino flaggelates and cyanobacteria is important for many applications. Instruments exists that use spectral fluorometry (excitation at different wavelengths) for this task. However, due to the complexity of the algal pigments (dependence on algae species etc.) this methods is only semi-quantitative and has to be setup for the specific sea area. Most instruments are too insensitive for open ocean measurements. A promising approach could be the measurement of absorption instead of fluorescence (see above).
Phytoplankton and zooplankton by morphology
Instruments are available working with two principles 1) Flow-cytometry and 2) Flow-through video/photo. Both systems are too complicated to operate unattended on FerryBoxes (huge amounts of data to be evaluated manually after the cruise). They need major improvements in order to work in a reliable way.
The major problems are:
a) The size range is two small to cover the whole spectrum of plankton,
b) the long-term stability (unattended) is poor.
However, the most important demands are the development of suitable software to first identify algal species from the videos/photos and make a reliable classification and second to improve the relationship between morphology and algal groups.
Algal species by genetic sensors
Different biochemical/genetic methods exists in the lab (antibody reactions, nucleic acid biosensors etc.) with sophisticated electrochemical/optical detection systems. Few systems are already semi-automated but still need water sampling, filtration and cell disruption. These very high specific methods (alga species but also algae groups) have a very high potential and are extensively used in medicine and biochemistry. These methods should be promoted for fully automation and use on FerryBoxes.
Organic micro-pollutants by genetic sensors
The methods summarised above could as well be applied for the measurement of toxic organic micro-pollutants. At the time being the sensitivity is not enough for marine applications.