Within the HYPOX project, marine environment monitoring activities in locked systems involved observatories located in different areas around the world. The observatory implementation, observation strategies and main obtained conclusions are explained below.
Koljö fjord Observatory (Sweden): this compact, movable and flexible cabled observatory aimed to provide a platform for carrying out long-term in situ chemical sensing tests. In case of oxygen measurements, this allowed finding long-term trends and assessing causes and mechanisms for oxygen depletion and renewal. The area offered high variability in environmental conditions, like temperature (-0,5-25ºC), salinity (20-34), dissolved oxygen (0-150% saturation), pCO2 (100-4000 μatm), nutrients concentration, fouling episodes, etc. Due to this variability, a lot of parameters were monitored:
- Horizontal currents in the entire water column at 1 m resolution
- Acoustic signal strength in the entire water column at 1 m resolution
- Water level measured at two positions
- Oxygen measured at 5 depths
- Salinity measured at 5 depths
- Temperature measured at 11 depths
Loch Etive Cabled Observatory (Scotland): Loch Etive is a 30km long sea loch which has three inputs of fresh water and is opened to the ocean at the entrance. Bottom water is isolated from top by marine water, leading to a gradient between the lower basin of the loch (well mixed and oxygenated) and the upper basin (hypoxic). Overturning events just occur occasionally (according to models, near once every 16 months). Measurements were made in order to resolve overturning frequencies and subsequent oxygen dynamics of Loch Etive. A sensing instrument equipped with conductivity, temperature and dissolved oxygen (DO) sensors as well as a Doppler Current Meter, was positioned at 15m depth. Also, an Aanderaa RDCP600 instrument equipped with Conductivity, Depth and Temperature (CTD), Dissolved Oxygen (DO) and a profiling Current meter was positioned at 125m depth.
The deployment period of the observatory lasted 10 months. Within this period three overturned events were identified by sharp and rapid increases in the oxygen concentration, among other symptoms. Even though a clear trend of oxygen decline was observed , bottom waters didn’t show hypoxic conditions because of these overturning episodes..
Amvrakikos Gulf (Greece): this is an embayment with a maximum depth of 65 m, which is connected to the Ionian Sea through a narrow channel (the Preveza Straits). The Gulf has a positive water balance, with inputs from rivers and precipitations and an output due to evaporation. The main objectives of these studies were to obtain information about the water column structure in the Gulf on seasonal basis, the water circulation along the Preveza straits, the surface sediments (through isotopic analyses) and the salinity and density gradient from sea-surface to bottom.
Four oceanographic surveys in Amvrakikos Gulf took place from 2009 to 2011. During these periods, measurements of physicochemical parameters such as temperature, salinity, dissolved oxygen, pH, Oxidation Reduction Potential (ORP), turbidity, dissolved CH4 and H2S were carried out on 19 sites using two multiparameter devices, a methane sensor and a H2S sensor. Additionally, Gulf currents were measured in two locations of Prevenza Straits. Results showed that the water column is divided into two major homogeneous layers, a brackish surface and a saline bottom layer, which seem to control the vertical distribution of dissolved oxygen in summer. In winter the Western basin appears to be well oxygenated. Similarly, the pH distribution in the water column is controlled by this two layer structure.
Aetoliko Lagoon: this Greek lagoon has a maximum depth of 30 m and has a deeper basin in the north zone. Measured parameters where similar to those from Anvrakikos Gulf. Data confirmed a two-layer structure with dissolved oxygen continuously decreasing below 7m (June and November) or 12m (February and April). On the other hand, high dissolved Methane and SH2 concentrations were found in the whole lagoon, with a gradient increasing concentration from surface to seabed.
Katakolo Bay: The GMM (Gas Monitoring Module) benthic observatory was used for monitoring O2 in this thermogenic gas seep area. More exactly, long-term monitoring was carried out inside the Katakolo harbour, due to the vast seepage occurring in the harbour that enables studying O2 versus gas seepage. Several oxygen, methane and hydrogen sulphide sensors with a unique time reference and a dedicated data-acquisition system were used. Results revealed the competition between oxygen and methane through methane peaks associated to oxygen drops.
Lake Zurich and Lake Lugano (Switzerland): Lake Zurich data’s set included measurements for O2, temperature, SiO2, CO2, pH, conductivity, NH4-N, NO2-N, NO3-N, Cl, PO4 and P, but only O2, T, pH, conductivity and turbidity was measured in Lake Lugano. Profiles were taken at a fixed location in the deepest part of the lakes.
Data were spatially and temporally standardized before data analysing; thus, interpolation and averaging were applied to obtain samples at evenly spaced depths and time intervals. In this way, seasonal hypoxia episodes could be identified.
Lake Zur (Switzerland): in this place, a newly developed Profiling Analyzer (PIA) for determining ultra-low oxygen concentrations was applied above the oxic-anoxic interface in the water column. In this interfaces (which represent zones of intense biogeochemical cycling in fresh water bodies) it’s very difficult to determine fluxes across the interface and possible redox cascades because of the difficulty for setting the exact localisation of the anoxic front and the low sensor resolution for low oxygen concentrations (below 1 μmol L-1). By using this new system, the oxic-anoxic interface could be located on a spatial scale of centimetres, with a detection limit between 6 and 12 nmol L-1.
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