TEST SET-UP AND ENVIRONMENTAL CONDITIONS

The test principle has been developed through several similar projects (ref. NIVA reports). The idea is to establish a series of replicate experimental systems (Figure 2.1), which are maintained in easily accessible indoor basins (Figure 2.2). Each system is referred to as a benthic chamber and the biodegradation environment inside the chambers is made to resemble the conditions at the North Sea seabed as closely as suitable to the purpose.

The chambers had an area of 48 x 48 cm and a height of 35 cm. The four walls of the chamber was dismounted and brought on board FF Trygve Braarud, UiO, for sampling of test communities at 200 m depth in the Oslofjord nearby Solbergstrand. The acrylic chamber walls fit tightly into the steel box of a USNEL box corer. The box corer with the chamber walls were lowered to a penetration depth of 25-30 cm into the seabed. By reversing the winch the drag of the wire releases a spade which digs into the sediment below the sample before bringing the entire 48x48x30cm section of the seabed to the surface. Onboard the boat, a bottom plate was inserted between the spade and the sample before

Oslofjord SW 60 m depth

Pump Water bath Chamber water

Perforated pipe Sampling cell

Cuttings layer Test sediment

Drain

Figure 2.1. Schematic drawing of benthic chamber used for experimental study on

biodegradation of pseudo oils on cuttings. Each chamber had a surface area of 50x50cm and a depth of 35 cm.

Outer Oslofjord

Figure 2.2. Schematic drawing of test set-up and water flow through the test basin. In the present test a second peristaltic pump and a third tray with another four chambers was added to the set-up shown above.

removal of the chamber from the steel box. A temporary lid was placed on top of the chamber and textile bands were strapped around each chamber to prevent the overlying water from spilling during handling and transportation.

At the Solbergstrand laboratory the boxes were placed at their respective locations in the trays The sediments appeared very similar with regard to colours (shades of grey and brown) and approximate number of animal structures. Typical depth between the rim of the box and the sediment surface was 15 cm, corresponding to a water volume of 34 l. Two boxes differed with regard to volume of

overlying water. Those two boxes which had water volumes of 27l and 40l, respectively, were chosen for control purposes. The remaining ten boxes were distributed at random in the three trays shown in Figure 2.2. The transportation lids were then replaced by tightly fitted chamber tops with outflow through a sampling cell, as shown in Figure 2.1.

As shown in Figure 2.2, two six-channel peristaltic pumps maintained separate flows of seawater from the header tank through each chamber. Thus, throughout the experimental period, the chamber water was continuously renewed with a turnover ranging from 12 to 32 hours, depending on oxygen consumption rates.

A laminar type internal circulation system was maintained by submersed, aquarium pumps driving water through a perforated pipe positioned along one side of the chamber. By timer-control, the

6

Figure 2.1. Temperature and salinity in header tank during experimental period.

Table 2.1. Test conditions in sediments and overlying water.

Parameter Status

Sediment type Marine, soft clay, transplanted from 200m depth “ size fraction<63µm 92-97%

Sediment redox state Oxygen always available at sediment surface Oxygen saturation 50-100% in overlying water

Mean temperature 7.69°C (n=371) Mean salinity 34.39 PSU (n=371) Current velocities up to ca 10 cm^.s^-1

Current generation 15 minutes every two hours

Illumination Dim to dark, bright light only during sampling and inspection

pumps were activated for 15 minutes every two hours. The pumps generated characteristic current velocities of 5-10 cm^.sec^-1. No visible resuspension of cuttings or sediments were ever observed to result from the internal circulation system.

Test conditions are summarised in Table 2.1. Temperature, salinity and oxygen concentration was recorded automatically, every 12 hours, on sensors located in the header tank. As shown in Figure 2.1 and Table 2.1, during the entire experimental period from 01.07.95 to 7.01.96, salinity and

temperature in the source water ranged 31.8-35.0 PSU and 6.2-11.3°C, respectively. As shown in Figure 2.1 an influence of more surficial watermasses occurred during the 90-130 days time interval.

The event was probably triggered by rather unusual weather conditions during the period. The

increase of water temperature was accompanied by a temporary increase of oxygen consumption rates in control chambers and in most of the treated chambers (Figure 3.7).

The extreme values of temperatures and salinities observed in the chambers during the 90-120 days period are unlikely to occur at the offshore seabed location. However, the difference between the

mean salinity and the salinity at depths in the North Sea will be negligible with regard to biological impacts. The temperature at the Brage sea-bed may be a few degrees lower than the experimental mean temperature of 7.7°C. Because of temperature adaptation, it is not clear that such a temperature difference will have any effect on the actual degradation rates.

Temperatures in the benthic chambers were kept within ±0,5°C of the source water. By flow

adjustments determined by oxygen consumption rates in each chamber, oxygen concentrations were maintained at concentrations 10-50% lower than the concentration in the source water.

Particle size fraction smaller than 63 µm was determined in samples of the top 3 cm layer of the two control chambers collected on day 1. Of the sample collected in CON 4, 97.1% of the particles were washed through the 63µm mesh size sieve, as compared to 92.1% of the sample from CON 10.