Poland - IOW

Material and Methods 19
Due to low NH + 4 concentrations in water column samples the diffusion method was not
suitable and therefore the NH + 4 distillation method after (Velinsky et al., 1989) was applied.
As explained for the diffusion method MgO is added to the sample to shift the equilibrium of
NH + 4 -NH 3 towards NH 3 . During distillation the NH 3 evaporates from the sample and is
collected in a measuring cylinder containing a weak acid which traps NH 3 as NH + 4 again.
This distillate is then mixed with a molecular sieve which collects all NH + 4 within the solution.
The molecular sieve was filtered onto a precombusted GF/F filter (450 °C for 3 h) and dried at
60 °C over night. The molecular sieve was removed from the filter and transferred into a tin cup.
For the benthic chambers 4 to 6 δ 15 N values are missing due to low NH + 4 concentrations and
sample volumes for the samples from the start of incubation.
POM and Organisms
Benthos samples were dried and homogenized with mortar and pestle before measurements
on the mass spectrometer. Small animals were measured as a whole directly after drying.
From calcareous organisms like mussels and barnacles the shells were removed and the
remains acidified prior to measurements. Filters for particulate organic matter (POM) and
plankton samples were dried and measured directly. All samples were transferred into tin cups
for analyses of δ 15 N, δ 13 C and C/N values.
2.5 Mass spectrometric analyses
Tin cups were pressed into pellets and measured in an elemental analyser (flash EA) coupled
to an isotope ratio mass spectrometer (IRMS, Finnigan Delta S). The samples are combusted
with additional oxygen at 1020°C (flash combustion) converting all inorganic and organic
nitrogen and carbon into gases. The carrier gas helium transports these combustion gases
through a reduction furnace (650°C) where NO x is reduced to N 2 . To remove water before
transporting the gases to a gas-chromatographic column which separates N 2 from CO 2 , the
helium flow passes a water trap. A subsample (~ 1%) is transferred into the IRMS were the
gases are ionized and accelerated before entering the magnetic sector. The ions are separated
depending on their charge ratio and hit a collector. A detector transforms the electric signal.
All isotope values are noted relative to reference gases (N 2 , CO 2 ,):