Poland - IOW

Discussion 50
concentrations. OM was not measured in the sediments of the sampling site in Hel. However,
since they consist of sand OM concentrations should rather low. Shallow sandy areas in the
Baltic sea contain only about 0.1 % particulate organic carbon (POC), which is a part of OM
(Leipe et al., 2010) and thus a low content of 0 to 1 % organic carbon in coastal sandy
sediments seems reasonable (Deutsch et al., 2010).
DON concentrations in pore water were mostly between 15 and 20 µmol L -1 in the pore water
with some peaks of 40 to 50 µmol L -1 . Overall they did not change significantly with depth.
Burdige & Zheng (1998) found a similar pattern with similar concentrations in marine
sediments consisting of silty sands with low OM content. They found higher DON
concentrations (>100 µmol L -1 ) in sediments with higher OM content. DON concentrations
were very similar on all sampling sites independent from SGD and they did not increase
significantly with depth. Therefore SGD does not seem to be a significant source of DON, but
DON seems to originate from the water column, which showed similar concentrations as pore
water. Some experimental studies showed that anoxic conditions led to a decrease in pore
water DON concentrations (Enoksson, 1993; Hansen & Blackburn, 1991). It has therefore
been assumed that, under anoxic conditions, the mineralization of dissolved organic matter
(DOM) may be less efficient (Hansen & Blackburn, 1991). This could be an explanation why
the anoxic submarine groundwater in Hel did not contain higher DON concentrations.
4.1.5 DIN in the SGD
Only in the upper 10 cm of the sediment nitrite-nitrate was found mainly not exceeding 5
µmol L -1 . Nitrite-nitrate was overall decreasing with depth to 0 µmol L -1 . The absence of
nitrate was not surprising since the SGD on the Beach of Hel was found to have highly
reducing conditions (Vogler et al., 2011). Nitrate cannot occur in reducing conditions together
with high ammonium concentrations and also nitrite was not found in the pore water either.
The SGD is therefore no source for nitrite or nitrate.
Samples from groundwater lances show increasing ammonium concentrations with increasing
depth correlating negatively with salinity. Therefore SGD at Hel is clearly a source of
ammonium. Concentrations increased to amounts higher than 200 µmol L -1 . During
November 2009 they even reached 5.8 mmol L -1 . Ammonium was therefore the dominant
nitrogen species in the SGD. Studies on groundwater which also found high ammonium
concentrations suggest a contamination (Böhlke et al., 2006). Investigations of Chincoteague
Bay, Maryland, USA, also revealed ammonium as one major nitrogen species in both fresh
and saline sub-estuarine groundwater next (Bratton et al., 2009). Ammonium concentration