Environmental Engineering Reference
In-Depth Information
4.1 The Oxic Water Column as a Source of Methane to
the Atmosphere
Methane concentrations in the non-seep oxic water column (055) decrease
from 50 nM at around 76 m to concentrations of
<
10 nM at the sea surface.
At the seep site (038) methane concentrations are 10 times higher above the
seafloor in 80 m with values of up to 550 nM (Fig. 3). However, due to aerobic
consumption of methane, surface water methane concentrations (uppermost 10
m) at the seep site are only 1.6 times higher than at the non-seep site with values
of up to 16 nM. Oxidation rates at the seep site (up to 1.6 nM d
−
1
) were on
average approximately 30 times higher relative to the reference station (0.001
to 0.05 nM d
−
1
). The relative turnover was 97 % at the seep and 87 % at the
reference site. The oxidation of methane could also be traced using the stable
carbon isotopic composition of the methane at the seep station. Here, a constant
increase in the δ
13
C
CH
4
values at the seep station from -67 ‰ VPDB (bottom)
to -49‰VPDB (5 m below water surface) over the entire water column clearly
shows the preferably usage of light
12
C methane by the aerobic methanotrophs
(Fig. 3).
The percentage of aerobic methanotrophs from the total cell number deter-
mined by DAPI varied between 0.1 to 4.5 % at both sites. Intriguingly, only
methanotrophic bacteria of type I were detected with the exception of one
sample from 100 m water depth at the seep site, where type II methanotrophs
represented 2 % of total cell counts. This is at odds with earlier studies reporting
that methanotrophs of type I and II are equally abundant in the water column
[12]. Furthermore, Gal'chenko and coworkers found methanotrophs of type I
and II representing up to 10 % of the total cell counts in the water column,
whereas our findings indicate that they represent less than 5 % of the total cell
counts (see results for possible explanation).
Comparing surface water methane concentrations with the methane con-
centration expected assuming atmospheric equilibrium [51], we find that the
surface water at the seep and reference sites is 3 to 5 times supersaturated with
respect to methane and therefore both stations act as a source for atmospheric
methane. This is in agreement with other investigations that have measured
methane fluxes from the Black Sea water column to the atmosphere [1, 42].
The latter authors found an air-sea methane flux above a shallow seep area of
0.96-2.32 nmol m
−
2
s
−
1
that is 3 times higher than calculated for the surround-
ing shelf (0.32-0.77 nmol m
−
2
s
−
1
) and 5 times higher than assessed for open
Black Sea waters (water depth
>
200 m, 0.19-0.47 nmol m
−
2
s
−
1
). Hence, we
can conclude that the gas seeps of the upper slope and shelf, where methane
emanates in to the oxic water column, contribute substantially to the methane
emission. The total number of active seeps at the upper slope and shelf is
