Environmental Engineering Reference
In-Depth Information
Thiomargarita
and
Beggiatoa
are also abundant south of Walvis Bay between
24
◦
S and 25
◦
30'S near Sylvia Hill and between Conception Bay and Walvis
Bay. The distribution maps represent quasi-averages over 7 years of observation.
It is important to note that significant intra- and interannual fluctuations in
abundance exist. These are integrated into the maps for the areas where stations
were revisited several times.
Field observations have consistently shown that
Beggiatoa
is absent from
areas with bottom water hydrogen sulphide.
Thiomargarita
, in contrast, can
tolerate hydrogen sulphide at least temporarily at millimolar concentrations
and has been found to survive in environments repeatedly containing bottom
water hydrogen sulphide [7]. This is confirmed by the distribution map of
Thiomargarita
and the areas where bottom water hydrogen sulphide has been
found on the Namibian shelf (Fig. 8c). The difference in distribution between the
two types of large sulphur bacteria may have to do with the slower metabolism
of
Thiomargarita
relative to
Beggiatoa
, when growing with nitrate. The large
size of
Thiomargarita
may allow them to store larger amounts of nitrate and
survive for longer time periods under adverse conditions.
5.4 Temporal Variation
We assessed the intra- and interannual variability at a selected station in shal-
low depth (28 m), with shallow gas saturation (SEDLAB Station 1). Between
May 2001 and May 2004, concentrations of porewater methane, hydrogen sul-
phide, and water column oxygen were determined nearly every two months
(Fig. 9a-c). At this sampling frequency marked fluctuations in oxygen levels in
the water column were detected, in concert with variations in methane concen-
tration and with fluxes of hydrogen sulphide. There was no apparent periodicity
in the observed fluctuations, and except for one period oxygen concentrations
in the bottom water were below 22 µM. The most conspicuous feature was
the extreme oxygen depletion over the whole water column, with drops in sur-
face concentrations of oxygen to values as low as 67 µM. A relatively stable
chemocline was only present during the austral summer 2001/2002 (October
2001 until May 2002). During this period, two low-oxygen periods occurred in
December 2001 and in May 2002. Oxygen-rich water intruded after May 2002,
but was interrupted by another low-oxygen period in October 2002, which was
also terminated abruptly. Subsequently, oxygen levels dropped in the bottom
water and the chemocline rose gradually throughout the year 2003 reaching the
shallowest depth in March 2004. Data acquisition stopped in May 2004 with
an apparent return to better ventilated conditions.
During time periods when a shallow chemocline was present, methane con-
centrations increased abruptly in the sediment so that the depth of methane
