Rapp. Comm. int. Mer Médit., 36,2001
194
Introduction
The Rogoznica Lake is a small, intensely eutrophied saline lake, sit-
uated on the eastern coast of the Adriatic Sea. It has an area of about
5300 m
2
and a maximum depth of 15 m. The Lake has no visible con-
nection with the surrounding sea, but lake tides are detectable on the
cliffs indicating that underground water connection exists. Despite
permanent water exchange between the Rogoznica Lake and the sur-
rounding sea through the porous karst, anoxic conditions prevail in
deeper layers of the Lake, probably due to remineralization of organ-
ic matter produced in the period of intensive primary production.
Anoxic deep water is rich in sulfur (up to 900µM), especially in the
form of sulfide or elemental sulfur (1).
The Rogoznica Lake is particularly interesting from biological
point of view because of its specific ?ora and fauna. Specifically, in the
Lake there are living some phytoplankton species that are relatively
rare in surrounding sea (Hermesinum adra
t i
c u m, Eunotia sp.,
Prorocentrum arcuatum) and only one copepode species (Acartia ital-
icaSteuer) (2, 3). Although, very poor qualitatively, phytoplankton
and zooplankton communities are quantitatively very rich.
Furthermore, in some periods a monospecific bloom appears in the
Lake and probably contributes to the mass mortality that occasionally
occurs in the Lake. The last mass mortality of the organisms in the
Lake occurred in September 1997. The presence and distribution of
total and sulfur photoautotrophic bacteria in the condition of mass
mortality of the organisms in the Lake (holomictic period) and
throughout the time scale of the anoxic water renewal (period of strat-
ification) were studied.
Materials and methods
Measurements and samplings were done biweekly from the begin-
ning of October until the end of December 1997 at 2 m intervals from
the surface to the bottom (12 m).
The thermohaline features were measured by CTD multi probe
IDRONAUT OS316.
Enumeration of total number of bacteria was made by epi?uores-
cence microscopy using the standard AODC technique (4). Abundance
of phototrophic sulfur bacteria were estimated from fresh samples
using auto?uorescence microscopy (5).
Results and discussion
The holomixis observed right after mass mortality in the Lake was
followed by development of anoxic conditions with proportionally
high concentration of reduced sulfur compounds (6,2). Maximum
number of total bacteria has been occurred during the holomixis right
after the mass mortality of the organisms in the Lake due to the inten-
sive processes of remineralization of dead organisms. An average of
total bacteria was 2.2x10
7
cells ml
-1
with maximum values in the sur-
face layer and minimal values at the bottom. The presence of sulfur
bacteria has been observed in relatively high concentration throughout
the water column as well (average 3.7x10
9
- cells ml
-1
), but their verti-
cal abundance gradually increased from the surface to the bottom.
Presence of anaerobic photosynthetic sulfur bacteria could explain
proportionally high abundance of elemental sulfur in anoxic water col-
umn of the Lake since the bacteria partially transform sulfide to ele-
mental sulfur (1).
Two weeks after the mass mortality of organisms occurred (sam-
pling of 14
th
October), the values of total bacteria drastically felt to an
average of 5.11x10
6
cells ml
-1
and contemporaneously the number of
sulfur bacteria decreased up to an average values of 0.49x10
6
cells
ml
-1
.Vertical distribution of both total and sulfur bacteria was similar,
i.e. they were uniformly distributed through the whole water column.
It is significant that the presence of sulfur bacteria was evident
throughout the water column in high concentrations until 27
th
October,
that was during the holomixis (period of the low oxygen concentra-
tion). According to the salinity, temperature and oxygen vertical pro-
files in the Lake after 27
th
October, stratification of the water column
progressed and vertically stratificated water column was kept until the
sampling of 29
th
December. In that period the concentration of oxygen
was increasing in the Lake, specially in the surface layers what is a
direct result of phytoplankton community activity (6). With an
increase of oxygen content of the Lake the number of sulfur bacteria
drastically decreased and their vertical distribution was changed. In
the upper oxygenated layers where the oxygen saturation is particu-
larly high, there were no sulfur bacteria at all, whereas in the layer
under the thermocline they were far less presented than in the period
of holomixis (average value of 0.02x10
6
cellsml
-1
).To resume, with
the increasing of oxygen content the abundance of sulfur bacteria
decreased and their vertical distribution changed depending of the ver-
tical distribution of O
2
and H
2
S. Similar results have been established
by Cohen et al.(7), studying the distribution of sulfur photosynthetic
bacteria in Solar Lake, and Mazumder and Dickman (8) in Crawford
Lake suggesting that the development of the photosynthetic sulfur
bacteria communities was closely related to the simultaneous estab-
lishment of an anoxic zone with increasing H
2
S concentrations toward
the bottom.
According to the results shown above, it could be stressed the role
of autotrophic sulfur bacteria in the process of photosynthesis during
holomyxis (anoxic period). In such conditions an autotrophic sulfur
bacteria have very important ecological advantage regarding their
capability to utilize high concentrations of H
2
S for the process of pho-
tosynthesis.
References
1. Ciglenecki, I., Kodba, Z. and Cosovic, B.,1996. Sulfur species in
Rogoznica Lake. Mar. Chem.53, 101-111.
2.Vilicic, D., Marasovic, I. and Kucpilic, G.,1996/97. The Heterotrophic
Ebridian Micro?agellate Hermesinum adriaticumZach, in the Adriatic
Sea.Arch. Protistenkd. 147, 373-379.
3. Krsinic, F., Caric, M., Vilicic, D. and Ciglenecki, I., 2000. The
calanoid copepod Acartia italica Steuer, fenomenon in the small saline
Lake Rogoznica, (eastern Adriatic coast) J. Plank. Res. 22: 1441-1464.
4. Hobbie, J.E., Daley R.J. and Jasper, S.,1977. Use of Nucleopore filters
for counting bacteria by ?uorescence microscopy.Appl. Environ.
Microbiol.33, 1225-1228.
5. Macisaac, E.A. and Stockner, J.G.,1993. Enumeration of Phototrophic
Picoplankton by Auto?uorescence Microscopy. In: Kemp, P.F.et al.,
(eds), Handbook of Methods in Aquatic Microbial ecology. Lewis
Publishers, USA, pp. 176-187.
6. Baric, A., Grbec, B., Kuspilic, G., Marasovic, I. and Nincevic, Z.,2001.
Physical, chemical and biological characteristics of a small saline lake
during meromictic and halomictic conditions. Scientia Marina (in press).
7. Cohen, Y., Krumbein, W.E. and Shilo, M.,1997. Distribution of
photosynthetic microorganisms and primary production. Limnol.
Oceanogr.22, 609-620.
8. Mazumder, B.A. and Dickman, M.D.,1989. Factors affecting the
spatial and temporal distribution of phototrophic sulfur bacteria. Arch.
Hydrobiol.116, 209-226.
Rapp. Comm. int. Mer Médit., 36,2001
TOTAL AND SULFUR BACTERIA DURING HOLOMICTIC PERIOD AND PERIOD OF STRATIFICATION IN
THE SALINE ROGOZNICA LAKE (CENTRAL ADRIATIC)
Nada Krstulovic* and Mladen Solic
Institute of Oceanography and Fisheries, Split, Croatia - krstulovic@izor.hr
Abstract
Establishment of the anoxic water column with increasing H
2
S concentrations in the Lake was followed by the development of a dense
population of photosynthetic sulfur bacteria. The data presented in this paper discuss the relationship between photosynthetic sulfur
bacteria and total bacteria during the holomixis and during the period of stratification in the Lake.
Key words: total bacteria, sulfur bacteria, holomixis, stratification
