Diatoms as indicator of climate change over the last 200+ years at Lake Tanganyika,
Kigoma, Tanzania
Student: Yovita Mallya
Mentor: Curt Stager
Introduction
Diatoms are single-celled microscopic organisms belonging to the class Bacillariophyceae. They feature a
distinctive highly ornamented cell wall composed primarily of silica. Each cell wall or frustule is made of
two valves and the belt-like elements that hold the valves together, called a girdle. In ecological studies of
diatom communities, the valves are used as a taxonomic tool. Systematically there are two groups of
diatoms, the radially symmetrical centric diatoms and the bilaterally symmetrical pennate diatoms. Diatoms
may be planktonic, benthic or facultative planktonic. The majority of planktonic species are centrics. The
study of diatoms can be an important tool in the reconstruction of the paleolimnology and paleoclimate
conditions of large lakes since diatom communities are sensitive to climate induced changes in lake
conditions. Furthermore, diatoms can be well preserved in lake sediments due to their siliceous cell walls
(Cohen, 2003). The present study is aimed at the reconstruction of climatic changes of Lake Tanganyika for
the past to 200+ years. This reconstruction was accomplished by studying the diatoms in core LT03-03
taken from Kigoma Bay, Lake Tanganyika.
Field work
The core studied (LT03-03) was taken in the Kigoma Bay at 112 m water depth in July 2003 (see Geo
Intro, Fig. 1). The sample site was chosen so that is was isolated from sediment influx. Total core length
was 51cm and the top sediments were undisturbed. Coring was done during the cruise of the Geo-team on
the M/V Maman Benita using a short gravity corer.
Laboratory work
Preparation of diatoms:
Core LT03-O3 was sub-sampled every centimeter. The mud was taken from the core and mixed well.
Afterwards, a small portion of it was taken by using a small spatula and put into a test tube. This was done
for every 2 cm between 0 and 20 cm, and every 5 cm for the rest of the core. Water was added, followed
by 1 ml of hydrogen peroxide, and mixed well. The tubes were put into a beaker half-filled with water and
heated for one hour on a hot plate in order to clean the diatoms from organic matter. Samples were cooled
to room temperature and centrifuged for five minutes to obtain a concentrated diatom solution free of
peroxide. This solution was re-suspended using clean water. One ml of the suspension was distributed
evenly over a cover glass, which was left to dry on a hot plate and then mounted in Permount on the slide.
Diatom analysis was performed using a Leica light microscope at a magnification of 1000x. The different
diatom groups were identified to genus level. The identification was done using a taxonomical work on the
diatoms of Lake Tanganyika (Cocquyt 1998) and the assistance of Christine Cocquyt. A total of 200 valves
were counted per slide if the diatom concentration allowed it. The complete valves was counted as well as
the broken ones if more than half of it was present including the central raphe endings. For 'long'
Nitzschia's however, the broken ends were counted and their numbers divided by two to obtain the real
number of that group. During the diatom investigation sponge spicules were also counted.
The numbers of diatoms were transformed into percentages for convenience.
Results
Nineteen diatom genera were identified and are listed in Table 1.
Appearing at 5 cm, 'long' Nitzschia's dominate the rest of the core from 10 cm on with 65% of the total
relative abundance (Fig.1). The majority of the observed diatoms are pennates, which include Pinnularia,
small 'Nitzschia', Cymbella, Navicula, Caloneis and Gomphonema as the most important. These genera can
be divided in two major groups according to their ecology: benthics and pelagics. The benthic diatoms
dominate the bottom of the core, whereas the pelagic ones (long Nitzschia's) dominate the top of the core.
Five phases could be distinguished in the core as shown in Fig.2.
Starting at the bottom of the core, phase one (25 cm to 50 cm) is dominated by 'long' Nitzschias. Their
abundance increases from 65% at the bottom of his zone to 77% at the top. Benthics, on the other hand,
reached their highest relative abundance (35%) at the bottom (Fig. 3). They became less abundant at the top
of this zone at 25 cm depth, decreasing from 35% to 23%. Moreover, the highest diatom diversity was
observed in this zone of the core studied. Sponge spicules were less important and never exceeded 10% of
the total relative abundance.
Phase two (at 20cm) is dominated by 'long' Nitzschia's (90%). Benthic diatoms (10% of the relative
abundance) were less diverse than in the first phase. No sponge spicules were observed.
Phase three (12 cm to 19 cm) is characterized by an increasing importance of Pinnularia from the bottom
of this zone to the top. 'Long' Nitzschias are still dominant (between 47 and 85 %). Sponge spicules
increased from the bottom to the top, with a maximum of 25 %.
Phase four (10 cm) is composed of only two genera, 'long' Nitzschias and Pinnularia. Both are equally
represented (50 % of the relative abundance).
Phase five (at 5 cm) is dominated by sponge spicules. No diatoms were observed in this layer.
The general trend of carbonates was an increasing one from the bottom up to the top of the core. A detailled
study was undertaken by James Apolkarpi (this volume).
Discussion
High carbonate values at the top of the core (James Apolkarpi, this volume) may be due to twentieth
century global warming. Tropical lakes, and especially Lake Tanganyika, are very sensitive to global
climatic changes. Besides temperature increases, less wind induces lower mixing and causes more stability
in this meromictic lake.
There are several possible explanations for the presence of numerous spicules in the top nine centimeters of
the core. It might be due to a lower lake level, as the sponges are typical benthic organisms. However, the
high percentage of broken spicules in phase five indicates that they have undergone transport. This
transport can be caused by higher input from the rivers, which is an indication of higher rainfall. The
absence of diatoms points to a recent decrease in lake productivity.
The presence of Pinnularia in phase four is an indication of higher oxygen concentration in the lake or
input from tributaries. This is due to a higher circulation in the epilimnion and a higher rainfall,
respectively. The low relative abundance of benthic diatoms and the predominance of planktonic genera
('long' Nitzschias) in phase three show that during this period winds were weaker, hence there was low
stirring/mixing of the lake. It might also indicate an increase in lake level, probably due to heavy rainfall,
but that implies that the littoral zone must be far away from the sampling site.
Phases one and two showed a decrease in pennate diatoms towards the top. The high diversity in benthic
diatoms on the bottom of the core (50 cm) might be due to dry conditions and decrease in lake level (low
stand). Low stand of the lake facilitate light penetration trough the bottom hence growth of benthic
diatoms. High lake productivity in the littoral zone may also contribute to high diversity of diatom
communities.
Conclusions
The decrease in lake productivity for the recent decades is influenced by global climatic change, especially
the increase in temperature. The diatom results of this core study have shown two high stands of the lake
level, influenced by climatic changes. Indications for a decrease in monsoon wind strength for the recent
years were found.
Future research
More study on sediments and effective dating of the cores by means of plant remains or AMS 14C of bulk
material is needed. This is important for reconstructions of paleoclimates and conservations of unique lake
communities.
Acknowledgements
Special thanks are given to the Nyanza Project for funding the project. Also thanks to my mentor Curt
Stager and co-mentor Christine Cocquyt for their support during my research work and to Kiram Lezzar
and Christine Gans for their assistance during the fieldwork.
References
Cocquyt C. 1998. Diatoms from the Northern Basin of Lake Tanganyika. Bibliotheca Diatomologica 39:
276 pp, 56 plates.
Cohen A.S. 2003. Palaeolimnology; The history and evolution of lake systems. Oxford University Press.
Nyanza Project Report 2000
11%
6%
15%
Pinnularia
Nitzschia(long)
Nitzschia(small)
sponge spicules
Benthic
3%
65%
Figure 1: An overview of diatom and sponge spicule count for core LT03-03 from Kigoma Bay.
Diatom Variation with Depth
100.0%
80.0%
Sponge spicule
Pinnularia
Benthics
Nitzschia(small)
Nitzschia(long)
60.0%
40.0%
20.0%
0.0%
0
5 10 11 12 13 14 15 16 17 18 19 20 25 30 35 40 45
Depth (cm)
Figure 2: Distribution of diatoms as observed for core LT03-03 from Kigoma Bay.
Figure 3: Overall count of diatoms and sponge spicules indicated in percentages along core LT03-03 from
Kigoma Bay.
Table 1:
DIATOM ASSEMBLAGE CORE LT03-03 FROM KIGOMA BAY
Genus name
Nitzschia(long)
Nitzschia(small)
Cymbella
Navicula
Amphora
Caloneis
Diploneis
Achnanthes
Anomoeoneis
Gomphonema
Gomphonitzschia
Gomphocymbella
Cocconeis
Gyrosigma
Cymbellonitzschia
Pinnularia
Surirella
Cymatopleura
_Rhopalodia
Cyclotella
Sponge spicule
Core depth (cm)
5
10
12
0
50
47.1
0
0
0
0
0
0
0
0
0
0
0
5.88
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
50
11.8
0
0
0
0
0
0
0
0
0
0
0
0
100 0
35.3
13
59.3
0
3.7
0
0
0
0
0
0
0
0
0
0
0
0
18.5
0
0
0
0
18.5
14
50
0
0
0
0
0
0
0
0
0
0
0
0
0
0
20
0
0
0
15
15
15
76.5
0
2.94
0
0
0
0
0
0
0
0
0
0
0
0
2.94
0
0
0
0
17.6
16
59.6
0
8.51
4.26
0
0
0
0
0
2.13
0
0
0
0
0
2.13
0
0
0
0
23.4
17
78.9
0
5.26
8.77
0
0
0
1.75
0
1.75
0
0
0
0
0
0
0
0
0
3.51
0
18
64.7
3.92
15.7
3.92
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
11.8
19
85.1
1.15
3.45
5.75
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
2.3
2.3
20
92.3
1.54
2.05
0
0.51
1.03
0.51
0
0
1.54
0.51
0
0
0
0
0
0
0
0
0
0
25
80
4.2
1.4
2.8
0.7
0
0
0
0
0.7
0
0
0
0
1.4
0
0.7
0
0
0
8.3
30
78.3
4.82
0
3.61
1.2
2.41
0
0
0
0
0.6
0.6
0.6
0.6
0.6
0
0
0
0
0
6.63
35
75
8.11
2.7
4.05
0.68
0
0.68
0.68
0
0.68
0
0.68
0.68
0
0
0
0
0
0
0
6.08
40
73.2
4.58
3.27
7.19
0.65
0.65
1.96
0.65
0.65
0
0
0.65
0
0
0
0
0
0
0
0
6.54
45
71.84
8.738
2.913
2.913
0
0
0
0
0
0.971
0
0
0.971
0
0
0
0
0.971
0
0
10.68
50
68.3
11.8
3.82
10.3
0
0
0.38
0.38
0
0.76
0
0
0.38
0.38
0
0
0.38
0
0.76
0
2.29