Relationship of ostracode abundance and taphonomy to environmental variables on
the Luiche delta platform, Lake Tanganyika, East Africa
Student: Jana M. Van Alstine
Mentors: Dr. Andrew Cohen & Dr. Kiram Lezzar
Introduction
Ostracodes are very common benthic invertebrates found primarily at the sediment-water interface. Their
abundance, widespread occurrence, and high preservation potential make them ideal tools for studies
linking past and present environments. Ostracodes have a bivalved carapace made of CaCO3 and chitin
(Thorp, 1999). After death, the valves are subject to the environmental stresses of the areas in which they
are transported and/or deposited, predominantly related to energy, water, and sediment chemistry. Studies
based on this linkage of valve taphonomy of modern ostracodes can, in principle, be used to interpret past
environmental conditions from core records.
In this study, subfossil ostracodes accumulated over the past 50 years in the surficial sediment were
analyzed in order to examine relationships between taphonomic and abundance variables, and
environmental factors. Ostracode valves were examined for redox staining, breakage, and abundance, and
compared against variables of depth, geography, proximity to river mouth, total organic and inorganic
carbon, and percent sand. Samples were taken from five northwest-southeast transects, shallow to deep
profiles, across the Luiche delta platform in the Kigoma region of Lake Tanganyika, Tanzania.
Lake Tanganyika in East Africa is a perfect “natural laboratory” in which to study the role of depositional
environments in affecting taphonomic variables in large lakes. The lake is long-lived with an extensive
sediment record that contains valuable information about regional climate, surrounding terrestrial
environments, biotic evolution within the lake, geology, and the hydrodynamics of the lake; this setting is
well suited for studies of potential paleoenvironmental indicators. Lake Tanganyika is located within the
western branch of the East African Rift Valley, 4oS - 8oS, bordered by the countries of Tanzania, Burundi,
the Democratic Republic of the Congo, and Zambia. It is a large, deep lake, running 600km from north to
south, and at 1500m of water depth is the second deepest lake in the world (Coulter, 1991).
Study Site
This study site for this project was the delta platform of the Luiche River, located on a platform margin
south of Kigoma, Tanzania. This delta platform was chosen as a study area because of its accessibility,
proximity to Kigoma, and also to possibly link this study with cores taken in 1999 by the Nyanza Project
(Castaneda,I., 1999) in the future. The laboratory work for this study was conducted in Kigoma at the
Tanzanian Fisheries Research Institute (TAFIRI).
Methods
Field Methods:
Samples were taken from aboard research vessels R/V Echo and M/V Maman Benita using a Ponar Grab
Sampler with maximum 600 cm2 sampling surface area. The top five centimeters of sediment were taken
and stored in sealed glass jars. Samples were taken from water depths ranging 9.4m to 131.0m in
approximately 10m depth intervals from five transects across the Luiche delta platform. GPS coordinates
of all samples were recorded. In shell lag samples there was very little sediment. As a result these samples
could not be used for volumetric abundance studies. For these samples the grab sampler was rinsed and the
water directed into a 106 micrometer sieve in order to catch the minor amounts of sediment to be used for
taphonomic variable percentages. The transects run northwest-southeast; from north to south the transects
are labeled one through five, two transects are located north of the river mouth, one in front of the river
mouth, and two south of the river mouth (for map, see figure 1, William E., this volume).
Laboratory Methods:
Abundance data - A quantity of preserved raw sample sufficient to contain 100 to 200 ostracode valves was
weighed using a microbalance, washed through a 106 micrometer brass sieve, and rinsed into a scored petri
dish. All ostracode valves (greater than 50% intact) within the sample were counted under an Olympus
SZH stereo microscope and recorded. Using the valve count, wet weight, and water content (Lewis, this
volume), the ratio of valves per gram of dry sediment was calculated using the equation: valve count/ [wet
weight - (wet weight x water content)]. Then the values were converted to percentages, using the maximum
abundance value as 100%, in an Excel spreadsheet.
Grain size analyses were carried out on these samples as well in separate studies. Percent total organic
carbon from the analyses were used in this study (see Lewis, this volume).
Taphonomic indicators - Preserved raw samples were washed through a 106 micrometer brass sieve and
rinsed into a separated plastic storage jar. A portion of the sieved sample was scooped into a scored petri
dish and the taphonomic indicators were counted under an Olympus SZH stereo microscope. The
taphonomic indicators counted were: valves as carapaces vs. disarticulated valves, % oxidation staining, %
reduction staining for samples in all transects, and broken valves (100% > broken valve ≥ 50%) vs. whole
valves for samples in transects 1, 3, and 5. When one of the factors reached 100 counts, counting stopped
for that comparison and the ratio was recorded. The ratios were converted into percentages in an Excel
spreadsheet.
Results
Abundance
Ostracode abundance was plotted against water depth (figure 1). The maximum abundances of ostracodes
occurred at approximately 40m of water depth for all transects, with smaller ratios of valves per gram dry
sediment in shallower and deeper water. The abundance ratios decreased rapidly below 60m of water depth
for all transects. The abundance curve for transect 1 is not properly represented, because abundance ratios
could not be calculated for shallow water shell lag samples at 20m, 32.4m, 36.4m, and 39.1m (samples
GB02, GB03, GB04, and GB05, William, this volume). Had these values been calculated, I believe that
the trend of transect 1 would follow the others more closely and peak at approximately 40m as well.
Percent abundance was compared to the percent total organic carbon (TOC) present in the sample (figure
2). The maximum ostracode abundances occurred where there was approximately 1%-2% TOC for all
transects. Abundances were reduced at lower TOC values and at higher TOC values with a drop when
TOC percentages reached a value above 2.5%.
Oxidation Staining
Oxidation staining of ostracode valves occurred more often at shallow water depths, with a decrease in
staining percentages below 40m of water depth (figure 3). In transects 1 through 4, the percent of valve
oxidation staining peaked at 10m and at approximately 40m. In transect 5, the oxidation percent reached its
maximum at the shallowest water depths and then continuously decreased as water depth increased. The
percentages of valve oxidation staining decreased from north to south over the delta platform (figure 4).
Reduction Staining
The percent of valves with reduction staining were compared to water depth (figure 5) and by transect
(figure 6). Staining peaked at different depths for the different transects between the depths of 30m and
60m. Transects 1 and 5, located furthest from the river mouth, had peak reduction staining at greater water
depths. There was a general increase in valve reduction staining from north to south, with an exception at
the river mouth where there was a slight decrease.
Valve Breakage
Broken valve counts were carried out on samples from exclusively on Transects 1, 3, and 5 because of a
lack of time. The values were plotted against water depth by transect (figure 7). The percent of broken
valves declined with increasing water depth. The broken valve percentages were higher at greater water
depths for transect 3, but the change in valve breakage going into deeper water remained consistent
between all transects. Valve breakage was also analyzed by polynomial regression best fit, which indicated
a strong inverse relationship between breakage and water depth (figure 8).
Discussion
Strong relationships were shown to exist between the taphonomic and environmental variables in this
study. The results revealed a complex system of relationships on the delta platform between taphonomy,
water depth, energy, geography, water and sediment chemistry, and organic matter.
The results showed ostracode abundance to be related to both water depth and TOC, both of which are
closely but inversely correlated with oxygen concentrations in the water column. TOC also provides a food
source for ostracode populations so its relationship to ostracode abundance is complex. The optimal levels
of TOC were between 1.19% to 2.44%, at values below or above this range the abundance numbers
decreased, notably at values greater than 2.5% (figure 2) (for more information on the TOC, see Lewis, this
volume). TOC was plotted against water depth; the preferred % TOC occurred around 40m (figure 9).
This was the same depth that the maximum ostracode numbers were found at across the platform (figure 1).
Lower TOC values suggest there may not be enough food to support a large population. Deep water is
more anoxic and organic rich. High values of TOC would indicate, and are often correlated with a poorly
defined sediment-water interface. These environments are not preferred by ostracodes (Thorp, 1999) and
abundance would decline.
Reduction staining of the ostracode valves showed relations to water depth, %TOC, geography, and
distance from the mouth of the Luiche River. Reduction staining was most abundant from 30m to 60m of
water depth; transects 1 and 5, more distal from the mouth of the river, had maximum staining at greater
water depths than those more proximal (transects 2, 3, and 4). After the maximum staining frequency was
reached, the percentage of stained valves dropped quickly (figure 5). This may actually be a function of
transport mechanism. Most of the valves observed in very deep water were small juveniles with no
staining. Rather than having these valves transported along the bottom of the lake, they may have been
rafted out to the deeper water. The lack of staining on the valves may be the result of the short amount of
time the valves have been lying in the sediment. There was also a general trend of increasing reduction
from north to south across the platform, though there was a drop in reduction staining at transect 3 at the
mouth of the river (figure 6). Both % TOC and the distance from the mouth of the river could jointly
control these patterns. The river water was rich in organic material relative to the lake water it was entering
(figure 10). This denser water may have bypassed close to the river mouth, resulting in a smaller
percentages of reduction staining in this area, and sinking as it moved away from the outlet causing deeper
reduction maximums at more distal locales. Lake currents may have swept the denser water southward,
creating a pseudo-oxygen gradient in the region of the Luiche delta platform.
This gradient was shown by oxidation staining on ostracode valves as well, the more reduced area in the
south, and increasingly oxic to the north (figure 4). This phenomenon could also be explained by a natural
oxygen gradient in the Luiche delta platform region; CTD profiles could be obtained to test this hypothesis.
Oxidation staining also had a strong relation to depth, staining percentages decreased with increasing
depths (figure 3). The data showed two peaks, one at 10m, one at approximately 40m of water depth for
transects 1-4. Transect 5 had the least amount of valve oxidation staining overall, and had its maximum
within the first 20m of water, then continually decreased. Currents could be bringing oxygen down these
depths, which would bring the means to produce the staining.
Valve breakage was highly correlated with water depth (R2 = 0.7904). The percentage of broken valves
decreased as water depth increased (figure 8). As mechanical energy is reduced, there is a lower
probability that the valves will be disturbed on the lake bottom. Energy is lower in deeper water, and very
high in shallow zones such as the shoreline where there is wave action, or at the mouth of a river where a
turbulent flow enters a body of water that is relatively still. This relationship was shown with transects 1,
3, and 5. Transect 3 was located nearest to the mouth of the river; the percent of broken valves was greater
at deeper water depths, but the relationship between breakage and water depth remained consistent with the
other transects (figure 7).
Future Work
More grab samples should be taken and analyzed in the same ways as done here to improve our
understanding of taphonomic variables. Ratios of juveniles to adults, as well as species identification of the
ostracodes would be very helpful. It is apparent from my observations that species richness and the number
of adults decrease with increasing water depth. With these analyses, relationships may be established that
will be valuable in other paleointerpretations.
Counts will be done on transects 2 and 4 for valve breakage, because of the strong correlation of this
variable in the other transects with water depth. The polynomial regression model developed in this current
study can be used in conjunction with valve breakage counts from core samples taken by the 1999 Nyanza
Project on the Luiche delta platform to produce a paleo-lake level curve. By comparing this curve with
independent estimates of paleo lake level over the same interval we will be able to test the robustness of
this new paleoenvironmental and paleoclimatic indicator for Lake Tanganyika.
Acknowledgements
First and foremost I would like to thank my mentors: Andy Cohen for taking the time out of his incredibly
busy schedule to mentor this project, sharing his unbelievable knowledge of ostracodes, bad jokes, and
statistical patience; Kiram Lezzar for his wisdom of rift lakes, optimism and attitude. I am sincerely
grateful to Lindsay Anderson and Catherine O’Reilly for their support and borrowed ears, David Kinyanjui
for good company, and Michelle Olsgard and Joe Sapp, my fellow troopers in pulling an all-nighter at
TAFIRI in the final stretch of the project. Thanks to Christine Lewis and Leah Morgan for their data, and
to the crew of the R/V Echo. I would also like to recognize the NSF grant ATM 9619458 supporting the
Nyanza Project.
References
Castaneda,I., Erchak, I., Harper, M., Total organic matter, carbonate, and grain size determination in two east-west coring transects,
Luiche and Malagarasi River Deltas, Lake Tanganyika, East African Rift Valley, The Nyanza Project 1999 Annual Report, 1999.
Coulter, G.W. 1991. Lake Tanganyika and its Life. Oxford University Press. New York, New York.
Lewis, C. J., Elucidating the interplay between tectonic and climatic controls on modern depositional processes in the Luiche delta.
The Nyanza Project 2002 Annual Report.
Morgan, L., Tectonic controls of sedimentary pathways and depocenters: Canyon conveyor belts and ridge rubbish on the Luiche
River Platform Margin, Lake Tanganyika. The Nyanza Project 2002 Annual Report.
Thorp, J.H., and Covich, A.P. 1999. Ecology and Classification of North American Freshwater
Invertebrates. Academic Press, Inc.
TOC vs. Abundance
O stracode Abundance
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va lve s/ g dry se d
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2000.00 4000.00 6000.00 8000.00 10000.00
100.00
0
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Trans ect 1
Trans ect 2
Trans ect 3
% Abundance
80.00
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40.00
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Trans ect 4
Trans ect 5
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120
Transect 1
Transect 2
140
Transect 4
Transect 5
T
Figure 1: Ostracode abundance (valves/ g dry sediment) vs. water depth (m). Data is separated by transects; the
maximum abundance is at approximately 40m of water depth for all transects. Transect 1 is missing abundance data
for samples in depth range 20m to 39.1m. Abundance values over the entire platform drop off uniformly below 60m of
water depth.
Figure 2: Percentage TOC vs. % abundance. Maximum abundance values peak at % TOC range 1.19-2.44, implying
that this is the favored range of organic material for ostracodes (%TOC values from Lewis, this volume).
Oxidation staining of valves vs. water
depth: Luiche Delta Platform
Oxidation Staining by Transect
% Valves
0
20
40
60
80
60
100
0
50
Transect 1
40
Depth (m)
Transect 2
Transect 3
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Transect 4
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Transect 5
% valves stained
20
40
30
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10
100
0
1
120
2
3
4
5
transect
140
Figure 3: Percentage of valves with oxidation stains vs. water depth (m). Data is separated by transects;
the northern transects (1 & 2) have a higher percentage of oxidation staining than those in the south.
Transects 1-4 have two peak oxidation depths: 10m & ~40m. Transect 5 has the least amount of staining,
peaks in the shallow water depths, then continually decreases in staining percentage.
Figure 4: Percentage of valves with oxidation stains sorted by transects. Here the north to south trend in
oxidation is very clear: more oxic in the north, decreasing to the south.
Reduction staining of valves vs. water
depth: Luiche Delta Platform
0
20
40
60
80
100
Re duction Staining by Transe ct
% valves stained
0
80
70
20
Depth (m)
40
Transect 1
60
Transect 2
50
Transect 3
Transect 4
60
Transect 5
40
30
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80
10
0
100
1
2
3
4
5
t ra nse c t
120
Figure 5: Percentage of valves with reduction stains vs. water depth (m). Data is separated by transects;
the transects in the south (4 & 5) have a higher percentage of oxidation staining then those in the north.
The transects located farthest from the river mouth have peak reduction stains at greater water depths than
those closer to the river outlet.
Figure 6: Percentage of valves with reductions stains sorted by transects. There is a general trend of
increase in reduction staining from north to south with the exception of transect 3 at the river mouth.
B rok en valves vs. w ater depth by transect:
Luiche D elta Platform
% V alves
0.00
10.00
20.00
30.00
40.00
50.00
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70.00
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20
Depth (m)
40
60
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N orth T ransect (1)
100
R iver M outh
T ransect (3)
South T ransect (5)
120
140
Figure 7: Percentage of broken valves vs. water depth (m) sorted by transect. There is a decrease in valve
breakage with increasing water depths. Transect 3 (river mouth) has higher % breakage at the same water
depths when compared to transects 1 & 5, but follows the same trend.
Broken valves vs. water depth: Luiche Delta Platform
% b ro ken valves
0 .0 0
0
10 .0 0
2 0 .0 0
3 0 .0 0
4 0 .0 0
50 .0 0
6 0 .0 0
70 .0 0
8 0 .0 0
9 0 .0 0
20
40
60
y = -0.0003x 3 + 0.0509x 2 - 3.5827x + 107.98
R2 = 0.7949
80
10 0
12 0
14 0
Figure 8: Percentage of broken valves vs. water depth in a scatter plot fitted with a second degree
polynomial regression line. R2 value indicates a strong correlation.
TO C vs. De pth
TOC by Transect
%TOC
0.00
2.00
4.00
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8.00
10.00
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Tra ns e c t 1
Tra ns e c t 2
Tra ns e c t 3
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Tra ns e c t 4
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Tra ns e c t 5
120
140
7.00
% TOC
40
6.00
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4.00
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2.00
1.00
0.00
1
2
3
4
5
Transect
Figure 9: Percentage TOC vs. water depth (m). There is a general increase of % TOC with increase in
water depth. The ostracode preferred % TOC was ~1.5%, which falls at approximately 40m of water
depth.
Figure 10: Percentage TOC sorted by transect. The highest values of TOC are in Transect 3, in front of the
river mouth. TOC percentages decrease to the north and south, implying organic input from the river.