Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 71–80
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Palaeogeography, Palaeoclimatology, Palaeoecology
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o
Paleoecological response of ostracods to early Late Pleistocene lake-level changes in
Lake Malawi, East Africa
Lisa E. Park a,⁎, Andrew S. Cohen b
a
b
Department of Geology and Environmental Science, University of Akron, Akron, OH 44325-4101, USA
Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA
a r t i c l e
i n f o
Article history:
Received 15 January 2009
Received in revised form 24 February 2010
Accepted 27 February 2010
Available online 10 March 2010
Keywords:
Lake Malawi
Ostracoda
Lake levels
Climate change
a b s t r a c t
Ostracod species from at least seven genera were recovered from drill cores recently collected at Lake
Malawi, a large (29,500 km2) and deep (706 m) African rift valley lake located in the southern African tropics
(9–14° S). These genera include Limnocythere, Candonopsis, Ilyocypris, Sclerocypris, Gomphocythere, possible
Allocypria? and an unknown Cypridosine. Taphonomic variables such as the percentage of valve breakage,
adult individuals, carbonate and oxidized coatings as well as associated mineralogy, can be used to delineate
lake-level low and highstands. This record of lake-level fluctuations is correlated with paleoecological
changes in ostracod communities throughout the core record of the past ∼ 145 ka. Two major assemblages
found within the Lake Malawi cores include a Limnocythere-dominated shallow, saline/alkaline assemblage
that occurred between 133 and 130 ka and between 118 and 90 ka and a deeper water Cypridopsine
assemblage that lived in waters 10s to 100s of meters in depth and dominated the ostracod assemblage
during intervals of lake-level transitions (136–133 ka, 129–128 ka and 86–63 ka), between the occurrence of
the littoral assemblage and the appearance of indicators of bottom water anoxia. Changes in occurrences and
abundances indicate variations in paleoecological affinities related to lake chemistry and oxygenation of
bottom waters. The characteristics of the different lineages influence how that lineage is likely to respond to
environmental variability such as lake-level fluctuations. Clades that include large numbers of endemic
species in Lake Malawi tend to be specialists and thus more susceptible to environmental perturbations. On
the other hand, cosmopolitan species within the lake all appear to be generalists, suggesting that they have
high tolerances to environmental variability and therefore, these genera tend to be monospecific within the
lake and are less likely to have radiated within the basin. The distribution of these ostracods can be used to
evaluate hypotheses about the environmental history at the landscape-scale and their potential influence on
species' distribution and diversification histories.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
1.1. Malawi and lake-level changes
Orbitally-induced climatic change has been responsible for a
number of high-amplitude lake-level fluctuations documented from
paleorecords throughout Africa (Gasse and Street, 1978; Gasse et al.,
1989; Beuning et al., 1997; Gasse, 2000; Johnson et al., 2001). These
changes have been faithfully recorded in the sediment record of large
and small lakes alike—but the longer records, such as that recovered
from the ancient Lake Malawi have the most extensive and highly
resolved record, archiving not only the physico-chemical changes in
the lake basin but the paleoecological response to these events (Fig. 1)
(Cohen et al., 2007; Scholz et al., 2007).
Lake Malawi is a large (29,500 km2), deep (706 m) meromictic lake
that is characterized by two barriers to vertical mixing—a thermocline
at 40–100 m depth and a chemocline at approximately 230 m depth
(Patterson et al., 2000). Today, Lake Malawi is hydrologically open and
drained by the Shire River, although 82% of total water loss is through
evaporation (Beadle, 1981). It is currently a dilute (240–250 μS/cm
conductivity) water body with negligible nutrient concentrations in
surface waters, which promotes very high water clarity (Patterson and
Kachinjika, 1995). The modern lake's watershed experiences a mesic
climate, with highly seasonal rainfall (800–2400 mm/yr) and lowland
vegetation that is dominated by wet Zambezian woodlands that are
replaced by evergreen and subsequent montane forests with increasing
elevation (Cohen et al., 2007).
Prior investigations show that Lake Malawi has undergone many
lake-level fluctuations throughout its history. The Malawi region is
estimated to have experienced extreme aridity episodically between
135 and 70 ka, with diminished climatic and ecologic variability after
⁎ Corresponding author.
E-mail addresses: lepark@uakron.edu (L.E. Park), cohen@email.arizona.edu
(A.S. Cohen).
0031-0182/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.palaeo.2010.02.038
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L.E. Park, A.S. Cohen / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 71–80
early Late Pleistocene megadroughts (McGlue et al., 2007; Scholz
et al., 2007).
Unlike the smaller lake-level falls that have been suggested for the
LGM, the early Late Pleistocene lowstands to a relatively shallow lake
would have eliminated most rocky shoreline habitat in the lake,
converting the majority of the lake's shoreline to muddy or sandy
bottoms and sub-dividing the basin (Cohen et al., 2007; Scholz et al.,
2007). This would have had enormous implications for community
dynamics during this time period as sub-basins were isolated and lake
physiography and chemistry were completely altered. In addition, the
megadrought low-stand demonstrated for Lake Malawi has provided
a robust constraint on genetic divergence of cichlid fish and other lake
faunas.
1.2. Effect of lake-level changes on faunas
Ancient lakes like Malawi and its sister Great Lake, Tanganyika
have long been recognized as providing great opportunities for the
study of community structure, particularly as it relates to the origin
and maintenance of species diversity (Coulter, 1991; Martens, 1997;
Cohen, 2000; Alin and Cohen, 2003). The extraordinary diversity
within the ancient lakes of the world often has been attributed to both
basin longevity and lake-level fluctuations. In most cases, basin
longevity alone cannot explain the complex and sometimes
conflicting evolutionary histories of the major clades within these
lakes; and most studies have concluded that the lake-level changes
experienced within these basins help explain the patterns of
speciation and adaptive radiations for which they are so famous
(Brooks, 1950; Fryer and Iles, 1972; Beadle, 1981; Wannik and White,
2000). The role that lake-level fluctuations have had in creating and
maintaining the high degree of endemism in Malawi and other
ancient lakes, as well as in possibly driving extinction events has also
been linked to the ecological changes seen through each basin's
history as communities respond to these environmental fluctuations.
The cichlid fish and thiarid gastropods have documented complex
ecological responses to lake-level changes that are linked to their
diversity patterns (Sturmbauer and Meyer, 1992; Danley and Kocher,
2001; Sturmbauer et al., 2001; Genner and Turner, 2005; Genner et al.,
2007). These studies suggest that ecological dynamics, particularly as
they relate to extrinsic driving forces such as lake-level fluctuations
play an important role in the overall diversity of these clades. These
dynamics can be inferred from paleoecological records. Lake-level
changes have had significant impact on basin morphometry, water
salinity, mixing regimes, and habitat heterogeneity which in turn
likely impacts species distributions and community structure.
1.3. Ostracods in the African rift lakes
Fig. 1. Location map of Lake Malawi with Sites 1 and 2 indicated. Bathymetric contours
are 100 m.
70 ka (Cohen et al., 2007; Scholz et al., 2007). During these arid
intervals Malawi became a shallow, alkaline/saline lake, with lake
levels at least as low as − 580 m relative to modern, and the lake's
surrounding watershed became a semi-desert. Although the Last
Glacial Maximum (LGM) aridity is clearly demonstrable throughout
tropical Africa, these findings suggest that its impact on lake and
terrestrial ecosystems would have been smaller than the preceding
Among the crustaceans found within Lake Malawi and other
ancient lakes, bivalved ostracods are one of the most important and
diverse groups—having extensive radiations not just in East African
Rift (EAR) lakes but in almost all of the ancient lakes in the world. The
calcium carbonate shells of these microcrustaceans allow them to be
preserved in core samples and therefore permits a more extensive
understanding of their long-term ecological responses within these
lake basins.
Until the late 1990s, there had been only four major studies of
modern ostracods from Lake Malawi (Daday, 1910; Sars, 1910; Fryer,
1957; Martens, 1990). Between 1994 and 1998, Martens (2002)
conducted a modern census of all ostracods in Lake Malawi from
several cruises parallel to trawls for demersal fish. This research was
part of a larger INCO-DC project on the trophic ecology of the
demersal fish community of Lake Malawi (contract number:
ERBIC18CT970195) in which the ostracod census was part of Task 3
which examined the diversity, structure, seasonality and production
of invertebrate communities within the lake. The primary objectives
L.E. Park, A.S. Cohen / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 71–80
of the larger project were to provide trophic models to quantify
energy flows through the demersal fish community of the lake and to
determine fishing pressure in order to evaluate the accuracy of fishing
statistics in the lake. The ostracod census conducted by Martens
(2002) provides the most up-to-date recording of all non-insect
invertebrates found within the lake. In his study, nearly one hundred
benthic samples were taken for ostracods that included dredging,
PONAR grab and hand net samples using SCUBA. Martens used the five
dredge samples with the highest diversity for his analysis. Prior to this
study few studies existed (Daday, 1910; Sars, 1910; Fryer, 1957;
Martens, 1990)—and only 20 ostracod species (including 3 endemic
species) had been documented from the lake. After the 2002 study,
these numbers tripled.
Ostracods are the most diverse group of crustaceans in the lake
known with 63 currently described species, N62% of which are
endemic to the lake (Martens, 2002). Unlike Lake Tanganyika which
has flocks of cytherids and candonids, there have been no genera or
species from these clades reported from Malawi. The main radiations
are within the Cypridopsinae, Limnocythere sp.l., Gomphocythere and
cypridid clades. Chrissia species that are found within the modern lake
are not endemic and are found in associated water bodies—primarily
inflowing rivers.
Little is known about the origin and maintenance of ostracod
diversity in Malawi. Controlling factors might be linked to dispersal
ability, salinity and depth tolerances, feeding and/or reproductive
strategies and extrinsic factors such as lake-level fluctuations, changes
in water chemistry and depth as well as the development of subbasins. Therefore it is important to explore the relative roles of each,
particularly as they are related to paleoecological changes throughout
the lake's history. In particular, determining how species might react
to a particular environmental factor or the increased variance of that
factor is important because different species may have different
responses to resource availability. There are obvious trade-offs
associated with a species' ability to respond to varying environmental
conditions and to compete for resources in the absence of that
variability. There are species that have low resource requirements in
the absence of variability. These species are typically the most
negatively impacted by variation in resource levels (Chase and
Leibold, 2003). Species that are unaffected by environmental
variability or that benefit from variable resource availability have
the ability to store consumption obtained during periods of high
resource availability. Each ostracod species in Malawi most likely has
its own response to environmental conditions and can drive the
species ecological and evolutionary dynamics. To examine these
responses we have studied the patterns of ostracod community
change and its possible correlation to the paleolimnological history of
Lake Malawi using the long drill core records now available from that
lake.
2. Methods
2.1. Paleoecological response to lake-level change
In 2003, the Lake Malawi Drilling Project collected seven cores from
deep water sites in the lake. Site 1 (4 cores), in the central part of the
lake, is located at 11° 17.66′S, 34° 26.15′E in 592 m water depth. Site 2 (3
cores) is in the northern part of the lake basin (10° 01.1′S 34° 11.2′E) in
359 m water depth. Core collection, basin dynamics, geochronology and
sedimentation are described in Scholz et al. (2007). One of these cores,
MAL 05-1C from the deep water drill site, has been sampled and its
ostracod paleoecology briefly reviewed in a previous paper (Cohen et al.,
2007). This paper adds new and more complete data to the analysis—
particularly with respect to the ostracod occurrence, abundance and
taphonomic data. Samples for ostracods (n = 471) were taken at 16 cm
intervals (∼300 yr sample step resolution) and wet sieved with
deionized (DI) water using a 125Ø mesh stainless steel sieves. One
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aliquot was weighed wet, oven-dried and re-weighed to determine
water content, and this calculated water content was applied to the
second (wet sieved and undried) aliquot to determine its dry weight.
Sieved residues were counted for total number of ostracods per sample
(expressed as #/g dry weight), % genera based on 100 valve counts, and
taphonomic condition (% adult—as a proxy for size sorting, whole
carapace, broken, carbonate coated, and reduction/oxidation stained).
Fossil identifications were confirmed using a Phillips XL30 ESEM and
compared to the modern material from Malawi and other EAR basins.
2.2. Statistical analyses
The core sample data included a variety of data types that made it
preferable to analyze via principal component analysis (PCA). Patterns
were delimited and correlations were tested between taphonomic
variables and species' distributions. First axis loadings are discussed
below. Time series autocorrelation was conducted to identify
periodicities and other spectral characteristics in a time series and
to identify smoothing and cyclicity. Detrended correspondence
analyses were performed on abundance data of the major faunal
elements including the Limnocythere, Candonopsis and Cypridopsines
as a way to reduce the large multivariate ecological data set and
determine the strength of the environmental gradient. A Gaussian
response model to generic abundances along the temporal gradient
was developed using a least-squares algorithm. This allows for
determining changes in abundances throughout the core, and
identifying the peak abundances along the temporal gradient.
3. Results
There are two major ostracod assemblages found within the Lake
Malawi cores—a shallow, saline/alkaline lake assemblage and a
deeper water assemblage. The saline/alkaline lake assemblage
consists of monospecific Limnocythere sp. (with subordinate Candonopsis, Ilyocypris, and Sclerocypris in some samples) that typifies
shallow littoral conditions in highly alkaline (pH 9–9.5, alk N 30 meq/l)
and saline (K20 N 4000 µS/cm) African lakes. These Limnocythere
assemblages are characterized by high adult/juvenile ratios, low
levels of decalcification, common carbonate coatings and frequent
valve abrasion, all typical of reworking and accumulation in the
littoral zone of large African lakes. These taphonomic trends are seen
in Fig. 2 which also shows that this type of assemblage dominates
episodically between 133 and 130 ka and between 118 and 90 ka in
Core 1C. It is notable that African cosmopolitan genera that are
generally restricted to freshwater conditions are absent from all core
samples examined (for example Stenocypris spp. which are present in
modern dilute Lake Malawi).
The second major ecologically-based assemblage is a deeper water
fauna that is dominated by weakly calcified, juvenile valves of the
endemic Cypridopsine flock (Fig. 2). These individuals are frequently
abundant and show little or no signs of coatings or abrasion and most
likely lived in waters 10s to a few 100s of meters depth. This type of
assemblage occurs in transitional zones of the core (136–133 ka, 129–
128 ka and 86–63 ka) between the occurrence of the littoral assemblage
and the appearance of indicators of bottom water anoxia (Figs. 2 and 4).
Deltaic stillstands from the 62–64 ka interval at the −200 m level,
coupled with the deep water ostracod assemblage from the same time
interval demonstrate that Lake Malawi must have been ventilated to
considerably greater depths (∼350–400 m) than at present, slightly
greater than the maximum mixing depths observed in southern Lake
Tanganyika today. The Limnocythere and Cypridopsines are negatively
correlated as illustrated in the autocorrelation in Fig. 5. In general, if the
autocorrelation sequence consists of somewhat smooth data, the
correlation will remain relatively high for small lag times, but as lag
time exceeds the interval of interdependency, the autocorrelation will
drop. In general, the two genera do not occur together, but rather occur
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Fig. 2. Distribution of taphonomic variables throughout Core 1C from Lake Malawi. Ostracod abundance has been log transformed and normalized per gram. The dominant genera of
Limnocythere and Cypridopsines are indicated. The percent broken valves, whole carapaces, adults as well as carbonate, reduced and oxidized staining are all indicated. PC1 diatom
curve is of the combined diatom plus screen wash data sets (Stone et al., 2011–this issue). Lightly shaded areas represent shallower lake levels and dark shaded areas are deeper water.
in counter dominance to each other in accordance with various high and
low lake stands as independently indicated by the taphonomic
characters associated with these assemblages (Fig. 2).
Comparing the changes seen within the core with the taxa found in
modern Malawi, it is apparent that there is both a shallow and deep
water assemblage within the lake. The modern Limnocythere flock in
Malawi is unusual for African lakes; but highly speciose Limnocythere
lineages exist in other lakes around the world, mostly in highly
alkaline and slightly saline systems. The comparably speciose deeper
water Cypridopsine lineage also occurs in other African lakes like
Tanganyika. The co-occurrence of both assemblages in several parts of
the core may indicate transitional times of environmental instability.
Fig. 3. Principle component analysis of taphonomic variables associated with Core 1C. Correlations are plotted in inset.
L.E. Park, A.S. Cohen / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 71–80
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Fig. 4. Distribution of taxa throughout Core 1C. Ostracod abundance has been log transformed and normalized per gram. Percent Cypridopsines, Limnocythere, Candonopsis as well as
other species. PC1 diatom curve is of the combined diatom plus screen wash data sets (Stone et al., this issue). Lightly shaded areas represent shallower lake levels and dark shaded
areas are deeper water.
3.1. Ostracod taphonomy and mineralogy
Taphonomic variables such as % broken valves, whole carapaces,
adults, carbonate coating, iron and oxidation staining were plotted
against depth and the PC1 of the diatom curve (which is the proxy for
lake-level changes; Stone et al., 2011–this issue). The % broken valves
variable does not appear to be directly correlated with lake-level
fluctuations, but the percent of whole carapaces and adults does
appear to increase during the times when Limnocythere and Candonopsis species dominated the assemblage between 133 and 130 and
between 118 and 90 ka (Fig. 2). The percent of reduced Fe staining
increased upcore between 86 and 63 ka, during an interval when lake
levels were starting to rise towards modern levels. This corresponds
with the dominance of the Cypridopsine species, most of which are
endemic and not defined to species level (Martens, 2002, 2003). The
number of juveniles also increases along with the dominance of
Cypridopsines (Fig. 2), probably as a result of the death of immature
cohorts during episodic anoxia at or near the oxicline in the deeper
water settings where these assemblages formed. These variables are
all correlated with the PC1 of the diatom assemblage, which is used as
the major proxy for lake-level fluctuations.
Principal component analyses of these variables indicate that %
whole carapaces, adults and oxidized coatings are strongly correlated
(Fig. 3). Slightly over 34.6% of the variance is explained by this axis,
which we interpret, based on the loadings, as being fundamentally
related to lake level and (indirectly) available moisture (high positive
loadings correlate with low lake levels and aridity).
Fig. 5. Time series autocorrelation between the Limnocythere and Cypridopsine lineages
throughout the core showing a negative correlation of the two clades through time.
Fig. 6. Detrended correspondence analysis of the major clades of ostracods throughout
Core 1C based on % of fauna recovered at each core interval.
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Fig. 7. Genus packing (Gaussian) response model along the temporal axis. The solid line
represents all genera other than Candonopsis, Limnocythere and the Cypridopsines; the
double dots and dashed line are the Candonopsis; the dashed line represents the
Limnocythere and the small dashed line represents the Cypridopsines.
Plotting the log of ostracods/g against the % of Cypridopsines,
Limnocythere, Candonopsis and other species reveals an asynchronous trend between the Limnocythere species flock and the
Cypridopsines (Fig. 4).
Modeling the gradient packing of major genera (with weighted
averaging) fits generic abundance data along a temporal gradient and
demonstrates for this data that these generic assemblages have changed
through time and have experienced optimal conditions at different
times through the basin's history (Fig. 5). Specifically, the Candonopsis
sp. and other genera were optimized between 135 and 112 ka, followed
by the Limnocythere between 112 and 90 ka, and then the Cypridopsines
between 98 and 90 ka (Figs. 4, 6 and 7). Community assemblages and
their dominant genera changed through time during various time
intervals and those lineages tracked environmental changes through the
basin's history. The DCA results support the different assemblages along
the environmental gradient (Fig. 6). DCA1 has 74.5% of the variance and
DCA2 has 14.5%. Detrending separates the genera into three distinct
groupings—Limnocythere, Candonopsis, and Cypridopsine species. The
environmental gradient is lake level along the temporal axis.
Although it is evident that lake levels affect ostracod community
composition and individual clades respond to changes in water depth,
basin physiography, and water chemistry, it is not as clear how individual
species or lineages may respond to environmental variability. Correlation
analyses indicate that the endemic elements of the ostracod fauna—the
Limnocytherid and Cypridopsines are the most sensitive to lake-level
fluctuations and are negatively covarying (Figs. 4 and 5). The species of
cosmopolitan genera are not as strongly influenced by environmental
variability, suggesting that their distribution and diversity are reflected in
the overall paleoecological associations and diversity patterns in the lake's
history.
4. Discussion
The fossil ostracod species recovered from the Lake Malawi drill core
were exclusively benthic/epibenthic and required oxygenated waters in
which to live—conditions that only exist in the lake today in water
depths above 200–250 m (Martens, 2003). The ostracods recovered
from Core 1C typically occur in sediments deposited prior to 63 ka.
These assemblages are assumed to have been in situ and not transported
downslope via gravity flows or mixing because of their fragile nature as
well as high number of juveniles. Based on experimental work done by
Park et al. (2003) a high juvenile:adult ratio indicates little or no
downslope transport within these faunas. Additional evidence relates to
the anoxic zone that is typically undersaturated with respect to CaCO3.
Based on these criteria as well as the understanding of the maximal
depths of the oxygenated zone in nearby Lake Tanganyika, we interpret
the occurrence of abundant ostracods at these core intervals to indicate
water depths in Lake Malawi that were at least ∼280 m shallower than
at present (Cohen et al., 2007). The ecological affinities of the two major
assemblages of ostracods found in the cores support this interpretation
and lead to further insight into the effect of lake-level fluctuation on
origination and maintenance of species diversity.
Within the modern faunas of Lake Malawi, depth appears to be
more important in determining similarities between faunas than
geographical location, even if the sampling locations are several
hundred kilometers apart from one another (Martens, 2002).
Differential depth zonations of the ostracod clade mimic the patterns
seen in the earliest radiation of cichlid fish—between the rock and
sand dwelling lineages (Danley and Kocher, 2001). The origin of the
two major flocks—the shallow water Limnocytherids, which are
adapted to more saline/alkaline conditions, and the deeper water
Cypridopsines is unknown and may or may not have resulted from
multiple invasions of each group into the lake basin. Subsequent
diversification of these initial lineages was most likely promoted by
repeated lake-level fluctuations and species selection.
Several ostracod taphonomic variables such as the percent adult
and whole carapaces, % whole carapaces, % carbonate coated valves
and oxidized coated valves seem to be associated with lake-level
fluctuations. Examples of these coatings can be seen in Plate IA, B, G
and J. Reduced coatings can be seen in Plate ID. Some algal borings can
be seen on the Candonopsis species in Plate IF but these were not that
common as taphonomic features.
The relative amounts of mica, siderite, vivianite, and pyrite within
the sand fraction as well as the percent of reduced-stained ostracod
valves provide clues to lake-level fluctuations that in some cases are
related to changes in circulation and water body stratification (Fig. 2).
Mica flakes accumulate in the distal portions of deltaic deposits in
lakes, often with finer grained mud as a result of their hydrodynamic
characteristics. Siderite, vivianite and pyrite are all redox sensitive
minerals, that form commonly in anoxic water, although poorly
oxygenated conditions can occur in both deep (below oxicline) and
shallow water (e.g. deltaic marshes) settings. The latter, in particular
is a common setting for pyrite formation, where sulfates are readily
available for reduction. Pyrite and mica abundances were positively
correlated in the PCA analysis (Fig. 2), perhaps as a result of cooccurrence in deltaic deposits or from reducing conditions as a result
of lake-level fluctuations.
4.1. Comparing Lake Malawi to Tanganyika
Documented ostracod diversity differs considerably between Lake
Malawi and its closest counterpart, Lake Tanganyika. Whereas generic
diversity is identical (n = 21 genera), the total number of species in
Tanganyika is estimated to be well over 100 species versus Malawi at 63.
Also, the morphological disparity of ostracods from Lake Tanganyika is
much greater than in Lake Malawi. Furthermore, alpha (within-sample)
diversity is also typically much higher in Lake Tanganyika ostracod
faunas (Martens, 2002). A much higher proportion of the Tanganyikan
Plate I. Plate indicating typical examples of ostracods found throughout core MAL05-1C. A—Right valve of Cypridopsine cf sp A from MAL05-1C 24E 3 78.2–79.5; B—Left valve of
Cypridopsine cf sp G from MAL05-1C 24E 3 78.2–79.5 cm; C—Right valve of Cypridopsine cf sp G from MAL05-1C-24E3 58.1–59.1 cm; D—Right valve of Cypridopsine sp. S from
MAL05-1C-17H2-78–79 cm; E—Right valve of Cypridopsine sp. R from MAL05-1C-24E3 58.1–59.1 cm; F—Left valve of Candonopsis sp from MAL05-1C-24E-3 58.1–59.1 cm; G—Right
valve of Limnocythere sp 4 from MAL05-1C-24E3 58.1–59.1 cm; H—Right valve of possible Allocypria sp. 1 from MAL05-1C-24E3 58.1–59.1 cm; I—Left valve of Sclerocypris sp juv from
MAL05-1C-24E3 58.1–59.1 cm; J—Right valve of Gomphocythere sp. 1 MAL 1C 24E3 78.2–79.5 cm. Scales are all 100 µm and as indicated. Letters A, B, G and J are examples of
carbonate coating.
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Table 1
Species list of modern and fossil ostracods found within Lake Malawi. Endemic species
are indicated by *. Species found in modern Lake Malawi are indicated by +. Fossil
species found in the core material are indicated by ♦. Note that many of these species
are not described and identified only by informal letter and numerical designations that
refer to Martens (2002) report.
Species list
Class Ostracoda Latrielle, 1806
Subclass Podocopa G.W. Müller, 1894
Order Podocopida Sars, 1866
Suborder Podocopina Sars, 1866
Superfamily Cypridoidea Baird, 1845
Family Candonidae Kaufmann, 1900
Subfamily Candoninae Kaufmann, 1900
♦ + Candonopsis africana s.l.
Subfamily Cyclocypridinae Kaufmann, 1900
+ Cypria lenticularis Müller, 1898
+ Physocypria castanea (Brady, 1904)
Family Cyclocyprididae
♦ possible Allocypria sp.1
♦ possible Allocypria sp.2
Family Cyprididae Baird, 1845
Subfamily Cypricercinae
+ Cypricerus inermis (Brady, 1904)
+ Neocypridella fossulata (Daday, 1910)
+ Strandesia laticauda Daday, 1910
Subfamily Cypridopsinae
+* Cypridopsinae n.gen. cunningtoni Sars, 1910
♦ + * Cypridopsinae n.gen. sp. A
♦ + * Cypridopsinae n.gen. sp. B
+* Cypridopsinae n.gen. sp. C
+* Cypridopsinae n.gen. sp. D
+* Cypridopsinae n.gen. sp. E
+* Cypridopsinae n.gen. sp. F
♦ + * Cypridopsinae n.gen. sp. G
+* Cypridopsinae n.gen. sp. H
+* Cypridopsinae n.gen. sp. I
+* Cypridopsinae n.gen. sp. J
+* Cypridopsinae n.gen. sp. K
+* Cypridopsinae n.gen. sp. L
+* Cypridopsinae n.gen. sp. M
+* Cypridopsinae n.gen. sp. N
+* Cypridopsinae n.gen. sp. O
+* Cypridopsinae n.gen. sp. P
+* Cypridopsinae n.gen. sp. Q
♦* Cypridopsinae n.gen. sp. R
♦* Cypridopsinae n.gen. sp. S
+ Cypridopsis vidua (Müller, 1776)
+ Plesiocypridopsis fulleborni (Daday, 1910)
+ Zonocypris costata (Vavra, 1897)
Subfamily Cyprinotinae Bronstein, 1947
+* Cyprinotinae n.gen. sp.1
+* Cyprinotinae n.gen. sp.2
+* Cyprinotinae n.gen. sp.3
Subfamily Herpetocypridinae Kaufmann, 1900
Acocypris platybasis
+ Chrissia fasciculata (Daday, 1910)
+ Chrissia fullborni (Daday, 1910)
+ Chrissia marginata (Daday, 1910)
+ Chrissia perarmata (Brady, 1904)
+ Chrissia sinuate (Müller, 1898)
+ Chrissia stagnalis (Daday, 1910)
+* Humphcypris sp.1 Martens, 1998
+ Stenocypris major (Baird, 1859)
Subfamily Megalocyprinae
♦ Sclerocypris sp.
Family Ilyocyprididae Kaufmann, 1900
+ Ilyocypris propinqua Sars, 1910
Family Notodromadidae Kaufmann, 1900
Subfamily Oncocypridinae, DeDeckker, 1979
+ Oncocypris mulleri (Daday, 1910)
Superfamily Cytheroidea Baird, 1850
Family Limnocytheridae Klie, 1938
Subfamily Limnocytherinae Klie, 1938
+* Limnocythere jocquei Martens, 1990
♦ + * Limnocythere sp.1
♦ + * Limnocythere sp.2
♦ + * Limnocythere sp.3
Table 1 (continued)
Species list
♦ + * Limnocythere sp.4
+* Limnocythere sp.5
♦ + * Limnocythere sp.6
+* Limnocythere sp.7
♦ + * Limnocythere sp.8
+* Limnocythere sp.9
+* Limnocythere sp.10
♦* Limnocythere sp.11
♦* Limnocythere sp.12
♦* Limnocythere sp.13
♦* Limnocythere sp.14
♦* Limnocythere sp.15
♦* Limnocythere sp.16
Subfamily Timiriaseviinae Mandelstam, 1960
+* Gomphocythere emrysi Martens, 2003
+* Gomphocythere huwi Martens, 2003
+* Gomphocythere irvinei Martens, 2003
+* Gomphocythere lisae Martens, 2003
+* Gomphocythere piriformis Martens, 2003
♦* Gomphocythere sp.1
Superfamily Darwinuloidea Brady and Norman, 1889
Family Darwinulidae Brady and Norman, 1889
+* Alicenula inverse Rossetti and Martens, 1998
+* Alicenula serricaudata Rossetti and Martens, 1998
+ Darwinula stevensoni Brady and Robertson, 1870
+* Penthesilenula gr.incae Rossetti and Martens, 1998
SUPERFAMILY CYPRIDOIDEA
* Indicates endemic species to Lake Malawi.
+ Indicates present in modern Lake Malawi.
♦ Indicates fossil species that occurs in cores.
species (18%) is found in associated water bodies in Tanganyika
compared with Lake Malawi (9.5%), although the Malawi fauna remains
much less studied than that of Tanganyika (Martens, 2002). The pattern
of greater ostracod diversity in Tanganyika relative to Malawi is part of a
broader pattern, as Tanganyika has about twice as many documented
non-insect invertebrate species as Malawi. Since Tanganyika is older
than Malawi, the age of the lake has been invoked as a reason for this
discrepancy (Martens, 2002). Basin age is also hypothesized to correlate
with endemicity, since Lake Tanganyika has a high rate of ostracod
species endemism (70%). However, as noted above, the effect of basin
age is confounded by the impacts of much more recent lake-level
fluctuations that almost certainly differ between the two lakes. Of
particular significance in this regard are the much more profound
ecological impacts of Early Late Pleistocene megadrought events on Lake
Malawi versus Tanganyika, which may or may not have caused much
more extensive selective extinction in the former lake (Park et al., 2000).
If true, this could possibly have contributed to the much higher levels of
morphological disparity observed in the modern Tanganyika fauna
relative to Lake Malawi.
High generic diversity is common in lakes, with many genera
represented by a single species. Having high species diversity within
single lineages is far less common within lacustrine ecosystems. While
Malawi and Tanganyika share the same total number genera, they
share only five in common (23.8%) and remarkably only one species—
Darwinula stevensoni. Examining the species present in modern Lake
Malawi, most appear to be within two genera, the Cypridopsinae n.
gen. and Limnocythere (Plate I and Table 1). The origins of these clades
within the Malawi basin most likely came from the surrounding
watersheds with subsequent endemic speciation.
Examining the ecological tolerances/preferences of the ostracod
lineages within Malawi suggests that key differences may help to
explain overall diversification patterns that may not be related to lake
longevity or habitat variability. Genera and species found in modern
Lake Malawi tend to be more cosmopolitan—the lake exhibits far less
endemism in ostracods than other ancient lakes and include taxa such
as Cypridopsis vidua, Candonopsis africana, various species of Oncocypris, Physocypria, Ilyocypris, Stenocypris, Sclerocypris, Strandesia and
L.E. Park, A.S. Cohen / Palaeogeography, Palaeoclimatology, Palaeoecology 303 (2011) 71–80
Zonocypris that are commonly found in lakes throughout the world.
These genera are common in less permanent water bodies in Africa
and none of them are found in Lake Tanganyika. The two major flocks
within modern Malawi, the Cypridopsine and Limnocythere groups,
are associated with different ecological conditions—specifically water
depth and salinity/alkalinity tolerances that may have influenced
their respective radiations.
We cannot yet determine how rapidly the Malawian ostracod
clades radiated, pending a thorough phylogenetic analysis of both
fossil and modern faunal molecular genetic data. The rates of the
cichlid fish and thiarid gastropod evolution within the Malawi basin
appear to have varied throughout their histories as one would expect
as did the ostracods. Likewise, the type of change most likely varied
between anagenic to cladogenic processes, responding to disparities
in extrinsic forcing mechanisms causing differential species sorting
and selection. Ostracod clades comprising eurytopic species may have
experienced low extinction rates and would likely have been widely
distributed geographically, and would probably display high retention
of plesiomorphies. The species radiations seen in the Limnocythere and
Cypridopsine flocks could have resulted from the short term
acquisition of autapomorphies that arose due to lake fluctuations in
response to climatic change. Water level fluctuations have been
shown to be important modulators of speciation processes in tropical
lakes, particularly ancient ones such as Malawi and Tanganyika,
because they temporarily form or break down barriers to gene flow
among adjacent populations and/or incipient species (McCune and
Lovejoy, 1998). There is building and substantial evidence that
climatic phenomena synchronized the onset of genetic divergences
of lineages in various species flocks such that recent evolutionary
history seems to be linked to the same external modulators of
adaptive radiation (Sturmbauer et al., 2001; Genner et al., 2007). The
interplay between the intrinsic characters of different species and
lineages and the extrinsic driving force of lake-level fluctuation needs
to be further examined to better understand the role of niche
optimization and ecological distribution of ostracods in this speciose
ancient lake.
5. Conclusions
Ostracod species from at least seven genera were recovered from
drill cores recently collected at Lake Malawi, a large (29,500 km2) and
deep (706 m) African rift valley lake located in the southern African
tropics (9–14° S). These genera include Limnocythere, Candonopsis,
Ilyocypris, Sclerocypris, Gomphocythere, possible Allocypria? and an
unknown Cypridosine. Taphonomic variables such as the percentage
of valve breakage, adult individuals, carbonate and oxidized coatings
as well as associated mineralogy, can be used to delineate lake-level
low and highstands. This record of lake-level fluctuations is correlated
with paleoecological changes in ostracod communities throughout
the core record of the past ∼ 145 ka. Two major assemblages found
within the Lake Malawi cores include a Limnocythere-dominated
shallow, saline/alkaline assemblage that occurred between 133 and
130 ka and between 118 and 90 ka and a deeper water Cypridopsine
assemblage that lived in waters 10s to 100s of meters in depth and
dominated the ostracod assemblage during intervals of lake-level
transitions (136–133 ka, 129–128 ka and 86–63 ka), between the
occurrence of the littoral assemblage and the appearance of indicators
of bottom water anoxia. Changes in occurrences and abundances
indicate variations in paleoecological affinities related to lake
chemistry and oxygenation of bottom waters. The characteristics of
the different lineages influence how that lineage is likely to respond to
environmental variability such as lake-level fluctuations. Clades that
include large numbers of endemic species in Lake Malawi tend to be
specialists and thus more susceptible to environmental perturbations.
On the other hand, cosmopolitan species within the lake all appear to
be generalists, suggesting that they have high tolerances to environ-
79
mental variability and therefore, these genera tend to be monospecific
within the lake and are less likely to have radiated within the basin.
The distribution of these ostracods can be used to evaluate hypotheses
about the environmental history at the landscape-scale and their
potential influence on species' distribution and diversification
histories.
Acknowledgements
This project was funded by the U.S. National Science Foundation—
Earth System History Program (EAR-0602350), the International
Continental Scientific Drilling Program, and the Smithsonian Institution. Initial core processing and sampling were carried out at LacCore,
the National Lake Core Repository at the University of Minnesota. The
authors wish to particularly thank D. Gauggler, R. Markus and C.
Johnson, T. Astrop and C. Boush.
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