Walia Special Edition on the Bale Mountains (2011) - Zoologische ...

total nitrogen, chlorophyll a, conductivity and pH) show considerable variation between seasons
(i.e. they are all distinctly lower during the wet season except for pH which is higher). The lakes
are naturally eutropic (average TP of 103.3 µg/L in the dry season and 43.0 µg/L in the wet season;
average TN of 1400.3 µg/L in the wet season and 658.8 µg/L in the dry season), and they appear
to be at the boundary between P and N limitation (average mass TN/TP of 13.55 during the dry
season, and 15.33 during the wet weason; Guilford and Hecky 2000; Sterner 2008). Within the
area of investigation, basalt (often olivine-basalt) is predominant and likely responsible for the low
conductivity of most the lakes (surface-water conductivity ranging from 17 to 457 µS/cm in the dry
season, and 17 to 263 µS/cm in the wet season). In at least one case (Hara Laki), alkaline trachyte
contributes to the considerably elevated conductivities (2410 µS/cm during the dry season, and
912 µS/cm during the wet season). All lakes have a neutral to mildly basic pH (ranging from 7.09
to 9.40 in the dry season, and 7.53 to 9.56 in the wet season), and well-oxygenated bottom waters.
They can all be classified as continuous warm polymictic (sensu Eggermont et al. 2007). Surfacewater
temperatures at the time of sampling ranged from 2.6°C to 13.2°C during the dry season, and
7.2 and 16.3°C during the wet season. Surface-water temperatures are highly dependent on the time
of sampling, but for lakes visited at basically the same hour (Hara Lucas, Hara Lakota, Koromi
Lake, Hara Laki, and Dimtu Lake), they are always higher during the wet season. We also noticed
nighttime freezing on several occasions. Based on the available data (more water chemistry data as
well as vegetation monitoring data will become available shortly), we can already conclude that the
Bale Mountain lakes have distinctly different properties during wet and dry seasons (i.e. seasonal
variations within one lake are often as large as variations among lakes). The only exception is Garba
Guracha, the only lake with a water depth exceeding 2m (the lake is hence more buffered against
environmental fluctuations).
Preliminary surveys of the plankton by Löffler demonstrated a large similarity between
lakes. He found that virtually no phytoplankton or rotifers were present in any of the lakes and
ponds. He also noted that the zooplankton was strikingly uniform and was comprised exclusively of
paleartic species (like Daphnia obtusa, Arctodiaptomus (Rhabdiaptomus) n. sp., and Megacyclops
viridis). The benthic Cladocera (mainly genera Macrothrix, Leydigia, Alona, Alonella, Chydorus),
benthic copepods (with the exception of Paracyclops fimbratus) and ostracods, on the other hand,
were found to belong to African subspecies/species or genera. In-depth study of the Bale Mountain
Daphnia populations by Kotov and Taylor (2010), however, revealed misidentification and showed
‘Daphnia obtusa’ to be an endemic obtusa-like species (called Daphnia izpodvala) whose closest
affinity is to the North American Daphnia puleata. Moreover, since the other Cladocera species
were not identified to the species level, zoogeographical interpretations made earlier (emphasizing
the palearctic character of Bale’s plankton and the continuous distribution of many of the species
along the African mountain “backbone”) may not be totally correct. Our biological analyses are
still in progress, but a quick screening of a small selection of samples revealed at least 12 species
with various biogeographical affinities (Alona affinis, Alona guttata, Alona cf. rectangula, Alona
costata species complex, Alona intermedia, Alonella excisa, Chydorus sphaericus species complex,
Chydorus parvus, Daphnia izpodvala, Leydigia sp., Macrothrix hirsuticornis species complex,
<strong>Walia</strong>-<strong>Special</strong> <strong>Edition</strong> on the Bale Mountains 177