The ostracod assemblage in the Sea of Galilee: the 2012 data
The ostracod assemblage of the lake is clearly dominated by C. torosa which accounts for ca. four fifths of the total assemblage. However, only four samples exclusively included valves and/or carapaces of C. torosa. The noded and smooth forms of C. torosa do not show a systematic distributional pattern in the Sea of Galilee. Both occur over the full depth range from shallowest locations (0.3 m) down to 32 m depth where ostracod remains were recorded. However, abundances of the smooth form C. torosa f. littoralis are significantly higher above a water depth of ca. 12 m in comparison to deeper locations, possibly as a result of the larger influence of saline springs at the lake shores and littoral locations within the lake (Frenzel et al. 2012; Fig. 5). Ilyocypris hartmanni reaches higher abundances in a moderate depth range of ca. 5–12 m. The other member of the genus, I. cf. nitida, shows higher numbers in shallow waters of 5 m and less, and also in a range between 14–18 m.
Number of ostracod remains and living specimens in surface samples from the Sea of Galilee. Number of valves (A); number of carapaces (B, bubble size similar to those in A with a carapace representing two valves); number of taxa (C); portion of noded C. torosa in relation to all C. torosa (D); specimens of C. torosa f. torosa (orange) and N. angulata (green) collected alive (E); and specimens of C. torosa f. littoralis collected alive (F); white crosses in A and B indicate absence
In contrast, abundances of Darwinula stevensoni are typically higher in the intermediate depth range from 5–15 m. Neglecandona angulata occurs over the full depth range where ostracod remains were recorded but higher abundances were recorded only at a few locations in a range of 16–25 m. Humphcypris subterranea was more sporadically recorded, mostly in shallower waters down to 8 m. Highest abundances were found in the delta region of the Jordan River at the northern lake shore, likely indicating that the species not only prefers springs and streams flowing from springs but flowing waters in general (Hartmann 1964; Mischke et al. 2014a). A somewhat similar pattern (shallow water depth, higher abundance in the delta region of the Jordan River) is observed for Cypridopsis vidua, Isocypris beauchampi and Pseudocandona sp. (Figs. 5–6). Valves and carapaces of these taxa possibly and at least partly originated from the flowing waters on the Jordan River fan. In contrast, Heterocypris salina, Herpetocypris sp., P. cf. mastigophora and C. kingsleii were recorded predominantly or exclusively near the southeastern margin of the lake which is shallow with a relatively flat lake floor and dense emerged vegetation of reeds and sedges near and on the shore. Loxoconcha galilea, only reported from the modern lake and the lake’s deposits of last glacial maximum (LGM) age so far, was found at only three widely-spaced locations at depths between 5–16 m (Figs. 5–6; Lerner-Seggev 1968; Mischke et al. 2014a).
Of the 15 taxa recorded in our survey, seven species (P. cf. mastigophora as P. producta, a younger synonym) were already reported from the Sea of Galilee in the study by Lerner-Seggev (1968). A few, poorly preserved and thus, not thoroughly examined specimens of Candona and Candonopsis were noted by Lerner-Seggev (1968) too. These likely correspond to specimens identified from the 2012 materials as Neglecandona angulata (species formerly assigned to Candona and recently combined with Neglecandona; Meisch et al. 2019), as Pseudocandona sp. (with many species formerly assigned to Candona and combined with Pseudocandona in the 1970s and later on; Danielopol 1973; Meisch 2000), and as Candonopsis kingsleii. Assuming these three taxa were recorded by Lerner-Seggev (1968), only H. subterranea, H. salina, C. vidua, Herpetocypris sp. and I. beauchampi were apparently not identified in the study from the 1960s. These five taxa were mostly recorded in the Jordan River delta region or near the southeastern shore of the lake which were not included in the littoral samples or two depth transects of Lerner-Seggev (1968). Thus, the higher number of taxa recorded in the 2012 survey probably results from the higher number of samples collected all over the lake rather than because of recent introductions to the lake. The comparability of both studies is also limited due to the use of dip nets in littoral areas and an Emery-type grab and Ockelman dredge at larger depth in the 1960s, and an Ekman-Birge grab in 2012. Lerner-Seggev (1968) stated that the results of her “research are qualitative” whilst specimens per 100 mL of surface mud were counted in the newly conducted study. However, taxa recorded in the 1960s were all identified in the 2012 material too, suggesting that the ostracod assemblage of the Sea of Galilee did not experience significant changes. This inference is surprising, given the large variations in the volume of water extracted from the lake via the National Water Carrier of Israel and the resulting lake-level and water-chemistry changes, and the significant changes in the nitrogen and phosphorus fluxes in the lake during recent decades (Nishri and Hamilton 2010; Rimmer and Nishri 2014; Gophen 2020).
Detailed data for the distribution of the recorded ostracod taxa in the Sea of Galilee. Panels are arranged with most abundant taxon to the upper left and least abundant to the lower right (note the three different abundance scales). Taxa codes in Table 1
Comparison of the data for 2012 with the ostracod assemblage from the last ice age
Although clearly dominant in the 2012 samples, C. torosa is even more abundant in the ice-age sediment samples collected from the archaeological site Ohalo-II at the southwestern shore of the lake in comparison to the accompanying taxa (Figs. 2, 7). Noded specimens of C. torosa are significantly less abundant in the Ohalo deposits, indicating that conditions were probably more brackish in the LGM lake (Frenzel et al. 2012). However, drier conditions are not necessarily implied for the LGM due to the well-documented precipitation-driven higher discharge of saline brines from deep sources in the Sea of Galilee region and the resulting direct relationship between catchment precipitation and lake salinity (Goldschmidt et al. 1967; Gvirtzman et al. 1997; Rimmer et al. 1999). Higher precipitation in the region during the LGM was recently inferred from another archaeological site at the Jordan River upstream of the Sea of Galilee which confirmed earlier reconstructions of highest lake levels in the Dead Sea Basin (Torfstein et al. 2013; Rice et al. 2023; Bunin et al. 2024). However, there are a few additional ostracod taxa recorded at Ohalo-II which were not found in samples from 2012. Among them are typical freshwater species such as Gomphocythere ortali and Candona candida and other species commonly found at slightly more brackish locations such as Sarscypridopsis aculeata or Neglecandona neglecta (Fig. 7). In addition, valves of Prionocypris zenkeri had been recorded at Ohalo-II, a species typically occurring in slowly flowing streams with dense aquatic vegetation (Meisch 2000).
Comparison of mean relative species abundance data for the ostracod assemblage from the Sea of Galilee in 2012 and that of the trench samples from the archaeological site Ohalo-II at the southwestern shore of the lake, dated to ca. 23 ka. (only samples with a minimum of 50 valves (disarticulated and articulated) included)
These additional species, although recorded with very low numbers, probably result from the larger number of samples investigated for the LGM location, the integrated record of ca. 5 ka duration, from ca. 25 − 20 ka, and the proximity of the shore and transport of ostracods into the lake from nearby springs and streams (Fig. 7; Mischke et al. 2014a).
Assessment of post-mortem transport of ostracod remains to the seasonally anoxic zone of the lake
The numbers of recorded ostracod valves and carapaces per 100 mL of collected surface sediment vary over a large range at a lake depth of 18 m and shallower (range, average and standard deviation for valves: 4-393, 120, 109; for carapaces: 0–73, 20, 20; Fig. 4). In contrast, valves and carapaces are significantly less abundant at lake depths larger than 18 m (range, average and standard deviation for valves: 0–42, 6, 10; for carapaces: 0–4, 1, 1). This distributional pattern with a relatively abrupt change at ca. 18 m depth suggests that most of the benthic environment of the hypolimnion, the seasonally anoxic lake floor beneath 18 m depth, is not occupied by living ostracods during the period of water-column mixing in winter. A homogenous water temperature in the water column of the Sea of Galilee and mixing typically occur from mid-December to mid-March but weather conditions may shorten or extend this period to ca. three weeks or four months, respectively (Nishri et al. 1998). The observed significant change in the numbers of ostracod remains at 18 m depth corresponds to the position of the thermocline in the lake, reported to occur between 17–19 m (Marti and Imberger 2008; Imberger and Marti 2014). However, one live specimen of C. torosa f. littoralis was recorded at 19 m and two animals at 22 m lake depth during the sampling campaign in January 2012 in the northern part of the lake (Fig. 3). Thus, the uppermost part of the lake floor beneath the thermocline might be here and there inhabited by ostracods due to the shorter duration of anoxic conditions at shallower water depth (Fig. 2). Alternatively, the colder inflowing waters of the Jordan River possibly cause an underflow of well-oxygenated waters in this region of the lake, supporting ostracods at a lake depth which is typically not oxic in regions more distal from the river delta. In general, the paucity of ostracod valves and carapaces on the lake floor below the depth of the thermocline provide evidence that most of these valves and carapaces were transported to the hypolimnion of the Sea of Galilee by wind-driven currents and waves. Valves were absent at only two of the 68 sampling locations with lake depths of 37.3 and 39.6 m, situated in the central and deepest part of the lake (Fig. 2). In contrast, carapaces were not recorded at ten locations including a single position above the thermocline, located at 1.8 m depth near the northeastern shore of the lake (Fig. 2). The lower abundance of carapaces below the thermocline in comparison to valves shows that the lighter and relatively disc- or bowl-shaped disarticulated valves are apparently more efficiently transported over longer distances by currents. Studies by Reyment (1960), Kilenyi (1971) and Kontrovitz (1975) suggested that ostracod valves are probably less easily mobilized than carapaces by currents but once brought into suspension, may travel longer distances than carapaces. Zhai et al. (2015) and Mao et al. (2021) recognized that the post-mortem transport of especially juvenile ostracod valves is significant in three large brackish lakes on the Mongolian Plateau in Inner Mongolia (Northern China) and a freshwater lake in the Tianshan Mountains in Xinjiang (Northwestern China). Their lakes are all shallower than 14 m, and a similar post-mortem dispersal may apply, especially for juvenile valves in the shallower parts of the Sea of Galilee. However, the paucity of valves and carapaces in the surface deposits of the Sea of Galilee at greater depth shows that reworking and transport of ostracod remains to the more central and deeper part of the lake is insignificant. Thus, ostracod valves from locations in lake basins unaffected by most-mortem transport represent the local habitat characteristics as the main control of the recovered species-assemblage composition unless the specific basin is affected by conditions such as frequent mass movements (Wirth et al. 2011; Simonneau et al. 2013).
Table 1
Ostracod taxa from the Sea of Galilee with numbers of animals collected alive, valves, carapaces and locations the taxon was found (total number of locations: 68)
Taxon
|
Code
|
Animals
|
Valves
|
Carapaces
|
Sampling locations
|
Cyprideis torosa forma torosa (Jones, 1850)
|
Ctorft
|
10
|
3842
|
421
|
63
|
Cyprideis torosa forma littoralis (Jones, 1850)
|
Ctorfl
|
13
|
1235
|
446
|
63
|
Ilyocypris hartmanni Lerner-Seggev, 1968
|
Ihar
|
|
412
|
48
|
41
|
Ilyocypris cf. nitida Lerner-Seggev, 1968
|
Icfn
|
|
293
|
25
|
35
|
Darwinula stevensoni (Brady & Robertson, 1870)
|
Dste
|
|
124
|
68
|
33
|
Neglecandona angulata (G.W. Müller, 1900)
|
Nang
|
1
|
245
|
6
|
41
|
Humphcypris subterranea (Hartmann, 1964)
|
Hsub
|
|
58
|
|
8
|
Limnocythere inopinata (Baird, 1843)
|
Lino
|
|
22
|
13
|
19
|
Heterocypris salina (Brady, 1868)
|
Hsal
|
|
23
|
|
2
|
Cypridopsis vidua (O.F. Müller, 1776)
|
Cvid
|
|
12
|
|
6
|
Herpetocypris sp.
|
Herp
|
|
8
|
|
5
|
Isocypris beauchampi (Paris, 1920)
|
Ibea
|
|
6
|
|
2
|
Pseudocandona sp.
|
Pseu
|
|
3
|
1
|
4
|
Loxoconcha galilea Lerner-Seggev, 1968
|
Lgal
|
|
3
|
1
|
3
|
Potamocypris cf. mastigophora (Méthuen, 1910)
|
Pcfm
|
|
4
|
|
3
|
Candonopsis kingsleii (Brady & Robertson, 1870)
|
Ckin
|
|
1
|
|
1
|