| In order to understand the temporal and spatial variation of Daphnia species resting-eggs density in the sediment of Lake Chaohu, the lake sediment samplings was collected in August 2014. Lake Chaohu was divided into three regions(namely, the eastern zone, middle zone and western zone) according to the morphological characteristics and nutritional status of the lake. Three Daphnia species, D. similoides, D. galeata and D. pulex, are identified in Lake Chaohu.According to 210 Pb method, the age of the 20 cm sediment layer in the eastern zone of Lake Chaohu was about 50 years. With the increase of the sediment layer, the densities of ephippia containing resting eggs and total ephippia of three Daphnia species showed an obvious trend which increased firstly and then decreased. Maximum densities of ephippia containing resting eggs(2.88±1.44) ×103 ind. /m2 and total ephippia(4.99±2.26) ×103 ind. /m2 of D. similoides appeared in the 6 cm of sediment layer. The maximum densities of the ephippia containing resting eggs(0.54±0.63) ×103 ind. /m2 and total ephippia(1.32±1.83) ×103 ind. /m2 of D. pulex were respectively in 9 cm and 11 cm. Maximum densities of the ephippia containing resting eggs(1.93±1.91) ×103 ind. /m2 and total ephippia(5.72±1.20) ×103 ind. /m2 of D. galeata occurred in 11 cm.The age of the 20 cm sediment layer in the middle zone of Lake Chaohu was also about 50 years, and with the increase of the sediment layer, the densities of ephippia containing resting eggs and total ephippia of three Daphnia species in the middle zones of the lake showed also obvious trend which increased firstly and then decreased. The maximum densities of both ephippia containing resting eggs(6.08±2.00) ×103 ind. /m2 and total ephippia(12.03±2.74) ×103 ind. /m2 of D. similoides appeared in the 6 cm of sediment layer. The maximum density(0.84±0.10) ×103 ind. /m2 of ephippia containing resting eggs of D. pulex occurred in 7 cm, and the maximum density of total ephippia(4.03±3.58) ×103 ind. /m2 was in 8 cm. The maximum densities of both ephippia containing resting eggs(2.05±0.89) ×103 ind. /m2 and total ephippia(4.33±3.80) ×103 ind. /m2 of D. galeata were in 7 cm. No ephippia of D. pulex and D. galeata were observed below 17 cm in the middle zones of the lake.The age of the 20 cm sediment layer in the western zone of Lake Chaohu was in 1981. The densities of ephippia containing resting eggs and total ephippia of D.similoides and D. pulex increased firstly and then decreased in the western zone of the lake. The maximum density(2.47±3.96) ×103 ind. /m2 of ephippia containing resting eggs in D. similoides appeared in the 6 cm of sediment layer while its total ephippial density(8.42±1.98) ×103 ind. /m2 occurred in 4 cm. There was no ephippia of D. similoides between 16 cm and 20 cm. The maximum densities of both ephippia containing resting eggs(1.50±2.14) ×103 ind. /m2 and total ephippia(5.90±6.34) ×103 ind. /m2 of D. pulex appeared in 4 cm. No ephippia of D. pulex were observed between 14 cm and 20 cm. The densities of ephippia containing resting eggs and total ephippia of D. galeata showed a gradual decrease trend with the increase of sediment layer, and their maximum densities were respectively(2.71±1.99) ×103 ind. /m2 and(9.69±7.68) ×103 ind. /m2. No ephippia of D. galeata were observed between 12 cm and 20 cm.In the eastern zone of the lake, pearson’ correlation analysis showed that the relationship between total phosphorus(TP) concentration in the sediment and total ephippial density of D. similoides was very significant(P=0.001), and there was significant relationship(P=0.006) between total nitrogen(TN) concentration and the density of ephippia containing resting eggs of D. similoides. In the middle zone of the lake, very significant relationships(P<0.01) between TP concentration and total ephippial densities of three Daphnia species were observed while there were very significant relationships(P<0.01)between TN concentration and ephippia containing resting eggs of three Daphnia species. In the western zone of the lake, significant relationship(P<0.05) between TN concentration and total ephippial density of three Daphnia species were observed while there were significant relationship(P<0.05) between TP concentration and the density of ephippia containing resting eggs. One-way ANOVA(LSD test) showed that very significant differences(P<0.01) of ephippia containing resting eggs and total ephippia of D. similoides between the middle and eastern zone of the lake were observed as well as the western zone of the lake, while there was no significant difference(P>0.05) between the eastern and western zone of the lake. However, no significant differences(P>0.05) of ephippia containing resting eggs and total ephippia of D. pulex and D. galeata among three zones of the lake were observed. |
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