Effects of Hypoxia and Hypomagnetic Field on Morphometric and Life-History Traits in Freshwater Cladoceran Daphnia magna

The intensity of climatic changes and human activities is increasing every year. The general consequence of these processes for freshwater ecosystems can be a dissolved oxygen decrease. There is also a possibility of a reduction in geomagnetic field intensity due to a reversal of the Earth’s magnetic poles. It is assumed that the magnetic poles’ reversal may proceed relatively quickly and coincide with global climatic changes. To evaluate the influence of these processes on aquatic organisms, we studied the effects of different dissolved oxygen levels (2 mg/L, 5 mg/L, and 8 mg/L) under the geomagnetic field (51.7 ± 0.2 μT) and hypomagnetic field (0 ± 0.2 μT) on the model freshwater crustacean Daphnia magna Straus. It was found that reduced oxygen levels and the hypomagnetic field led to a decrease in the sizes of parental females, a reduction in the number of produced offspring, and an increase in the period between broods. The newborns from the first brood in the hypomagnetic field were larger than that from the geomagnetic field. The dissolved oxygen level and magnetic environment affected the age of the first brood release and caudal spine length. The results imply that the probable coincidence of the geomagnetic pole reversal and the decrease in the dissolved oxygen level due to global climatic and geophysical processes will have a more negative impact on freshwater crustaceans than the occurrence of these processes at different times.