Per Theodor (or Teodor) Cleve, Swedish chemist, mineralogist and marine biologist, studied at the University of Uppsala and in Paris and was awarded a doctoral degree in 1863. During the next years he studied complex metal compounds, ending his series of studies with a detailed work in English, On ammoniacal platinum bases (1972). He then began a series of analyses of the rare earth metals, particularly cerium, erbium and lanthanum. After preparing many new compounds of these metals he was able to prove that they were trivalent. He also demonstrated the tetravalence of thorium. His work on the rare earths led to the discovery of two new elements, holmium and thulium. He also published several papers on the chemistry of naphtalene. The mineral cleveite (a variety of pitchblende containing uranium oxide and rare-earths) was named after him. His chemical publications included Dictionaire de chimie (1883) and Analyse chimique qualitative (1885), as well as many papers (in French, Swedish and English).
After working in Paris for some time Cleve was appointed assistant professor in chemistry at Uppsala and also taught chemistry at the Stockholm Technological Institute from 1870 to 1874. He then became professor in general and agricultural chemistry at Uppsala. Later he chaired the Nobel Prize committee for chemistry from 1900 to 1905.
Cleve was, however, also an expert on marine plankton (minute animals and plants floating in the seas), and particularly diatoms (unicellar algae of the class Bacillariophyceae), which he described in numerous papers between 1867 and 1891. During a period of a few years he reported on the plankton of Finland (1891), Spitzbergen (1899), the North Sea (1899, 1902), Greenland (1900), and the Malay archipelago (1901). Hence J.D.F. Gilchrist*, government biologist of the Cape Colony, sent him the plankton recovered during the marine biological survey of South African coastal waters around the turn of the twentieth century. Cleve reported on the zooplankton in a series of papers under the general title "The plankton of the South African seas", in Marine investigations in South Africa. His four papers dealt with the Copepoda (a subclass of Crustacea; 1905, Vol. 3, pp. 177-210), Vermes (flatworms; 1908, Vol. 4, pp. 125-128), Halocypridae, and Cypridinae (families in the subclass Ostracoda of Crustacea; ibid, pp. 129-132 and 133-138). This work started the descriptive phase of biological oceanography in southern Africa. Among other findings Cleve reported that many of the species of Copepoda found south and west of the Cape Colony also occur in the northern hemisphere. This finding supported his hypothesis that the waters of the temperate North Atlantic originate in the Benguella Current, rather than in the Gulf Stream, supposedly passing as an under-current below the waters of the tropical Atlantic. He also found evidence that some plankton forms from East Africa may be carried in the mingled waters of the Agulhas and Antarctic currents to the west coast of America.