The strontium isotope distribution in water and fish within major South African catchments.

Abstract

Strontium has 4 naturally-occurring isotopes (⁸⁴Sr, ⁸⁶Sr, ⁸⁷Sr, ⁸⁸Sr) all of which are stable (Faure, 1986). The correlation between the ⁸⁷Sr/⁸⁶Sr isotope ratio of lake water and fish fin spine tissue was investigated in 23 lakes within 4 major South African catchments. Data showed that fish within a specific lake all have the same Sr isotope ratio in their fin spine tissue regardless of species, age, sex and condition. The origin of the dissolved Sr fraction in lake water can be either from the natural weathering of upstream geological units or from an upstream anthropogenic source. The Sr isotopic ratios of the water samples were, however, constant over a multi-year period suggesting that the main source may be the more consistent geological environment. The Sr isotope ratio of river and lake water generally increases along the course of the rivers within the tertiary catchments of the areas investigated. In large rivers like the Vaal, where pollution also plays a role, the pattern is much more complicated. In the Olifants River catchment, Lake Middelburg, Lake Witbank and Lake Doornpoort have a similar Sr isotope ratio, which is distinct from Lake Bronkhorstspruit. Lake Loskop which is downstream from these lakes has a Sr isotope ratio between these two extremes, indicating mixing of water from upstream sources. Similarly Lake Arabie (Flag Boshielo), which is even further downstream, shows a Sr isotope composition between the composition of Lake Loskop and the lakes in the Elands River.