Magnesium isotopes reveal bedrock impacts on stream organisms.

Magnesium is an essential element to aquatic organisms and understanding the origin of Mg is important for understanding their growth. Ultimately, Mg in streams is derived from the chemical weathering of bedrock in the catchment. In this study, we used Mg stable isotope ratios (δMg) to test whether stream organisms reflect lithological sources in stream catchments. In November 2017 and May 2018, we sampled aquatic insects and small gobies from six temperate streams in the Lake Biwa area (central Japan). Three of these streams had up to 38% limestone in their catchment (limestone streams), and three streams lacked limestone (non-limestone streams). We hypothesised that stream organisms from limestone streams had significantly lower δMg values compared to those of the same organisms from non-limestone streams. Aquatic insects from limestone streams had an average of 0.78‰ lower δMg values than those of the same organisms from non-limestone streams, thereby indicating a lithological control on the δMg of aquatic insects. Aquatic insects often showed an offset to higher δMg values compared to those of stream water, thereby pointing to a Mg-enriched diet as an additional Mg source to water and/or Mg isotope fractionation during Mg accumulation. Instead, stream water was the main Mg source for small gobies, as their bones reflected the δMg of water. We concluded that δMg could trace Mg sources of aquatic organisms, and the same methodology can be applied to other metals.