Toxins are produced by algae or plankton, and when those get at high concentrations, you can get sufficient quantities of toxins produced by these algae into the marine environment, and that can work its way up the food chain.
But given that the three countries already collaborate closely in many areas—they make up, with Canada and New Zealand, the “Five Eyes” intelligence-sharing pact—it is the novelty of Australia’s nuclear submarines that has garnered the most attention.