Loss of Nutritionally Essential n-3 PUFA in Riverine Benthic Macroinvertebrates Following an Extreme Rainfall Event

The Anastasia River (southern Sakhalin Island) is a key salmon spawning ground, where summer storm floods can drastically alter benthic communities that form the diet of fish. This study assessed the impact of heavy rainfall on the benthic macroinvertebrates in the lower reaches of the river by analyzing taxonomic composition, biomass, and fatty acid (FA) profiles of dominant taxa before and after a flood event. A catastrophic decline in biomass was observed (from 35.7 ± 4.4 g m−2 to 1.74 ± 0.68 g m−2), alongside a significant shift in community structure. Crustaceans (dominated by Eogammarus kygi), with a unique FA profile rich in long-chain n-3 and n-6 polyunsaturated fatty acids (PUFAs), were the primary bearers of high nutritional value. All crustaceans exhibited omnivorous diets, with river crabs (Eriocheir japonica) having a broader spectrum than conspecifics inhabiting the marine littoral zone. Amphipods were key processors of allochthonous matter. The flood caused not only a quantitative but also a severe qualitative reduction in community nutritional value, with the content of physiologically crucial n-3 and n-6 PUFAs dropping by a factor of 25 and 15 on average, respectively. The disproportionately high loss of n-3 PUFAs indicates that the qualitative degradation of food resources by extreme floods may be an underestimated factor limiting the post-flood recovery of fish populations.