Recent declines of macroalgal forests due to climate change imply that the aquaculture production of macroalgae may also be negatively affected by ocean warming. It has previously been shown that nutrient enrichment can offset the negative impact of warming on the survival and growth of the cultivated macroalga Undaria pinnatifida, although it can also increase the risk of herbivory by unrecognized mesograzers. Observations of several hundreds or thousands of the isopod Cymodocea japonica around U. pinnatifida cultivation ropes in autumn, prompted the analysis of their size-frequency distribution, relationship between body size and consumption rate, and combined effects of elevated temperature and nutrient enrichment on the consumption rate of the algae by the isopod in the current study. Although the body size of the isopods collected in autumn was smaller than those collected in winter and spring, the consumption rate of 600 small isopods was similar to the rate of a large individual. Additionally, the consumption rate doubled in response to an increase in temperature from 15 to 18°C and nutrient enrichment, and tripled with their combination. These results suggest that ocean warming combined with nutrient enrichment increase the risk of herbivory during U. pinnatifida cultivation, especially during isopod outbreak periods.
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