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Abstract:The proliferation of drifting macroalgae and the accompanying occurrence of the impoverishment of zoobenthic species has become a major problem worldwide. We examined the temperature dependency of the effects of macroalgae on the survival of Manila clams in Nakaumi Lagoon in western Japan. We conducted (i) semi-monthly field surveys to clarify seasonal changes in macroalgal abundance and clam density and (ii) 7-day macroalgal enclosing and excluding experiments in two temperature settings (mean water temperatures: 19 and 29°C) to test temperature-induced differences in algal effects on bottom-water redox conditions and clam survival. Clam density decreased during summer (July-September), when increased coverage and thickness of drifting macroalgae was observed. In contrast, clam density increased from autumn to spring as macroalgal abundance decreased. Further, reducing conditions (Eh<0) were only detected in bottom waters in the presence of macroalgae during summer (water temperature >29°C), suggesting that drifting macroalgae induced hypoxia under summer temperatures and consequently led to clam mortality. The 7-day algal enclosure experiment resulted in reducing bottom water conditions when mean water temperature was 29°C. Further, clam survival decreased in the algal enclosure at 29°C, whereas no such changes were detected at 19°C. These results indicate that the effects of drifting macroalgae on the geochemical environment and clam survival are temperature dependent; therefore, global warming may enhance the likelihood of macroalgae-induced hypoxia with its associated adverse consequences in marine life.