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.
