Combined Effects of Drift Macroalgal Bloom and Warming on Occurrence and Intensity of Diel-Cycling Hypoxia in a Eutrophic Coastal Lagoon

Recently, eutrophication-induced macroalgal bloom and elevating temperature caused by climate change have become major threats to benthic organisms by causing coastal hypoxia. However, combined effects of drift macroalgae and warming on the occurrence and intensity of hypoxia are not well understood, although these anthropogenic stressors have co-occurred. We conducted 10 seasonally replicated 7-day algal enclosure/exclosure experiments at a shallow coastal zone in the eutrophic Nakaumi Lagoon, western Japan, to evaluate the combined effects of drift algae and water warming on occurrence and intensity of diel-cycling hypoxia. Experimental units were 1x1m plots of the sandy bottom fenced within 1m height with plastic mesh that excluded or included drift algae, with automated dissolved oxygen (DO) sensors deployed at the sediment surface. DO fluctuated over a diel cycle for both algal treatments, and algal presence and elevated temperature additively amplified the diel DO cycle. Algal presence and elevated temperature synergistically increased the occurrence and intensity of diel-cycling hypoxia. The occurrence of hypoxia, including anoxia, increased non-linearly in the presence of algae when mean water temperature exceeded similar to 25 degrees C, whereas such drastic increase in the hypoxia occurrence was not observed in the absence of algae. Furthermore, the daily minimum DO declined more steeply with warming under algal presence than the absence of algae. These results suggest that coastal areas in the lagoon are now seriously threatened by simultaneous progressions of eutrophication linking to algal bloom and global warming.