The role of terrestrial humic substances on the shift of kelp community to crustose coralline algae community of the southern Hokkaido Island in the Japan Sea

Crustose coralline algae dominates coastal rocky shores (crust-dominated community) in certain world coastal zones after the disappearance of kelp forest community. Local fisheries suffer greatly when this occurs. Well known ecological hypothese to explain this phenomenon such as sea urchin excessive grazing and anomalously high temperature events do not satisfactorily explain the shift in algal composition in the Japan Sea coast. Previously we have shown that coralline algae has a competitive edge over other algae under the extremely low iron levels associated with deforestation in the Japan Sea along the coast of the southern Hokkaido Island. In the present study we present direct evidence that forest-derived humic substances inhibit crustose coralline algal spore germination and also promote macroalgal oogonium formation. Our results strongly suggest that suburban development associated with deforestation in the Japan Sea coast watershed reduced the flux of humic substances into coastal water and in turn created an environment in which coralline algae dominate over other algae. (C) 1999 Elsevier Science B.V. All rights reserved.