CHANGES IN DISTRIBUTION OF ULVA SPP. WITH SEASONAL AND TIDAL CYCLES ON TIDAL FLAT IN SETO INLAND SEA

Abstract

 Green tides, in which Ulva spp. overgrow and deposit on the coastline, cause various adverse environmental effects. We investigated seasonal and tidal changes in the distribution of Ulva spp. using a drone and an underwater camera for 2 years on Ohama beach of Innoshima Island in the Seto Inland Sea.

 Ulva spp. were distributed from the tidal flat (datum level: 0 m or more) to the sublittoral zone (datum level: -2.2 m) year-round. The Ulva spp. distribution on tidal flats changed due to the movement of the shoreline with the tide. Ulva spp., which had accumulated over a wide area of the tidal flat at low tide, moved to the shore side while accumulating near the shoreline as it moved due to the rising tide, but remained near the change point of the slope of the tidal flat even at high tide. In addition, the Ulva spp. distribution changed seasonally, and the coverage area expanded from spring (March‒April) and decreased from mid-August. The Ulva spp. distribution changes not only seasonally but also according to tides and topographic slopes, so it was considered necessary to consider all these factors to accurately estimate the abundance of these species.