A numerical assessment for the effect of filling a sea caldron on seaweed expansion and habitat environment for aquatic species.

Tomoki Izumi, Takaaki Kanaguchi, Masayuki Fujihara

Monday 29 june 2015

14:35 - 14:50h at Europe 1 & 2 (level 0)

Themes: (T) Hydro-environment, (ST) Ecohydraulics and ecohydrology

Parallel session: 2G. Environment – Ecohydraulic

An assessment for the effect of a fishing ground newly constructed by filling a sea caldron, whose bottom is generally formed by muddy sediment and thus which often has low productivity of fisheries, on seaweed (Sargassum horneri) expansion and habitats for sea grass (Zostera marina) and Cephalochordate (Branchiostoma belcheri) around the new fishing ground is conducted by using numerical hydrodynamic model and Habitat Evaluation Procedure (HEP). At first, tidal current in the target area of Seto Inland Sea, Japan, is reproduced by using a multi-level density flow model and then the current after filling the sea caldron is estimated. From these results, change of the flow field between before and after filling it becomes clear and consequently distribution of resuspended particle sizes on the sea bottom is estimated based on the threshold of sediment movement. After filling, maximum tidal current become 10 to 30 cm/s faster than before and the resuspended particle size is estimated 0.2 to 0.4 mm at the top of the filled area. Next, for estimation of the seaweed expansion, germlings released from the observed natural Sargassum bed are tracked based on the resultant tidal current by the use of an Euler-Lagrangian transport model and settling area of them is estimated. The result shows that the germlings can reach a part of the filled area. Thus if adhesive substrates are properly arranged at the reached area, the self-formation of the Sargassum bed would be expected. Lastly, Habitat Suitability Index (HSI) is developed for the evaluation of habitat environment for Zostera marina and Branchiostoma belcheri. Based on earlier studies and our field observation, velocity on the sea bed and the Shields number are used for Suitability Index (SI) of Zostera marina; median particle diameter of the sea bed and mud content are used for SI of Branchiostoma belcheri. These results show that HSIs at one of three observation stations for Zostera marina and one of two for Branchiostoma belcheri slightly rise and those at other stations slightly degrade. Through the procedure mentioned above, useful information about change of hydrodynamic condition, seaweed expansion and habitat environment would be given for designing stage of this kind of project.