大型褐藻アラメおよびカジメの群落形成に関する実験生態学的研究 | A study on the experimental ecology of establishment of marine macrophyte population of Eisenia bicyclis and Ecklonia cava (Laminariaceae, Phaeophyta).

An urgent necessity for conservation and maintainance marine macrophytebed， especially kelp forest consisting of Eisenia bicyclis and Eclonia cava（Laminariaceae， Phaeophyta） becomes evident for healthy human activity andcontinuous recruitment of marine resources，In addition， creation of amenityarea in the sea is increasingly recognized in relation to the urban development.The macrophyte bed in the shallow sea plays an important role in this context，and basic knowledge on ecology of these macrophyte community is indispensablefor the development of techniques concerning the creation of macrophyte bed withminimum influence by man． The aim of this study is to provide ecologicalknowledge on the establishment of Eisenia and Ecklonia populations on the hardsubstrata artificially settled on the sea bottom.　 Three environmental factors， namely the light intensity， sand cover on thesubstratum and grazing pressure by herbivorous benthic animals， have definitiveeffects on development and structure of macrophyte vegetation． The effects ofthese factors were preliminarily assessed separately in the field choosing suitable sites as well as laboratory experiments． Eklonia vegetation was surveyed in a submarine cave in relation to the light intensity. Density and plant size diminished and disappeared at light intensity of 1 % full light.In the presence of dense sea urchin population, Eisenia cannot grow well on the coast of Fukushima Prefecture. The substratum covered seasonally with drift sand exclude the Eisenia and Ecklonia plants. The structure and distribution of Eisenia and Ecklonia were surveyed in aarea to the west of Miura Peninsula， Kanagawa Prefecture， Pacific coast of central Japan， where dense population is established in the favorable environment with low intensity of grazing． Eisenia occupies rather shallow range of -2 to -4m deep and Ecklonia forms population on hard substrata of -5 to -18m. 　Fluctuation of sand accumulation on the hard substrata affects the colonization and continuation of Eisenia and Ecklonia populations. Successive observation in the field confirmed that the deep sand cover on the hard substrata caused high level of mortality and eliminated the Eisenia and Ecklonia plants. Vertical distance from the surface of sandy bottom to keep each　population’s stable continuance was +30cm in the case of Eisenia and +20cm in the case of Ecklonia populations by the yearly average values. Laboratory experiments showed that the fibrous foldfast and growing point of Ecklonia are easily damaged by sand deposition than those of Eisenia.Species of Sargassum, other brown algal macrophyte, can tolerate the sand cover than Eisenia and Ecklonia. This affects the structure of macrophyte vegetation near sandy bottom, where a species of Sargassum grow in place of Ecklonia.Artificial substrata of concrete blocks of various size were settled on the sandy bottom at about -10 m deep to follow up the process of establishment of vegetation on them. Ecklonia population developed on the blocks, with a half of density and slightly smaller plant size as on the natural habitat, in three years after setting. Smaller blocks were often covered by sand. The temporal coverage by sand on the blocks caused certain modification of vegetation structure. Smaller blocks supported shorter Ecklonia plants than taller and larger blocks.Several Eisenia individuals grew on the blocks mixing with Ecklonia.　This might be caused by the higher water turbidity, stronger wave action, drift sand and insolation than on the natural rocks.The results obtained in this study provide a basic information in creation of artificial macrophyte beds, in selecting suitable sites and proper planning of artificial substrata.