Dissipative properties of various kinds of turbulent phenomena are investigated. Two expressions are derived for the rate of entropy increase due to thermal and viscous dissipation by turbulence, and for the rate of entropy increase in the surrounding system; both rates must be equal when the fluid system is in a steady state. Possibility is shown with these expressions that the steady-state properties of several different types of turbulent phenomena (Bénard-type thermal convection, turbulent shear flow, and the general circulation of the atmosphere and ocean) exhibit a unique state in which the rate of entropy increase in the surrounding system by the turbulent dissipation is at a maximum. The result suggests that the turbulent fluid system tends to be in a steady state with a distribution of eddies that produce the maximum rate of entropy increase in the nonequilibrium surroundings.

The relationship between coral distributions and physical variables was investigated in Amitori Bay, Iriomote Island, Japan. Field observations were conducted to obtain data on coral distributions, sea temperature, sea salinity, wind speed, and river flow rate. The observed data were then used in ocean and wave model numerical simulations and soil particle tracking analysis to obtain the spatial and temporal distributions of wave height and the numbers of soil particles. The main results of this study indicate that wave height and the number of soil particles have a significant correlation with coral distribution. Higher wave heights result in greater coverage of tabular coral and lower coverage of branching coral. A greater number of soil particles relates to lesser coverage of tabular coral. On the contrary, the number of soil particles is not correlated with branching coral coverage. The potential habitats for tabular corals largely depend on the species. Acropora hyacinthus is distributed in conditions with low numbers of soil particles, A. digitifera is distributed in conditions with low numbers of soil particles and high wave heights, and A. millepora is distributed in conditions with high numbers of soil particles and low wave heights. Averages of diversity index of the coral types at the mouth and inner parts of the bay are lower than average of the whole region, but average of diversity index at the intermediate part of the bay with the intermediate physical disturbances is higher than it, which seems to support the intermediate disturbance hypothesis.

SUMMARYThe mechanism of transitions among multiple steady states of thermohaline circulation is investigated from a thermodynamic viewpoint. An oceanic general-circulation model is used to obtain the multiple steady states under the same set of wind forcing and mixed boundary conditions, and the rate of entropy production is calculated during time integration. Three states with northern sinking and four states with southern sinking are shown to exist in the model by perturbing the high-latitude salinity. It is found that for transitions among southern sinkings the transition tends to occur from a state with a lower rate of entropy production to a state with a higher rate of entropy production, but the transition in the inverse direction does not occur. These transitions can thus be said to be irreversible or directional , in the direction of the increase of the rate of entropy production. For transitions between northern sinking and southern sinking, the rate of entropy production can either increase or decrease depending on the direction of the perturbation. The decrease is found to be associated with a collapse of the northern sinking circulation by a certain amount of negative salt (positive fresh water) perturbation to the northern hemisphere. After this collapse, a new southern sinking circulation develops, and the corresponding rate of entropy production increases. All these results tend to support the hypothesis that a nonlinear system is likely to move to a state with maximum entropy production by perturbation.

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2 東海大学 沖縄地域研究センター(〒907-1541 沖縄県八重山郡竹富町上原870-277) 3 正会員 (研)防災科学技術研究所 水・土砂防災研究部門(〒305-0006 茨城県つくば市天王台3-1) 4 東海大学 海洋学部海洋生物学科(〒424-8610 静岡県静岡市清水区折戸3-20-1) 西表島崎山湾におけるサンゴ分布と物理環境との関係を明らかにすることを目的として，調査研究を実 施した．まず，湾内72地点における群体形状別サンゴ被度分布と群集タイプ別サンゴ分布域の現地調査を 行い，次に，湾域の夏季と冬季の平均的な気象場における数値シミュレーションによる海洋流速場と土粒 子数分布の計算結果を解析し，現地調査で得られたサンゴ分布と比較した． 結果は，以下の通りである．1) サンゴ生育場所の底面流速が大きいほど，サンゴ被度が高い．また， 群集タイプにより，底面流速が異なる．2) サンゴ生育場所の土粒子数が多いほど，サンゴ被度が低い． また，土粒子数が多い場所では，主としてウミショウブが生育している．