Effects of Atmospheric Exposure on Periphytic Algal Communities in an Artificial Stream

27 Mar Effects of Atmospheric Exposure on Periphytic Algal Communities in an Artificial Stream

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Abstract

人工水路に設置した基質に生育した珪藻類のMelosira variansが優占する付着藻類群集を水路の外に移動し，大気暴露がクロロフィルaとして測定された現存量に及ぼす影響について検討した．晴天下で5時間大気暴露した群集と，24時間あるいは48時間大気暴露した群集の現存量の最大減少率に相違はなく，5時間の大気暴露でも藻類は明瞭に減少した．晴天下で5時間大気に暴露した群集の現存量は，水路に戻してから7日間の実験期間を通して大気暴露前に比べて少ない状態が維持された．それに対して，河床材料が移動しない規模のフラッシュ放流を受けた群集では3日後の現存量は減少したが，7日後にはフラッシュ放流前より多くなった．大気暴露後の藻類現存量コントロールの効果が長く持続したのは，大気暴露により乾燥して基質に固着した生物膜が，水中で徐々に剥離するためであることが無機物量の変化から明らかにされた．曇天下で5時間大気に暴露した群集では，暴露前と暴露後の7日間を通して現存量の変化はほとんどなかった．晴天下と雲天下における藻類現存量の変化の相違から，大気暴露時の天候により現存量コントロールの効果が明瞭に異なることが示された．

English Abstract

This study was intended to elucidate effects of atmospheric exposure on stream-side for biomass of periphytic algal communities, as estimated with chlorophyll a. Melosira varians-domi ted communities, developed on artificial substrates, were investigated in an experimental stream. No distinct difference was found in the maximum reduction rate of the biomass between algal communities exposed to the atmosphere under sunny weather for five hours and those exposed for 24h or 48h. Throughout the seven days after reinundation, the biomass of the communities after atmospheric exposure on stream-side under sunny weather for five hours was maintained at a lower level than before exposure to the atmosphere. In contrast, regarding communities exposed to artificial discharge with no bed-motion, although the biomass decreased after three days, it was greater after seven days than the level before discharge. The effects of controlling the biomass of algal communities were maintained longer after exposure to the atmosphere, as indicated by changes in the ash dry mass that were caused by adherence of the dried periphyton mat on the substrates. That adherence results from exposure to the atmosphere, but the layer gradually peels off in the water current. With exposure of the communities to the atmosphere under cloudy weather for five hours, a slight difference in the biomass was apparent throughout the seven days before and the seven days after exposure. Differences of changes in the biomass of algae in the current in sunny and cloudy conditions indicate that the effects of controlling the biomass differ depending on the weather at the time of atmospheric exposure.