Effects of Palaemon Paucidens on Zooplankton Community and Water Quality

27 Mar Effects of Palaemon Paucidens on Zooplankton Community and Water Quality

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Abstract

千葉県成田市北須賀の印旛沼漁業協同組合の敷地に同じように造成された 2 つの植生再生実験池では，一方は沈水植物が再生・繁茂し透明度が高い水界に，他方はアオコが発生し透明度が低い水界になった．後者では前者の 10 倍のスジエビが捕獲された．そこで，実験池内に 8 基の隔離水界を設置し，スジエビの在・不在を操作し，動物プランクトン群集と水質に与える影響を調べた．水質については，実験開始直後からスジエビ在の隔離水界で濁度，懸濁態物質 (SS)，全窒素 (TN)，全リン (TP)，クロロフィル a (Chl. a)，および溶存態有機炭素 (DOC) の値が有意に高くなった．ミジンコ類の総密度は，スジエビ在の隔離水界で有意に減少した．逆に，ワムシ類の密度は，有意に増加した．ミジンコ類のうち，大型および遊泳性のミジンコ類 (Daphnia 属，Diaphanosoma 属，Scapholeberis 属）の密度は，スジエビ在の隔離水界で有意に減少したが，小型の底生性ミジンコ類 (Alo 属，Chydorus 属) の密度については，有意差はなかった．ただし，スジエビ在区の栄養塩が実験開始直後に増加したのに比べ，スジエビ不在区でのミジンコ類の密度の増加は，遅めにあらわれた．そのため，スジエビの存在は，まず生物攪拌と栄養塩回帰を促し，その後大型甲殻類動物プランクトンを捕食することによるカスケード効果が加わり水質を悪化させ，浅い湖沼や池のレジームシフトを誘導することが明らかになった．

English Abstract

The effects of shrimp (Palaemon paucidens) on water quality and the zooplankton community were investigated by using eight enclosures in a restoration experiment pond of the Inbanuma Fishermen Cooperative Association in Kitasuga, rita, Chiba (Japan). In shrimp enclosures, NH4-N, total nitrogen (TN), and chlorophyll a (Chl a) concentrations increased soon after the start of the experiment, indicating shrimp excretion and bioturbation caused increased NH4-N, TN, and Chl a concentrations. Suspended solids (SS), TN, total phosphorus (TP), Chl a, and dissolved organic carbon (DOC) were significantly higher (P < 0.001) when shrimp were present. The total density of cladocerans and rotifers decreased significantly when shrimp were present but increased in the absence of shrimp. Among cladocerans, total densities of Daphnia, Diaphanosoma, and Scapholeberis decreased significantly (P < 0.05) when shrimp were present; however, the densities of Alo and Chydorus, which are small-sized benthic animals, did not differ significantly in the presence or absence of shrimp. The results of this study show that the presence of shrimp decreased water quality because nutrient recycling was accelerated by shrimp excretion and bioturbation, in addition to the trophic cascade effects, i.e. zooplankton community structure changes from shrimp through feeding on large cladocerans.