南海北部浮游桡足类对浮游植物的摄食压力

Copepod grazing pressure on phytoplankton in northern South China Sea

于2004年2月（冬季）和8月（夏季）在南海北部使用肠道色素法研究了浮游桡足类群落对浮游植物现存量的摄食压力。将底到表垂直拖网（网孔径200μm）获得的桡足类分为中型（〉500μm）和小型（200—500μm）两个体长组。冬季：中型桡足类的丰度为103—2343（平均633）ind／m^3，肠道色素为0．15～2．71（平均1．31）ng／ind，肠道排空率为0．011（A4）～0．019（c1）min^-1，个体摄食率为3．13—58．48（平均28．36）ng／（ind·d），群落摄食率为1902—62369（平均18679）ng／（d·m^3），对浮游植物生物量的摄食压力为每天0．25％-6．39％（平均3．04％）。小型桡足类的丰度为418—6250（平均1691）ind／m^3，肠道色素为0．18—3．44（平均1．19）ng／ind，肠道排空率为0．012（A4）-0．016（C1）min^-1，个体摄食率为3．67—70．04（平均24．34）ng／（ind·d），群落摄食率为1540—158561（平均44227）ng／（d·m^3），对浮游植物生物量的摄食压力为每天0．44％-15．70％（平均6．59％）。浮游桡足类群体对浮游植物生物量的摄食压力为每天1．02％-20．10％（平均9．63％）。夏季：中型桡足类的丰度为111—1298（平均621）ind／m^3，肠道色素分别为0．22—1．58（平均1．03）ng／ind，肠道排空率为0．017min^-1，个体摄食率为5．52—39．92（平均25．95）ng／（ind·d），群落摄食率为4411-26667（平均12878）ng／（d·m^3），对浮游植物生物量的摄食压力为每天0．23％-6．52％（平均2．52％）。小型桡足类的丰度为756—8804（平均2990）ind／m^3，肠道色素含量为0．09—2．92（平均0．87）ng／ind，肠道排空率为0．020min^-1，个体摄食率为2．73—87．77（平均26．26）ng／（ind·d），群落摄食率为9309—139817（平均47191）ng／（d·m^3），对浮游植物生物量的摄食压力为0．86％-37．79％（平均10．99％）。浮游桡足类群体对浮游植物生物量的摄食压力为1．09％-39．95％（平均13．51％）。数据表明，2月份和8月份桡足类群体对浮游植物现存量的摄食压力较低，桡足类摄食不是南海北部浮游植物死亡的主要原因。

To evaluate the role of planktonic copepods in the carbon cycling in northern South China Sea, using gut pigment method, copepod grazing pressure on phytoplankton stock was studied during 10 February --6 March representative of winter and 26 August --6 September representative of summer, 2004 in northern South China Sea （ 110.99 -- 116.79°E, 18.48 -- 23.12°N）. Zooplankton samples were towed in duplicate with a net in 200μm mesh at each station. One set of the samples was fixed to determine copepod abundance; another set was classified into large, medium and small size groups in 1000, 500 lxm and 200 lxm respectively. The contents of copepod guts, gut evacuation rate and gut pigment content were determined except for the large group because of insufficient statistical amount. Results show that in winter, gut pigment content, gut evacuation rate, individual grazing rate and abundance of medium copepods were 0.15 --2.71 ng/ind, 0.011 --0. 019 min-1, 3.13 --58.48 ng/（ ind-d）, 103 --2343 ind/m3, respectively; and the medium copepod grazed at rate of 1902 --62369 ng/（ d·m3）, which in other words, exerted daily grazing pressure at 0.25 %- 6.39 % on phytoplffnkton living stock. Similarly, those of small copepods were 0.18 --3.44 ng/ind, 0.012 --0,016 min-1, 3.67 --70.04 ng/（ ind·d）, 418 --6250 ind/m3, respectively; and the small group grazed at rate of 1540 --158561 ng/（ d·m3）, or 0.44%- 15.70% per day on the stock. Altogether, the daily grazing pressure of the copepod community （medium and small） on the phytoplankton stock was 1.02%--20.10% . In summer, gut pigment content, gut evacuation rate, individual grazing rate and abundance of medium copepods were 0.22 -- 1.58 ng/ind, 0. 017 min-1, 5.52 - 39.92 ng/（ ind·d）, 111 -- 1298 ind/m3, respectively ; and the medium copepod grazed at rate of 4411 --26667 ng/（ d·m3） or daily 0.23 %- 6.52% on the stock. Similarly, the corresponding values for the small group copepods were 0.09 - 2.92 ng/ind, 0. 020 min - 1, 2.73 -- 87.77 ng/（ ind·d）, 756 - 8804 ind/m3, respectively, and they grazed at rate of 9309 --139817 ng/（ d·m3）, or alternatively, a daily grazing pressure of 0.86% -37.79% on the stock. The combined daily grazing pressure from copepod community （ medium and small groups） on the stock was 1.09%--39.95% . No seasonal variety of copepod grazing pressure on stock was observed between the two seasons. The grazing pressure by copepod therefore should not have been the main cause of phytoplankton loss.