亚热带养殖海湾皱瘤海鞘生物沉积的现场研究

皱瘤海鞘是亚热带海域分布最广和数量最多的附着生物种类之一.2012年1月-7月在典型的亚热带养殖海湾-大亚湾大鹏澳海域,利用沉积物捕集器现场测定了皱瘤海鞘(Styela plicata)的生物沉积速率,并测定了生物沉积物中有机物(OM)、总碳(TC)、总氮(TN)、有机碳(OC)和有机氮(ON)含量.结果显示:皱瘤海鞘的生物沉积速率变化范围为每天145.5-1011.8m/个,平均每天516.0mg/个,海鞘的生物沉积速率变化范围为每天154.8-1065.8 mg/g干重,平均每大463.3 mg/g干重.海鞘生物沉积物中OM、TC、OC、TN和ON含量分别为14.38％、10.80％、2.87％、3.06％和0.86％,高于自然沉积物中的含量,分别为13.39％、7.36％、2.32％、2.29％和0.67％.其中TC和ON含量要显著高于自然沉积物(P＜0.05).皱瘤海鞘的OM、TC、OC、TN和ON的生物沉积速率分别为每天74.20,55.73,14.80,15.79和4.43 mg/个.实验期间附着在浮筏养殖设施和养殖牡蛎壳上的海鞘密度变化范围为54.9-222.1个/m2,平均147.5个/m2,养殖海域单位面积的海鞘生物沉积速率平均为每天76.1 g/m2,是自然沉积速率(平均每天62.7 g/m2)的1.21倍,其中OM、TC、OC、TN和ON的平均沉积速率分别为每天10.94,8.21,2.18,2.32和0.65 g/m2.据此可推算,大鹏澳筏式牡蛎养殖海区(约103 hm2)皱瘤海鞘的年生物沉积物负荷为29000 t,其中OM,TC,OC,TN和ON分别4100,3100,820,870和240 t.研究结果说明,海鞘等附着生物在大规模浅海贝类养殖中对养殖生态环境的影响也不容忽视.

An in situ study on biodeposition of ascidian (Styela plicata) in a subtropical aquaculture bay, southern China

A worldwide increase in farming of filter feeding bivalves and a growing awareness of the likely environmental impacts stem from biodeposition lead to extensive and well recognized studies. Whereas the suspended longline culture with massive and intensive facilities built (e.g., buoys, anchors and ropes) and bivalve shells in the farming area supply the footstone for the fouling organisms such as the ascidians and large number of breeding depended on them, which can be extremely abundant in areas of large-scale bivalve culture. Therefore, the fouling organisms may also influence the environment by remarkably increasing their amounts. There is an urgent need to examine the possible effects of biodeposition from the fouling organisms. The ascidian Styela plicata is one of the most predominant fouling species in tropical and subtropical bays. S. plicata is a filter feeding species like the bivalves. However, untill recently, the biodeposition of S. plicata was unknown. In order to evaluate the influence of S. plicata on the coastal environment, the biodeposition rate of ascidian S. plicata was investigated in Dapeng Cove, located in the southwest of the Daya Bay, a subtropical bay in southern China. Intensive oyster Crassostrea gigas longline culture activities were conducted there. The ascidians were sampled from the culture facilities and shells of the oyster Crassostrea gigas. The biodeposition rate was measured in situ in suspended culture areas by sediment traps from January to July, 2012. The contents of organic matters (OM), total carbon (TC), total nitrogen (TN), organic carbon (OC) and organic nitrogen (ON) in biodeposits and in natural sediments were analyzed and compared. Results showed that the biodeposition rate of S. plicata ranged from 145.5 to 1011.8 mg·ind^-1·d^-1, with an average of 516.0 mg·ind^-1·d^-1.The mean contents of OM, TC, OC, TN and ON in the biodeposits was 14.38%, 10.80%, 2.87%, 3.06% and 0.86%, respectively, and were higher than those of 13.39%, 7.36%, 2.32%, 2.29% and 0.67% respectively, in natural sediments. In particular, the contents of TC and ON in the biodeposits were significantly higher than those in the natural sediments (P<0.05). The biodeposition rate of OM, TC, OC, TN and ON by S. plicata were 74.20, 55.73, 14.80, 15.79 and 4.43 mg·ind^-1·d^-1, respectively. The density of S. plicata in the longline culture area ranged from 54.9 to 222.1 ind/m², with an average of 147.5 ind/m². The OM, TC, OC, TN and ON deposited by S. plicata was 10.94, 8.21, 2.18, 2.32 and 0.65 g·m^-2·d^-1, respectively. The average biodeposition rate of ascidian S. plicata in the farming area was 76.1 g·m^-2·d^-1, 1.21 times higher than the natural sedimentation rate (62.7 g·m^-2·d^-1) in the longline culture area. An estimated amount of 29000 t of biodeposits, 4100 t of OM, 3100 t of TC, 820t of OC, 870 t of TN and 240 t of ON can be produced by all the ascidians in suspend culture area in Dapeng Cove in one year. The results indicated that S. plicata has a high biodeposition rate and the species can be an important contributor to aggregating mineral and organic matter from water column to surface sediment, which may impact the physical, chemical and biological conditions of benthic environment. Therefore, when assessing the effects of suspended longline culture of bivalves on the coastal ecosystem, not only the farming bivalves, but also the fouling organisms especially the ascidians should be considered.