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.