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 200 μm 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 μm and 200 μm 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 phytoplankton 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.