Reproduction, Population Recruitment And Life History Of Calanus Sinicus In The Yellow Sea

As the key zooplankton species of the Yellow Sea and East China Sea,Calanus sinicus links the primary production to the higher trophic levels, occupying the central position of the marine ecosystem. Base on the 9 cruises in the Yellow Sea and 11 cruises in the Jiaozhou Bay during March 2006 to August 2007 and several experiments in laboratory, we studied the seasonal variations of the reproduction, population recruitment and oil storage in C. sinicus, hoping to illustrate the population dynamics, clarify the driving factors and construct the conceptual model of its life history.In March-April, C. sinicus showed relatively high gonad maturity and fecundity, and active population recruitment over the Yellow Sea, matching the favorable food and temperature conditions. While the fecundities were still high in the nearshore areas, the population on the continental shelf of the Yellow Sea showed lower fecundity and some features in preparation for the over-summering in the Yellow Sea Cold Water Mass (YSCWM), such as the dominance of C5s and accumulation of lipids in the oil sac. During summer, both the reproduction and population recruitment were not successful in the Yellow Sea, especially in the YSCWM where the gonad development was arrested in the immature stages GS1-GS2 and no spawning was recorded. In October, the reproduction and recruitment in the tidal front areas were active, which might explain the recovery of the nearshore population in autumn-winter. After November, with the increasing of the vertical mixing, the YSCWM gradually disappeared. C. sinicus started to reproduce and recruit at very low rates until the next March-April when the active reproduction begins. Based on the fecundity, hatching success and female abundance, we calculated the potential population recruitment rate of C. sinicus. On the continental shelf, recruitment was restricted in March-April when active recruitment occurred and in winter when the recruitment was very low. In the nearshore areas, however, the population recruitment could occur in every season except for summer. In general, the spring recruitment was most important for C. sinicus population in the Yellow Sea.We studied the relationships between reproduction of C. sinicus and the possible influencing factors. The fecundity was closely related to the food conditions represented by the phytoplankton biomass and the ciliate abundance but not to the oil sac volume. Under starvation, the reproduction of C. sinicus could only sustain for 3-5 days, while it would recover spawning after a few days of food addition. The oil storage was consumed during starvation but could not prevent the decrease of egg production. Based on these results, we conclude that the energy for reproduction was mainly from the recent diet, but not from the inner lipid storage which might be a kind of energy buffer coping for the unfavorable conditions.Fecundity was positively related to the prosome length of females, which was possibly mediated by the relationship between prosome length and clutch size. There was no significant relationship between the fecundity and temperature, however, temperature could affect the egg production indirectly by influencing the metabolic rates and prosome length.It was worthy to note that there was close and significant relationship between the egg production rate of C. sinicus and its gonad maturity. This relationship might enable us to estimate the fecundity by examining the preserved samples and compare the difference in long term studies such as the climate change effect on copepods.Spring diatom bloom was a major seasonal feature in the Yellow Sea and the adjacent areas. Based on the results of several cruises, we found that the effects of diatom bloom on the reproduction of C. sinicus were complex. In March 2006, the diatom bloom consisted by Skeletonema costatum occurred in the eastern Jiaozhou Bay. During this bloom, the fecundity of C. sinicus was high but its hatching success was only ~50%. The diatom (Landeria annulata) bloom occurred in the same area of the Jiaozhou Bay in February 2007, during which time the hatching success was near 100%. In April 2006, the fecundity and hatching success were only moderate during a diatom bloom consisted by Thalassiosira pacifica. In April 2007, a diatom (Skeletonema costatum) bloom occurred in the central areas of the Yellow Sea. The average fecundity of C. sinicus was as high as 27.8 eggs/fem/day, which was much higher than the previous studies. Generally, the hatching success and nuapliar survival was also high. We conclude that this diatom bloom may stimulate the population recruitment for the population on the continental shelf of the Yellow Sea.We studied the regional and seasonal variations of the lipid accumulation in C. sinicus C5. The oil sac volume was generally small in the nearshore areas in the Yellow Sea. However, the seasonal variation of oil sac volume in the population on the continental shelf showed close match with over-summering. The maximum oil sac appeared in May-June when the population prepared for over-summering. Then the oil sac volume gradually decreased possibly consumed by the metabolic needs during the dormancy. In December, the oil storage decreased to the same level as the nearshore population. The oil accumulation restarts in next April when food conditions get better. Besides the energy source for the dormant population, the lipid storage might also provide a cue for the dormancy, which has critical importance during the life history of C. sinicus.Since there were distinct water masses, population abundance and structure, reproductive characteristics, population recruitment and the oil storage strategies in the nearshore areas and the continental shelf in the Yellow Sea, we discussed the life history in the two areas separately. The conceptual life history model of C. sinicus on the continental shelf was as follows. There were possibly 4-5 generations per year. In late December, the dominant C5 began molting and maturing. The offspring of this over-summering generation composed G0, which developed during the winter in food-poor environment and might produce G1 which could mature in March, when the food conditions began to turn better. During March-mid May, there were likely 2 new generations, G2 and G3. When the food environment favors, there would be a new generation G4 after mid-May. G3 and G4 were supposed to enter dormancy after June. It was likely that a small part of the C5s from the G3 and G4 would molt to be adults, while a great portion of the C5s remain under diapause until the termination of over-summering in December, when the C5s of G3 and G4 molted to adults and started the new generation. The gonad maturity and lipid storage could match this conceptual model very well and the 2 parameters showed sensitivity to the environment, so we conclude that the 2 parameters could be applied for the studies concerning the effect of climate change on population and life history.We also discussed the population recruitment patterns, possible generations and life histories of the nearshore population which had not been fully studied. Compared with the population on the continental shelf, the nearshore population had generally higher fecundities and turnover rate. Besides, the nearshore areas provided several spawning and feeding grounds, it was meaningful to fully understand the ecology and life history of the nearshore C. sinicus population in the future.