| Pelagic appendicularians are important secondary producers in marine ecosystem, second inabundance only to copepods. Owing to efficient feeding, frequently regenerated filtering organ（house）, high fecundityand short generationduration, theyare commonlyknownas a ‘shortcut’in marine food web. They can response to short-term environment changes more rapidly thanother zooplankton groups. Organic matters accumulated through non-selective feeding can betransported to high trophic level through predation, and outsides the water column in forms ofcarcass or discarded houses. Oikopleura dioica is the only appendicularian species presented inthe Jiaozhou Bay. In order to illustrate the tempo-spatial distribution of O. dioica and evaluate itsenvironmental adaptation, its annual variation in abundance and body size were investigatedmonthly at12stations. Its grazing pressure on the standing stock of phytoplankton was estimatedwith synchronous environmental data. We also compared the relationship between somatic andgonad growths with that obtained under controlled laboratory conditions.Our results indicated that O. dioica was most abundant in summer and scarce in spring in theJiaozhou Bay. From June to October, O. dioica swarmed in the north part of the bay with thehighest station-specific abundance of2575.0ind/m3, whereas it was observed only at2stationswith abundances lower than3.0ind/m3during March–May. The biomass of O. dioica was lowthroughout the year, and the highest biomass expressed in carbon was only810.8μgC/m3.However, daily food consumption of O. dioica was equivalent to0.07-10.81%of thephytoplankton standing stock. Based on its abundance and grazing impact, O. dioica is animportant primary consumer in the Jiaozhou Bay, which was always neglected in previous studies.Our results indicate that it’s a key zooplankton group for further understanding of the food webstructure of Jiaozhou Bay marine ecosystem.The monthly averaged body and gonad lengths varied between325.3-521.4and39.8–99.2μm, respectively. The gonad/bodyvolume ratio was the highest of0.17inJanuary, and lower than0.08in the other months. During bloom period from July to October, the population was consistedentirely immature stages. In the other months, proportion of mature stages was highest in January（22.5%）,and lower than7%intheothers.Comparingto the laboratoryresults, O.dioicareached shorter body and gonad lengths, and matured in shorter body size in the Jiaozhou Bay, despite thehigh natural Chl a concentrations. It was indicated that the growth and population recruitment of O.dioica can be resource limited in natural environments, under competition from other herbivores.In consistent with previous results, high O. dioica abundance in the Jiaozhou Bay wassupported by rapid environment change in flood season and high food availability in neriticwaters. However, comparing to laboratory results, shorter gonad length was achieved at samebody length under natural conditions, indicating resource limit in the Jiaozhou Bay. As the foodconcentration in our study was much higher than the reported lower threshold in laboratory,increased energy demand was expected in natural environments. Although the existing evidencefor population regulation is supportive only in predation, its population size in our study waslimited mostly by food supply. Energy demand can increase in natural environments throughpredator avoidance behavior, metabolic consumption under fluctuated conditions, increasedhouse renewal rate etc. Competition from other herbivores can also elevate the maintenance foodconcentration in natural environments. Combined with the sample observation and synchronousenvironmental data, our research indicated the tempo-spatial distribution traits (high in summer,low in spring, eutrophicated inner part preferred), high grazing pressure on the phytoplankton,limited growth and requirement of which revealed its ecological adaptation traits and providedbasic data in deeper understanding the ecological role of the appendicularians. |
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