Response Of Oikopleura Dioica To Rapid Environmental Fluctuation In The Jiaozhou Bay

The appendicularian Oikopleura dioica is a filter-feeding gelatinous zooplankton with global distribution.This species holds a series of distinctive characters,such as simplified body plan,short generation time,high reproductive rates,small genome and efficient feeding ability,which make it attractive to both biologists and ecologists.Influenced by climate change and human activities,coastal eutrophication is becoming more and more serious,the population size of O.dioica shows a trend of explosive growth.The questions,why does individual miniaturization appear in eutrophic coastal waters,and whether the population increase can change from an exponential to a logistic model to avoid a final sudden collapse are still unanswered.Thus,it is necessary to evaluate the rapid response of O.dioica to the fluctuating environmental conditions.For one hand,it is of great practical and theoretical significance to understand the population dynamics and recruitment mechanism in eutrophic coastal waters,and for another hand,it can provide data support for exploring the impact of O.dioica on the eutrophic marine ecosystem.First,we studied the adaptability of O.dioica to nutrient fluctuations by a continuous survey(twice a week)in the Jiaozhou Bay from August 5 to September 27,2011.By analyzing the spatio-temporal variation of nutrients,chlorophyll a concentrations,and O.dioica abundance,the response time and pattern of population size to phytoplankton were revealed.Furthermore,by comparing the body sizes of individuals under natural and laboratory conditions,the reproductive mechanism and significance of early maturation under natural conditions were discussed.Second,we studied the regulation mechanism of intraspecific competition on population size via laboratory incubations in July 2017.To replace competitive pressure with the per capita food supply(PFS),by analyzing the PFS effects on somatic growth,reproduction performance,and the intrinsic rate of natural increase(r),the mechanism of population size regulation was revealed,and the population dynamics in natural conditions were discussed.Finally,we studied the effect of O.dioica on nutrient consumption via field experiment in the Jiaozhou Bay from October 22 to October 29,2015.By comparing the pathways of nutrient consumption and the scale of phytoplankton blooms between the field experiment and the field investigations,it will provide a basis for further research on the control of eutrophication and phytoplankton blooms by O.dioica in the future.In the investigations of the Jiaozhou Bay during flood seasons,the abundance of O.dioica was significantly positively correlated with the biomass of nano-and pico-phytoplankton(<20?m).They swarmed in the north part of the bay near the estuary,with the highest station-specific abundance of 28107 ind.m^-3.Our results showed that the eutrophication of seawater caused by rainfall and land-based input had led to a rapid increase in the biomass of nano-and pico-phytoplankton,which further led to an exponential growth of O.dioica population in a short time through the transmission of the food chain.The population size peaked with the blooms of nano-phytoplankton,and fell to a low value when micro-phytoplankton became prevalent,this can be used to indicate environmental changes in ecological study.Affected by feeding preference,O.dioica caused huge feeding pressure on nano-phytoplankton,which was conducive to the restoration of micro-phytoplankton as dominant species,and thus promoted the large-scale phytoplankton community.In addition,in the eutrophic environment of the Jiaozhou Bay during the flood seasons,mature individuals of O.dioica attained body lengths ranging from 115 to 619?m,which were much shorter than the laboratory population at the same temperature.Compared with the body/gonad length relationship established in laboratory studies,the natural population in the Jiaozhou Bay had greater gonad length at the same body length.The difference of gonad length between natural and laboratory specimens was-1-6?m for matures and 25-31?m for immatures at various food regimes.Our results showed that the miniaturization of O.dioica in eutrophic coastal waters was achieved by shortening generation time and reducing fecundity.Considering that maturation at smaller body size and shorter generation time arising therefrom can result in higher intrinsic rates of natural population increase,it suggests that early maturation favors a rapid response of O.dioica to fleeting phytoplankton blooms.Our results explained why smaller body size is usually achieved in natural environments comparing to those in laboratory,and revealed the response and impacts on size structure of phytoplankton communities.For O.dioica cohorts that were cultivated in various food and density regimes,the growth and reproduction performance were affected by the food concentrations and population densities,which were significantly related to PFS.Somatic growth,represented by body length,was saturated above the PFS of 8?g C ind^-1,and below this threshold,individuals reached small body and gonad lengths,and maturation was rarely observed during the incubation period.Mature individuals were present at all food concentrations,but only above another PFS threshold,4.5?g C ind^-1.Our results showed the r values were regulated by competition pressure via variabilities in maturation duration and the proportion of mature individuals in the cohorts.When the minimum PFS was satisfied in the designated generation time,no competition within the species,the r value tended to be regulated by the spawning proportion in the population.Otherwise,prolonged development duration and decreased r values were expected.Our results explained how natural populations maintain themselves when food is scarce,respond rapidly to phytoplankton blooms,and avoid overproliferation at the end of bloom periods.More importantly,for the first time,a mechanism was proposed in this study for efficient responses to fluctuations in food availability.In the field experiment of nutrient enrichment,no significant zooplankton grazing on the phytoplankton was observed,and the rapid consumption of nutrients was caused by the phytoplankton's explosive proliferation.However,in the eutrophic environment of the Jiaozhou Bay during flood seasons,a significant O.dioica grazing on the phytoplankton was observed,and the rapid consumption of nutrients was caused by the trophic cascade interactions,thus the occurrence of phytoplankton blooms were avoided.Our results showed O.dioica population can make a greater contribution to the consumption of eutrophication and fleeting phytoplankton blooms.This provides a good research direction for the in-depth understanding of the ecological significance of O.dioica.