The Influence Of Nitrogen And Phosphorus Concentrations And Its Ratio On Population Dynamics And Sexual Reproduction In Two Cladocerans

The influence of nitrogen and phosphorus concentrations and its ratio on population dynamics and sexual reproduction in Moina irrasa and Daphnia carinata were studied in this paper.The results were as follows:1. The body length at the first pregnancy of M. irrasa and D. carinata increased with increased phosphorus concentration, whereas the time at the first pregnancy (mature time) decreased. Although phosphorous concentration had no significant effect on the number of offspring at first brood of M. irrasa, it significantly increased for D. carinata. Maximum population density of M. irrasa (1244±226ind./(200mL)) and D. carinata (363±13ind./(200mL)) appeared in5mg/L and10mg/L phosphorous groups, respectively. The maximum male density of M. irrasa (30±22.7ind./(200mL)) appeared in low phosphorous concentration (0.05mg/L), while it occurred at the higher phosphorous concentration groups for D. carinata.The maximum cumulative ephippia number of M. irrasa (509.5ind.) and D. carinata (62.3ind.) appeared in0.05mg/L and1mg/L phosphorous concentration groups, respectively. The maximum percentage of ephippia containing resting eggs of M. irrasa (69.3%) appeared at the low phosphorous concentration group (0.05mg/L). The percentage of ephippia containing resting eggs of D. carinata increased gradually with increased phosphorous concentration, and the ratio were above70%under higher phosphorous concentrations (5-10mg/L). Our results suggested that phosphorus-deficiency could advance and induce the production of ephippium in two cladocerans, and the higher male density promoted the resting egg formation of cladocerans.2. The body length at the first pregnancy of M. irrasa and D. carinata increased with increased nitrogen concentration, whereas the mature time decreased firstly and then increased. The number of offspring at first brood of M. irrasa dropped firstly and then increased with increased nitrogen concentration, but it significantly increased for D. carinata. Maximum population density of M. irrasa (250±44ind./(200mL)) and D. carinata (371±11ind./(200mL)) appeared at1mg/L and247mg/L nitrogen groups, respectively. The maximum male density of M. irrasa (28.3±9.7ind./(200mL)) appeared at low nitrogen group, while the maximum value of D. carinata (63.3±4.5ind.(200mL)-1) occurred at high nitrogen concentration group (247mg/L). The maximum cumulative ephippia number of M. irrasa (88.7ind.) and D. carinata (128.3ind.) appeared at0.5mg/L and1mg/L nitrogen groups, respectively. The maximum percentage of ephippia containing resting eggs of M. irrasa (98.3%) appeared in the low nitrogen concentration (0mg/L). The percentage of ephippia containing resting eggs of D. carinata decreased gradually with increased nitrogen concentration, and the ratio were above95%under lower nitrogen concentrations (0-0.5mg/L). There were significant effects of nitrogen concentration on the cumulative ephippia number of M. irrasa and D. carinata. Our results suggested that nitrogen-deficiency could also induce the production of ephippium in cladocerans.3. The mature time at the first pregnancy of M. irrasa and D. carinata increased with increased ratio of nitrogen and phosphorus, whereas the body length and No. offspring at first reproduction decreased. The body length at first brood under lower N: P ratio groups were slightly bigger than the higher N:P ratio groups. Mean maximum of no. offspring at first reproduction of D. carinata (12.3±1ind./per female) and M. irrasa (17.1±3.1ind./per female) appeared in lower N:P ratio groups (N:P=5), respectively. Our results suggested that lower N:P ratio could promote the growth and reproduction at first in M. irrasa and D. carinata.