微型生态系统中浮游动物正磷酸盐排泄率的估算

在考查罗非鱼或鲢、鳙下行影响的微型生态系统实验后期,对其唯一的浮游动物──大型的特定正磷酸盐(PO4-P)排泄率按不同大小级别和培育时间进行了测定,同时估算了该浮游动物种群的PO4-P排泄率。结果表明,大型的特定PO4-P排泄率依其体重的增长呈指数式下降,并且随着培育时间的增加而大大降低。有鱼系统中大型种群的PO4-P排泄率及其与浮游植物PO4-P摄取率的比值都显著地小于无鱼系统,而放养罗非鱼或鳙的系统之二项估算值又明显地比鲢单养系统低。看来,大型种群的磷排泄在鲢单养系统的PO4-P再生中起有一定的作用,而在放养罗非鱼或鳙的系统中所起的作用很小。  

ESTIMATION OF ORTHOPHOSPHATE EXCRETION RATE OFZOOPLANKTON IN FRESHWATER MICROCOSMS

In the later period of the experiments to investigate the top-down effects of Oreochromis niloticus or Hypophthalmichthys molitrix and Aristichthys nobilis in freshwater microcosms, the specific orthophosphate(PO4-P) excretion rate of Daphnia magna, the sole species of zooplankton in the microcosms, was measured for the different size classes of the animal and in the different incubation time. Moreover, the rate of PO4-P cxcretion (Epz) by D. magna population in the microcosms was estimated with the equation Epz= Espi Bi, where Esp=the spceific PO4-P excretion rate, B=biomass (dry weight), and n=size classes of the animal. The results indicated that the specific PO4-P excretion rate of D. magna decreased exponentially with the increase of individual weight and reduced markedly as incubation time increased. And the rate of PO4-P excretion by D. magna population and its ratio to the rate of PO4-P uptake by phytoplankton in the microcosms stocked with fish were significantly lower than those in the fish-free microcosms, whereas the estimated values of the above indices in the microcosms with Oreochromis niloticus or Aristichthys nobilis were obviously smaller than those in the microcosms containing Hypophthalmichthys molitrix alone. It appears that phosphorus excretion by D. magna population have made some contributions to PO4-P regeneration in the microcosms stocked with Hypophthalmichthys molitrix alone but contributed inconsiderably to the recycling of phosphorus in the microcosms with Oreochromis niloticus or Aristichthys nobilis under the given experimental conditions.