Kinetics of phosphorus in Daphnia at different food concentrations and carbon:phosphorus ratios

Abstract

We examined the assimilation efficiency, excretion, and efflux of phosphorus (P) in adults and juveniles of Daphnia magna under different food levels (2-40 mg P L 21 ) and dietary carbon : phosphorus (C : P) ratios (90-930 in molar) with Chlamydomonas reinhardtii as food. The P assimilation efficiencies calculated by regression analysis were 38-85% and 66-89% for adults and juveniles, respectively, and were constant at food concentrations .24 mg P L 21 , but increased significantly when the diet shifted from P-sufficiency to P-deficiency. The mass-specific excretion rate of adults and juveniles was 1.1-33.2 ng P mg dry weight (DW) 21 h 21 and 3.0-63.4 ng P mg DW 21 h 21 , respectively, and was influenced by the food concentration and decreased with an increase in dietary C : P ratio. The efflux rate constants of the adults and juveniles were 0.182-0.298 d 21 and 0.096-0.185 d 21 , respectively. Food concentration did not affect the efflux, but an increase in dietary C : P ratio reduced the P efflux, suggesting stoichiometric regulation. Among the different routes involved in P loss from Daphnia, molting was the most important, contributing 44-75% of the total loss for the juveniles and adults. The mass specific loss rates were 13-54 ng P mg 21 h 21 and 45-110 ng P mg 21 h 21 . The relative and absolute P loss from each compartment (except the dissolved P release in adults) was independent of food concentration. Increasing the dietary C : P ratio decreased the mass-specific release rates by molting, dissolved P release, and reproduction, indicating the animals' endeavor to maintain P stoichiometric homeostasis.The significance of herbivorous zooplankton in supplying required phosphorus (P) to bacteria and phytoplankton in lake systems has stimulated numerous studies on P releases by different species of zooplankton (Peters and Rigler 1973;Scavia and Mcfarland 1982;Wen et al. 1994). Very high P excretion by zooplankton has been reported in the literature (e.g., several percent of the total body P content per hour, Lehman, 1980, or as high as 50% of the ingested P, Olsen et al. 1986). Model studies by Peters and Rigler (1973) and Wen et al. (1994) indicated that the P release was generally correlated with the abiotic (e.g., temperature) and allometric parameters. However, direct excretion is not the only pathway in P recycling by zooplankton. Zooplankton can feed on phytoplankton in surface waters and produce sinking fecal materials, which are remineralized by bacteria or contribute to the vertical flux of particulates (Sarnelle 1999). In addition, the P flux associated with living crustaceans may occur by molting, since a considerable percentage of body P is carapace bound (Vrede et al. 1999). Molting of carapace-associated P may represent a substantial drain on the animal's body, e.g., 25 ng of P was lost from moulting in Daphnia (Hessen and Rukke 2000). On the other hand, as an intermediate level in the food chain, zooplankton play an important role in the supply of P to higher trophic levels. Thus, P...

Calcium influence on phosphorus regulation in Daphnia magna: implications for phosphorus cycling

“…The k e of P was higher for adults and for daphnids fed P-sufficient food. In contrast, k e was not affected by the food quantity at > 0.07 mg C l -1 (He & Wang 2007).The exuvia contained more P in the higher Ca environment, consistent with the higher contribution of molting to the efflux of P when Ca level was higher. Similarly, a higher proportion of Ca was lost through molting in Daphnia magna (Tan & Wang 2009).…”

“…On the contrary, higher AEs were observed with P-deficient algae (Chlamydomonas reinhardtii) and D. magna in the studies of He & Wang (2007. Juvenile daphnids have higher AEs of P than adults, possibly because they need large amounts of RNA to maintain their higher growth rate (He & Wang 2007).…”

“…On the contrary, higher AEs were observed with P-deficient algae (Chlamydomonas reinhardtii) and D. magna in the studies of He & Wang (2007. Juvenile daphnids have higher AEs of P than adults, possibly because they need large amounts of RNA to maintain their higher growth rate (He & Wang 2007).Although the weight-specific IR did not vary greatly over the large range of Ca levels, daphnids cultured in the Ca-deficient environment (0.5 mg Ca l -1 ) had lower IR. Porter et al (1983) reported that weight-specific filtering rates of cladocerans were negatively related to body size.…”

“…Nevertheless, food quantity, food quality and the life stage of cladocerans can influence their AEs. For example, the P AE in adult D. magna was high (85.4%) when food was scarce (0.07 mg C l -1 ), and decreased exponentially with the elevation of food concentration, and leveled off (38%) when the food concentration was above 24 µg P l -1 (0.84 mg C l -1 ) (He & Wang 2007). Ferrão-Filho et al (2007) reported that D. pulex and Moina micrura had significantly higher AE of P from P-sufficient (molar C:P = 140) Ankistrodesmus falcatus than from P-deficient (molar C:P = 1000) algae, which may be due to the digestion resistance of the P-deficient algae.…”

“…Unassimilated P is egested into the water in the form of fragile fecal pellets and then quickly becomes dissolved and readily available to phytoplankton (Andersson et al 1988). P assimilated into daphnids is lost mainly in 3 ways (He & Wang 2007 : (1) excretion into the dissolved phase (released P is directly usable for primary producers); (2) shedding molts, which may sink out of the epilimnion (P becomes temporarily unavailable to primary producers); (3) reproduction, leading to the growth of the daphnid population (suggested to be more important than molting as a P drain for phytoplankton; Sommer et al 2003). Any substantial change to the relative importance of the 3 routes of P loss may affect the P cycle.…”