Phosphorus Release Patterns and the Effects of Reproductive Stage and Ecdysis in Daphnia magna

2009

Aquat. Biol.

We examined the influence of environmental calcium on phosphorus biokinetics and regulation in Daphnia magna. When the Ca concentration in culturing media increased from 0.5 to 200 mg l -1 , the specific P content of D. magna feeding on P-sufficient algae decreased significantly from 1.43 to 1.05% of dry weight. There was a significant negative relationship between the specific Ca and P contents (% dry wt) of daphnids. However, measured biokinetic parameters, including the assimilation efficiency (AE) of dietary P, the weight-specific ingestion rate (IR), the uptake of inorganic P from water, and the efflux rate constant (k e ) of P, were similar over the wide range of Ca concentrations, with some exceptions at the lowest Ca level (0.5 mg l -1 ). Diet was the dominant source of P for daphnids, and only 1 to 2% of P was incorporated from water, based on the biokinetic calculation. With the elevation of the Ca level from 0.5 to 200 mg l -1 , the proportion of P lost through molting by D. magna feeding on P-sufficient diets increased 2.5-fold, whereas the dissolved release decreased 1.9-fold. P released into the dissolved phase is readily re-utilized by primary producers, while P contained within the shed molts sinks out of the epilimnion and becomes at least temporarily unavailable to primary producers; thus, the effects of Ca on molting and dissolved release have significant implications for P cycling in freshwater lakes. Our results suggest that the availability of P to primary producers can be reduced by sedimentation of P-rich daphnid molts in high-Ca lakes, and Ca concentration in the water needs to be considered in studying the stoichiometric regulation of P in freshwater cladocerans. KEY WORDS: Phosphorus regulation · Phosphorus cycling · Phosphorus biokinetics · Effects of calcium · Daphnia magna Resale or republication not permitted without written consent of the publisherThe relative importance of different routes of phosphorus loss from Daphnia magna (via excretion, reproduction and molting) is affected by the calcium level in the water. Photo: Ke Pan OPEN PEN ACCESS CCESSAquat Biol 5: [1][2][3][4][5][6][7][8][9][10][11] 2009 of P is distributed within the carapace, possibly bound with Ca. Sterner et al. (1993) found that D. obtusa on a P-deficient diet has problems in molting. Shedding molts accounts for ~70 and ~50% of P loss from juvenile and adult D. magna over a 3 to 4 d period, respectively (He & Wang 2007). The link between Ca and P in D. magna is also indicated by the crystalline calcium phosphate composition of the shell of its ephippia (Kawasaki et al. 2004). Ca levels in the water considerably affect the calcification of the carapace of daphnids (Alstad et al. 1999) and, thus, their total Ca content (Jeziorski & Yan 2006, Tan & Wang 2008. It is therefore tempting to ask whether the content and distribution of P in daphnids is also affected by the aqueous Ca level, considering the close association of these 2 elements in these organisms. Among the various freshwater zooplankton s...

Section: Discussionsupporting

confidence: 54%

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

Tan¹,

2009

Aquat. Biol.

“…DOC was produced by excretion during the long-term depuration period, while DP release included both the excretion and the molting fluids (Peters and Rigler 1973), which was verified by the finding of peaks in the P release from molting Daphnia (Scavia and McFarland 1982). The DOC release rate also increased in the 2P animals, from 5.4% to 8.2-9.9% of body C. Consistently, Darchambeau et al (2003) found that the excretion rate increased from 5.7% to 13.4% of body C d 21 in the +P animals when they were shifted from +P to 2P diets.…”

Section: Discussionmentioning

confidence: 83%

Stoichiometric regulation of carbon and phosphorus in P‐deficient Daphnia magna

Limnology & Oceanography

2008

Daphnia magna were fed phosphorus-sufficient (+P) and P-deficient (2P) green algae Chlamydomonas reinhardtii (carbon : phosphorus ratio of C : P 5 90 and 930 in molar, respectively) for 5 d to produce different body C : P ratios. The dietary absorption as well as the elimination of body C and P were then quantified under contrasting dietary qualities (+P and 2P). The 2P animals fed with 2P algae had a higher absorption efficiency (AE) of both C (46%) and P (52%) than the control (+P animals fed +P algae) and the recovery group (2P animals fed +P food). During the physiological efflux, the 2P animals fed with 2P diet eliminated their body C at the highest rate (0.41 d 21 ) and their body P at the lowest rate (0.10 d 21 ) among the three groups of animals. Mass-specific C loss rates through dissolved release, respiration, and molting increased significantly, and the mass-specific P loss through dissolved release, molting, and reproduction decreased in the 2P animals compared with the +P animals, in agreement with the stoichiometric models. Consequently, the C : P ratio of dissolved release, molting, and reproduction all increased with the increase in P deficiency. The recovered Daphnia had medium values of AE, efflux rate constant, and mass-specific loss rates, indicating the reversibility of P limitation. Our study demonstrated that all the pathways (excretion, reproduction, molting, and respiration) may be involved in the stoichiometric regulation in Daphnia.

“…Sterner et al (1993) observed that the attachment of a molt to the posterior margin of new carapace caused difficulty in moulting in Daphnia fed P-deficient algae. It has also been suggested that some carapace-bound P may be lost during or after the molting process, which is accompanied by an increase of excretion (Scavia and Mcfarland 1982) and lower specific calcium in cast-off exuviae (Vrede et al 1999). Thus, the P loss through molting may even be underestimated in our study.…”

Section: Discussionmentioning

confidence: 99%

“…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).…”

mentioning

confidence: 99%

“…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).…”

mentioning

confidence: 99%

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

Limnology & Oceanography

2007

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...