Phosphorus distribution in three crustacean zooplankton species

The distribution of phosphorus (P) was assessed in homogeneously 33P-labeled Daphnia magna, Daphnia galeata and Eudiaptomus gracilis. The specific P contents were 1.48, 1.41, and 0.50% of dry weight (DW), respectively. The results support the view of low intraspecific variability in P : DW ratios in crustacean zooplankton. The fraction of P allocated to nucleic acids, phospholipids, and other P compounds was assessed in D. galeata and E. gracilis. In both species, the major pool, 35–69% of the total P content, was associated with nucleic acids. This fraction decreased with both body size (age) of D. galeata and E. gracilis and with the reproductive rate of D. galeata. D. magna and D. galeata revealed similar patterns of P allocation between body, carapace, and eggs. The carapace contained approximately 14% of the total P content. If most P is not reabsorbed from the exoskeleton prior to molting, the molting will result in a substantial P drain both from the individual and, at times, from the entire planktonic system. Zooplankton production has long been considered to be mainly energy (or carbon) limited, but recently much attention has been paid to the importance of food quality for the growth of crustacean zooplankton. One of the potentially important factors, which may place constraints on zooplankton growth, is phosphorus (P) (Hessen 1992; Urabe et al. 1997). Typically, daphnids have low carbon : phosphorus (C : P) ratios, while copepods have high C : P ratios in their somatic tissues (Andersen and Hessen 1991; Hessen and Lyche 1991). Furthermore, the sestonic P concentration is often below predicted and observed thresholds for P limitation of zooplankton growth (Hessen and Andersen 1992; Sommer 1992; Urabe and Watanabe 1992). This implies that in order to maintain a nearly homeostatic C : P ratio, a grazer must balance its net intake of elements relative to its bodily demands. At high C : P ratios in the food, some proportion of C (‘‘excess C’’) must be disposed off, invariably reducing the growth rate. Similarly, the relationship between the P content in zooplankton and in their food also has implications in terms of the amount of P that is recycled by zooplankton (Sterner et al. 1992). Expected major pools of P in crustaceans are nucleic acids (deoxyribonucleic acid [DNA] and ribonucleic acid [RNA]), phospholipids, small metabolites (e.g., adenosine triphosphate [ATP], adenosine diphosphate [ADP], reduced nicotinamide adenine dinucleotide [NADH], and reduced nicotinamide adenine dinucleotide phosphate [NADPH]), and calcium-associated P (hydroxyapatite) in the exoskeleton. Only limited information exists on the occurrence and dynamics of these compounds in crustaceans. The distribution of P between these pools, as well as between reproduction and somatic growth, reflects metabolic and physiologic constraints but is also a consequence of the animals’ life histories (Elser et al. 1996). The allocation pattern will also have implications for P flow within the planktonic food web because of differences in turnover time and bioavailability between different pools of P. While the high specific P content of Daphnia relative to that of other zooplankton, particularly relative to that of copepods, is settled, there is no obvious reason why this is so. If a high specific P content may lead to reduced growth rates during periods with Pdeficient food, there should be advantages to counterbalance this. The most obvious advantage would be that high specific P reflects a high content of RNA, promoting rapid growth rates (Hessen 1990; Elser et al. 1996; Main et al. 1997). There may also be a significant portion of P allocated to the carapace. The specific calcium content of the Daphnia carapace is particularly high (Hessen unpubl. data), and this may require a corresponding part of P for calcium-P bindings. Assuming that not all of this P can be retained during the molting, this carapace-associated P could represent a substantial drain of P not only for the individual Daphnia but potentially for the entire water body. In this paper, the patterns of P allocation between nucleic acids, phospholipids, and other P compounds are quantified in three crustacean zooplankton species, Daphnia magna, Daphnia galeata, and Eudiaptomus gracilis. In addition, the distribution of P between somatic tissues and eggs is assessed in D. magna and D. galeata. Consequences for P fluxes in lakes are discussed. D. magna Straus (from the culture collection of the Norwegian Institute for Water Research), D. galeata Sars (isolated from Lake Erken), and E. gracilis (Sars) (from Lake Norrviken) were fed 33P-labeled algae in batch cultures in 1to 5-liter glass or polycarbonate bottles. D. magna was fed Selenastrum capricornutum, D. galeata was fed either exponentially growing Rhodomonas lacustris (‘‘Daphnia galeata population A’’) or stationary phase Cryptomonas cf. marsoni (‘‘Daphnia galeata population B’’), and E. gracilis was fed exponentially growing R. lacustris. Selenastrum were grown at 188C and 70 mE m22 s21 in Z8 medium (Staub 1961). Rhodomonas and Cryptomonas were grown at 158C and 35 mE m22 s21 in 3 3 L16 medium (Lindström 1991) modified with soil extract and B vitamins. In all experiments, 33P was added to the algal cultures as carrier-free 33P-orthophosphate (DuPont NEZ-080 for Selenastrum and Amersham BF1003 for Rhodomonas and Cryptomonas). The initial specific activity ranged between 10 and 20 mCi 33P l21 in each algal culture; these are tracer amounts in comparison with the 31P content of the nutrient media. Algae were added to the zooplankton cultures every day or every second day. The algal concentrations in the zooplankton cultures were not determined. For Selenastrum and Rhodomonas, the food abundance was likely sufficient (above the incipient limiting level), but for Cryptomonas, the food concentration may have been suboptimal during the last days before harvesting the daphnids. Animals were fed 33P-labeled algae for at least 10 d in order to make them uniformly labeled. This is 2.5–5 times longer than the time required for homogeneous labeling of Daphnia with 32P or 33P (Peters and Rigler 1973; Hessen and Andersen