Mineral Limitation of Zooplankton: Stoichiometric Constraints and Optimal Foraging

Abstract

Nutritional deficiencies are a very common phenomenon, and consumers generally face food that is not optimally suited for their needs. Especially herbivores are habitually confronted with food of inferior quality, usually a result of too‐low nutrient concentrations in plant material. Waterfleas of the genus Daphnia are good model organisms to study the effect of inferior quality food, and how animals deal with this. We tested the effect of algae to which we had given different phosphorus contents on both life history and feeding parameters of Daphnia magna. Phosphorus content of the algae strongly affected both the growth rate and the feeding activity of the daphniids. Feeding activity increased with declining food quality (increase in C:P ratio of the algae), whereas growth rates were maximal at intermediate C:P levels. We conclude that the direct limitation of phosphorus is a very important factor determining food quality for zooplankters. Daphniids counterbalanced lower P content of their food by spending more C (energy) on acquiring this limiting resource. This implies that when Daphnia are given phosphorus‐limited food both the addition of phosphorus and the addition of carbon (energy) should increase the growth rate of the animals (co‐limitation). The influence of the phosphorus content of the food on the feeding activity of Daphnia offers a mechanistic explanation for the observed homeostasis in daphniids.

Increased carbon dioxide availability alters phytoplankton stoichiometry and affects carbon cycling and growth of a marine planktonic herbivore

“…Furthermore, A. tonsa do not store large amounts of lipids. Thus, the routes and mechanisms by which this disposal of excess C is achieved and the nature and fate of these C excretory products may vary between species, and, as we have seen, even between life stagers within species (Darchambeau et al 2003;Jensen and Hessen 2007;Plath and Boersma 2001;Sterner et al 1998;Anderson et al 2005;Hessen and Anderson 2008). The manner in which the consumers exude the C potentially affects both the consumers themselves and their environment.…”

“…Disposing of surplus C by respiration is a common mechanism, reported from a range of aquatic consumers including Daphnia, the copepod A. tonsa and the flagellate Oxyrrhis marina (Hantzsche and Boersma 2010; Malzahn Jensen and Hessen 2007). Among the mechanisms proposed for the increase in respiration rate are increased activity, such as increased appendage beat rate (Jeyasingh and Weider 2007;Plath and Boersma 2001), so-called 'wastage' respiration (Zanotto et al 1997) and respiration decoupled from other metabolism in the form of increased heat production (Trier and Mattson 2003). The increased metabolic activity underlying these mechanisms for the disposal of leftover C appears to come at a cost to the consumer (Hessen and Anderson 2008;Raubenheimer and Simpson 1999), although the exact nature of these costs is still not entirely clear (see Hessen and Anderson 2008).…”

“…In addition, several post-absorptive regulation mechanisms exist. Surplus carbon can be excreted through different mechanisms, ranging from increased activity (Plath and Boersma 2001) and respiration (Darchambeau et al 2003;Malzahn et al 2010) through changes in the digestion efficiency of carbon (DeMott and Tessier 2002). Whatever the mechanisms used, keeping homoeostasis and thus getting rid of excess carbon often comes at a cost to the herbivore and hence results in decreased growth and reproduction (Boersma 2000;Malzahn et al 2007).…”

How Daphnia copes with excess carbon in its food

“…It is not clear how this increased respiration (and thus extra gain of energy and ATP) may affect the animals in the long run. Plath and Boersma (2001) observed highly increased appendage beat rates of D. magna feeding on P-deficient algae in comparison with when feeding on algae richer in P. The authors suggested that the predicted resulting increase in filtration rate, an energy (thus C) costly process, offers a mechanistic explanation of the observed relative homeostasis of the animal. Our data confirm that the increase in the respiration rate is a process used by this species when facing a dietary nutrient deficiency, but they offer no identification of the energy expending process.…”

“…Direct excretion of DOC (Gardner and Paffenhöfer 1982) could be an interesting way for daphniids to get rid of previously assimilated C-rich, P-poor macromolecules like proteins (Elser et al 1996), but this process has never been quantitatively studied. Likewise, although suggested by Plath and Boersma (2001), the ability of daphniids to increase their respiration rates when feeding on C-rich algae has never been studied directly. Thus Sterner's (1997) humorous remark: "the animal doing extra work in its environment (swimming?)…”