Interplay between Energy Limitation and Nutritional Deficiency: Empirical Data and Food Web Models

Abstract

Abstract. Food quality may play an important role in consumer population dynamics. The frequently large differences in elemental and biochemical composition observed between autotrophs and their grazers suggest that food quality may be of particular importance for herbivores. Under nutrient-depleted conditions the carbon-to-nutrient ratios of autotrophs can increase to such an extent that consumers become nutrient rather than energy limited. Estimating the importance of this effect in situ in pelagic food webs is complicated by the omnivory of many consumers and by rapid nutrient recycling. Isolated predatorprey studies inadequately represent this interaction; instead, an ecosystem perspective is required. We used seven years of data from large, deep Lake Constance to develop seasonally resolved flux models of the pelagic food web and analyzed the balance between energy and nutrient constraints. The carbon (C) and phosphorus (P) flows were simultaneously quantified and balanced. C represented food quantity/energy. P was taken as a surrogate of food quality, because algal C:P ratios exceeded the threshold above which P limitation of herbivores is predicted by stoichiometric theory throughout summer and autumn.Primary production exceeded bacterial C production by a factor of 3, but autotrophs and bacteria took up approximately equal amounts of P during summer and autumn. As a consequence, the C and P supplies of suspension-feeding zooplankton were decoupled: Consumer C demands were largely met by phytoplankton whereas P was mostly obtained from bacteria and their protist predators. The degree of consumer P deficiency varied according to supplementation of their algal diet with P-enriched bacteria or bacterivores. This favored the occurrence of omnivores, i.e., organisms that minimized P deficiencies at the cost of enhanced energy limitation. In contrast with previous perceptions, P remineralization during P-depleted summer conditions was dominated by bacterivorous flagellates, carnivorous crustaceans, and fish, which fed on prey with an elemental composition similar to their own, whereas herbivores contributed only 30% of P cycling despite their large biomass and C production. Our results suggested a co-limitation of predominantly herbivorous consumers by C and P and a mutual dependence of the two types of deficiency at the individual and system level. This pattern is not specific to pelagic systems but appears to be applicable across ecosystem types.

Examination of the role of the microbial loop in regulating lake nutrient stoichiometry and phytoplankton dynamics

“…For example, in BAC + DIM, as a fraction of algal uptake, microzooplankton excretion was predicted to account for 10 % of C, 22 % of N and 11 % of P returned for mineralisation, which was significantly larger than that supplied from algal excretion for N and P (but not for C), and therefore different from the relative proportions consumed through bacterial grazing (18 % for C, 15 % for N and 20 % for P). This highlights the dissimilarity and decoupling of the C, N and P cycles and the importance of nutrient adjustments that occur during these microbial interactions, and is consistent with empirical work (Gaedke et al, 2002). The parameter sensitivity analysis indicated that both microzooplankton biomass and bacterial growth were particularly sensitive to the excretion fraction of the ingested material (K Ze ) grazed by microzooplankton.…”

“…The latter refers to the dynamics of the heterotrophic bacteria and the microzooplankton grazers (defined here as size less than 125 µm that account for rotifers, ciliates and juvenile macrograzers; Thatcher et al, 1993) -often termed the "microbial loop". This has been shown to play an important role in shaping carbon fluxes in lakes and in enhancing nutrient cycling at the base of food webs (Gaedke et al, 2002), including in Lake Kinneret which is the focus in this study (Stone et al, 1993;Hart et al, 2000;Hambright et al, 2007;Berman et al, 2010).…”

Source and supply of terrestrial organic matter affects aquatic microbial metabolism

“…Comparative studies have revealed that BP is positively correlated with net primary productivity (NPP) across fresh-and saltwater ecosystems (Cole et al 1988), but is usually 70 to 90% less than NPP (Ducklow et al 2002, Gaedke et al 2002. This observation suggests that bacterial metabolism may be tightly coupled to the local production of photosynthetically derived DOM.…”

Coupling of growth rate and body stoichiometry in Daphnia: a role for maintenance processes?

“…Such flows are largely driven by the activities of planktonic biota, including bacterioplankton, phytoplankton, and micro-and macro-zooplankton (Gaedke, Hochstadter & Straile, 2002). Recent developments in the field of ecological stoichiometry have highlighted the importance of elemental imbalance (e.g.…”