Phosphorus limitation and excess carbon in zooplankton

Abstract

Organisms rely on a series of chemical reactions, which are constrained by the availability of key chemical elements, such as carbon (C), nitrogen (N), and phosphorus (P). Ecological stoichiometry provides a tool for analyzing how the balance of elements required by organisms affects food-web dynamics. Ecological stoichiometric theory suggests that the balance between supply and demand of elements is determined by the conversion efficiency from resources to organisms.Autotrophs and heterotrophs commonly face unequal access to and uptake of elements. The stoichiometric variability of autotrophs is based on their ability to maintain the balance of elements required for growth. This creates a challenge for their grazers. Phytoplankton can adjust their P content to ambient nutrient concentrations, while zooplankton cannot store excess nutrients. Ecological stoichiometric theory thus suggests that zooplankton have relatively fixed stoichiometry compared with phytoplankton.Nutrient limitation is common in aquatic systems. Stoichiometric imbalances between phytoplankton and zooplankton mean that zooplankton rarely find optimal food sources, and phytoplankton production is in excess. P availability potentially limits zooplankton growth, because of the high C:P ratio in phytoplankton relative to zooplankton demand. Based on the Liebig minimum principle, organisms are normally limited by a single nutrient, while everything else is in excess. Under P deficiency, excess C cannot be allocated to zooplankton somatic growth, and the net intake of C must balance the C:P ratio of zooplankton. Thus, when zooplankton encounter nutritionally imbalanced foods the elements in excess are released in order to maintain homeostasis. Excess C, released by zooplankton results in two biochemical challenges: (1) to sequester the limiting element and (2) to either store or dispose of the element in surplus.Zooplankton must resort to various physiological solutions to cope with these challenges. As a first option, zooplankton can reduce their C assimilation efficiency but maintain their P assimilation efficiency. Alternatively, after assimilation, excess C may be stored in C-rich compounds. Finally, assimilated excess C could also be disposed of through respiration or extracellular release. Excess C released by zooplankton reduces C transfer efficiency and sequestration in aquatic ecosystems.In aquatic ecosystems, C sequestration largely depends on the balance between uptake and demand for key nutrient elements. These feedback mechanisms have arisen only because organisms must obey stoichiometric rules at the cell and body levels, which greatly constrain the range of element values in ecosystems. Thus, the fate of C in ecosystems is determined by the absolute and relative demands for N and P of each organism. Limiting elements are utilized for growth and transferred in food chains with high efficiency, while non-limiting elements must be disposed of. Therefore, low C:P phytoplankton communities subject to high turnover rates and high productivity are selectively channeled into zooplankton. When zooplankton face high C:P foods, excess C is returned to the environment. Hence, nutrient-deficient phytoplankton constitute poor food, influencing the entire food web and adversely affecting secondary production at all levels.Excess C processed by zooplankton has far-reaching implications for ecosystem food-web functioning and C sequestration. Studies of the fate of excess C in zooplankton would increase the understanding of energy flow and material cycling in aquatic ecosystems. This paper reviews the reasons for P limitation and excess C in zooplankton, principal routes for the disposal of excess C, and the ecological effects of this. In addition, the paper aims to provide insight and a theoretical foundation for related studies in China.