Olsen Y, Andersen T, Gismervik I, Vadstein O
Planktonic N and P nutrition, Essential nutrients limitation, Conceptual model, Inverse modelling, CNP flow networks, Nutrient release, Growth efficiency, Elemental CNP, stoichiometry, mesocosm, 38 m3, Hopavagen, Norway
The objective of the present study was to examine how N and P availability interactwith C metabolism in marine heterotrophic plankton and whether or not heterotrophic groups arelikely to be subjected to essential nutrient limitations in natural coastal waters. The nutrients studiedwere mineral P and N; the latter is a proxy for essential amino acids. We present a general theoreticalframework and criteria for evaluating essential nutrient limitations in heterotrophs. Datawere derived from a comprehensive mesocosm experiment, where food-web flows were estimatedusing inverse modelling. Bacteria of the mesocosm communities were severely P-deficientthroughout. The heterotrophic nanoplankton was most likely limited by another essential nutrientor by food C availability. Ciliates were most likely P-limited, whereas copepods might experienceP limitation, but were more likely limited by another essential nutrient or food C in the presentexperiments. The N contents of the food were close to the theoretical requirements for copepodsand ciliates. All planktonic groups released dissolved inorganic nitrogen (DIN), and all exceptbacteria released dissolved inorganic phosphorus (DIP). Our results suggested that P limitationmight be readily experienced by bacteria, ciliates and perhaps also by copepods, but not by heterotrophicnanoplankton in natural North East Atlantic coastal waters. A predator must balance itsenergy metabolism and growth under the variable availabilities of essential nutrients, and we proposethat C growth efficiency is a dynamic variable mainly dependent on the availability of themost limiting nutrient. We support the view that C availability alone cannot be used as a proxy forthe food limitation of bacterial and zooplankton growth. Specific essential nutrients should beregarded as potential limiting factors, as for phytoplankton. The variable nutritional requirementsof heterotrophic predators will then represent a major driver of heterotrophic species diversity,allowing a broad diversity of heterotrophic species in plankton communities.