Marine heterotrophic bacteria, protozoan and metazoan zooplankton may experience protein N or mineral P limitation in coastal waters

The objective of the present study was to examine how N and P availability interact with C metabolism in marine heterotrophic plankton and whether or not heterotrophic groups are likely to be subjected to essential nutrient limitations in natural coastal waters. The nutrients studied were mineral P and N; the latter is a proxy for essential amino acids. We present a general theoretical framework and criteria for evaluating essential nutrient limitations in heterotrophs. Data were derived from a comprehensive mesocosm experiment, where food-web flows were estimated using inverse modelling. Bacteria of the mesocosm communities were severely P-deficient throughout. The heterotrophic nanoplankton was most likely limited by another essential nutrient or by food C availability. Ciliates were most likely P-limited, whereas copepods might experience P limitation, but were more likely limited by another essential nutrient or food C in the present experiments. The N contents of the food were close to the theoretical requirements for copepods and ciliates. All planktonic groups released dissolved inorganic nitrogen (DIN), and all except bacteria released dissolved inorganic phosphorus (DIP). Our results suggested that P limitation might be readily experienced by bacteria, ciliates and perhaps also by copepods, but not by heterotrophic nanoplankton in natural North East Atlantic coastal waters. A predator must balance its energy metabolism and growth under the variable availabilities of essential nutrients, and we propose that C growth efficiency is a dynamic variable mainly dependent on the availability of the most limiting nutrient. We support the view that C availability alone cannot be used as a proxy for the food limitation of bacterial and zooplankton growth. Specific essential nutrients should be regarded as potential limiting factors, as for phytoplankton. The variable nutritional requirements of heterotrophic predators will then represent a major driver of heterotrophic species diversity, allowing a broad diversity of heterotrophic species in plankton communities.