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Selective recycling in marine ecosystems: nitrogen, phosphorus and plankton

In the world of plankton, the predator can also starve its prey! Zooplankton organisms do not only consume phytoplankton, they also modify its habitat by means of their storage or release of the nitrogen or phosphorus needed by microalgae for their growth.


In the sea as in every other ecosystem, the chemical elements making the organic molecules are transferred along the food chain: plants draw them from the water to synthesize their own molecules and consumers (both herbivorous and carnivorous) find them in their food. To allow organisms to develop normally, nutrients must be available in adequate proportions related to their nutritional needs, without the lack of any one of them being a limiting factor. Marine phytoplankton is most often limited by the availability of nitrogen (N), phosphorus (P), iron (Fe), or silicon (Si) for diatoms which have a siliceous test.

Phytoplankton is consumed by zooplankton, all the species of which do not have the same overall chemical composition as their prey. As a consequence, zooplankton rejects indifferent proportions the elements ingested in excess relative to its needs. This selective recycling of some elements modifies the initial composition of the environment, which in turn influences phytoplankton, first link in the chain. For instance, copepods (small planktonic crustaceans) are generally richer in nitrogen and less rich in phosphorus than the phytoplankton it feeds on. The phosphorus in excess in the diet will be rejected in the environment, thereby modifying the nitrogen-phosphorus ratio (N:P) with consequences on the dynamics of phytoplankton communities.


Zooplancton sample (photo P. Pondaven)

While well known in freshwater environments, these effects have been little studied in the sea; this study is the first one to test, in controlled laboratory conditions, the influence of zooplankton on growth and nutrient limitation of phytoplankton.

In a first experiment, phytoplankton growth was measured under different nutrient conditions (normal, enriched in N or in P) and submitted for five days to predation by two zooplankton species. A second step then aimed to find whether phytoplankton was limited by nutrient availability in this water enriched by zooplankton-excreted nutrients (after elimination of zooplankton organisms); a supply of N, P or both was therefore tested to check its effect on phytoplankton (no effect of the supply means that the added nutrient is not limiting in the environment).


The two-step experimental design


Surprisingly, it is when zooplankton (copepods or rotifers) is added to the cultures that phytoplankton biomass is the highest. Another effect is that the presence of these planktonic consumers also modifies the nature of limiting factors. These differences stem from the recycling of nutrients by consumers. Depending on whether they are copepods or rotifers, this fertilization is more efficient in conditions enriched in nitrogen or in phosphorus. These two zooplankton groups recycle the nutrients differently, in particular because of their growth rate which determines theirs needs in phosphorus. With a slower growth, copepods require less and excrete more phosphorus in the water: in nitrogen-rich conditions, the relative scarcity of phosphorus is compensated by copepod excretion and this stimulates phytoplankton production. In nutrient-poor ("oligotrophic") marine regions, such as subtropical gyres or Mediterranean Sea, these processes could control primary production. Conversely, the excretion of rotifers contains less phosphorus (thus more nitrogen in proportion), and therefore stimulates primary production only in conditions where nitrogen is lacking relative to phosphorus.


The thermoregulated phytoplancton culture (photo P. Pondaven)

This whole set of experimental results show that nutrient recycling by zooplankton influences the nature of limitation face by phytoplankton, and leads to a co-limitation involving both nitrogen and phosphorus. But the processes leading to these limitations are complex: phytoplankton is not subject only to the direct impact of zooplankton on nutrient rejection, but also to a "trophic cascade" effect involving interactions with populations of other kinds of organisms like bacteria or protists.

The paper

G. Trommer, P. Pondaven, M. Siccha, H. Stibor, 2012. Zooplankton-mediated nutrient limitation patterns in marine phytoplankton: an experimental approach with natural communities. Marine Ecology Progress Series 449: 83–94.
See the first page


The authors

G. Trommer, P. Pondaven, and H. Stibor conducted this study within theLaboratoire des sciences de l'environnement marin (Lemar) of IUEM, in collaboration with M. Siccha from Institute of Earth Sciences (Israel)


The journal

Created in 1979, Marine Ecology Progress Series ("MEPS") is one of the most prominent scientific journals in marine ecology. Its 20 to 25 yearly issues cover all aspects of this field: microbiology, botanics, zoology, ecosystems, biological oceanography, ecosystem-based fisheries management, pollution, environmental protection, conservation, resource management.



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