Valentina Valdès Castro

Dissolved organic matter (DOM) is an important reservoir of nutrients in the upper ocean and is generated by excretion, grazing, enzymatic hydrolysis and cell lysis. However, only a small fraction of DOM has been identified and little is known about its reactivity. Among contributors to DOM in the sea, zooplankton are the main grazers and they are the key link for transferring N and P from autotrophic to heterotrophic organisms in the various trophic levels, exercising control on the structure of food webs through direct predator-prey interactions, and modifying the available pool of nutrients by excretion products. These processes are important for the recycling of nutrients in the pelagic system, and the principal compounds excreted by zooplankton are: ammonium, dissolved organic nitrogen, urea and dissolved organic phosphorus. In this regard, it has been reported that composition and concentration of nitrogen (N) and phosphorus (P) excreted by zooplankton are affected by the quantity and nutritional quality of the food ingested and with impacts on the local regenerated production. Furthermore, the processes and mechanisms through which N and P compounds released by zooplankton can determine and influence the food web and structure of the microbial community, in terms of genetic diversity, are unclear. My PhD will focus on the role that trophic interactions, involving primary and secondary producers of the ocean, can play in the recycling of N and P in the pelagic environment.

Three hypotheses will be tested:  The type of food ingested (prey composition) by zooplankton determines the type of N and P compounds to be excreted with implications for the variability in the N and P demand by the autotrophic community.  The dominant type of N and P compounds released by zooplankton excretion can cause shifts in the microbial community structure.  Stoichiometry of zooplankton biomass varies as a function of food type ingested (prey composition) over a seasonal scale.

The approaches to tackle these hypotheses will be experimental. For this, the N and P compounds excreted by zooplankton under different diets will be identified and quantified, at the same time determining the role of these compounds in shaping the structure of the microbial community and its variability. The contribution of N and P excreted by zooplankton to the N and P demand of phytoplankton community will also be estimated, as to evaluate the impact of these excreted products on local regenerated production. Finally, the nutritional status of zooplankton organisms through changes in their particulate organic matter (POM) C: N ratio, mediated by seasonal variations in food abundance and its composition will be assessed. This study will take place in the coastal upwelling area of Central Chile (36°S), and in the oligotrophic area of the Mediterranean Sea, Banyuls Sur Mer, France. The results from this study will enable us enhancing our understanding of the processes and mechanisms through which N and P compounds released by zooplankton can determine and influence the food web and structure of the microbial community, and how these processes can be modulated upon variable environmental conditions over seasonal and regional scales.