INTERANNUAL VARIABILITY OF THE PLANKTONIC FOOD WEB IN THE GULF OF TRIESTE:AN INVERSE MODELLING APPROACH

During the last 20 years ecology has evolved toward a vision of the ecosystem as a system of interactions where the center of interest are interactions between them, quantified by flows of matter and energy. The quantitative mapping of food web flows based on empirical data is crucial and is a difficult task in ecology: it is difficult to comprehend the nature of complex food webs through direct observation, because the number of flows of matter and energy involved can be overwhelming. Furthermore it is generally difficult to measure or quantify the exchange of mass or energy between the compartments of a system. The planktonic food web of the Gulf of Trieste was studied using linear inverse modelling. We computed both the LIM – MN and the LIM – MCMC following literature criterion, to get the best solution for our planktonic food web case. We implement a new compartment, the growth compartment, to have a more realistic view of the food web, trying take in account the possibility of compartment growing. We compared the two methods solutions with the a priori scientific knowledge of the Gulf of Trieste to determine which could be defined the best way of solving the food web fluxes. The LIM – MN select the best solution minimizing the sum of squared flows, while the LIM – MCMC select the solution by an exhaustive sampling of the solution space. Both the solution respect the inequalities and all the ecological request and therefore are valid. The LIM- MN usually underestimates all the fluxes, while the LIM – MCMC gives a good representation of the flows ranges, rarely overestimating not very well bounded fluxes. Growth fluxes behave well, especially the LIM – MCMC ones, and give a good response as they are in agreement with the scientific knowledge of the Gulf of Trieste. Implementing this type of compartment could be a good compromise in using steady state conditions. It is our opinion that LIM – MCMC solution is the best way to depict all the planktonic food web fluxes of the Gulf of Trieste. Values of fluxes represent better what is the a priori ecological knowledge found in the literature. In particular, the solution explains the important role of bacterial compartments that are fundamental for the Gulf of Trieste marine life, as they channeled most of the carbon inside the food web. The so-called microbial loop is very strong, and poses the microzooplankton as the main actor in channeling carbon from lower ecotrophic level to higher ones, including fishes. Moreover, thanks to the values found in this study, the Gulf of Trieste planktonic ecosystem could be compared with the more productive ecosystem at a world scale.