Quo vadis ecosystem?

The description of ecosystems\u2019 evolution is a challenge for many scientists. Several attempts have been made, mostly using compartments and flows, to describe the structure of ecosystems and holistic indicators or goal functions (GF) to describe the state and the trend of their evolution. Under a steady flow of energy from the environment, an ecosystem can increase the biomass by enlarging the number of individuals and/or their body weights until a certain steady state level. In this situation neither the structure (e.g. species composition) nor the pattern of flows linking its compartments vary and both biomass and flow reach an \u201coptimal\u201d level. Benefits compensate the costs of the ecosystem evolution. A question is open: when can an ecosystem be considered to be in an optimal state and how does it reach this state? The answer to this question is also useful to solve the problems related to the concept of reference state as posed in the Water Framework Directive (WFD, EU, 2000/60/EC). In order to answer this question the characteristics of a suitable indicator are discussed and two state indicators are proposed, based on the assumption that costs and benefits of the ecosystem growth have to be in balance. The first one, the benefit/cost indicator (BC), is a function of well known state indicators, that fully satisfies the properties required although is difficult to compute and complex to understand. Nevertheless, from the researcher point of view, this indicator gives interesting insights into system behavior. The second, the supply demand balance (SDB), is an indicator based on two main assumptions, the first one being that an ecosystem can be represented by a network of compartments and flows, the second that the metabolic rates scale across species approximately as the (3/4) power of mass. The SDB indicator summarizes the distance of an ecosystem from an optimal state in a single number. Under these assumptions the SDB indicator can be regarded as a measure of ecosystem state. It is easy to compute and simple to understand. SDB looks like a good indicator both for scientific and practical uses to understand where the ecosystem is going. Finally the applicability of SDB is investigated by computing its values for 33 trophic networks describing the food webs of different aquatic ecosystems

Defining and modelling the coastal zone affected by the Po river (Italy)

One of the most important sources of pollution in coastal zones (CZ) is certainly that one produced by human activities in the associated river basin. Understanding the linkage between water quality in CZ and river catchments is important in order to better assess CZ processes and to evaluate different management options aimed at improving the coastal ecosystem state. CZ water quality targets as identified by the Water Framework Directive (EC 2000/60) require an accurate study of the effects of pollutant loads coming from river discharge. In order to evaluate the impacts of human activities in river catchments on the associated coastal zone, a sound definition for this geographic area is needed. Many definitions for this area have been proposed in different contexts. The definition is generally built upon a particular goal, and is henceforth highly variable according to the different purposes. In this paper a general methodology allowing to discern those areas of the sea that are directly influenced by fluvial discharge is presented. The methodology is based on the variation pattern of sea water characteristics, and provides a numerical evaluation of this influence. In particular an analysis based on salinity as tracer, results in a sound definition of this area. The methodology has been applied on the case study of the Po river. Due to the significant nutrient loads discharged by the river, the CZ associated with Po is affected by severe eutrophication phenomena that have important consequences on the ecosystem and on the socio-economy of the area. In order to study the impacts of nutrients loads carried by the river, a water quality model (WASP6) has been implemented. The model simulates the seasonal variability of nutrient concentrations, phytoplankton biomass and dissolved oxygen. Using the CZ model is possible to compare the effects of variations of nutrient loads on the biochemical (short term) and ecological (long term) quality of the coastal environment. This is accomplished by feeding nutrients loads forecasted for different scenarios by the catchment model (MONERIS) as forcing functions to the CZ model. This way the effect of the different catchment management scenarios are propagated to the CZ model, and the trophic conditions of the coastal ecosystem evaluated using TRIX. This study has been developed in the context of the European project EUROCAT

Modelling nutrient emissions from river systems and loads to the coastal zone: Po River case study, Italy

The nutrient emission model MONERIS (MOdelling Nutrient Emissions into River Systems) is applied to the Po catchment, a large (>70,000 km2), densely populated, highly agriculturally exploited and industrialized landscape. The catchment is located in northern Italy. The Po River discharges into the northwestern Adriatic Sea. Model runs cover the period 1991\u20132000. The purpose is to model the catchment in 2001, estimating nutrient emissions and natural background in the basin and loads to the coastal area. The model was calibrated with data for the period 1990\u20131995. After validation with data for the period 1995\u20132000, the model is used to evaluate future catchment management scenarios. MONERIS is a spatially distributed parameters steady state model with a time scale of 5 years. The emissions considered are originated from diffuse and point sources and delivered trough various pathways (groundwater, erosion, overland flow, atmospheric deposition, urban systems and WWTPs). In order to estimate nutrient loads to the river system, MONERIS includes a retention model. An overview of model input requirements, data needs and related problems and solutions adopted is presented in the paper. Simulated and measured data of several sections along the river are compared for calibration and validation. The relative importance of different nutrient generation pathways are evaluated. Finally, forecasted yearly nutrient loads at the outlet of PO basin for the years 2001, 2008 and 2016, consequence of different basin management scenarios, are presented. The results are ready to be supplied to a water quality Coastal Zone Model, allowing us to evaluate significant switches in trophic state conditions of the coastal ecosystem [see Artioli, Y., Bendoricchio, G., Palmeri, L., this issue. Defining and modelling the coastal zone affected by the Po River (Italy). Ecol. Model.]