Comparison Between the Energy Flow Networks of Two Habitats in the Venice Lagoon

Abstract

The energy flows of two habitats are quantified and compared by using a trophic network model. The habitats are representative of two typical subsystems of the Venice Lagoon: the seagrass meadows, which represent a mature stage in the ecological succession of lagoon environments, and the Tapes philippinarum fishing grounds, which are subjected to mechanical clam harvesting. The results obtained for the`seagrass habitat' are consistent with other field studies, confirming the`key-stone' role played by phanerogams in a lagoon environment. The results for the`Tapes habitat' show that the energy flow from the sediment to the water column is extremely important. Indeed, the high T. philippinarum biomass cannot be supported solely by the primary production of the local phytoplankton. Instead, this biomass could be supported by a supply provided by the sediment resuspension caused by intensive clam fishing. These findings suggest the presence of a positive feedback between fishing effort and T. philippinarum standing crop. ProblemOver the last decades the Venice Lagoon has been the target of numerous studies and research projects that address a wide range of questions and belong to many different scientific fields (Lasserre & Marzollo, 1996; web site Sistema lagunare veneziano, 1994). Nevertheless, these mainly bio-ecological studies were often limited spatiotemporally and lacked a more global ecosystemic approach such as the one addressed P.S

An inverse model for the analysis of the Venice lagoon food web

“…The total system throughput (TST), the overall production, the net primary production are slightly higher, but comparables, with the results presented in Libralato et al (2002). As explained in Section 2.2, TST corresponds to the sum of energy flows in the ecosystem, and is an indicator of its activity.…”

“…On the contrary, total catches estimated by the present model are well below the value reported by Libralato et al (2002) for the fraction of the lagoon occupied by R. philippinarum during the 1990s, and more similar to those estimated for seagrass meadow habitat. These differences in primary production and catches are confirmed by the value estimated by IM for the gross efficiency (catches/net primary production), which is below the lower boundary reported by Libralato et al (2002).…”

“…Regarding the ratio primary production/total respiration, which is an indicator of the fate of assimilated food, the value calculated by the IM lies in between the two values reported by Libralato et al (2002). On the contrary, total catches estimated by the present model are well below the value reported by Libralato et al (2002) for the fraction of the lagoon occupied by R. philippinarum during the 1990s, and more similar to those estimated for seagrass meadow habitat.…”

“…In the case of the Venice lagoon this activity is supported by a high net primary production. With respect to this, the mean value of our estimate, to which a high standard deviation is associated, exceeds of 50% the maximum production estimated by Libralato et al (2002), and is 4 times higher than the primary production estimated by Leguerrier et al (2003) for benthic and pelagic systems of the Brouage mudflat (conversion from mass to energy was carried out according to the average phytoplankton caloric content by Platt and Irwin (1973)). …”

“…The most relevant features of the lagoon food web are summarized in Table 3, which presents a set of well known ecological indexes and compares them with the findings presented in Libralato et al (2002) concerning two 27 compartment food web models, which were balanced for two typical habitat of the lagoon of Venice, namely bare-bottom clam fishing grounds and seagrass meadows. These models were constructed using a data set collected in the 1900s and balanced using the Ecopath software.…”