Ecological stages of the Venice Lagoon analysed using landing time series data

Abstract

The time series of landings in the Venice Lagoon from 1945 to 2001 were analysed with the aim of explaining the ecosystem changes occurred. The comparative analysis of the total landings and mean Trophic Level (mTL) time series allowed to identify four different stages in the lagoon ecosystem. The first period, from 1945 to 1973, was characterised by increasing trends in the landings and their mTL. The second one, from 1974 to 1989, showed a decrease in the landings but still an increase in the mTL. The third period, from 1990 to 1998, had again a positive trend in the landings, but the mTL showed a sharp decline. After 1998, a slight decreasing trend in both mTL and landings was observed: the analyses of the artisanal fishery landings only date back to 1995 this effect. The presence of four distinct periods was also confirmed by the analysis of the trends of other indices estimated using landings data: the Fishing in Balance index (FiB), the Trophic Efficiency (TE) and the Pelagic on Demersal landings ratio (P/D). In the first period, the increasing fishing pressure, along with no evidence of ecosystem crisis, suggested that an increased nutrient discharge was supporting it; analogously, the bottom-up effects had driven the dynamics of the ecosystem also in the second period, when the decrease in nutrient loads caused a shift of the primary producers from planktonic to macrobenthic. The spreading of the Manila clam, a non-native species, and the development of its massive mechanical exploitation have been the main forces driving the ecosystem during the third period, for which, however, no signs of crises were detected. The fourth period showed evidence of the ''fishing down the food web'' effect. Possible causes of such an effect were investigated and allowed us to conclude that not overfishing, but the effects of mechanical harvesting of the Manila clam had caused relevant impacts on habitat and benthic communities, concluding that the present level of exploitation of the stock of Manila clam is not sustainable in the long term. Our findings were also compared with the general evolution of enclosed seas, subjected to high nutrient loads, fishing pressure and invasion by non-native species. D

Changes in trophic flow structure of Independence Bay (Peru) over an ENSO cycle

“…A similar role has been identified for the introduced Manila clam Tapes philippinarum in the Venice Lagoon system (Pranovi et al, 2003). Furthermore, it was suggested that this strong top-down control of phytoplankton by T. philippinarum may be responsible for the system not returning to a phytoplankton-based trophic web (Libralato et al, 2004), although this seems unlikely in Independence Bay given the constant refreshment of productive waters that enter from outside the bay.…”

Climate driven changes in subtidal kelp forest communities in NW Spain

“…The use of commercial fisheries has been widely employed as an indicator for ecosystem change (e.g. habitat modification and changes in primary productivity) (Libralato et al, 2004;Okada et al, 2005), and climate change (Qui et al, 2010). In particular, studies demonstrate a positive relationship between kelp and decapod abundances (Johns and Mann, 1987;Daly and Konar, 2010;Irigoyen et al, 2011).…”

Reaction of an estuarine food web to disturbance: Lindeman’s perspective

“…During this period, the mean trophic efficiency of the lagoon was extremely low (i.e. 0.5%; Libralato et al 2004). Christian et al (2009) described a de crease in the mean trophic efficiency of the Neuse River estuary after a severe episode of hypoxia.…”

“…Analysing the scientific literature on the topic, stress is usually associated with low system efficiency. For example, between 1974 and 1989, the Venetian Lagoon experienced a high level of eutrophication (Libralato et al 2004). During this period, the mean trophic efficiency of the lagoon was extremely low (i.e.…”