O. Giovanardi

An Ecological Imbalance Induced by a Non-Native Species: The Manila Clam in the Venice Lagoon

Among the 19 non-native species of marine invertebrates which have invaded the Venice Lagoon and have established populations, Ruditapes philippinarum, deliberately introduced in 1983, is surely the most successful species. According to the hypothesis that alien species invasion could be favoured by an altered ecological, chemical or physical state of the system induced by anthropogenic disturbance, R. philippinarum turned out to be ‘the right species at the right moment’. By comparing historical data (1968, 1985, 1990) with 1999 data, changes in macrobenthic community, in particular bivalve molluscs, of the lagoon induced by R. philippinarum introduction and subsequent clam exploiting activity were assessed. It has been possible to describe a sharp reduction, both in terms of distribution area and density, of all other filter feeder bivalves. Moreover, by using the clearance rate of the most abundant bivalve species in 1990 and 1999 (Cerastoderma glaucum and R. philippinarum, respectively), it was possible to estimate that the filtration capacity, expressed as l h−1 m−2, has more than doubled. This has altered the functioning of the ecosystem, resulting in a stronger benthic–pelagic coupling. In this context, R. philippinarum attains control of the system. Considering all this, it is possible to state that the Venice Lagoon ecosystem has entered into a new state, probably more resistant but less resilient, with implications for future management choices.

Need for restricting bivalve culture in the southern basin of the Lagoon of Venice

At present, one of the environmental emergencies in the Lagoon of Venice is the impact of short-necked clam (Tapes philippinarum) fishery, which is practically an unregulated fishery. Although one of the proposed solutions would be the restriction of Tapes fishery to licensed areas, high seeding density can cause undesired effects on the environment. In this study several hydrobiological variables are compared between small areas of the Lagoon of Venice traditionally used for bivalve culture (clam, T. philippinarum and mussel, Mytilus galloprovincialis), and areas in the southern basin with seagrass meadows. Labile and suspended organic matter in the water was higher in areas with bivalve farming than in Zostera areas (undisturbed control). The same pattern was recorded for contents of total organic matter and acid volatile sulphides. The biomass of microplankton in farming areas was quite high (0.8–2.7 g m−3). Mesozooplankton was extremely abundant, particularly at night, when its biomass was 1–2 orders of magnitude higher than during the day. Its composition was different in the culture areas and in Zostera areas. The biomass of Tapes in culture beds and their filtering capacity were also estimated.

Recolonization dynamics in areas disturbed by bottom fishing gears

Results of two investigations on the effects of disturbance on benthic communities in lagoon and coastal areas, caused by bottom fishing-gears (‘hydraulic dredge’ for clams and ‘rapido’, a kind of beam-trawl for soles and scallops employed in the Northern Adriatic sea), are given. Such gears, although characterised by different features and targets, have similar effects on the sea bottom: both produce deep furrows (7–13 cm for the ‘rapido’, up to 20 cm for the ‘hydraulic dredge’), thus affecting the texture of the bottom. In 1992 (‘hydraulic dredge’) and in 1995 (‘rapido’) two different research projects were carried out; samples of benthos were collected immediately after the passage of the gears and at fortnightly-intervals, in treated and control areas. This allowed study of the modifications of the macrobenthic communities and investigation of the short and medium-term (dredge: 60 days, ‘rapido’: 15 days) progression of the recolonization processes in the disturbed areas. These dynamics have been analysed by giving emphasis to the species and to their time-space fluctuations. It has been found that characteristically ‘non-opportunistic’ species can assume an opportunistic behaviour during the initial phase of the recolonization processes of the disturbed areas.

Bloom of picocyanobacteria in the Venice lagoon during summer-autumn 2001: ecological sequences

A dense bloom of picocyanobacteria with biomass of 10–50 g m−3(wet weight) and numerical density 5–20 × 106 cells ml−1 broke out in the Lagoon of Venice in July 2001. The share of picocyanobacteria of the total phytoplankton varied in the Venice lagoon in July–September from 60 to 98% depending on the vicinity of the site to the channel’s driving tidal currents. The washout of the picocyanobacterial biomass occurred during the ebbs to the shelf zone of the adjacent Adriatic sea. The biomass of picocyanobacteria in coastal Adriatic water was at that time up to 1.7 g m−3(w.w.) with the share of picocyanobacteria ranging from 70 to 90%. The rest consisted of small phytoflagellats. The contents of suspended and labile organic matter in water increased during the bloom by a factor of 5–15. The photosynthesis rate in upper water layers rose by about 2 orders of magnitude, attaining 3–5 g C m−3 day−1, with a decomposition rate of 2–3 mg O2 l−1 day−1. The residence time of inorganic phosphorus standing stock in water was found to be as short as 6–12 min. The populations of micro- and mesozooplankton were found to be inhibited in areas of intensive bloom. A significant mortality of key species for the local fishery, e.g. the Manila clam, Tapes philippinarum, was recorded in the lagoon in September–October.

Common, rare or extirpated? Shifting baselines for common angelshark, Squatina squatina (Elasmobranchii: Squatinidae), in the Northern Adriatic Sea (Mediterranean Sea)

Historical baselines are needed to reconstruct long-term changes in marine animal populations and enhance our ability to articulate management recommendations. We reconstructed common angelshark (Squatina squatina) abundance in the Northern Adriatic Sea over the last two centuries by integrating different sources of formal and informal information. The wide amount of information collected helped assessing if the species is actually extirpated from the area, as stated in previous studies. According to naturalists’ accounts and historical documents, in the nineteenth and early twenty-first centuries the species was so abundant to sustain targeted fisheries, and large quantities of S. squatina were sold in the main fish markets. In the 1960s, the species collapsed and got economically extinct. Even if it was never caught in the area through scientific surveys during the period 1948–2014, from fishermen interviews emerged that the species is not extirpated. However, only 50% of interviewees caught S. squatina at least once and they were significantly older than the fishermen that never caught it (shifting baseline syndrome). Moreover, the size of the fish caught significantly decreased through time, indicating the depletion of larger individuals. Our integrated approach can be applied to any poorly assessed species so that appropriate international conservation measures can be prioritized.

Comparison between artisanal and industrial fisheries using ecosystem indicators

Artisanal fishery in the lagoon of Venice is a multi-target activity with an old tradition. It was the only fishing activity since a new one with most features of an industrial fishery flourished following the introduction of the Manila clam in 1983. To compare the two fishing activities, a set of ecosystem indicators (landings, catches, discards, biomass of the system, mean Trophic Level of the system and exergy) obtained by a model approach, was applied. The model used was a mass-balance model of the lagoon ecosystem developed with the software package Ecopath with Ecosim. The 73 scenarios obtained by changing the fishing effort of the two different fisheries were used to explore the impact of fishing activity on the ecosystem. The results showed that the two activities are strongly interrelated, even if they do not exploit the same resources, and that the mechanical clam harvesting is the driving force able to affect the ecosystem state fundamentally. All this produces a strong conflict between the two kinds of fisheries. The ecosystem and social optimisation depend mainly upon a reduction of clam fishery, while the optimisation of the economic aspects is strictly linked to the maintenance of this fishing activity.

The first assessment of marine debris in a Site of Community Importance in the north-western Adriatic Sea (Mediterranean Sea)

At present, few studies have investigated the marine litter abundance, composition and distribution on rocky bottoms due to sampling constraints. We surveyed by means of the ROV imaging technique a system of biogenic rocky outcrops classified as a Site of Community Importance in the Adriatic Sea. A mean density of 3.3 (±1.8) items/100m2 was recorded, with a strong dominance of fishing- and aquaculture-related debris, accounting for 69.4% and 18.9% of the total, respectively. The abundance of litter over the rocky bottoms was significantly higher than that on soft substrates, and its spatial distribution proved to be related to hydrographic factors. Litter-fauna interactions were high, with most of the debris (65.7%) entangling or covering benthic organisms, in particular habitat constructors such as the endangered sea sponge Geodia cydonium. Unless appropriate measures are undertaken to address this problem, the abundance of marine litter in the area is likely to increase.

Analysis of Long-Term Changes in a Mediterranean Marine Ecosystem Based on Fishery Landings

In the Mediterranean Sea, structured and standardized monitoring programs of marine resources were set only in the last decades, so the need to analyze changes in marine communities over longer time scale has to rely on different sources. In this work, we used seven decades (1945-2014) of disaggregated landing statistics for the Northern Adriatic Sea (Mediterranean) to infer changes in the ecosystem. Analysis of landings composition was enriched with the application of a suite of ecological indicators (e.g., trophodynamic indicators, such as the primary production required to sustain the catches - PPR; size-based indicators, such as the large species indicator - LSI; other indicators, such as the elasmobranchs-bony fish ratio – E/B ratio). Indicators were further compared with main ecosystem drivers, i.e., fishing capacity, nutrient loads and climate change. Species most vulnerable to fishing (i.e., elasmobranchs and large-sized species) dramatically declined at the beginning of the industrialization of fishery that occurred right afterwards World War II, as can be inferred by the negative drop of LSI and E/B ratio in the mid-1950s. However, until the mid-1980s landings and PPR increased due to improvements in fishing activities (e.g., the introduction of more efficient fishing gears) increasing fishing capacity, high productivity of the ecosystem. Overall, long-term effects of fishing were buffered by an increase in productivity in the period of high nutrient discharge (up to mid-1980s), but still drove significant changes in fish community structure. From the mid-1980s, a reduction in nutrient load caused a decline in productivity but the food-web structure was already modified and unable to support, or recover from, such unbalanced situation, resulting in the collapse of landings. This collapse is coherent with alternative stable states hypothesis, typical of complex real systems, that implies drastic interventions that go beyond fisheries management and include regulation of nutrient release for recovery. The work highlights that, despite poor capabilities to track species dynamics, landings and applied indicators might help to shed light on the long-term dynamics of marine communities, thus contributing to place current situation in an historical framework with potential for supporting management.

Paleochannel and beach-bar palimpsest topography as initial substrate for coralligenous buildups offshore Venice, Italy

We provide a model for the genesis of Holocene coralligenous buildups occurring in the northwestern Adriatic Sea offshore Venice at 17–24 m depth. High-resolution geophysical surveys and underwater SCUBA diving reconnaissance revealed meandering shaped morphologies underneath bio-concretionned rocky buildups. These morphologies are inferred to have been inherited from Pleistocene fluvial systems reactivated as tidal channels during the post- Last Glacial Maximum transgression, when the study area was a lagoon protected by a sandy barrier. The lithification of the sandy fossil channel-levee systems is estimated to have occurred at ca. 7 cal. ka BP, likely due to the interaction between marine and less saline fluids related to onshore freshwater discharge at sea through a sealed water-table. The carbonate-cemented sandy layers served as nucleus for subsequent coralligenous buildups growth.

Fish and fishery historical data since the 19th century in the Adriatic Sea, Mediterranean

Historic data on biodiversity provide the context for present observations and allow studying long-term changes in marine populations. Here we present multiple datasets on fish and fisheries of the Adriatic Sea covering the last two centuries encompassing from qualitative observations to standardised scientific monitoring. The datasets consist of three groups: (1) early naturalists’ descriptions of fish fauna, including information (e.g., presence, perceived abundance, size) on 255 fish species for the period 1818–1936; (2) historical landings from major Northern Adriatic fish markets (Venice, Trieste, Rijeka) for the period 1902–1968, Italian official landings for the Northern and Central Adriatic (1953–2012) and landings from the Lagoon of Venice (1945–2001); (3) trawl-survey data from seven surveys spanning the period 1948–1991 and including Catch per Unit of Effort data (kgh−1 and/or nh−1) for 956 hauls performed at 301 stations. The integration of these datasets has already demonstrated to be useful to analyse historical marine community changes over time, and its availability through open-source data portal will facilitate analyses in the framework of marine historical ecology.