Impact of mussel farming on sedimentary geochemical properties of a Northern Adriatic area influenced by freshwater inflows

doi:10.1016/j.ecss.2013.06.001

Estuarine, Coastal and Shelf Science

Volume 129, 1 September 2013, Pages 49-58

https://doi.org/10.1016/j.ecss.2013.06.001 Get rights and content

Highlights

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Higher deposition of organic matter and anoxic condition under the mussel farm.
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Phosphorus represent a good tracer of the mussel biodeposition.
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Pelite normalization enforce the impact signal of the mussel farm in sediments.
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Grid sampling represent a good approach to evaluate the impact of mussel farms.

Abstract

The interactions of mussel aquaculture with physico-chemical properties and the biogenic elemental composition of the surface sediment were studied at a long-line farm, located in a 12 km² area, which included the farm (4 km²) characterized by relevant freshwater inputs along the North-Western Adriatic coastline. Negative redox potentials were detected at two stations directly affected by the farm. Total phosphorus content reached the highest concentration at stations located beneath the farm. OC/TP and TN/TP showed similar patterns, which was characterized by average values underneath the farm (respectively 45.5 and 4.5) that were significantly lower than those measured at the control stations. Our hypothesis, that these patterns could be related to organic matter enrichment of the surface sediment underneath the farm, due to intense mussel biodeposition, was confirmed by the mass accumulation rates, estimated to be respectively 23, 2.3, 0.94 and 0.26 g m⁻² y^-1 for organic carbon, total nitrogen, total and organic phosphorus. Data concerning the sediment chemistry were normalized with respect to the pelite fraction thus increasing the capability of detecting the environmental impact of the farm under variable background sedimentary fluxes.

Introduction

The Northern Adriatic Sea is a shallow basin under the influence of several riverine inputs, with the three rivers Po, Adige and Brenta contributing approximately 84% of the total freshwater load (Cozzi and Giani, 2011). High concentrations of inorganic nutrients and organic matter (OM), mainly supplied by rivers, can support a high primary production along the North-Western Adriatic coast (Pugnetti et al., 2004, Mangoni et al., 2008), making this area particularly suitable for mussel farming. In fact, 50% of the Italian mussels produced in the years 2000–2005 were farmed in the Northern Adriatic Sea, accounting for approximately 12% of the European mussel production (FAO, 2007).

Extensive bivalve mussel culture does not require any feeding and, therefore, its impact on bottom sediment is expected to be less than intensive and semi-intensive fish farming (Danovaro et al., 2004, Sarà et al., 2004, Holmer et al., 2007). However, mussels are filter-feeding organisms which remove large amounts of particulate matter from the water column and void a large part of particulate material, as faeces and pseudofaeces, towards the surface sediment (Ward and Shumway, 2004). This process may produce notable physico-chemical changes, due to organic matter enrichment (Kaspar et al., 1985, Mirto et al., 1999, Danovaro et al., 2004). The sedimentation rate under mussel farms can be up to twice that in surrounding areas (De Vittor et al., 2000, Christensen et al., 2003). OM enrichment may result in enhanced bacterial activity (Holmer et al., 2007), high oxygen consumption, reducing condition (in the worst cases, sediment anoxia) (Tenore et al., 1982, Kaspar et al., 1985) and severe alteration of the macrofaunal community (Kaspar et al., 1985). Furthermore, nitrogen and phosphorus released by farmed mussels through direct excretion, or via the remineralization of faeces and pseudofaeces, could impact biogeochemical cycles of this elements and, in turn, pelagic and benthic marine ecosystems (Dahlbäck and Gunnarsson, 1981, Dame et al., 1989, Christensen et al., 2003). In contrast other studies indicate that mussel farming has no detrimental effect on the benthic domain in shallow areas characterized by high hydrodynamic regimes (Danovaro et al., 2004). Therefore, the impact of mussel farms depend, in a complex way, on both the hydrodynamic circulation on the dynamics of remineralizing processes and on the structure of the local benthic and marine ecosystems. Hence, it is important to identify a reliable set of indicators, to be used in Environmental Impact Assessment studies as well as in routine monitoring surveys.

Here we present a comprehensive investigation, aimed at testing a set of indicators of the impact of long-line mussel farming, which were selected during the FP6 Project ECASA (Ecosystem Approach for a Sustainable Aquaculture) and tested at several sites. We focus on the efficiency of these indicators in the presence of potentially confounding factors, represented by the Adige, Brenta and Po river plumes, which may significantly contribute to particulate OM enrichment in the investigated area. This case-study was selected to advance the limited knowledge concerning the assessment of environmental impacts of mussel aquaculture in Mediterranean areas which are directly affected by terrigenous-continental inputs.

Section snippets

Study site

The study was performed at a long-line Mediterranean mussel (Mytilus galloprovincialis) farm located in the North-Western Adriatic, south of Chioggia harbour, 5 nautical miles from the coast (Fig. 1) and in a shallow area, 20–24 m deep. The farm covers an area of about 3 km² and has been operating since 1991 with an average annual production of approx. 600 tons (Pastres et al., 2008). Mussels are grown on ropes approximately 2 m long, which are suspended on long-line cables, and placed at

Sampling strategy

The locations of the sampling stations were defined on the basis of the results of a preliminary survey, aimed at assessing the geochemical and chemical characteristics of surface sediments (0–0.5 cm), which was carried out on December 2005. Sediment samples were taken at 20 stations, which were placed is a regular grid on an approximately 12 km² area which included the mussel farm (Fig.1).

Based on this preliminary map of the sedimentary variability of the area, and on the time-series analysis

Water circulation

The average current velocity was 12.6 cm s⁻¹ and currents were mainly directed south-southeastward in spring and southwards in summer. The dispersion around the mean was rather high (CV% = 79%), due to systematic variations induced by the tide. Previous time series of current velocity collected at site G (Fig. 1) in 2001–2002, 2 m above the seabed, showed that the main current was oriented towards southeast and the mean velocity was 6 cm s⁻¹ (Giovanardi et al., 2003). The main current was

Discussion

The Brenta and Adige rivers and the northern branch of the Po river delta, located near the study area, significantly affect the grain size distribution of bottom sediments underneath the mussel farm and in its surroundings. Their sedimentary supply explains the positive gradient of pelite and OC contents in sediments following the NE-SW direction, making difficult to identify and quantify the impact of mussel biodeposits on sediment grain-size and composition.

In this study, we did not observe

Conclusions

The complex hydrodynamic circulation and the presence of riverine inputs, required a comprehensive experimental design, aimed at identifying the impact of the mussel farm. The results show that mussel farming activities have a detectable impact, even in highly dynamic areas, where they have often been reported as not detrimental for the environment. The approach here described provided a better understanding of the origin and distribution of the organic matter in the whole area avoiding that

Acknowledgements

This study was carried out in the frame work of the ECASA FP VI EU project. We are grateful to Camilla Antonini for Arc Gis maps, Fabio Savelli for his assistance in laboratory analyses and VISMA crew for assistance during sampling activities. We dedicate this paper to Giuliano Gianni who greatly helped us in field work thanks to his experience.

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Impact of mussel farming on sedimentary geochemical properties of a Northern Adriatic area influenced by freshwater inflows

Highlights

Abstract

Introduction

Section snippets

Study site

Sampling strategy

Water circulation

Discussion

Conclusions

Acknowledgements

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Estuarine Coastal and Shelf Science

Marine Environmental Research

Geochimica et Cosmochimica Acta

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Continental Shelf Research

Marine Pollution Bulletin

Marine Geology

Continental and Shelf Research

Marine Geology

Science of the Total Environment

Estuarine Coastal and Shelf Science

Aquaculture

Estuarine Coastal and Shelf Science

Marine Pollution Bulletin

Mar. Chem.

Aquaculture

Seasonal and puri-annual variability of sediment accumulation and organic matter fluxes in the northwestern Adriatic shelf and its relationship with the frontal system

Chemistry and Ecology

A semiautomated method for the determination of inorganic, organic and total phosphate in sediments

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