Coexistence of urban uses and shellfish production in an upwelling-driven, highly productive marine environment: The case of the Ría de Vigo (Galicia, Spain)

https://doi.org/10.1016/j.rsma.2016.04.002Get rights and content

Highlights

  • Ría de Vigo: coexistence of environmental pressures and shellfish production.

  • Wind-driven upwelling controls the dynamics of the Ría de Vigo coastal ecosystem.

  • Pollution of the Ría de Vigo generates an intense social response.

Abstract

The Ría de Vigo, located at the northern border of the Iberia-NW Africa eastern boundary upwelling ecosystem, is an incised deep (45 m maxim depth) valley originated by the combined action of tectonic and erosive processes. Upwelling events promote a quick renewal (3–4 days) of the volume of water in the Ría, which is replaced by cold, oxygenated and nutrient-rich subsurface oceanic water, giving rise to nutrient fertilization of the embayment that translates into high primary production rates (ca. 3 g C m−2  d−1). This high productivity allows the development of an important shellfish exploitation activity. The Ría de Vigo has 529 rafts where 37 000 tons of mussels are produced annually. This territory has been subjected to an increasing human pressure since the early XX century deriving in an intense landscape transformation. The city of Vigo showed a six-fold increase in the number of inhabitants from 1900 to 2001 and more than 21% of the land in the study area is currently artificial. Therefore, urban and industrial activities coexist with local fisheries and shellfish mariculture that accounts for ca. 7% of the income and employment in this urban region. Incomplete depuration of wastewater in treatment plants, explain the relatively high contribution (22%) of sewage-derived nutrients to the total allochthonous fertilization in the Ría. Furthermore, and given the importance of shellfish production, microbial pollution originated from urban wastewater is considered as the main environmental threat of the Ría de Vigo. The dependence of the local economy on marine renewable resources together with the limitation of the natural productivity of the Rías imposed by anthropogenic pressure lies behind a significant fraction of the conflicts occurring in the region during the past two decades. The characteristics of Ría de Vigo, described in this review, confer this coastal ecosystem unique, conditions as a natural laboratory where the interaction between human activity and marine ecological processes can be studied within the framework of the World Harbour Project.

Introduction

The Ría de Vigo, located in the Northwest Iberian Peninsula (Spain), is one of the European harbours included in the World Harbour Project (WHP). The Ría de Vigo is not a heavily populated area, has not a large commercial port and is not an intensively polluted area nor a pristine environment when contrasted with the rest of WHP harbours. However, it is a perfect example of the coexistence of human populations, the economic activity associated to them, the consequent alteration of the marine ecosystem, including degradation of water quality conditions, the production of a considerable amount of shellfish and the presence of well preserved marine ecosystems, in a very reduced coastal area (156 km2) affected by wind-driven upwelling, therefore highly dependent on processes occurring at the global scale. In this regard, it is a suitable model system to explore the interaction among production ecosystem functions and anthropogenic environmental changes at different scales.

The coexistence of fishing and aquaculture activities in relatively polluted areas is a widespread phenomenon that has been intensively studied given its relevance for management and regulation purposes. Thus, for example, descriptions of mixed inputs of pollutants into St. Lawrence estuary (Pellerin and Amiard, 2009), discharges from mine activities in Greenland fjords (Søndergaard et al., 2011), anthropogenic inputs from industrial origin and urban effluents in the Bay of Muggia (North Adriatic Sea) (Moschino et al., 2016) or heavy metals pollution from anthropogenic activities in the southern coast of China (Zhang et al., 2012), in relation to the production of marine resources have been reported in the literature.

Human settlements and economic activities associated to the coastline of the Ría de Vigo date from before the Roman colonization period. However, the position of the city of Vigo as the most populated in the region and its role as an important regional communication node is quite recent, late XIX century, associated to the development of port infrastructures. The industrial revolution and ship motorization in the XIX century, brought about the need and the viability to build larger vessels which required greater depths in the ports, natural conditions already provided by the port of Vigo, where dredging was not and still is not necessary. In the second half of the XX century, the Port of Vigo became the main port of shipment of the Galician transoceanic migration, with ca. 1.2 million Galician people (ca. 40% of the current population of the region) migrating to South América from 1911 to 1970 (Campillo Ruiz et al., 1993). A sustained population growth was observed in the coastal municipalities of the Ría, particularly in Vigo, where a six-fold increase in the number of inhabitants occurred from 1900 to 2001. The urbanization of Vigo spread all along the coastal margin, as a consequence of the port activity, and also to the interior valleys were natural and agricultural land was transformed for the development of new industrial developments, largely related to the automotive industry. Currently, 300 000 inhabitants live in the City of Vigo and 428 000 in the eight municipalities surrounding the Ría.

The key geographical feature and identity element for the human populations inhabiting the area is, undoubtedly, the Ría. “Rías” are penetrations of seawater into the coast due the submersion of river watersheds that extend along the coastline. Von Richthofen (1886) was the first author who proposed the use of the term “Ría” to define a particular coastal typology characterized by the existence of a valley occupied by seawater, taking as an example the Galician Rías, their formation being related to tectonic and eustatic processes as well as regional and local singularities (Méndez and Vilas, 2005). They are incised valleys where only the inner brackish water zone can be considered an estuary (Evans and Prego 2003), being therefore divided into an inner zone close to the river mouth with typical estuarine characteristics and an outer, deeper zone, usually separated from the on-shelf ocean by an archipelago of small islands which constitute the natural area Atlantic Island National Park.

The Galician Rías in general, and the Ría de Vigo in particular, are highly productive ecosystems that allow the development of significant traditional fish and shellfish exploitation activities as well as an intense mussel raft aquaculture. This high capacity to produce biomass is related to their geographical location at the northern border of the Iberia-NW Africa eastern boundary upwelling ecosystem (Barton, 1998, Arístegui et al., 2009, Alvarez-Salgado et al., 2010). During upwelling events, subsurface (150–200 m) water upwells over the continental shelf entering the bottom layer of the Ría de Vigo, promoting a quick renewal of the water in the embayment (Alvarez-Salgado et al., 2000, Barton et al., 2015). The resultant inflow of shelf bottom water into the Ría, give rise to nutrient fertilization of the embayment triggering the production of high amounts of phytoplankton biomass.

The Ría de Vigo is impacted by diverse anthropogenic activities reflected, for instance, by increases in sedimentation rates over the last centuries (Pérez-Arlucea et al., 2005, Álvarez-Iglesias et al., 2007, Bruschi et al., 2013) or by increasing levels of some heavy metals, such as lead and copper, in the sediments (Prego and Cobelo-García, 2003, Howarth et al., 2005, Evans et al., 2011). However, the input of incompletely untreated urban sewage waters into the water column is likely to be the most important environmental problem faced by the socio-ecosystem of the Ría de Vigo given its potential ability to affect one of the most significant ecological services provided by the Ria, the annual production of 37 000 tons of mussels (Suris-Regueiro et al., 2014).

In this paper, a review of the main environmental, economic and social characteristics of the Ría de Vigo is presented with the aim of illustrating the potential of this harbour to contribute to the main goals of the World Harbour Project. For this purpose, we used existing knowledge accumulated in the scientific literature and also retrieved and processed available information to investigate temporal trends and magnitudes of land use changes and marine land reclamation and explored the relative contribution of different types of conflicts in the study area. We aimed at presenting the relationship among the hydrodynamics of the Ría de Vigo, controlled by coastal upwelling, shellfish production and human-derived environmental impacts.

Section snippets

Study area

The Ría de Vigo, located at the southern region of Galicia (Northwest Iberian Peninsula; Spain) is 32.8 km long, gradually widening from 0.6 km at the inner zone to 15.5 km at its mouth, covering a total surface of 156 km2 with a volume of water of ca. 3 km3. Its maximum depth is 45 m at the southern channel (Fig. 1).

The spatial distribution of surface sediment typology in the Ría de Vigo is related to bathymetry, to subsurface currents associated to rivers and to the hydrodynamics of water

Human pressure: economic activity and land use dynamics

The gross domestic product of the study area was 9500 million dollars in 2012, corresponding 82.5% of this value to Vigo. The traditional agriculture and fishing activities are now enriched by new industrial sectors, many of them related to the maritime industry (fishing, shipyards, port activities, etc.). The canning industry is a significant component of the economy of the Ría. In the last decades, the fish transformation and freezing industry grew intensively. Vigo is one of the largest

Conclusion

This review illustrates the role of Ría de Vigo as a model system where environmental pressures on the coastal ecosystem coexist with an important fish extraction and shellfish production activity and with almost pristine marine natural areas over a reduced spatial scale.

The progressive accumulation of human population in the Ría de Vigo led to a significant increase of its impact on the marine environment. Loss of breeding and nursery habitats of commercial species due to landfilling and

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