Decadal changes in the spatial coverage of Zostera noltei in two seagrass meadows (Ría de Vigo; NW Spain)

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Abstract

Seagrass meadows are influenced by anthropogenic activities occurring in the littoral, generally resulting in an intense regression in the last decades. This study examines the evolution of the Z. noltei coverage from 1989 to 2014 in two seagrass meadows located in the Ría de Vigo (Arcade and A Ramallosa), subjected to different degrees of coastline alteration, anthropogenic land cover, population growth and shellfishing activity. A spatial analysis of the distribution of Z. noltei was carried out using aerial photographs from 1989, 2004 and 2014 and field surveys. The results obtained showed that in Arcade, a meadow characterized by low watershed-scale anthropogenic pressure and absence of shellfishing on the seagrass, the Z. noltei coverage decreased by 48 % in the studied period. By contrast, in A Ramallosa, a meadow characterized by moderately watershed-scale anthropogenic pressure and presence of shellfishing on the seagrass, the Z. noltei coverage increased by 56%. The observed increase of Z. noltei coverage in A Ramallosa appears to be related to the combined effect of increase of finer sediment loads and a decrease of bivalve harvesting.

Introduction

Seagrass meadows are highly productive coastal ecosystems (McRoy and McMillan, 1977, Duarte and Chiscano, 1999, Hemminga and Duarte, 2000) that offer shelter for several commercial species (Beck et al., 2001, Nagelkerken et al., 2001), reduce water turbidity through enhanced sediment deposition (Duarte and Chiscano, 1999, Touchette and Burkolder, 2000, Koch, 2001, Short et al., 2007) and act as carbon sinks, contributing to carbon sequestration into the marine sediments (Short et al., 2007).

However, seagrasses have been declining worldwide due to the anthropogenic pressure on coastal areas (Short and Wyllie-Echeverria, 1996, Orth et al., 2006, Duarte, 2017) leading to the disappearance of nearly 29% of seagrass meadows in the last century (Waycott et al., 2009). Nevertheless, it is worth mention that seagrass recovery, especially of Zostera noltei, has been recently reported for different European embayments in the last decade (de los Santos et al., 2019).

Population growth, eutrophication, coastal urbanization and sea level rise have been reported as the major causes of this decline (Fonseca et al., 2000, Duffy, 2006, Orth et al., 2006). Shellfish harvesting is increasing worldwide (Narita et al., 2012) being an important socioeconomic activity that has grown intensively in the last decades in NW Spain (Cochón and Sánchez, 2005, Surís-Regueiro and Santiago, 2014). Mechanical impacts associated with bivalve harvesting have been also shown to led to seagrass decline (Short and Wyllie-Echeverria, 1996, Cochón and Sánchez, 2005, Orth et al., 2006, McLaughlin et al., 2007, Waycott et al., 2009).

Apart from physical habitat disturbances, anthropogenic organic pollution is also a major factor causing eutrophication and consequent seagrass decline (Zylicz et al., 1995, Hemminga and Duarte, 2000, Duarte et al., 2004, Sánchez-Lizaso et al., 2015). Watershed-derived organic matter inputs towards water bodies are enhanced by artificial land in the watersheds (Kaye et al., 2006, Mantas et al., 2016). Indeed, recent studies carried out in NW Spanish coastal watersheds showed that high population size and artificial land cover were related to higher contents of anthropogenic nitrogen both in the sediment and in the plant tissue of Z. noltei meadows (Román et al., 2019a, Román et al., 2019b).

The aim of this work is to assess the relationship between the potential anthropogenic stressors and the spatial coverage of Z. noltei at meadows located at two coastal embayments from NW Spain (Ría de Vigo). For that, we quantified the changes in the spatial coverage of Z. noltei between 1989 and 2014 at these meadows and the magnitude of anthropogenic stressors in watersheds and in the coast, which experienced different degrees of anthropogenic pressure.

Section snippets

Study sites

The seagrass meadows studied (Arcade and A Ramallosa) are located in the Ría de Vigo, a prolonged inlet located at NW Spain (Fig. 1), where fish and shellfish exploitation coexist with potential human threats to the coastline, such as land reclamation and outflow of untreated wastewater (Fernández et al., 2016).

The tidal range of this coastal area is mesotidal (Nombela et al., 2007), being characterized by a temperate and wet oceanic climate with a high pluviometry (>600 mm) (Rodríguez-Guitián

Results

In the Arcade watershed, the highest increases of artificial land appeared at the villages of Redondela and Arcade, whereas agricultural and natural land decreased slightly in the whole watershed. Agricultural cover was distributed sparsely in small plots, as the majority of crops belonged to nearby households traditionally exploited for private consumption (Fig. 3). The relative rate of increase of artificial land was positive, whereas rate of change in agricultural land was negative. By

Discussion

The results obtained in this investigation showed that totally opposite temporal patterns were observed in two Z. noltei meadows studied that were located in the same Ría and separated by a short distance. Unexpectedly, the higher population densities and artificial land cover in the watersheds were not associated to parallel decreases in Z. noltei cover in the sites studied. A direct anthropogenic pressure, such as the land reclamation caused by coastline alteration, was related to the loss of

CRediT authorship contribution statement

M. Román: Data curation, Formal analysis, Investigation,Methodology, Validation, Visualization, Writing - original draft, Writing - review & editing. E. Fernández: Conceptualization, Funding acquisition, Resources, Investigation, Methodology, Supervision, Validation, Visualization, Writing - review & editing. J. Zamborain-Mason: Data curation, Formal analysis, Investigation, Methodology, Validation, Visualization, Writing - original draft. L. Martínez: Data curation, Formal analysis,

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research was funded by the Ministry of Economy and Competitiveness, Spain through project REIMAGE (grantsCTM2011-30155-C03-01 and CTM2011-30155-C03-02). M. Román was supported by a PhD fellowship from the Xunta de Galicia, Spain .

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