A modeling approach to assess coastal management effects on benthic habitat quality: A case study on coastal defense and navigability
Graphical abstract
Introduction
Estuaries and coastal embayments are a preferential habitat for humans (Small and Nicholls, 2003), thereby causing major alterations to the sea-scape. Firstly, the increase in coastal populations, in combination with sea level rise and increasing intensity of extreme storms, is bringing a large part of the world's human population under the threat of coastal storm surges (McMichael et al., 2006). This has led to the still ongoing realization of a high number of dams, embankments and storm surge barriers in the richest countries (Fig. 1). With the growing prosperity of developing countries (where most of the endangered population lives) these measures will likely be more commonly adopted worldwide (Temmerman et al., 2013, Perkins et al., 2015).
Secondly, waterways have for centuries played an important role in trade, causing civilizations to develop in delta areas with good direct access to both the sea and the land behind. Nowadays, the handling capacity of estuarine ports is a crucial factor for the economic development (Halpern et al., 2008). The continuously growing global trade network and the ongoing increase of the commercial ships are pushing toward a more intensive dredging of the waterways to the harbors (Fig. 1). The global dredging market increased by nearly threefold over the past decade from $ 5.3 bn in 2000 to $ 14.7 bn in 2011, according to the International Association of Dredging Companies.
Due to anthropogenic needs, wet infrastructures are so ubiquitously diffused that they have been proposed as a main driver of change in coastal environments (Bulleri and Chapman, 2010). Dams, embankments and storm surge barriers provide coastal protection by damping the tidal energy. In contrast, the deepening of estuarine beds often facilitate the inland penetration of seawater, leading to a landwards increase of the tidal energy (Stive and Wang, 2003). In both cases, the ecological implications can be large, and should be taken into account for development plans (Nienhuis and Smaal, 1994, Swanson et al., 2012, Nordstrom, 2014, Perkins et al., 2015). It is well known that hydrodynamic forces and their morphodynamic consequences structure estuarine life (Snelgrove and Butman, 1994, Ysebaert et al., 2003, Cefalì et al., 2016). Alterations of the eco-hydro-morphological environment can have negative, or even catastrophic, social consequences when they affect essential ecological services (Adger et al., 2005, Danielsen et al., 2005, Diaz et al., 2006, Perkins et al., 2015).
Distribution patterns and shifts can be assessed using Species Distribution Models (SDMs), which are statistical tools that combine observations of species occurrence or abundance with environmental variables (Elith and Leathwick, 2009). The application of SDMs in assessing the distribution of marine species has increased considerably in the last years as a tool for ecosystem management and marine spatial planning (reviewed in Robinson et al., 2011, Reiss et al., 2015).
Despite this, there are several challenges associated with the study and prediction of complex alterations of the eco-hydro-morphological environment. A first challenge is related to the large internal heterogeneity of coastal environments. Estuaries and embayments are indeed characterized by strong gradients in depth, salinity, current velocity, sediment composition and other factors (McLusky and Elliott, 2004, Morais et al., 2016, Trancart et al., 2016). A management strategy at the scale of the system may lead to strong spatial divergence in response, where different subhabitats are affected in very different ways (Cozzoli et al., 2013). Secondly, it is not an easy task to forecast long-term morphodynamics (Lesser et al., 2004), neither to translate morphodynamic conditions into habitat suitability (Cozzoli et al., 2014). Non-linear dynamics, unaccounted variables and unexpected features that can arise as the system develops, and the assumption behind physical and ecological expectations can be mismatched. Thirdly, extensive data series of field collected observations, inclusive of both hydromorphological (e.g. elevation, current velocity, granulometry, salinity) and biological (e.g. abundance and composition of ecological communities) measurements, are virtually never available with an extent that is relevant compared to the morphodynamic scales (decades, De Vriend et al., 2011). This reduce the possibility to fit and field-validate predictive models.
Despite the large uncertainty in predicting the environmental consequences, the realization of new coastal infrastructure is an unrestrainable need of human society (Temmerman et al., 2013, Nordstrom, 2014, Perkins et al., 2015), and the infrastructures design must attempt to account for ecological aspects. Presently, incomplete knowledge of ecological impacts undermines predictive management that would otherwise allow for appropriate spatial planning in coastal infrastructure design (Perkins et al., 2015). In this perspective, the study of existing anthropogenically modified ecosystems is a precious source of information to support adaptive management and future decisions (Folke et al., 2004, Matthews et al., 2011).
In this paper we hence focus on the ecological effects of contrasting hydrodynamic modifications of two adjacent estuarine habitats subject to large-scale infrastructural works. For this purpose, the neighboring Westerschelde and Oosterschelde estuaries (Dutch Delta, SW Netherlands, Fig. 2) are an ideal model system. The two basins share a common location, origin and regional pool of macrozoobenthic species (Cozzoli et al., 2013). To a large extent, they had similar hydrodynamic characteristics until approximately 50 years ago, but in the meantime they have undergone very different anthropogenic modifications. Coastal safety is a prominent issue in both sites, but, due to different navigability requirements, two radically different approaches were followed to achieve this goal. The Oosterschelde was partially embanked by a storm surge barrier. The Westerschelde, due of its importance as shipping route to the port of Antwerp, kept an open connection with the sea. In this basin, coastal safety is ensured by heightening and strengthening the dikes along the estuary. In recent decades the Westerschelde was extensively dredged to allow the transit of larger vessels (Fig. 2).
We investigated the long-term effect of the habitat alterations for an important part of the estuarine natural community: the macrozoobenthos. Macrozoobenthic organisms are a central component within the estuarine food webs (Herman et al., 1999) and they can affect biogeochemical cycles on a global scale (Heip et al., 1995). Distribution models of macrozoobenthos communities are useful tools to detect anthropogenic impacts at the ecosystem level (Robinson et al., 2011, Reiss et al., 2015).
While most macrozoobenthic communities studies focus on local/short term disturbances (e.g. bottom disruption, increase turbidity, resuspension of pollutants, see Short and Wyllie-Echeverria, 1996), we compared the benthic habitat suitability before (1960) and after (2010) the major infrastructural works. Despite these two basins being intensively monitored during recent decades, the amount of field – collected biological observations is insufficient to directly reconstruct the benthic community evolution. Hence we integrated hydrodynamic and ecological modeling to investigate the effects of morphological/hydrodynamic alterations on a whole-basin scale, over a time span that is relevant compared to intrinsic morphodynamic time scales.
We are fully aware that by combining hydrodynamic model with community models, we increase the level of uncertainty relative to what could be obtained from long-term monitoring series (Reiss et al., 2015). However, given the major hydrodynamic and morphological alterations in both these estuaries, such modeling approach should be able to reveal the dominant trends.
Section snippets
Study area
The present-day geomorphology of the SW Netherlands is the result of the interplay between natural processes and large-scale human interference that dates back at least two millennia. From the Middle Ages onwards, land reclamation led to a gradual separation between the Oosterschelde and the Westerschelde. The Oosterschelde was definitively cut off from the Schelde river in 1897. Following the disastrous North Sea flood of 1953, a massive system of coastal defense was implemented (Delta Works,
Results
The fitted 95th quantile benthos distribution models (Table A 2, Table A 3, Table A 4, Table A 5) were successfully validated (Fig. A 1). The habitat suitability for the analyzed benthic community descriptors show large variations at the variation of the covariance structure between environmental factors (Figs. A 3–A 6). Macrozoobenthos overall biomass (Fig. A 3) individual size (Fig. A 5) and species diversity (Fig. A 6) are potentially higher in the intermediate intertidal at marine salinity.
Discussion
This study shows that following fundamentally different management options may entail long-term morphological and hydrodynamic alterations of the ecosystem, with consequences for benthic community. The enhanced hydrodynamics following deepening of the Westerschelde (Fig. 3) had negative implications for benthos (i.e., reduced subtidal habitat suitability, turnover toward opportunistic species in the intertidal habitat, Fig. 4). In contrast, the attenuation of hydrodynamics by creating a storm
Conclusion
The management of coastal and estuarine areas is complex and critical, as these areas host both in terms of ecology (McLusky and Elliott, 2004) and economy (Costanza et al., 1997) one of the most valuable ecosystems around the world, which at the same time belong to one of the most impacted environments (Barbier et al., 2011). The realization of wet infrastructure with multiple goals ranging from coastal defence to transportation will continue, being essential to ensure future human welfare (
Acknowledgments
This work was mainly funded by the Ecoshape/Building with Nature project. The NIOZ Monitor Taskforce was for a large part responsible for the fieldwork and the taxonomic analysis of the macrofauna samples. Rijkswaterstaat (executive body of the Dutch Ministry of Infrastructure and the Environment) was responsible for the funding of these activities in the framework of different national monitoring projects such as MWTL. The hydrodynamic modeling for the Westerschelde was funded by the Antwerp
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