Recent environmental evolution of regenerated salt marshes in the southern Bay of Biscay: Anthropogenic evidences in their sedimentary record

Abstract

Short sediment cores (up to 44 cm long) taken from salt marshes regenerated during the last 60 years in the Urdaibai Biosphere Reserve have been interpreted on the basis of microfaunal and geochemical determinations and historical data. Agricultural soils in the middle and upper estuary reaches were abandoned during the 1950s and entrance of estuarine water provoked a rapid natural environmental transformation of these anthropogenic areas. Increasing amounts of sand and benthic foraminifera were deposited at a very high sedimentation rate (average 16 mm yr⁻¹) during the 1950s and 1960s allowing well developed regenerated salt marshes to be rapidly established in these formerly occupied areas. During recent decades much lower sedimentation rates (average 2.5 mm yr⁻¹), abundant agglutinated foraminiferal assemblages and enrichment of heavy metals (Pb, Zn, Cu, Ni and Cr) due to industrialization are characteristic of these already regenerated environments. This rapid regeneration process (less than 10 years) is of great interest for environmental management of modern coastal zones where extensive reclaimed land could be easily restored to tidal wetlands under the current scenario of accelerating sea-level rise.

Introduction

Although land loss continues to be the most pervasive threat to coastal areas and salt marshes in Europe, in some regions anthropogenic actions such as drainage and embankment construction have been stopped and even reversed (Airoldi and Beck, 2007). This is the case of some coastal wetlands in the southern area of the Bay of Biscay, which were drained at the beginning of the 18th century for agricultural purposes and malaria eradication and subsequently abandoned during the mid 1900s due to rural migration to the cities (Cearreta et al., 2002). This abandonment and lack of dyke maintenance provoked the entrance of tidal, estuarine water and allowed their natural regeneration. There is considerable international interest in restoring tidal flow to dyked salt marshes (Portnoy, 1999, Sinicrope et al., 1990) in order to reestablish important wetland functions (waste processing, nutrient cycling and fertility, biodiversity, biological regulation or recreational among others) elucidated over the past few decades (Lotze et al., 2006). Worldwide experiences have noted previously that some accidentally restored sites, such as the abandoned agricultural fields, were colonized by marsh vegetation as rapidly as highly engineered projects (Williams and Faber, 2001). Observation of the rapid evolution of restoration sites where natural physical processes were unimpeded has now led practitioners to rely on encouraging natural physical processes (rather than engineered projects) as much as possible to restore ecologic functions in these environments (Williams and Faber, 2001). Zedler and Callaway (1999) concluded that soil organic matter in constructed wetlands would not reach the values of the reference natural salt marshes, and Craft et al. (2003) observed that constructed salt marshes were less effective in sequestering total organic carbon over the long term than natural salt marshes.

Current concerns regarding global sea-level rise (GSLR) associated with anthropogenic warming of the atmosphere and oceans and its societal and physical impacts on the coastal areas have resulted in increased interest on past environmental changes recorded in coastal environments. In fact, GSLR is affecting coastal geomorphology, erosion, and most specially wetland loss (Leatherman, 2001, Nicholls, 2004). The GSLR rate for the last century has been estimated at 19 cm based on a number of high-quality tide gauge records (Bindoff et al., 2007, Church et al., 2004, Church and White, 2006), which is three times higher than the previous century (Jevrejeva et al., 2008). More recent estimates of GSLR about ~ 3 mm yr⁻¹ became available based on satellite altimetry data beginning in 1993 (Cabanes et al., 2001, Cazenave and Nerem, 2004, Leuliette et al., 2004) suggesting even a greater acceleration over the last decades. This sea-level rise acceleration is increasingly stressing coastal ecosystems (Church et al., 2008). Potentially, sea-level rise would: 1) increase salt water intrusion landward, producing salinification of coastal aquifers and affecting the species composition and the ecologic function of wetlands; 2) enhance tidal creek incision and beach erosion, potentially forcing a coastal retreat; and 3) increase the potential impact of future storms. Under this scenario, wetlands appear to be especially vulnerable to sea-level rise (Harvey and Woodroffe, 2008).

The study of regenerated wetlands over time in response to tidal inundation and sea-level rise can potentially provide key information of future trends of coastal evolution under the current climatic scenario of global warming and accelerating sea-level rise.

The southern Bay of Biscay (Fig. 1) is formed by rocky cliffs interrupted by small estuaries where salt marsh environments develop. During the last centuries these salt marsh ecosystems have been occupied initially with agricultural purposes and lately to support modern urban and industrial development. These human activities have provoked their destruction, size reduction or degradation of their environmental quality. Rivas and Cendrero (1991) concluded that human occupation of salt marshes and other intertidal areas can be considered as the main geomorphological process in the southern Bay of Biscay during the last two centuries. The first significative change took place at the beginning of the 18th century when the need for fertile land and malaria eradication were powerful reasons to drain salt marshes. Then, the Cambó Law of 1918 promoted additional desiccation of coastal wetlands based on their suspected insalubrity (Gogeascoechea and Juaristi, 1997). During the 1950s these reclaimed areas were abandoned due to rural migration to the cities, and the lack of dyke maintenance provoked the entrance of tidal, estuarine water and allowed their natural regeneration.

Natural, regenerated and still-reclaimed salt marshes represent around 65% of the whole estuarine area of the Urdaibai Biosphere Reserve. Consequently, an adequate scientific knowledge of the recent geological processes acting on these areas is of maximum importance to the correct management of this protected environment. This study uses a combined high-resolution microfaunal–geochemical approach to examine the recent history of salt marsh evolution and environmental regeneration in the Urdaibai estuary. The main aim of this work is the identification and assessment of natural versus anthropogenic processes in this area by means of benthic foraminifera and heavy metal contents from sediment cores collected from previously reclaimed salt marshes. ²¹⁰Pb and ¹³⁷Cs determinations have also been undertaken to provide a chronology for environmental changes recorded in the Urdaibai estuary salt marshes. The response of previously reclaimed areas to tidal inundation is of great interest in order to plan scientifically sound adaptation measures to face current sea-level rise. Cearreta et al. (2002) initiated the geological study of natural regeneration process of previously reclaimed areas in the Basque coast focused on the nearby Plentzia estuary, and Santín et al. (2009) studied the consequences of the reclamation and regeneration processes on the organic matter content of different salt marsh areas in the Urdaibai estuary. This joint microfaunal–geochemical approach is very cost effective and of wide applicability to all temperate coastal wetlands.