“The present as the key to the past” or the importance of the ecology for revealing the history of the Ebro Delta

A recent study uses biological indicators to reconstruct deltaic habitats

version in Spanish and Portuguese

Deltas are dynamic environments where morphological changes occur at human time scales (50-100 yr), and where intense biological and geological interactions are still evident in the fossil record. When adding human impacts into this complex equation, as for example the transformation of natural wetlands into rice fields, deltas can be considered “natural laboratories” to study changes over historical (millennia) and recent (decades) periods. This information can be then used for proposing management measures to mitigate ongoing and future climate change impacts.

The Ebro Delta (NE Iberian Peninsula) is one of the most ecologically important coastal wetlands of the Western Mediterranean due to its faunal and vegetal biodiversity. In addition, it supports an important economic activity for the regional population by means of rice agriculture, tourism and coastal fisheries.

Despite that the ecology of the Delta has been largely studied since the 1980s, there is a lack of scientific understanding about its natural and human-driven evolution at millennial and decadal-centennial time scales. This necessity is also justified by the fact that hypothesis about the Ebro Delta’s origin was exclusively based on personal interpretations from Roman written records.

Paleoecology provides reference conditions for those degraded habitats, which can be used as a benchmarks to restore and mitigate human-induced and sea level rise impacts

Recently, a new paper has been published to study the present and past Ebro Delta habitats from a paleoecological perspective (the paleoecology is the ecology of the past). This work summarises the PhD thesis of Dr Xavier Benito, which was developed at IRTA (Research Institute for agricultural and food technology, Sant Carles de la Rápita – Tarragona – Catalonia). For each of the two temporal windows, different methods were applied: i) at the present, the utility of diatoms (unicellular algae) and foraminifera (unicellular protozoa) as ecological indicators of the habitats, and ii) in the past, the use of fossil foraminifera to reconstruct natural (origin and evolution) and human-driven changes (rice field colonization).

The leading author, Xavier Benito, claims that their results showed that Ebro Delta originated at least 8,000 years ago – since only deltaic habitats were identified in the sediments at that time, rejecting therefore the reining hypothesis about the delta’s Roman origin.

Habitats identified according to diatom composition and abundance in the Ebro Delta samples, and Holocene (last 12,000 years) evolution of the Ebro Delta as reconstructed from micropaleontological fossil record. 

These results have clear implications in the context of current climate change impacts. Basically, bringing the Delta’s habitat to a good ecological condition can increase their capacity to cope with future changes, such those resulting from climate change. We should consider that climate change projections predict an increase in sea level and a reduction in the Ebro’s discharge and sediment loading. The long-term perspective provided here shows how the most “natural” Ebro Delta could cope with past climate changes, which probably implicated higher sea level fluctuations than those projected by the end of the 21th century: more than 0.5 m (it should be noted that around 60% of the Delta lies below 0.5 m of surface elevation).

 Fossil microorganisms revealed that the Ebro Delta originated at least 8000 years ago

Fortunately, it is not needed to go back 8,000 years in the Delta’s history to find such natural habitats. The results also showed that just 150 years ago, before the proliferation of rice cultivation, habitats were hydrologically connected with the sea and characterized by high diverse foraminifera communities. These conditions can be used as a benchmark to assess which habitats were deviated from its natural reference conditions (high ecological status) as result of rice colonization in the Delta. Only then, stakeholders can use this information to restore habitats supported by natural fluxes of water and sediments in order to mitigate potential wetland loss due to sea level rise.

Reference: Benito, X., Trobajo, R. Ibáñez, C. & Brunet, M. Benthic diatoms and foraminifera as indicators of coastal wetland habitats: application to palaeoenvironmental reconstruction in a Mediterranean delta. Boletín de la Sociedad Española de Ficología ALGAS, 2016 (51).