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Evidence of North Africa’s Green Revolution Preserved in Sedimentary Organic Matter Deposited in Three Coastal Lagoons.
Citation:
OCZKOWSKI, A., R. J. Flower, J. R. Thompson, F. Ayachi, M. H. Ahmed, M. Ramdani, AND S. Turner. Evidence of North Africa’s Green Revolution Preserved in Sedimentary Organic Matter Deposited in Three Coastal Lagoons. ECOLOGICAL APPLICATIONS. Ecological Society of America, Ithaca, NY, 21(5):1708-1717, (2011).
Impact/Purpose:
Like many developing regions, agricultural efforts have intensified along the North African coastline. Coastal managers and scientists have become concerned about the impact upstream agricultural decisions may be having on their coastal ecosystems. We collected sediment cores from coastal lagoons in Egypt, Morocco, and Tunisia and found evidence of this accelerated watershed development in lagoons from two of the three countries. Our data confirm the concerns of local stakeholders that eutrophication has had dramatic impacts on these local ecosystems, particularly in the Egyptian and Tunisian sites.
Description:
Because of longer residence times and limited mixing in coastal lagoons, the impacts of anthropogenic nutrient loading to lagoon food webs are often more pronounced than in other coastal ecosystems. For these reasons, many lagoons also provide an excellent environment for the deposition and accumulation of organic matter (OM). Sediment cores were retrieved from three North African lagoons to provide records of recent environmental changes. We measured percentage nitrogen (%N), nitrogen stable isotope values (δ15N), and percentage organic matter (%OM), and we used radiometric dating techniques (210Pb, 137Cs) to examine the evidence for the intensification of upstream agricultural practices in sediment cores from Lake Manzala (Egypt), Ghar El Melh Lagoon (Tunisia), and Lagune de Nador (Morocco). With the exception of one core collected near a sewage outfall, sediments from Lake Manzala clearly reflected the impact of agricultural intensification following completion of the Aswan High Dam and delta barrages in the mid-1960s to early 1970s. Both %N and %OM more than doubled in three Manzala sediment cores, and δ15N values declined from 5‰ to <1‰. These changes reflect the increasing use of synthetic fertilizers (δ15N ≈ 0‰) from the 1960s to the present. Sediments from Ghar El Melh show a similar trend, with %N more than tripling, %OM increasing by 50%, and δ15N declining from 6‰ to 2‰ since 1965. These changes are consistent with the increasing use of water from a nearby river for crop irrigation and agricultural fertilizer use. Lagune de Nador receives relatively little agricultural drainage water, and core data did not show the same trends as Manzala and Ghar El Melh. Overall, the sediment core data from these systems reflect environmental shifts in the quantity, quality, and isotope signature of the deposited organic matter and confirm the concerns of local scientists and environmental managers that eutrophication has had dramatic impacts on the coastal ecosystems, particularly at the Egyptian and Tunisian sites.