Riverine and wet atmospheric nutrient inputs to the Southwestern Mediterranean region of North Africa

Abstract

This study describes the first simultaneous long-term effort to examine the nutrient inputs to the southern Mediterranean Sea from rivers and wet atmospheric deposition. Extensive daily rainwater sampling (280 samples) from the Annaba region (SW Mediterranean Sea, Algeria) and from river discharges (144 samples) at two river outlets feeding the Annaba Bay, the Seybouse River (SR) and the Mafragh River (MR),were collected and analyzed for dissolved nutrients from 2012 until 2017. During the 6-year study period, the Annaba region experienced contrasted hydrological conditions varying from heavy rainfall events during winter that have triggered large river flooding in 2012 and 2015 to the severe drought of 2016, which have profoundly affected both the atmospheric and riverine freshwater and nutrient inputs. The annual freshwater volume delivered to Annaba Bay averaged approximately 1.7 km3, of which 51% was from MR, 33% from SR, and 16% from precipitation. Precipitation over the Annaba region was associated with unusually high levels of DIP and DSi, resulting in deposition rates (0.54 mmol Si m−2 yr−1and 6.22 mmol P m−2 yr−1, respectively) that are several times higher compared to the average values reported for the Mediterranean region. In contrast, both the DIN and DON deposition rates were relatively low (16.2 and 4.7 mmol N m−2 yr−1, respectively) compared to the values reported for the Mediterranean region. Interestingly, the levels of nitrogen compounds in rainwater were similar to those in the MR waters. For all nutrient species analyzed in this study, SR waters always contained higher nutrient levels compared to those in MR and rainwater. The majority of nutrient loading entering Annaba Bay was delivered through the riverine inputs, averaging 2744, 962 and 92 t yr−1 for DSi, DIN and DIP, respectively. The wet atmospheric deposition contributed only 2.5% of DSi, 10% of DIN and 7% of DIP total annual flux. The riverine stoichiometric N:P and Si:N ratios were imbalanced in most cases, averaging 28 and 0.84, respectively. The N:P and Si:N ratios in rainwater were more balanced, particularly during the dry season when Saharan airflow dominated the region and supplied more DIP and DSi.