Morphological and chemical variability of colloids in the Almeria-Oran Front in the eastern Alboran Sea (SW Mediterranean Sea)

Across the different water types (Mediterranean, Frontal, and Atlantic waters) in the Almeria-Oran Front (Eastern Alboran Sea, SW Mediterranean Sea), we determined organic carbon in the particulate (60-0.2 mum), colloidal (0.2 mum-1,000 Da) and dissolved (<1,000 Da) fractions. The morphological and chemical variability of colloids was studied by means of analytical electron microscopy. Particulate and colloidal organic fractions accounted for 6 to 9% and 1 to 2%, respectively, of the total organic carbon. Visual examination of colloids revealed three morphologically and chemically differentiated morphotypes (globules, aggregates of rounded entities, and spherulitic aggregates) whose distribution, abundance, and chemical composition vary with the depth and between water types. Globules dominated 5 m deep at all stations; always contained Ca, P, S, and Cl; and were marked by the presence of other elements such as Si, Fe, or Mg depending on the water type. By contrast, at 40 in deep, Mediterranean, Frontal, and Atlantic waters were characterized by different morphotypes: Fe-P-enriched aggregates of rounded entities in Mediterranean waters; globules with variable abundances of Si, Fe, Ca, P, S and Cl in Frontal waters; and Mg-rich spherulitic aggregates in Atlantic waters. These morphological and chemical differences in colloids across the front raise questions about their origin, their aggregation, and the availability of elements for biological activity.