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 wate rs) 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 electro n microscopy. Particulate and colloidal organic fractions accounted for 6 t o 9% and 1 to 2%, respectively, of the total organic carbon. Visual examina tion of colloids revealed three morphologically and chemically differentiat ed morphotypes (globules, aggregates of rounded entities, and spherulitic a ggregates) whose distribution, abundance, and chemical composition vary wit h the depth and between water types. Globules dominated 5 m deep at all sta tions; always contained Ca, P, S, and Cl; and were marked by the presence o f other elements such as Si, Fe, or Mg depending on the water type. By cont rast, at 40 in deep, Mediterranean, Frontal, and Atlantic waters were chara cterized by different morphotypes: Fe-P-enriched aggregates of rounded enti ties in Mediterranean waters; globules with variable abundances of Si, Fe, Ca, P, S and Cl in Frontal waters; and Mg-rich spherulitic aggregates in At lantic waters. These morphological and chemical differences in colloids acr oss the front raise questions about their origin, their aggregation, and th e availability of elements for biological activity.