Association of macroaggregates and metals in coastal waters

• Macroaggregate matrix has higher metal/metalloid contents compared to colloids.
• Higher colloidal Me/Corg. ratio indicate its higher “binding potential”.
• Me/Corg. ratios reflect the background concentrations and colloid composition.
• Metals in higher molecular weight (MW) fraction are markedly retained.
• Dissolved metals, are mostly bound in the low MW colloidal fraction.

Macroaggregates (macrogels) occurring episodically in the northern Adriatic, offer a rare opportunity to study interactions between marine organic macromolecules and metals and metalloids (Me). Macrogels are formed by agglomeration of dissolved heteropolysaccharides, mostly of phytoplankton origin. Filtration and centrifugation were used to discriminate between macrogel matrix and interstitial water colloids. The interstitial fraction was then used in an incubation experiment to follow the progress of microbial degradation of colloids. The colloids were ultrafiltered through membranes and each size class of colloids (> 30 kDa, 10–30 kDa and 5–10 kDa) was analyzed for metal, metalloid, organic carbon (Corg) and total nitrogen (Ntot) content. Higher Me concentrations and lower Me/Corg ratios in matrix, which are an indicator of Me bound to organic matter, suggest that Me in a matrix can also be bound onto Me containing particles. All the studied Me in macroaggregate colloidal fraction, except Se, proved to be bound to large macromolecules (> 30 kDa), mainly glycoproteins and aminopolysaccharides. During the rapid degradation of all, except the 5–10 kDa fraction, the Me associated with the > 30 kDa fraction were markedly preserved. Consequently, the Me/Corg ratios increased due to the loss of Corg and relative increase of metal-binding sites. The observed differences between our Me/Corg ratios and the Irving–Williams series can be explained by variable background concentrations of the studied Me in the northern Adriatic. Comparison between Me levels in macroaggregate colloids and dissolved marine colloids, considered to be macroaggregate precursors, revealed similar values. Distribution coefficients (Kd) between colloidal and truly dissolved Me were lower than Kd for particulate matter, suggesting that organic colloids, including macroaggregates, exhibit rather weak Me binding capacity and that Me are not distributed homogenously in the various particle pools in the northern Adriatic coastal waters.