Contributions of colloidal and high molecular weight dissolved material to alkalinity and nutrient concentrations in shallow marine and estuarine systems

We investigated the contribution of high molecular weight and colloidal material to total alkalinity, dissolved organic and dissolved inorganic nutrient concentrations in samples from a temperate nearshore environment (Tomales Bay, California), a tropical nearshore environment, (Australia Institute of Marine Science, Townsville, Queensland, Australia) and a coral reef environment (Davies Reef, Australia). Amicon UM-10 ultrafilters (10 000 daltons or 3 nm nominal cutoff) were used to remove high molecular weight material, including colloids, from conventionally filtered samples (Whatman GF/C glass fiber filters, Tomales Bay; or 0.45-μm pore size mixed cellulose ester Millipore filters, Australia). We found no evidence of a high molecular weight component of total seawater alkalinity. Ultrafiltration removed 3% of reactive phosphorus, 71% of organic phosphorus, 28% of organic carbon, 1% of ammonium and 27% of organic nitrogen from GF/C filtrates of Tomales Bay samples. Reactive silicate and nitrate plus nitrite concentrations were unaffected by ultrafiltration. Microscopic examination of Tomales Bay samples revealed that 75–100% of the bacterioplankton present in the unaltered sample passed through the GF/C filter. Calculations based on published values of the carbon content and carbon: nitrogen: phosphorus ratio of bacterioplankton cells indicated that their contribution to organic carbon retained by the ultrafilter was negligible. However, bacterioplankton accounted for 12% of the organic nitrogen and 27% of the organic phosphorus in GF/C filtrates and 29% and 39% of the organic nitrogen and organic phosphorus, respectively, removed by ultrafiltration. Organic phosphorus removed by filtration through GF/F glass fiber filters, through 0.22-μm pore size Millipore (mixed cellulose ester) filters, or through 1.0 or 0.2-μm pore size Nucleopore (polycarbonate) filters was equivalent to the calculated phosphorus content of the bacteria removed by the same filters.