Abstract 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.