The increased use of silver nanomaterials presents a risk to aquatic systems due to the high toxicity of silver. The stability, dissolution rates and toxicity of citrate- and polyvinylpyrrolidone-coated silver nanoparticles (AgNPs) were investigated in synthetic freshwater and natural seawater media, with the effects of natural organic matter investigated in freshwater. When sterically stabilised by the large PVP molecules, AgNPs were more stable than when charge-stabilised using citrate, and were even relatively stable in seawater. In freshwater and seawater, citrate-coated AgNPs (Ag-Cit) had a faster rate of dissolution than PVP-coated AgNPs (Ag-PVP), while micron-sized silver exhibited the slowest dissolution rate. However, similar dissolved silver was measured for both AgNPs after 72h in freshwater (500-600μgL(-1)) and seawater (1300-1500μgL(-1)), with higher concentrations in seawater attributed to chloride complexation. When determined on a mass basis, the 72-h IC50 (inhibitory concentration giving 50% reduction in algal growth rate) for Pseudokirchneriella subcapitata and Phaeodactylum tricornutum and the 48-h LC50 for Ceriodaphnia dubia exposure to Ag(+) (1.1, 400 and 0.11μgL(-1), respectively), Ag-Cit (3.0, 2380 and 0.15μgL(-1), respectively) and Ag-PVP (19.5, 3690 and 2.0μgL(-1), respectively) varied widely, with toxicity in the order Ag(+)>Ag-Cit>Ag-PVP. Micron-sized silver treatments elicited much lower toxicity than ionic Ag(+) or AgNP to P. subcapitata. However, when related to the dissolved silver released from the nanoparticles the toxicities were similar to ionic silver treatments. The presence of natural organic matter stabilised the particles and reduced toxicity in freshwater. These results indicate that dissolved silver was responsible for the toxicity and highlight the need to account for matrix components such as chloride and organic matter in natural waters that influence AgNP fate and mitigate toxicity.