Composite blends of polyester containing plastic materials, and a starch-based polymeric material that increases the biodegradability of the polyesters of such a composite in simulated or actual marine conditions (e.g., simulated by ASTM D-6691). Enhanced rate or extent of biodegradation may also be exhibited in simulated or actual land-based disposal conditions. The starch-based polymeric materials are substantially amorphous, and are homogenously blended with the polyester plastic materials. While polyester plastics such as PBAT, PLA, PCL, and/or PBS may exhibit some biodegradability characteristics when composted and/or disposed of in landfill conditions at elevated temperatures, they exhibit limited if any biodegradability when disposed of in a marine environment. Even conventional blends of starch with such polyesters do not exhibit any significant marine biodegradability with respect to the polyester components therein. Third party testing shows the present blends exhibit substantially complete biodegradability of the entire composite under marine conditions, after about 400 days.