Changing business conditions and the effects of regulatory changes such as Basel III and the Volcker Rule have impaired over-the-counter dealer markets such as those that trade bonds, the traders said. In addition, the trading volume of stock markets now is dominated by high-frequency dealers, the report said. The report, The New Liquidity: Investment Implications of Structural Market Changes, noted that the high-frequency traders differ significantly from traditional dealers.

"While they deliver fast execution and tight spreads for small orders, they supply less stability and less reliability for highly demanding orders," said David Brooks, a co-author of the report and managing director of global equity trading at The Boston Company Asset Management, LLC, a BNY Mellon investment manager. "They also are less reliable and stable for all orders during periods of market stress."

The major source of dealer liquidity historically came from human specialists on traditional exchanges or from the trading desks of broker/dealers, the report said. The increased automation of exchanges decimated the ranks of the specialists on traditional exchanges, while reduced margins, general banking deleveraging, stricter capital requirements and a greater focus on core businesses have dramatically reduced the capabilities of the trading desks, the BNY Mellon traders said.

Credit markets have been affected more than the equity markets because the bond markets are more diffuse as corporations have many different bond issues, and some could go days or weeks without being traded, the report said.

"Investors need to be prepared for higher volatility because of growing liquidity constraints, especially in certain parts of the bond markets," said Lynn Challenger, a co-author of the report and managing director of trading at Mellon Capital Management Corporation, a BNY Mellon investment manager. "They should consider their liquidity needs in both benign and turbulent markets and how how they will source it."

Another defensive move suggested by the traders is for investors to maintain tighter tolerance ranges around asset allocation targets so they have a smaller effect on the markets when they need to rebalance.

However, the fragmentation of liquidity also is likely to create transitory distortions in prices, providing opportunities for those with readily available capital and the ability to deploy it before the opportunity disappears, said Amy Koch, a co-author of the report and managing director of fixed income trading at Standish Mellon Asset Management Company, a BNY Mellon investment boutique.