NEW YORK and LONDON, Dec. 4, 2012 /PRNewswire/ -- The unreliable availability of market liquidity since the onset of the 2008 financial crisis has made investors' portfolios more vulnerable to periods of market stress, but it also has provided opportunities for those who can exploit pricing dislocations that could result, according to a white paper written by the heads of trading from three BNY Mellon Investment Management investment boutiques. 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.