NEW YORK ( TheStreet) -- Shares of online brokers TD Ameritrade ( AMTD), E*Trade Financial ( ETFC) and Charles Schwab ( SCHW) were mixed Wednesday, as concerns persisted that trading volume continued to decline in September.

TD Ameritrade on Tuesday said its average daily client trades per day totaled 309,000 in August, down 28% from the year-age equivalent period and off 6% sequentially. Its rivals, Schwab and E*Trade also had disappointing DARTs for August; both firms saw trades decline about 5% sequentially, they said last week.

TD Ameritrade's average spread-based balances rose 27% from a year ago to $54.8 billion as of the end of August, it said. The broker's average fee-based balances of $66.4 billion, rose 16% in the same timeframe. Its client assets totaled $332 billion as of Aug. 31, an increase of 15% from the year earlier but down 2% from July.

Analysts attributed the trading slowdown to lower volatility as compared to August 2009 as well as general seasonality. However, the financial crisis of the past three years also appears to have sent many retail investors to the sidelines.

While the online brokers have made forays into more general financial services and wealth management offerings for their retail customers, trading commissions still make up a significant portion of fee revenue.

Trading volume in September has continued to fall, according to Rich Repetto, an analyst at Sandler O'Neill & Partners. As of September 13, DARTs were offl between 4% and 7% from the end of August, Repetto estimates in a TD Ameritrade-specific note on Wednesday.

Consolidated share volume to-date is down 4.4%, Repetto says.

However, the slower activity was "likely depressed earlier in the month due to Federal and religious holidays, and we have seen a slight pick-up in consolidated share volume (Tapes A, B, & C) the past week that we expect to continue for the balance of the month," Repetto writes.

He lowered retail DART estimates for TD Ameritrade to be flat month over month. Previously he expected TD Ameritrade's trades to increase 10% from August.

On the other hand, Madeline Schnapp, TrimTabs director of macroeconomic research, is skeptical that retail investors will move cash on the sidelines back into equities after being burned in the past decade's asset bubbles (tech and housing).

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