NEW YORK (AP) ¿ Standard & Poor's Rating Service on Tuesday lowered its outlook on Spectra Energy Corp., citing concerns over the impact of low commodity prices on the natural gas transport and storage company.

The outlook was reduced to "Negative" from "Stable." S&P affired its long-term corporate credit rating at "BBB+," which is investment grade.

In a note to investors, S&P analyst William Ferara wrote that about three-quarters of Spectra's cash flow comes from relatively stable assets with fixed fees. However, he noted that low commodity prices are impacting segments such as Spectra's DCP Midstream LLC.

Ferara said he expects Spectra Energy's field services unit to produce about $550 million of earnings before interest, taxes depreciation and amortization, or EBITDA, this year. That's down from $986 million last year after a $128 million reduction in the fourth-quarter of 2008, he noted.

Ratings on Spectra may also be lowered if the Houston-based company pursues acquisitions that introduce business risk, Ferara wrote.

Shares of Spectra rose 5 cents to close at $14.44 Tuesday.

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