investing.com - It's been a volatile year for the U.S. stock market, but investors remain largely positive about the investment climate, even if they expect more volatility to come, according to a new poll.The Wells Fargo (NYSE:WFC) Gallup Investor Optimism Index hit 103 in the second quarter. Though that's down from a 117 reading in the first quarter, it's the sixth straight quarter that the index was over 100. Despite the overall optimism, 55% of investors were either somewhat or very concerned about future market volatility. Only 5% had no concern.What's more, a high percentage of investors expect volatility to continue in 2018 with the worst of it yet to come.Investors were concerned with other issues as well, according to the poll.The biggest concern was the political climate in Washington. Almost eight of 10 surveyed were either very or somewhat concerned.Concern about the size of the federal budget deficit was a close second. The least source of worry for investors was the economy. About 50% expressed some degree of concern.