The 100 largest U.S. public employee pension systems earned just $14.3 billion on their investments in the first quarter, their worst performance since September 2015, according to new U.S. Census Bureau data.

The pension funds' total assets dipped by $27.8 billion, or 0.7 percent, from the prior quarter to $3.78 trillion. It was the first time totals have fallen since September 2015.

Total contributions into the funds also dipped, from both employees and government employers, while benefit payments rose slightly, according to the Census data.

The underwhelming first quarter performance was driven by several factors, particularly equity volatility, according to Roy Eappen, a senior analyst at Wells Fargo Securities.

It was a stark change from the certainty that prevailed in the fourth quarter of 2017, when fiscal and monetary policy and tax reform legislation all provided a level of clarity, Eappen said.

The start of 2018 can be seen as coming off the "high" of 2017, which was "a very good year," he said.

In fact, the total dollar figure of returns on public pension investments in 2017 was nearly $528 billion, a sum exceeding any annual total in at least the prior 20 years, according to a Reuters analysis of Census data.

Early this year, discussion of a possible trade war with China, lower than expected retail sales and revisions to gross domestic product all combined to heighten market uncertainty and undercut equities, Eappen said.

The S&P 500 Index S&P took a nosedive in February, erasing nearly all the gains it made in January to end the quarter up just 0.3 percent.

Meanwhile, the CBOE Market Volatility Index also jumped in February and stayed somewhat elevated through March.