Few asset-management companies have consistently raked in assets like BlackRock (NYSE:BLK), the largest asset manager in the world, with more than $4.3 trillion in assets under management at the end of 2015.

The company's institutional index funds, and iShares exchange-traded funds are extraordinary asset gathers, collecting high-margin fee-earning assets that drive BlackRock's top line. In 2015, all but the company's institutional products added assets under management, and AUM grew by 4% from inflows, driving fee growth of 6%.

Can the run continue into 2016?

Challenges in 2016
One of the challenges of investing in asset-management companies is that the ups and downs of the financial markets inherently affect a company's results. When stock prices drop, AUM naturally falls. Fee revenue drops, too.

For BlackRock, this could be the biggest short-term challenge to its results. Larry Fink, the company's CEO, took to CNBC to explain his outlook on the markets, warning that stocks could drop another 10% as the market reassesses its investments in 2016. The market was down about 6% year to date when Fink made that assertion, about where it sits today.

Beyond the movements of the broad market, which are virtually impossible to predict, BlackRock's next biggest challenge is holding the line on pricing and fees on its funds.

BlackRock's iShares ETF products are part of bitterly competitive fee wars going on across the industry. In just another sign that fund fees will continue their march toward zero, Vanguard recently decided to drop fees on its most-popular index funds to just 0.01% per year for its largest investors.

Though fees aren't the only reason investors are moving to ETFs -- liquidity and accuracy in tracking an index matter, too -- lower expenses are certainly a big driver of fund flows. That its most capable competitor, Vanguard, doesn't have a profit motive is a long-term challenge to the company's lead in exchange-traded funds.

Opportunities in 2016
Given the market's performance, a second interest-rate increase by the Federal Reserve appears less likely today than it would have seemed just months ago. Rising rates remain an important earnings driver for BlackRock, as the company waived about $112 million in fees on low-yielding money market funds for its clients in the first nine months of 2015.

Only about 45% of that loss was offset with savings elsewhere. Rising rates would presumably allow BlackRock to remove its fee waivers and begin collecting fees on its line of money market funds. This represents an immediate increase in revenue with no incremental expenses – revenue that should drop straight to the bottom line.

In short, it's unlikely that, after several years of rising stock prices, 2016 will be BlackRock's best year yet. It's already the largest asset manager in the world, and thus its performance has more to do with how the markets perform rather than its skill in collecting and managing assets.

Jordan Wathen has no position in any stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.