It follows news last week that SSE was reviewing all its offshore wind commitments, including major wind farm projects in Scotland, including one near Islay, another in the Firth of Forth and at Hunterston.
SSE blamed the UK government's decision not to earmark two of its other major offshore developments for support in an initial subsidy programme.
Now industry body Scottish Renewables, which represents more than 330 companies working in the renewable energy sector, has obtained figures from offshore wind developers with projects in Scottish waters about their investments in Scotland during 2013. These suggest £28.9 million was spent in 2013 compared to £63.6m in 2012.
Lindsay Leask, senior policy manager for Scottish Renewables, said: "Uncertainty throughout the industry is growing, as none of the major projects planned for Scottish waters have had their planning applications determined, and the details around accessing market incentives are still unclear."
She said Scotland had all the ingredients to develop a world leading offshore wind industry: "Yet a reduction of some 55% (in investment) in a year is cause for concern not only for the sector, but should also be a worrying sign for the Scottish and UK governments too."
"We'll be working hard over the coming weeks to ensure the developers gain the certainty they need to unlock further investment."
A spokeswoman for the Department of Energy and Climate Change said: "Ten projects have been told they are provisionally affordable, but it's not a done deal. We expect investment contracts to be signed and laid in Parliament in spring 2014."