It is one of the great natural harbours in the world.

Viking  longships used it and so did the British Grand Fleet and of course it is the last resting place for the Kaiser's scuttled German fleet

But the actual water that is in Scapa Flow is the liquid backcloth to a very different dispute, and a crucial vote on the subject  by Orkney Islands Council is imminent.

The council plan to change  20-year-old environmental controls which ban ships, discharging any ballast water loaded elsewhere into the waterway. It has been rumbling on for years.

Earlier this month  a special meeting of the Orkney Islands Council's  policy and resources committee, debated the subject for two hours but resolved to press ahead with the change.

The new policy would  allow ballast water  previously exchanged in the North Sea to be released into Scapa Flow.  Ships with onboard ballast water treatment plants will not only have to exchange the water before entering Scapa, but will also have to treat it before it is released.

However, opponents say the bottom line is that a quantity of untreated ballast water will be discharged into Scapa Flow, raising fears of alien species appearing.

There are concerns that the likes of the Chinese Mitten Crab could make an entrance, having  already  appeared  in the Tyne. It acts as a predator and competes for the food on which the indigenous species rely.  Meanwhile  if Zebra Mussels get a grip they could choke out the oxygen from the seabed.

There are also fears for the brackish water of the Loch of Stenness barely separated from  the north of Scapa Flow. It has been designated a Site of Special Scientific Interest as well as a Special Area of Conservation. Its varying salt content supports a range of biological and botanical life.

But the Council has to date paid £454,000 for specialist environmental, scientific and legal advice and say the new controls will be more than sufficient

However, the committee's decision must be endorsed by the full council which meets on December 10.  Even then it could well still go to the Scottish Government for approval because of the opposition of official watchdogs.

A the start of the December 10  meeting councillors  will hear from opponents of the change including wildlife tour guide Steve Sankey. He will be accompanied by representatives of RSPB Scotland and the Orkney Fisheries Association.

It is money that is driving the change . This latest round of the dispute came to  a  head in the summer of last year when councillors received  a report from their officials. It warned that the local authority was losing out on the money it could charge for ship to ship transfers of oil in Scapa Flow.

Its marine services department had a minimum charge of £26,250, for each such operation, but it could easily  earn £41,000 from a transfer involving 100,000 tonnes.

However the report said the majority of such opportunities were being lost to locations which did not have ballast water management policy constraints, "... namely  Sullom Voe (Shetland) , Southwold (Suffolk) or at anchor off Denmark, due to the cost and inefficiency which results from the current policy for Scapa Flow...."

The report said that in April and May 2012 alone there were five operations which could have earned the council around £200,000, but were lost to other ports because of the ballast controls.

When ships are loaded with cargo/oil there has to be a discharge of the ballast water carried by the vessels to maintain stability when not loaded.

So oil tankers must periodically halt loading, move out of Scapa Flow and discharge some or all ballast, before returning to resume loading.

Last year's report  concluded  "This imposes considerable costs on ship operators in terms of time and fuel."

It estimated that it added upwards of £100,000 to the cost of the whole operation, thus making it un-economic.

But the Orkney Manifesto Group (OMG) of three councillors  oppose the policy change and are convinced  a new ballast water management policy would perpetuate the risk of invasive alien species affecting Scapa Flow and its environs

OMG Convener, Councillor Alistair Gordon said recently: "This could, in theory at least, also affect the Loch of Stenness Special Area of Conservation. The proposed new ballast water management procedure has already been found unacceptable by government watch-dogs, Scottish Natural Heritage (SNH) and the Scottish Environment Protection Agency (SEPA) and so the policy will have to go to Scottish ministers for their final approval."

Councillor Gordon added "We felt during the debate on the new policy that it was being driven by the necessity to save money on the loss-making Scapa Flow Oil Port, which is within Orkney Marine's budget. This year, the loss is running at £1.7 million."

He said that rather than change the ballast management, there should be urgent talks  with the Flotta terminal's operator, Talisman Sinopec. At the same time opening discussions with SNH and SEPA and the Scottish Government as to what is acceptable, in their view, by way of ballast water management.

"We do not believe that our proposal to ban ballast water discharge in the Flow in the meantime need threaten Flotta terminal operations."

As always in environment versus industry disputes, the  stakes are high for both sides. This one could well have a few years left in it yet.