SNH start crucial lagoon research

It’s a breathtaking experience even looking at an Ordnance Survey Map of the north east  of the Isle of North Uist in the Outer Hebrides. It defies logic and imagination, presenting a lacework of land and water that makes it impossible to decide which defines the picture.

Is is a seascape – a series of islets and reefs in shallow water; or is it land so permeated by water of one kind or another that it has to be defined as a landscape?

The latter is the answer, although the eye can never accept it – or deny it – and any entrepreneurial business running helicopter or seaplane tours to see this from the air would meet a lively market for its services.

This unique place is a European designated site of Special Scientific Interest. What makes it special is known but not researched – a situation Scottish Natural Heritage (SNH) is about to change.

The lacework of this low lying coastal land is a mixture of  boggy freshwater lochans and complex inlets from the sea. The main one of these is Loch Portain, coming in from the Minch and taking a dog leg north to the townland of Lochportain. It is navigable and yachts do use it – but only the knowing. From a map or chart, it is all but indistinguishable as a single inlet loch.

Many of the lochans on the land are infiltrated by the rising tide.

This entire complex relationship between land and water, fresh and saline, creates the series of lagoons which SNH aim to learn much more about in a long term and innovative research project. This will also be used as a benchmark of ways of managing European designated sites.

SNH has just invested in six data loggers which record a number of environmental variables every five minutes.

These devices can be left in place for up to a year before they are retrieved and it is these babies that are headed for the laceland of North Uist in a data downloaded.monitoring programme of environmental conditions in its saline lagoons.

The loggers are shortly to be tested in a complex of lagoons at Clachan for 24 hours. – two tidal cycles.

Assuming that this throws up no problems,  they will then immediately go to Loch an t-Sruith Mhoir to be installed there and left alone for a full year. After this, they will be recovered from the site, their data downloaded and the deciphering and analysis begun.

Stewart Angus, SNH’s coastal ecology adviser who is leading the programme, says: ‘The Uists have some of the finest lagoons in Europe and Loch an t-Sruith Mhoir is one of the best of all. It supports a number of unusual species that only live in a few lagoons – lochs that have both fresh water and sea water inflow’.

Lagoons, climate change and a mystery

Of worldwide interest is developing how an understanding of how our inevitably rising sea levels will affect coastal habitats. Knowing how salt water functions in the Uist lagoons is a key to that understanding.

And the North Uist lagoons have their special mysteries in this.

Salinity  – and temperature – affect the density of seawater, leading to stratification – with, here, layers of fresher water lying above layers of heavier salinity. The boundary between these layers is known as the halocline.

In normal conditions, as the tide rises the saline water flows over the edge of the lagoons and gently slips down to the bottom, below the fresher surface water.

Winter storms however, produce rougher and higher tides and these generally mix up the layers of water in the lagoon.

However, in one known North Uist lagoons – Loch Obisary (Loch Obasaraigh), it is known that this does not happen. The water layers remain separate in the face of storm conditions. As yet, no one knows why this is – and SNH hope, eventually, to be able to account for it.

Referring, in part to this, Stewart Angus says: ‘Our work in 2005 revealed there are complex relationships between surface water and ground water in the Uists that are extremely important in the context of rising sea levels’.

Despite the work already done, Mr Angus is not blind to the extent of the challenge ahead. He says: ‘However, our understanding of these lochs is so poor that we do not even know what the tidal range is, though it is believed to be in the order of 50cm.  Water levels change with the tide by just 50cms compared with the usual four metres on open Uists coats.  The loggers will directly record temperature, depth, conductivity, and pH and will also give salinity via specialist software.’

He sees the research to be done in a way that visually relates to the physical reality of North Uist – as a jigsaw. If this study is successful and if the data loggers live up to their promised performance, the intention is to keep moving them around to other specific locations in the area, slowly putting a picture together that will help in developing a strategy for how best to respond to climate change.

The research will focus first on salinity in winter and then more widely on the nature of the complex relationship between fresh and saline water.

Rare species

The Uist lagoons  – and pockets of Loch Sween in Argyll which experience lagoon conditions  – also play host to some very rare species.

One is a rare stone wort – which looks and feels hard but is a plant. A huge oddity here is that fact that, apart from a few lagoons, it has only been found ion a brickworks in Peterborough. Now there’s a conundrum.

Then there are spire snails, whose sub species are so hard to tell apart it takes a DNA test to be certain.

With these as rare inhabitants of the Uist lagoons, members of groups of shrimp species – Gammarus (the Uist variety swims sideways); and Mysids, a very small shrimp. We understand that it is the Mysids that are found in just a few pockets of Loch Sween.

The data loggers, their year alone and the public

Stewart Angus is openly nervous about leaving the data loggers alone in Loch an t-Sruith Mhoir for an entire year. They are made for this sort of work and their batteries have two years capacity – but its a long time to wonder what’s going on. Supposing it turns out to be a scientific dead letter box?  As Mr Angus says, this is the only way they’ll get adequate data out of the Uist lagoons so they’re  going for it.

Although the equipment will be left in the open it will be placed in such a way as to minimise storm damage. There is one boat on the loch, operated by the local estate which has committed to warning anglers about the location of the devices.

Described as ‘resembling fluorescent light tubes’ we should tell those with the same sort of imaginations as our own – immediately conjuring images of floating Jedi light sabres – not to go tramping over the North Uist coastal bogs in the hope of the highly exotic. They look like fluorescent light tubes turned off and if you see them, they’re not oddly located industrial dumping.

Stewart Angus’ says: ‘We are asking people not to approach the devices if they come across them as this will disrupt the readings.’

And a quirky side issue

Back to the halocline – anyone who has just seen the recent television series following a group of five would-be submarine commanders as they went through the four week long ‘Perishers’ test in HMS Turbulent , out of Faslane in Argyll, will have come across the ‘thermocline‘.

Water strata can be determined by temperature as well as salinity, with warmer layers at the top. In this case the thermocline is the boundary between warmer and cold layers.

It is characterized by a negative sound speed gradient, making the it important in submarine warfare. Submarines  – as some did during the ‘Perishers’ selection challenge – can hide below it because it reflects active sonar and other acoustic signals so enemy ships cannot ‘see’ them.

And later on in the SNH research in North Uist, we may learn a lot more more about the performance of the halocline, and particularly in winter.

In the meantime – give side-stroking shrimps in Loch Sween a wide berth.

NOTE: The images accompanying this article are copyrighted to SNH and are by Sue Scott of SNH. They were taken in 2009 as part of a study commissioned by SNH into the ecology and distribution of the bird’s nest stonewort. They show:

• Top: an underwater view of Loch an t-Sruith Mhoir
• Centre: the  halocline in Loch an Duin, North Uist. The brackish (mixed salt and fresh) water lies on top (because it is less dense) than the more saline water below. The study will reveal how this stratification behaves in the study lochs – the loggers will be positioned to have one below and one above the halocline at each of the three stations
• Above: a lagoon cockle in Loch an Duin, with bird’s nest stonewort around it and a few spire snails. Bird’s nest stonewort is now only found in Loch an Duin and Loch an t-Sruith Mhoir in the UK. Lagoon cockles are only confirmed in Scotland from lagoons in the Uists and Loch Sween.