A salt solution...?

Next week sees the start of a major project at RSPB Conwy, undertaking one of the biggest changes to the reserve since its creation almost 20 years ago.  We need to tackle an invasive non-native plant, marketed as Australian swamp stonecrop or New Zealand pygmyweed, but usually referred to by its scientific name, Crassula helmsii.

Away from its native Antipodean home and with nothing to keep it in check, Crassula spreads rapidly through waterbodies and around the margins.  At Conwy, we lost our educational wildlife pond to the weed in 2009 but subsequently found Crassula in the Shallow Lagoon (in front of the coffee shop).  It has since spread to the Deep Lagoon.  The only proven way of eliminating Crassula is with prolonged flooding with saltwater.

What we’re doing
The lagoons are a couple of metres above the height of the estuary, so just opening up the sea wall isn’t an option. On the highest tides in early March, three huge pumps will fill the Shallow Lagoon from pipes across the seawall, opposite the coffee shop. 

Access arrangements
We'll have to close the estuary track for a few days while this operation is underway, but will keep all the hides open.  Between Monday 5 and Monday 12 March, there will be no access along the estuary track between the Benarth Hide and the car park, so the circular walk will be partly-closed (if you go to Benarth Hide, you'll have to come back the same way).

Will it work?
Providing we can maintain sufficiently high water levels and keep the water salty enough, we should eliminate Crassula from the shallow lagoon, as has been achieved successfully at our Old Hall Marshes reserve in Essex.

We have just a few days when the tides are high enough to pump water onto the lagoon, and we will raise the water about a metre above its current level.  It may prove necessary to top-up the lagoons with more water from the estuary later in the year.  We can also pump from the Afon Ganol, but this is fresh water and so raising the water level this way will reduce the salinity. 

What does it mean for wildlife?
To eliminate the Crassula, we'll be keeping water level in the Shallow Lagoon high and salty for at least a year, which will mean some changes for wildlife.  We've assessed the impacts on our key species, and expect that most waterbirds won’t mind.  Some ducks, such as shoveler, that prefer freshwater may not visit, but we may get ducks such as pintail.  While the water levels remain high, there will be less mud for waders to feed, but this is a temporary measure.  Initially, the reeds may not like it, but they'll be dying back for the winter soon and it's an impressively tough plant that is quite capable of growing in brackish conditions, so we hope they will recover.

We’ll also have to fill the bridge pond with saltwater, just in case there’s Crassula in there, as it is connected to the Shallow Lagoon.  That’s bad news for any dragonfly larvae, but we created several new freshwater ponds earlier this year as alternative habitat.

The long-term
We’ll continue to monitor salinity, water levels and Crassula throughout the year.  We’re aiming for salinity levels between 15 and 20 parts per thousand (ppt).  By comparison, sea water is 35 ppt, but the water in this part of the estuary is less salty.   Once we have filled the Shallow Lagoon, we will have a better idea about whether it’s feasible to fill the deep lagoon.  This is a much bigger operation as we’d have to empty the fresh water first, and have to pump a lot of water during just a handful of high tides. The Crassula in the Deep Lagoon is above the waterline, so we can treat it with a herbicide until we decide whether flooding with seawater is possible.  This means that we’ll need to keep the water level at its current, reduced level for now.

We have still to determine the long-term management of the lagoons.  The lagoons at Conwy have some other problems, besides the Crassula: it’s proved challenging to maintain water levels during extended dry periods and the mud lacks invertebrate food for breeding and roosting waders.  The initial flooding will give us a chance to determine whether maintaining brackish conditions is the best option in the long-term.  We'll be keeping the blog updated with news of the work, and with such a significant change, it will be interesting to see the transformation through 2012-13.

We would like to thank Environment Agency Wales and the Countryside Council for Wales for their help with this project.