Wednesday, 24 August 2016

Oysters and Sea Lettuce and Sand Furrows

A few days ago, the JEP carried this story:

BOLD new proposals to tackle Jersey’s growing sea lettuce problem, which would involve cutting huge troughs and ridges into St Aubin’s Bay, have been unveiled today.

Environmental lobbyists Save Our Shoreline have suggested cutting a number of 400-metre-wide channels into the sand.

They claim that the move would accelerate the flow of the outgoing sea water close to the sea-bed and pull the lettuce out to sea, preventing it from ‘matting’ at the top of the beach.

David Cabeldu, researcher for SOS Jersey, together with Tony Legg, of Jersey Sea Farms, who are both putting forward the proposal, were due to meet Infrastructure Minister Eddie Noel this afternoon.

The reader might have been alarmed by this dramatic picture, but in fact, the proposals were nothing like as large-scale and gigantic as suggested in that story.

Jacqui Carrell, writing to the JEP, corrected this impression

The JEP article ‘SOS propose accelerating flow of outgoing sea water’ [18 august 2016] is incorrect about the width of the proposed channels. They will have given readers the impression the channels are going to be very wide and all over St Aubin’s Bay! SOS Jersey would like to reassure everyone that the proposed channels are not going to be 400 metres wide, but 400 metres LONG and ONE furrow wide.

To clarify, these furrows would be low and temporary and dug at specific angles to the tidal flow. Sea lettuce will wash up and down with each tide, but these channels should prevent it from gaining a hold in the intertidal areas and other specific areas and, as a result, stop its accumulation at the top of the beach.

The furrowing will be done early in the season and repeated if necessary; this approach is meant to be a short term, partial solution to the sea lettuce problem until the Jersey native oysters programme starts to show results. We are mindful of tidal flows and of the local flora and fauna, and furrowing plans reflect these.

We would also like to clarify that SOSJ have acted as facilitators in the proposed Jersey native oyster project and haven’t the resources to carry out further surveys. We will issue no proposals: that is for the relevant States department to do. We will monitor and help when possible, but Infrastructure are now in a position to continue the sea lettuce project in collaboration with Tony Legg of Jersey Sea Farms.

Why Oyster farming?

Oysters feed by filtering water to remove phytoplankton and other particles floating in the water. This action helps to maintain water clarity and quality and to cycle nitrogen and phosphorus, two nutrients that can be harmful to our bays. Increased filtration by healthy oyster populations can also prevent harmful algal blooms. These blooms can affect the health of the water and wildlife living in and around it.

But oysters need some removal of sea lettuce before they will prove effective. As noted in the Mill River, in Canada, in 2012, too much sea lettuce choked oysters. This is the story:

Hal Perry said sea lettuce is choking out oysters and costing fishermen their living. The underwater plant grows profusely when there is excess nitrogen in the waterway from nitrate run-off from farmers' fields, industry, cities, and sewage or forestry practices. As it rots, it sucks oxygen from the water.

Oyster fisherman Johnny Powers said he's never seen an oyster season this bad. "All dead, not a single living oyster amongst it," said Powers, as he pulled up a nearly empty oyster tong from the riverbed. "Most of the fishermen I talked to say there's anywhere from a 35 to 50 per cent mortality rate," he said. "So that's our income cut almost by 50 per cent."

A report in August this year confirms that the problem remains unresolved for Mill River estuary:

“Back in the late 70s, we use to dig clams on almost every point on the Mill River and Hill River, but now there’s not a clam to be seen,” said Mr Lane, adding when sea lettuce dies and goes to the bottom, it can smother clams as well as oysters, “Talk to oyster fishermen, they will tell you in the last five years that the amount of oysters they’ve caught that are dead has increased and the amount of live ones are decreasing.”

Ray Konisky, a Marine Ecologist, writing in UNH Magazine, Winter 2011 noted also how excessive sea lettuce could affect oysters:

"In the past, there was enough natural flushing, with tides going in and out, to handle any excess." Now there's too much—especially nitrogen, which has risen 42 percent just in the past five years. Algae blooms, including a large alga known as sea lettuce, can spread in thick green mats, making it hard for anything else, including oysters, to survive. It's a vicious cycle: Just when the bay needs its natural water treatment system more than ever, oysters are suffocating in sea lettuce and silt.”

Oyster can help restore the ecosystem, but they need some help. And both removal of excess sea lettuce and furrows may be one way forward.

1 comment:

Colin Machon said...

Hi Tony,
I have been involved in this project and your comments are welcomed but not entirely correct.
The oysters will not need the sea lettuce to be removed for a number of reasons.
- The lettuce (macroalgae) develops after the initial spring phytoplankton bloom, it is that bloom that the oysters crop and start to sequestrate the nitrogen and phosphorous from. Instead of a double belt of these nutrients from the decomposition of the microalgae and the newly delivered ambient nutrients, a far weaker solution is available.This continues the phytoplankton bloom but not the macroalgae one.
-The smothering effect you mention relates to oysters on the bottom ,in this case they are contained in raised structures albeit below the waterline.

Just for further information the furrows work in two ways ,with the survival of eelgrass as a strong requirement.This had been explained to the JEP but the challenge of thinking had proved too much.
The furrows will be drainage channels that in some respects will be self sustaining due to the nature of sand profiles at that level. At present the beach slopes at 0.6% ,that is not enough to shift the the Ulva the furrows tip this drain to 3% parallel to the shore and 1% perpendicular .This is a major boost to drainage with little effort.
The other effect is that any wave action at all will have a mini rip-tide effect , pumping the weed and water lower down the shore.

The Mailonline caliber of our press prevents rational explanation and only allows enough information for irrational explanations.

Thanks Tony

Yours Colin