The scent emitted by the algea’s predator may warn of toxins

The odour emitted by copepods can be used in a warning system for toxic algal blooms.

The method could provide advance warning of outbreaks up to seven weeks in advance.

‘You can imagine forecasting toxic algal blooms in much the same way as we forecast the weather,’ says Erik Selander, an ecologist at Lund University.

The copepod is only a few millimetres long, but it is the algae’s number one enemy.

Algae form the basis of the marine food web, comparable to green plants on land, and copepods are the most common zooplankton that feed on algae. Grazing copepods are therefore seen as a threat by the algae. When the algae detect the scent of copepods, they produce toxins in self-defence.

Protected by poison

‘It’s an effective defence; when they’re sufficiently poisonous, the grazers avoid them,’ says Erik Selander.

The method of measuring the odour compounds produced by copepods to predict outbreaks of toxic algae has been tested as part of a collaborative research project both in Gullmarsfjorden in Bohuslän and in Monterey Bay in California.

In the study, small sampling units were deployed to passively collect the odour compounds from the water; however, the odour from copepods also accumulates in mussels. As samples are regularly taken from mussels in the areas where food companies farm and fish for mussels and oysters, Erik Selander believes that the measurements could relatively easily be adapted, without significant additional costs, to also provide information on the concentration of odour compounds from copepods.

Causes diarrhoea

Better toxin forecasts would be of great value to the fish and shellfish industry, according to Erik Selander. On the west coast, there are outbreaks of diarrhoea-inducing algal toxins.

There are algal toxins that kill fish as well, in fish farms for example. It would be absolutely brilliant if we could be warned a couple of weeks in advance that the risk is high, so that we can take action.

So what do copepods smell like? It is not a smell that the human nose can detect, explains Erik Selander. The substance is only present in the water and, moreover, in very low concentrations – but the algae detect it immediately.