What are toxic phytoplankton and how do they occur?
The harvesting of mussels and other filter feeders, such as razors, has been restricted in Lamlash Bay, Whiting Bay and Pirnmill since May. COAST has been asked: “What exactly is a toxic phytoplankton bloom and why does it occur?”
In the marine environment, single-celled, microscopic plant-like organisms naturally occur in the surface waters of any body of water. These are phytoplankton sometimes known as microalgae, plant like organisms. Phytoplankton use sunlight to convert simple inorganic molecules, such as water and carbon dioxide, to complex organic compounds, such as protein, carbohydrates, and lipids.
The ocean waters that surround continental coastlines are the home of a large number of marine algae, making them the most productive areas for the commercial harvest of marine finfish and shellfish. They are important as they form the base of the food chain for many other marine organisms. However, about 2% of the thousands of species of marine phytoplankton are harmful or toxic and these can have negative effects on the environment, seabirds, fish and marine mammals; or on people who eat marine shellfish which concentrate these contained toxins.
Much of the time, microalgae are present at very low numbers, but when conditions are right, they can multiply rapidly and explosively, producing a noticeable discoloration in the water called a "bloom." These blooms can cover very large areas of the coastal ocean that are often visible from satellite observation of the earth. Large numbers of "blooming" algae can, at times, result in the presence of high levels of toxins or result in lower oxygen levels in seawater, thereby creating significant problems for marine life.
Examples of this sort of phenomenon, an explosion in numbers of organisms, are widespread in the natural world. Regular monitoring is carried out in the UK by local authorities to ascertain if a harmful algal bloom (HAB) is occurring.
In the spring in temperate and sub-polar waters, as the water warms, and winter tides and winds have stirred up the seabed, nutrients rise to the surface and there can occur a sudden bloom of phytoplankton such as various diatom and dinoflagellate species. In winter, waters are well-mixed, i.e., water is circulated from the bottom to the top of the water column because the water is relatively colder and therefore maintains a more uniform density). In the early spring, the upper water layers have enough nutrients circulated up from bottom waters to maintain phytoplankton growth but the phytoplankton are unable to grow rapidly as there is frequent mixing from wind and sunlight levels are not yet strong enough.
Light is a vital ingredient for growth, the essence of photosynthesis. However, as the ocean warms in the later spring, the warm water will remain at the top of the water column as it is less dense. This will create a layer of stratification in the water. At this time, the phytoplankton are maintained in waters with enough light and abundant nutrients, allowing their population numbers to grow explosively. Algae may be considered as blooming at concentrations of hundreds to thousands of cells per millilitre. Under these conditions the sea may turn green, yellow-brown or red. For some species this is known as the Red Tide.
In most cases the phytoplankton will use up the available nutrients in a matter of weeks or months, eventually dwindling in number in summer. Many species of diatom will sink to the bottom and create resting cysts, a passive stage in the life history, when nutrient concentrations run low. Also, it is not uncommon to see a succession of phytoplankton species reach their growth peaks at different times through the course of the bloom as different species will have optimal nutrient uptake at different concentrations.
The term Eutrophication is the enrichment in the primary productivity of an ecosystem with chemical nutrients containing nitrogen, phosphorus or both and is a natural phenomenon. Runoff from agriculture, discharge from septic tanks and sewers, waste materials of faeces and food from fish farms, in other words man made pollution, can all enhance the chemical nutrients in the water. So more phytoplankton are produced. It is a process of impact and changes those progresses through various phases in response to the intensity, nature and degree of nutrient loading. Phytoplankton blooms can therefore be promoted by polluted waters.
The growth of marine phytoplankton (both non-toxic and toxic) is generally limited by the availability of nitrates and phosphates, which can be abundant in coastal upwelling zones as well as in agricultural run-off. The type of nitrates and phosphates available in the system are also a factor, since phytoplankton can grow at different rates depending on the relative abundance of these substances (e.g. ammonia, urea, nitrate ion).
Whilst many of the algal blooms are noxious, producing smelly odours, discoloured water and gelatinous masses, some produce marine toxins that enter the food chain when consumed by shellfish and finfish. Mussels have the ability to concentrate and accumulate very large amount of these toxins without apparent harm to themselves.
In Lamlash Bay the dinoflagellate Dinophysis species is the HAB. They produce a toxin okadiac acid and dinophysis toxins. Diarrhetic Shellfish Poisoning (DSP) is a gastrointestinal illness. DSP symptoms usually occur within 30 minutes to a few hours after consumption of contaminated shellfish. Symptoms include diarrhea, nausea, vomiting and abdominal pain. It is suggested that long term exposure may promote tumour growth in the digestive system.
There have been no fatal cases of DSP reported and full recovery is expected within 3 days. Whilst there is a bloom of these algae present, shown by analysis of seafood and water samples, legal notice is given to prevent the taking of shellfish from an infected area. Once the concentration drops to a residual concentration two further samples must show the same reduction in density before an area is re-opened. Local authorities are responsible for issuing notifications (North Ayrshire Environmental Health, for issuing the notification for Lamlash Bay/Whiting Bay and Argyll and Bute Environmental Health for issuing notifications for Loch Fyne, which includes Pirnmill). Both come under the Food Standards Agency in Scotland (Shellfish Results Co-ordinator, Food Safety Monitoring and Policy).