Håndbog om giftige alger i badevand
Summary and conclusions
Massive algal blooms along the beaches are a recurring problem every summer in lakes and coastal areas. The blooms lead to aesthetic inconveniences which makes it unpleasant to bathe. A more serious problem is the toxins which some algae produce and which may present a health risk to bathers and holidaymakers visiting recreational areas.
Along the Danish beaches only blooms of blue-green algae (cyanobacteria) cause a recurring risk of health problems. Thus, the basis of this report is the presence of these algae and their toxins – the cyanotoxins. However, an assessment of the bathing water quality should include all types of toxic algae as well as blooms of other plankton algae and macrophytes. To a great extent, emergency plans, control programmes and actions will be independent on the type of alga.
There are no fixed safe-guard values for the presence of toxic algae in bathing water. WHO has realised the potential health hazard of blue-green algae blooms and has summarised the present knowledge and proposed guidelines for safe-guard values (Chorus & Bartram 1999). The recommendations are described in detail in Chapter 7 and partly incorporated in the Danish recommendations in Chapter 2.
Therefore, handling of the algal blooms must take place on the basis of the local authorities’ assessment of the actual situation, experience from earlier incidents and existing knowledge on the types of algae causing the blooms.
All blue-green algae blooms must be considered toxic unless investigations prove otherwise. Observed effects of blue-green algae in bathing water and drinking water prove that a massive presence of blue-green algae – where the water is discoloured – presents a health hazard. Danish investigations show that the majority of this type of blooms contains toxic species and cyanotoxins.
The risk cannot be quantified and depends on the type of cyanotoxins, the concentrations of algae, the sensitivity of the bather and the duration of the contact with the algae. The risk is considered small as long as people do not bathe for longer periods, do not take in alga-coloured water or ‘eat’ foam, and assuming they are not ill or suffer from hypersensitivity or allergy or in other ways are infirmed. There is no reason to assume that bathing causes chronic effects. The symptoms disappear in a few hours – a few days at most.
Water without visible algal growth does not usually contain high concentrations of toxins, and the risk of infections is on the whole absent. Exceptions are e.g. stagnant water just after the decay of a massive bloom, and water which is in contact with foam (fresh or dried). As a rule-of-thumb the toxins will disappear in a week or two. Current and mixing of the water reduce this period considerably.
There is no evidence that the blooms of other toxic algae cause problems in bathing water. In case of other types of blooms the risk must be assessed in each single case and must be based on the knowledge of the given species and any existing measurements of toxicity and toxins.
Massive amounts of filamentous macroalgae may also reduce the quality of the bathing water. The macroalgae are not poisonous and do not present a health hazard. Initiatives to reduce the quality of the bathing water should therefore only aim to reduce the aesthetic inconveniences of these algae. Such initiatives are not within the frames of this handbook.
According to the Danish ‘Bathing Water Instruction’ and the ‘Guide to Control of Bathing Water’, the responsibility for the control of the hygienic and aesthetic quality of the bathing water lies with the local authorities. The results of the control of the bathing water must continuously be submitted to the counties and the health inspectorate. Every year the counties are to summarise the results of the bacteriological surveys and report them to the Danish Environmental Protection Agency. The authorities are recommended to use the results to inform the public. In case the quality requirements have not been met, the local authorities – in consultation with the counties and the health inspectorate - must carry out further investigations and decide on the necessary measures, and possibly even forbid bathing.
Especially in case of algae, the ‘Bathing Water Instruction’ requires that the presence of massive algal blooms and particularly of toxic algae is controlled, and that a hygienic assessment of the blooms of toxic algae takes place, and that action is taken in accordance with the hygienic risk. Toxic blooms often give rise to situations where quick retrieval of data and knowledge, assessment of risks and submission of information to other authorities and the public are essential. To ease this process and ensure that the problems with algae are handled as rationally and operationally as possible, it is recommended that an Algal Protocol should be elaborated.
An Algal Protocol may support the local management in its continuous control of the bathing water and in acute situations with algal blooms. The Register should draw up guidelines stating which kind of information should be gathered, what should be done in a given situation, and who should do it, who should be informed, how and when.
Recommended contents of the Algal Protocol:
 | A plan of effort ensuring that the right persons are involved at the right time, that the basis for decision has been established, and that the decisions are made and carried out. |
| An information plan ensuring that the public has sufficient knowledge to be able to decide when it is safe to bathe and when not. |
| A surveillance plan describing the continuous surveillance and what must or can be done in connection with a bloom. |
The design and contents of the Protocol depend on the types and sizes of the algal problems occurring in the affected bathing areas.
The information plan must be central, as it is impossible for the authorities to know the bathing water situation at all times. The algae appear and disappear within few hours, they are transported and accumulated and spread, depending on wind and current; and the toxicity varies. A realistic control programme can never cover this heterogeneity in time and space.
It is recommended that the Protocol should be prepared in a teamwork between the county, the local authorities, and the health inspector.
This handbook treats each of these elements and puts forward proposals for the contents of such an Algal Protocol. In addition a summary description of the algae is presented as background information.