Algal bloom: investigate before blaming toxins

Green, red or brown water, scum driven against a shoreline, or fish ventilating at the surface all warrant a rapid response. None of these signs proves that cyanobacteria are present, that cyanotoxins have been produced, or that either caused a mortality event. A bloom can harm aquatic animals through several pathways, including oxygen depletion, sharp pH variation, gill injury and toxin exposure. It may also coincide with pollution, infection or thermal stress. The practical priority is therefore to protect water users, preserve evidence and investigate systematically before assigning a cause.

Abnormal colour is a warning, not a diagnosis

Phytoplankton proliferations are shaped by the waterbody. Nutrient loading, water residence time, stratification, light and temperature interact. The World Health Organization (WHO) notes that potentially concerning cyanobacteria are often visible as streaks, scums, greenish turbidity or occasionally reddish discoloration. It also warns that visual inspection alone can miss a dispersed, deep or wind-displaced population.

Appearance is not specific. Duckweed, filamentous algae, resuspended sediment and other microorganisms can create dramatic colour. Conversely, cyanobacteria may be abundant without forming a continuous surface layer. French health authorities state that a sudden colour change at a bathing site prompts cyanobacterial counts and, depending on the situation, toxin measurement. The principle extends beyond recreational-water regulation: observation should trigger measurements and sampling, not replace them.

Separate three field mechanisms

The first mechanism to assess is hypoxia. Dense biomass respires at night, while decomposition consumes additional oxygen. Conditions may therefore differ sharply between afternoon and dawn, surface and bottom, or an exposed shoreline and a sheltered cove. A single mid-afternoon measurement can be falsely reassuring. Fish gathering near an inflow, gasping at the surface or dying predominantly early in the morning support an oxygen-related hypothesis, but do not establish it on their own.

The second mechanism is toxicity. Some cyanobacteria can produce microcystins, anatoxins, cylindrospermopsins or saxitoxins, but not every bloom is toxigenic and identifying a genus does not predict toxin concentration. A systematic assessment by the French Agency for Food, Environmental and Occupational Health & Safety (ANSES) also found that cyanotoxin contamination in freshwater fish varies with species, diet, tissue and analytical method. A concentration in water cannot therefore be converted mechanically into a concentration in fish.

The third scenario is multifactorial. Heat, elevated pH, unionised ammonia, organic matter, infectious agents and contaminants may act together. Unusual colour and distressed fish then describe a situation, not a single cause. The investigation should keep several hypotheses open until field measurements, laboratory analyses and animal examination allow them to be ranked.

The first hours: protect, describe and measure

The immediate task is to reduce exposure without destroying evidence. Managers should mark affected areas, notify the site owner and relevant authorities, keep people and domestic animals away from scums, and avoid improvised disturbance or treatment. Applying an algaecide, mixing the water without documentation or immediately removing every fish can change bloom distribution and compromise interpretation. Any emergency action taken to protect animals should be logged with its time, location and intensity.

A field record should capture time, recent weather, wind, rainfall, flow or water-level changes, site uses and interventions during the previous days. Georeferenced photographs can document the extent and colour of the event, scum position, shoreline condition and animal distribution. For fish, record species, size classes, approximate numbers, behaviour, external lesions and spatial pattern before later collection reduces the event to a count of carcasses.

Immediate measurements should include temperature, dissolved oxygen, pH, conductivity and transparency, each linked to a time and depth. Measurements are more informative when repeated in an affected area and a comparison area, at the surface and near the bottom where depth allows. A dawn profile paired with an afternoon profile captures diel variation better than one isolated value. WHO field guidance specifically recommends connecting these measurements with water colour, odour, visible scum and observations reported by local stakeholders.

Sample without creating false representativeness

Bloom density can change by orders of magnitude across a few metres. The investigation must therefore define what each sample is intended to represent. A targeted scum sample documents a near-maximum concentration; it does not represent the average waterbody. A depth-integrated sample is better suited to estimating population biomass through the water column. Both may be necessary, but they must remain separate and be labelled with coordinates, depth, time and sampling method.

Contact the laboratory before collection to agree on containers, volumes, preservatives, transport temperature, holding time and analytical scope. Depending on the question, the plan may combine phytoplankton identification and quantification, chlorophyll or phycocyanin, selected cyanotoxins, nutrients and physicochemical parameters. If fish are affected, freshly dead or moribund specimens may be required for necropsy, histology, toxicology and infectious-disease testing. A decomposed carcass collected from the shore will rarely answer all of those questions.

Sampling also requires traceability and biosecurity. WHO recommends treating every bloom as potentially toxic, minimising water contact, wearing gloves and rubber boots, and avoiding aerosol exposure. Equipment must be cleaned between locations to prevent transfer of organisms or contaminants. General field and boat safety remains the priority, even in shallow water where turbidity can conceal hazards.

Interpret converging evidence, not isolated results

A defensible conclusion brings together four lines of evidence: timing, spatial gradient, water conditions and biological findings. An oxygen decline that precedes mortality and is strongest where biomass accumulates supports hypoxia. Detecting a cyanotoxin supports causation only when the sampled matrix, concentration, exposure pathway and lesions are compatible. Likewise, recovering a bacterium from a degraded carcass does not establish a primary infectious cause.

Human recreational use and consumption of caught fish remain matters for the relevant health and environmental authorities. Routine bathing-water classification is mainly based on indicators of faecal contamination; by itself, a favourable classification does not rule out a cyanobacterial hazard. Advice on swimming, fishing or consumption should therefore follow local rules and appropriate testing rather than the visual disappearance of scum.

Turn emergency evidence into prevention

After the event, managers can convert the case file into a surveillance plan: fixed locations, a seasonal schedule, dawn measurements during warm periods, internal triggers, a designated laboratory and a clear notification chain. Trends in nutrients, transparency, stratification and dissolved oxygen help identify recurrent risk conditions. Over the longer term, reducing phosphorus inputs and managing the catchment addresses eutrophication more reliably than repeated in-water interventions.

Vetofish can help environmental managers, local authorities and consultancies design an investigation plan, coordinate sampling of water and animals, interpret results together and formalise a site-specific alert protocol. The aim is to shorten the interval between the first warning and an evidence-based decision, without promising that colour, one toxin result or one laboratory finding will explain the whole event.