MARINE NUISANCES – Red tides – the HABs and the HABs-not


Red tides are an actual phenomenon of biblical proportions. Referred to as harmful algal blooms (HABs) by the scientific community, they are caused by a microscopic plant-like organism called dinoflagellates. When waters become eutrophic (high in nutrients), a dinoflagellate bloom may occur and their populations can become very large and dense, in a short space of time.

Dinoflagellates carry a reddish-brown pigment within the cell which, in the high concentrations found in bloom conditions, turns the sea red and this is the reason they have been called red tides.

Gibraltar has never had a recorded instance of a HAB but environmental conditions have been changing and increasing the likelihood of such an event. In the bay, we have eutrophic riverine inputs caused by runs off from farm fertilizer and pesticide use. Further, we have the inputs from overflow pipes from sewage systems which also carry high concentration of nutrients. This is all in a context where average sea surface temperature is rising slowly and the stage is set for what could be a disastrous occurrence for our coastline. The one redeeming factor is that water flow within the bay is pretty high and this has provided us with a life-line, so far.

The first issue with algal blooms is that when the plants die, the process of decomposition by bacteria uses up the oxygen in the water. As the concentration of oxygen falls, the ability of marine organisms to absorb it becomes increasingly stressful. This kills most of the fish life in the area as a result.

The second issue is that some species of dinoflagellates produce a harmful neurotoxin called brevitoxin and when it builds up in the tissues of oysters and other filter feeders, it is the cause of paralytic shellfish poisoning (PSP) in humans. When a high concentration of brevitoxin is consumed, PSP has been known to cause death in humans.

Further complications can arrive because the brevitoxin often becomes airborne. Inhalation of brevitoxins can produce severe asthma attacks in humans. In Florida, life guards are trained to spot the early bloom stages and warn people off the beaches. They often count dead fish numbers as they wash up on shore. This provides them with some anecdotal information as to the impact the bloom is presently having before it arrives at the shoreline.

So why the concern? HABs reduce air quality drastically whilst depleting dissolved oxygen in the waters. In Florida, when they have these blooms, there are tons of dead fish that wash up on shore, asphyxiated. HABs also contaminate the food sources we rely on making them toxic to eat. Worst of all, the occurrences and the size of the blooms are increasing with each passing year and there is little that can be done.

Prevention is certainly better than cure so here is a perfect example. Gibraltar already has some marine monitoring which is great, but not enough. The resolution required to create an early warning system is much higher than what we presently collect. Further, our water quality metrics are focused, in the main, around the bathing areas. The likely sites of these blooms are not these areas at all. The blooms often originate from the mouths of rivers or places where there are high nutrient inputs.

The blooms are most likely to occur when the water warms in late spring and early summer. It is likely to impact us during our summer bathing season, but, unlike the smelly seaweed, these blooms would make it highly dangerous to be on the coastline at all. Beaches would need to be closed, boat users would have to be very careful when passing through blooms, access to coast would have to be severely locked down, fish would die by the ton load and the cetaceans in the straits would be at risk of ingesting toxic fish and dying. It would be catastrophic. For objectivities sake, it is important to highlight that of all the nuisances within this series, this is probably the least likely. For a start, instances of HAB’s in the immediate area are non-existent but this doesn’t mean that the dinoflagellates could not be imported through ballast water from shipping. Florida’s blooms are primarily driven by Aeolian transport of Saharan dust which carries much needed iron to the western Atlantic. But this source of nutrient input is also not that easy to discount because of our proximity to the Sahara.

What is true is that without action to closely monitor the health of our waters, we are walking blindly into what is to come. D.O. Flynn once said “the haves and the have-nots can often be traced back to the dids and the did-nots”. In our case, let’s hope that it’s not the HABs, but rather, the HABs-not!

words | Lewis Stagnetto, The Nautilus Project