MARINE NUISANCES – Macro algae and factors that induce growth


The marine environment hosts a huge variety of algal species most of which are too small to see with the human eye. They are there, however, in greater numbers than we can imagine and modern satellites have been a key component in lifting the lid on their secretive world. Our beach activities bring us into contact with both the micro- and macro algae but we only ever notice the larger types. Macro algae is commonly referred to as seaweed and some large volumes of it brought a poignant issue to the surface, excuse the pun, over the course of last summer.

But what exactly is seaweed? In terrestrial terms, we can simply call them plants because they photosynthesise and are autotrophic (they get their energy from the sun). However, the difference is greater than the similarity. Terrestrial plants have specific structures like roots, stems and leaves, all of which are visibly absent from seaweeds. You could argue that kelp is the algae that most resembles modern terrestrial plants but to claim that kelp has leaves, is to misunderstand the subtle ties of algal growth.

One key difference is that seaweed absorbs nutrients from the surrounding water. Terrestrial plants can only do this at their roots and leaves, whilst algae absorb them from all parts. The cells that make up algae are unspecialised, jacks of all trades, whilst masters of none. But this difference is important to facilitating seaweed to grow at unprecedented rates providing that all conditions are right.

So, what are these conditions? Temperature variability in the ocean is much more stable than on land and in the Mediterranean, this varies less than 5oC from summer to winter. Water is also not an issue as seaweed grows in it. By contrast, light attenuation in the oceans is highly variable according to the seasons and this can have a crucial impact on suitable habitats for algal growth. Generally speaking, the more light the better, until algal cells reach their light saturation point. Once reached, extra light will not aid growth. Green algae have the highest light saturation point followed by brown algae and finally, red algae.

Like terrestrial plants, algae require nutrients which they take directly from the surrounding water and here we have another key factor which determines growth. Eutrophication is a term used to describe high levels of these nutrients in water bodies; it is more commonly referred to as water pollution. The level of eutrophication is the main factor in determining an algal bloom size, with higher levels promoting faster growth. The main nutrients for growth are Nitrogen and Phosphates and common sources for these are from agricultural run-off and untreated sewage.

Eutrophication process

Along with nutrient inputs, water circulation is an important component of nutrient concentration. Generally, dilution is the secret to pollution but in a water body which has a long residence time even a small trickle of nutrient input can have a large eutrophic effect.

How do these factors play out along Gibraltar’s coastline? In one particular area, we have seen many of the factors combine to create a real issue; Western Beach and forkweed (Dictyota dichotoma).

Forkweed is ubiquitous on every continent globally, including Antarctica but the increases in Western beach have raised community concerns recently. The huge concentrations of this algae required serious intervention this summer. What does the future hold for this beach? Looking at the variables paints a bleak picture, for me at least. Western beach is a shallow basin which facilitates good levels of light for most of the year. The exceptions to this might be in April and September when phytoplankton blooms raise turbidity. Nutrient levels at the beach are high due to the storm drain that inputs directly into the basin. Further, anecdotal evidence suggests that the residence time for water in Western beach is high, making the conditions eutrophic more often than not. Essentially, the key components for growth are there and they are unlikely to go away.

Western Beach seaweed removal

I often get asked “Why now?” and it’s a hard question to answer as all the data is not available. What is clear is that in biology, just because a habitat has the right conditions for an organism doesn’t guarantee that one will find it there. When habitats change, there can be a lag time before they move in. Once introduced though, the story changes and it is likely that this is what has happened at Western beach. The storm drain is a relatively recent occurrence for the basin. Even once it was built, the forkweed hasn’t moved in straight away, but rather, increased its range following the nutrient source until it has eventually arrived at Western beach. Once there, the forkweed has found ideal conditions to multiply and so it has, on the scale we saw last summer. Further, unlike the jellyfish issue, it is likely to continue happening each summer.

Solutions to this are of course complex. On paper, the quick fix is to close the storm drain or to divert it to another location. But the storm drain is a Spanish one and getting it blocked involves timely and expensive legal challenges which have no guarantees of success.

Increasing the water flow to the basin is practically impossible due to the fact that the runway is on one side and the Spanish mole on the other. Any flow solution would therefore demand both international co-operation, some kind of works on the runway a


nd a vast expense. Flow would help by decreasing the nutrient concentration and limiting the growth of the forkweed.

What is clear, is that the algae are not going to go away unless something drastic changes. As with all biological systems, my hope rests on nature redressing the imbalance we have created. Where algae go, herbivores tend to follow. These grazers should quickly multiply and control the levels of forkweed in the area. Urgent studies are required to identify the ecological status of the main grazers of forkweed in Gibraltar waters. We need to understand the impact these grazers are presently having on the mass of forkweed and how we might aid their numbers to bring this smelly matter to a close.

words | Lewis Stagnetto, The Nautilus Project