Messing up your visibility.
Whilst learning to dive in Gibraltar, one common complaint which constantly annoyed local divers was the lack of visibility compared to other parts of the world. “When I was in the Caribbean, I had twenty-five metres visibility!” was a popular pronouncement which seemed to imply that there was a problem with our local waters. After all, the coral reefs were the healthiest ecosystems on the planet evidenced by the number of species that live there, right? Not exactly!
From a human perspective, what is in the water is stuff which simply interferes with our vision, and this negatively affects our dive experience. But, take a water sample and look under the microscope and everything changes. The ‘stuff’ that we typically find is testimony to the wealth of life in our water from an algal perspective. One group which is of particular interest to researchers and are ubiquitous throughout the planet’s oceans, are the coccolithophores.
From the phylum Haptophyta, coccolithophores are single cell eukaryotic algae whose name is derived from coccus meaning spherical, lithos meaning rock and phore meaning carrier; spherical rock carrier. The reason for the strange name is down to their ability to take calcium carbonate from the water and turn it into beautiful plates which float on their cell membranes. The exact reasons they do this are not entirely understood, but light reflection and protection from predation are widely accepted theories.
There are around 300 known modern coccolithophore species with each one identifiable by the design and shape of the plates. Some are spoked like the wheels of a bike whilst others resemble the interlocking plates from a knight’s armour. Some species have trumpet like projections whilst others are simply a ball of spikes. The wide range of forms is simply breathtaking and certainly merit a quick Google search.
Being photosynthetic, Coccolithophores form the very base of the food web and play a crucial role in keeping the abundance of animals around Gibraltar’s coastline high. Similar to the role that grasses play in the great plains of Africa, these single cells are grazed on by predators like Copepods and consequently energy is transferred up the trophic levels, supporting more and more organisms.
They also play a vital role in carbon sequestration, much like terrestrial plants. Carbon dioxide freely transfers across the atmospheric oceanic boundary and the two are completely in equilibrium. As we increase anthropogenic carbon into the atmosphere, so are we increasing its concentration in the oceans. The net effect of carbon build-up is an acidification of the sea which is most widely publicised through the plight of coral reefs.
Coccolithophores, unbelievably, actually help regulate the build up of this carbon because they use it to make the plates on their membranes. This, combined with grazing pressures and natural mortality, create particulate matter commonly called ‘marine snow’ that sinks to the sediments far below the euphotic zone. This process removes the carbon from the surface waters and is a powerful force in regulating its build up. Their presence in the fossil record demonstrates that they have been helping to regulate Earth’s climate early into its natural history, and providing they survive this latest spell, are likely to continue its regulation well after we are gone.
By way of example as to how effective this carbon sinking can be, one must consider the white cliffs of Dover which have been built over millions of years by coccoliths accumulating in the sediments below surface blooms. Pretty impressive natural structure when considering they measure around ten microns.
These algae also play a significant role in producing approximately 50% of all the oxygen we breathe. It is comforting to know that despite our insistence at rainforest destruction, that there are some organisms which we will find hard to harvest in quite the same way, thus possibly preserving some of the planet’s oxygen producers.
These algae also have an interesting lifecycle; certainly if algae are your thing. They begin a growth phase as haploid holococcolithophore cells, typically in low nutrient concentrations. With time they develop into diploid hetrococcolithophore cells, normally as a result of higher nutrient availability. It is at this last stage that the cells switch from being K strategists to r-selected meaning their reproductive rate increases dramatically. Uniquely, they are ready to asexually reproduce in both stages of their lifecycle.
So, whilst it is true that algae mess up a diver’s visibility, it is also a testimony to more productive waters which support a high abundance food web. That certainly is good for divers! Further, increases in coccolithophores are associated with higher albedo for surface waters, which helps prevent heat absorption. That by itself is a climate change fighting strategy, and a hefty reason for appreciating the murky visibility – a thought well worth pondering over the next time you are in the water.