A blackwater breakdown


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The blackwater biotope is all the rage in hobby circles, but is there more to it than just staining the water? Chris Sergeant explores the dark and murky waters.

At face value, muted lighting over a biofilm-coated bogwood centrepiece, atop a sand-bed heavily littered with decaying leaves and botanicals, all creating the tinted water, looks to tick all the boxes for a ‘blackwater biotope’. But from a scientific perspective, it is technically a blackwater aquarium?

A recent episode of The Tint podcast saw Scott Fellman of Tannin Aquatics questioning whether the term ‘blackwater’ is now used as a generic descriptor for tinted, tannin-stained water. Given the many websites perpetuating this idea, he certainly has a point. With the ever-increasing popularity of blackwater biotopes, it feels only fair that we know exactly what is going on from an ecological perspective.

What is it?

Blackwater refers to soft, acidic water with a deep brown colouration, and while the tannins from decaying leaf litter, fallen botanicals and sunken wood play their part in its formation, their role is secondary. Instead, it’s the geology of a landscape that plays the focal role in its creation. I am certainly no limnologist, so it helps to use an example for context — in this case, the R o Negro. As one of the major tributaries of the Amazon, this blackwater river drains from the Precambrian Guiana shield, which is characterised by large areas of white sands, or podzols. These highly acidic soils form in forested landscapes, high in quartz and with a sub-surface layer high in metal oxides, notably iron and aluminium, and humus, yet low levels of minerals like calcium, magnesium, potassium, and sodium.

As the river drains through and flows over this substrate, high levels of organic acids, in the form of fulvic and humic substances, leach out from the soil and broken-down vegetation and dissolve within the surrounding water bodies, leaving the characteristically acidic (pH 3.0-5.5), ion-poor conditions, rich in dissolved organic carbon (DOC) associated with blackwater. In conjunction, the decaying riverbed debris releases a variety of organic and inorganic compounds, including various carbohydrates, phenolic compounds and tannins. While the tannic acids do contribute to the tea-stained aesthetic, it’s these humic and fulvic acids, along with the elevated DOC, that are the main source of colouration.

Read the rest of the feature in the February 2022 issue. Buy the latest digital edition and read instantly on your computer, mobile or tablet device.

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