Marine Aquariums: The Scoop Regarding Sandbeds and the Three Sediment Areas

I guess you could think of the benthic area of a marine aquarium as a mystery. You can also be very vague and puzzled as to why sandbeds can be such a vital part of an aquarium. If you think of a sandbed as three very distinct and diverse sediment layers, each with its own importance due to population and species of organisms that reside in these sedimentary layers. The three sedimentary levels are the top layer which is where the water and top of the substrate meet, known as the aerobic layer. The second layer is the anaerobic layer with the third and final layer or anoxic layer at the bottom of the tank.

Many of you may not be aware of the diversity and plethora of living organisms that reside amongst the three sedimentary zones. Some of these organisms live on, under or between the grains of substrate and then there are those organisms that are so small that they actually spend their entire life situated on one grain of sand for life not knowing of any other life form.

The top layer or the aerobic layer is made up of most, if not all, of the clean up crew inhabitants residing in a marine environment. Polychaete worms, microcrustaceans, sessile invertebrates, snails, crabs, aerobic bacteria and algae. Their is such an abundance of food stuff available for the plethora of life found in the first layer of sediment. Much of this food such as flake food, shrimp, biofilm, bacteria, plankton, detritus that accumulates on the bottom/ benthic area gets there either through diffusion, advection, water flow or animal life forms. Often rich in nutrients and diverse in animal life this layer is the most abundant in terms of life due to the oxygen and food surplus found there. In this aerobic area you also will find alot of competition for food as well as a predatory/prey relationship within the hierarchy of the food chain. The largest sediment organisms will be found in the aerobic areas, with smaller organisms found as the sediment layer gets deeper with less oxygen and nutrients. This scenario is best illustrated as the food chain or food web, where the larger animals feed upon the smaller animals found below them on the chain of life.

One very interesting note regarding this upper most layer of sediment is that oxygenated water, and food stuff will be drawn down lower in the substrate layers through advection and diffusion, but also through animal life. Take for instance the spaghetti and cirratulid hair worms, they are bottom dwelling worms that burrow in the substrate. Through their actions, they bring about some water movement to lower areas of the sediment as well as move food from the top of the substrate and move it down into lower sediment layers for other life forms to feed.

In the middle or anaerobic sediment layers their is very limited oxygen or none at all. Life does exist in this area however. Denitrifying bacteria reside in these areas providing beneficial breakdown of nitrates into non toxic nitrogen gas. Detritus is also broken down here as well as part of the nitrogen cycle where ammonia is broken down into a less toxic form known as nitrites. Nitrites are still very toxic and need to be broken down further into nitrates which is the least toxic form of nitrate before being released as nitrogen gas. This cycle plays out by the nitrosomanas and nitrobacter bacteria along with other forms of bacteria to help keep an aquarium functioning to is fullest.

Some soluble material will accumulate in the water and be removed by protein skimmers, while some gaseous materials will exit into the environment through respiration and the air/ water surface interface into the ambient environment. However, not all of these components are removed this way. The movement by many of the animals in the upper sediment layers are responsible for what little water movement their exists in the anoxic layer. Through this process, however, benefits do exist as toxic heavy metals such as sulfide and iron hydroxide are able to precipitate down deep in these anoxic areas as a result of chemical and bacterial processes which secure these dangerous gases/metals down deep so as not to impact life above. Hydrogen sulfide gas which looks like a grayish are in the bottom layer of the substrate is poisonous to many life forms above so the precipitation of these gases from leaching out causing dangerous results above is greatly appreciated.

Their is conflicting evidence and points of view regarding the stirring up of the sediment levels and also the releasing of the gasses trapped in the sediment layers. By stirring up the sediment, you are disturbing the organisms that live within these layers. It is best to leave them alone and possibly position a powerhead to provide efficient water flow to the benthic areas of your tank without disturbing life. Yes their are two ways that substrate will solidify which can cause problems and inhibit water flow and other processes from taking place due to elevated calcium levels causing a precipitation of calcium carbonate and the other being an influx of biofilm that can also make sediment less porous. By providing an assortment of clean up crew inhabitants and a diverse population of living organisms within the sediment layers and maintaining ideal water parameters, the three sediment areas will continue to work in a synonymous relationship with each other and provide you and your tank beneficial results now and for years to come.