AN OVERVIEW OF WHAT A SAND BED IS:
(PART-1)
Sand beds, essentially layers of sand of various thicknesses and arrangements, have come to be common fixtures in marine reef aquaria. The use of these sand beds has been correlated with significantly increased survivability of many organisms in reef aquaria, particularly when compared to the bare bottom tank arrangements that were in the vogue about a decade ago. Nevertheless, few hobbyists seem to realize why sand beds should contribute to the success of their tanks, and fewer yet seem to understand how those beds work.
Our aquaria are artificial ecosystems, or microcosms, representative of the real reef environment. There can be no doubt; our systems mimic relatively well many of the processes occurring in the natural world and, when populated by an appropriate group of organisms, many of the interactions occurring in natural habitats occur in aquarium systems. The approach of dealing with aquaria as artificial ecosystems has been criticized primarily on the basis that reef aquaria are patently artificial. However, such criticisms are both rather silly and quite wrong. Reef aquarium systems have to be quite good approximations of the real world, otherwise those animals that are kept in them would not be doing as well as they are. The organisms don't know they are not in the natural environment and any coral reef organism has evolved to deal with an environment that has as its limits those same limits as are found on real reefs.
The fact that we can deal with aquaria as good mimics of the real thing allows us to use "real world" or "scientific" data both to troubleshoot problems and to advance the techniques of animal husbandry. As we know that organisms must live within the ranges of their tolerances, we can recognize that problems will occur when something in an aquarium is far outside that range, such as the excessively high concentrations of poisonous heavy metals found in some artificial sea water mixes . Once that recognition is made, we can adjust for the problem and proceed. In this way, we may incrementally increase our understanding of the animals and what we need to do to maintain them.
If aquaria are artificial ecosystems, however, the component that is least artificial is the sand bed. This part of a reef aquarium, with little input from the aquarist, functions much as do the sandy areas near a real reef. That functionality is due to a rather complex interaction of physical and biological factors, but most of those interactions are unseen, and, I think, unappreciated by the average aquarist. Without those interactions our reef aquaria would simply fail. The fact that they don't fail is a tribute to the ease of constructing this one major functional analogue to an extremely critical coral reef community.
-Sediments In Reefs
Sand beds are constructed physically of sands, and sand is defined as unconsolidated sediments made of particles between one sixteenth of a millimeter and two millimeters in diameter. Coarser sediments are referred to as gravel, finer ones as silts and clays (Holme and McIntyre, 1984). Of course, in the real world there is a continuum of sizes found in these environments, and the sediments actually found in any one spot reflect not only the geological and biologic history of the area, but also the hydrographic regime of the area. In other words, what is present is the result of what is available that hasn't been washed away by the waves.
Sands surrounding natural coral reefs may be made of a number of substances. Around volcanic islands there are often regions of volcanic lava sand. Volcanic islands are the basis for most coral atolls and many fringing reefs, so lava sands are commonly found around reefs in nature. Coral reefs located near areas of river mouths or extensive areas of runoff often are surrounded by silica sands or fine sediments of other upland or inland sources. Of course, coral reefs may be surrounded by calcareous sands resulting from the breakdown of corals and other calcifying organisms. Calcareous sands may also be formed by the precipitation of particulate calcium carbonate in coral lagoons, one of the natural sources of oolitic sand. Calcareous sands may also be formed from the skeletal breakdown of other organisms, such as foraminiferans, bivalves, calcifying algae, or barnacles.
The chemical composition of the sands has a small effect on the organisms found in the sands, but that effect is minor compared to the effects due to differences in sediment particle size distribution. The sediment particles found in any given area are primarily due to the effects of sediment movements caused by wave action and water currents. Sediment density has some effect on what is present, but basically for any given sediment, finer particles will be found in areas with less water movement. Consequently, the pattern of sediments surrounding a coral islet that is one part of a coral atoll will be a complex tapestry of sediment sizes. In general, coarser sediments will be found in areas of higher current flow and wave action while finer sediments will predominate in areas of less kinetic energy. The absolute position of the sediments will often vary from season to season, particularly in intertidal and shallow subtidal areas. Tourists who always visit a given resort at a particular time of year are often quite amazed when they return to the resort six months out of sync with their usual pattern and find the sandy beach they expect to see has vanished, leaving a hard coral pavement instead. Movement of sediments is less in deeper waters but it still occurs. In fact, one major characteristic of natural sand beds is their mobility.
Sediments in any one spot may be characterized by several discrete parameters. The first parameter is the average, or mean, sediment particle size. The second factor is the shape of the cumulative sediment particle size distribution; if a sample of the sediment is taken and the diameters of the sediment particles measured, the resulting graph will be a bell or "normal" curve centered around the average particle size. How that bell curve deviates from an ideal statistical bell curve reveals a lot about the sediments. For example, at the extremes, the curve may be low, broad, and flat or quite narrow and high. In the former case, it indicates a wide variety of sediment particles in each sample which, in turn, indicates lesser effects due to waves or currents. In the latter case, the sediments will be almost all of the same size, indicating a lot of movement of sediment particles by wave action and the resulting "sorting" of them by size. Well-sorted sediments are quite characteristic of areas with high currents or strong wave action, while poorly sorted sediments with a wide variety of particle sizes are characteristic of calmer waters. The third important parameter is the amount of organic material found in the sediments. I have worked in areas where the organic content was effectively zero. At the other extreme, I have sampled some areas where the organic content of non-polluted sediments was as high as about twenty percent by weight. In polluted areas the organic content may be even higher.
The size distribution of sediments in any marine soft sediment area is critical to the determination of the organisms living in those sediments. Organisms live on, and between sand grains, and the mixture of the sizes of the grains is critical. Sand grains of inappropriate sizes may be too big to move or, conversely, too small to be stable. Additionally, the mixture of the various sizes determines the ease with which water moves through the sediments.
Here are a couple of links showing bacteria on individual sediments:
Ocean Explorer
http://www.rnw.nl/science/assets/images/020903bacteria.jpg
(GO TO PART 2)
(PART-1)
Sand beds, essentially layers of sand of various thicknesses and arrangements, have come to be common fixtures in marine reef aquaria. The use of these sand beds has been correlated with significantly increased survivability of many organisms in reef aquaria, particularly when compared to the bare bottom tank arrangements that were in the vogue about a decade ago. Nevertheless, few hobbyists seem to realize why sand beds should contribute to the success of their tanks, and fewer yet seem to understand how those beds work.
Our aquaria are artificial ecosystems, or microcosms, representative of the real reef environment. There can be no doubt; our systems mimic relatively well many of the processes occurring in the natural world and, when populated by an appropriate group of organisms, many of the interactions occurring in natural habitats occur in aquarium systems. The approach of dealing with aquaria as artificial ecosystems has been criticized primarily on the basis that reef aquaria are patently artificial. However, such criticisms are both rather silly and quite wrong. Reef aquarium systems have to be quite good approximations of the real world, otherwise those animals that are kept in them would not be doing as well as they are. The organisms don't know they are not in the natural environment and any coral reef organism has evolved to deal with an environment that has as its limits those same limits as are found on real reefs.
The fact that we can deal with aquaria as good mimics of the real thing allows us to use "real world" or "scientific" data both to troubleshoot problems and to advance the techniques of animal husbandry. As we know that organisms must live within the ranges of their tolerances, we can recognize that problems will occur when something in an aquarium is far outside that range, such as the excessively high concentrations of poisonous heavy metals found in some artificial sea water mixes . Once that recognition is made, we can adjust for the problem and proceed. In this way, we may incrementally increase our understanding of the animals and what we need to do to maintain them.
If aquaria are artificial ecosystems, however, the component that is least artificial is the sand bed. This part of a reef aquarium, with little input from the aquarist, functions much as do the sandy areas near a real reef. That functionality is due to a rather complex interaction of physical and biological factors, but most of those interactions are unseen, and, I think, unappreciated by the average aquarist. Without those interactions our reef aquaria would simply fail. The fact that they don't fail is a tribute to the ease of constructing this one major functional analogue to an extremely critical coral reef community.
-Sediments In Reefs
Sand beds are constructed physically of sands, and sand is defined as unconsolidated sediments made of particles between one sixteenth of a millimeter and two millimeters in diameter. Coarser sediments are referred to as gravel, finer ones as silts and clays (Holme and McIntyre, 1984). Of course, in the real world there is a continuum of sizes found in these environments, and the sediments actually found in any one spot reflect not only the geological and biologic history of the area, but also the hydrographic regime of the area. In other words, what is present is the result of what is available that hasn't been washed away by the waves.
Sands surrounding natural coral reefs may be made of a number of substances. Around volcanic islands there are often regions of volcanic lava sand. Volcanic islands are the basis for most coral atolls and many fringing reefs, so lava sands are commonly found around reefs in nature. Coral reefs located near areas of river mouths or extensive areas of runoff often are surrounded by silica sands or fine sediments of other upland or inland sources. Of course, coral reefs may be surrounded by calcareous sands resulting from the breakdown of corals and other calcifying organisms. Calcareous sands may also be formed by the precipitation of particulate calcium carbonate in coral lagoons, one of the natural sources of oolitic sand. Calcareous sands may also be formed from the skeletal breakdown of other organisms, such as foraminiferans, bivalves, calcifying algae, or barnacles.
The chemical composition of the sands has a small effect on the organisms found in the sands, but that effect is minor compared to the effects due to differences in sediment particle size distribution. The sediment particles found in any given area are primarily due to the effects of sediment movements caused by wave action and water currents. Sediment density has some effect on what is present, but basically for any given sediment, finer particles will be found in areas with less water movement. Consequently, the pattern of sediments surrounding a coral islet that is one part of a coral atoll will be a complex tapestry of sediment sizes. In general, coarser sediments will be found in areas of higher current flow and wave action while finer sediments will predominate in areas of less kinetic energy. The absolute position of the sediments will often vary from season to season, particularly in intertidal and shallow subtidal areas. Tourists who always visit a given resort at a particular time of year are often quite amazed when they return to the resort six months out of sync with their usual pattern and find the sandy beach they expect to see has vanished, leaving a hard coral pavement instead. Movement of sediments is less in deeper waters but it still occurs. In fact, one major characteristic of natural sand beds is their mobility.
Sediments in any one spot may be characterized by several discrete parameters. The first parameter is the average, or mean, sediment particle size. The second factor is the shape of the cumulative sediment particle size distribution; if a sample of the sediment is taken and the diameters of the sediment particles measured, the resulting graph will be a bell or "normal" curve centered around the average particle size. How that bell curve deviates from an ideal statistical bell curve reveals a lot about the sediments. For example, at the extremes, the curve may be low, broad, and flat or quite narrow and high. In the former case, it indicates a wide variety of sediment particles in each sample which, in turn, indicates lesser effects due to waves or currents. In the latter case, the sediments will be almost all of the same size, indicating a lot of movement of sediment particles by wave action and the resulting "sorting" of them by size. Well-sorted sediments are quite characteristic of areas with high currents or strong wave action, while poorly sorted sediments with a wide variety of particle sizes are characteristic of calmer waters. The third important parameter is the amount of organic material found in the sediments. I have worked in areas where the organic content was effectively zero. At the other extreme, I have sampled some areas where the organic content of non-polluted sediments was as high as about twenty percent by weight. In polluted areas the organic content may be even higher.
The size distribution of sediments in any marine soft sediment area is critical to the determination of the organisms living in those sediments. Organisms live on, and between sand grains, and the mixture of the sizes of the grains is critical. Sand grains of inappropriate sizes may be too big to move or, conversely, too small to be stable. Additionally, the mixture of the various sizes determines the ease with which water moves through the sediments.
Here are a couple of links showing bacteria on individual sediments:
Ocean Explorer
http://www.rnw.nl/science/assets/images/020903bacteria.jpg
(GO TO PART 2)