Dr. Rob Toonen

Section of Evolution and Ecology

University of California, Davis

Davis, CA 95616

Part 1 previously printed in the January 2000 issue of FAMA.

In a recent issue of FAMA there was an article entitled "Are Plenums Obsolete?" (Cohen 1999). I was glad to see that title, and expected to read an article suggesting that a plenum (which is essentially nothing more than an empty space screened-off beneath the gravel or sandbed in an aquarium) is not a necessary component of a successful natural nitrate reduction (NNR)- based reef tank. Upon reading the article, however, I was disappointed to discover it was little more than a product review for a new filter medium. I'm not saying that there is anything wrong with the product, but only that I had expected to see some review of the writings of biologists like myself, Ron Shimek, Eric Borneman & Jonathon Lowrie, who have been trying to point out that many of the most successful hobbyists base their systems on trying to emulate natural conditions found in the areas from which their animals originate. There have been a number of articles written about live sand in virtually every major aquarium magazine over the past couple of years, and the virtues and evils of a plenum have been vigorously debated by both sides of the dispute (in person, if not in print). I expected that someone had finally noticed that there are an active group of people who suggest that all the reported benefits of a plenum (and more) can be had simply by the inclusion of a deep sandbed in the aquarium or sump, and that the added complications of screening off a void space beneath those sediments is not only unnecessary, but may negatively impact the performance of the sediment system (relative to nature).

The article "Are Plenums Obsolete?" starts off much as I would start this one - by pointing out that there is no real agreement (even among the "experts") on the ideal conditions for establishing and maintaining a plenum system. There are also some questions regarding the efficiency and mechanism of action for these systems, because the void space of a plenum lacks a substrate on which the desirable bacteria can grow (other than the walls of the aquarium and the flocculant debris that filters into the void). The successful establishment of a so-called "Jaubert-style" system is further complicated by a number of difficult decisions you must make in establishing a plenum in any system, including: the volume of the plenum and size of the mesh used to separate it from the substrate, the depth, particle size and composition of the substrate used, and the correct order of sand/rock/livestock introduction as well as the proper "break-in" period of the tank. Finally, there are arguments made about storage of nutrients and the dangers of their release back into the aquarium, and a variety of other dangers that are supposed to be associated with the decision to include a plenum in the aquarium (some are reasonable, many more are not). I don't disagree with any of those points, but I would go one step further and point out that for every rule in biology, there is an exception. What I mean by that statement is that there will always be someone who by luck, skill, or sheer stubbornness will succeed in keeping a tank that, by all reasonable logic, should fail miserably; conversely, there will be the unfortunate soul who does everything "right" but still can't keep make their tank work. I'm sure that anyone who has been in the hobby for a long time will concede that as truth.

It was at this point that I part ways with the author of the previous article. I expected that given his introduction to that article, the next step would be to point out that in nature, the recycling of nutrients (such as nitrogenous wastes released by fish and corals) is accomplished largely by the seagrass beds and mangrove lagoons typically adjacent to reef areas (Lowrie & Borneman 1998, Toonen 1998-99). Lowrie and Borneman (1998) provide this quote from Ogden (1988) to illustrate the importance of interaction between coral reefs and the adjacent communities to the existence and persistence of coral reefs: "Mangrove and seagrass systems are sinks, trapping and accumulating organic and inorganic material and permitting the growth of coral reefs offshore (while) coral reefs buffer the physical influence of the ocean and permit the development...of lagoon and sedimentary environments suitable for mangroves and seagrasses."

Coral reefs and the nearby seagrass and mangrove habitats are closely linked ecosystems, and are largely dependant upon one another. As aquarists this fact should be of particular interest, because we can benefit in our efforts by taking advantage of both the processes that occur in the sea-floor sediments common to these adjacent habitats as well as those that take place on the reefs. The importance of sediments is often downplayed by aquarists, and several well-known authors have gone so far as to suggest that inclusion of sediments in the aquarium dooms a system to ultimate failure. Such statements fly in the face of basic scientific research that suggests sediment processes are not only beneficial to the existence of coral reefs, but may play a large part in the formation of the reefs themselves. In fact, Kinsey (1985) states that "gross production and calcification in coral reefs are, nevertheless, clearly dominated by benthic processes." Benthic (associated with the seafloor) marine organisms comprise a broad assemblage of diverse forms that are grouped together not by taxonomic relatedness, but rather on the basis of where they live. These benthic organisms are directly or indirectly involved in most of the physical and chemical processes that occur in marine sediments, and are therefore an essential part of the healthy function of coral reef ecosystems (Kinsey 1985, Day et al. 1989). If that is true (and the number of research papers in the scientific journals that support this assertion is enormous), then how can it be that the inclusion of an "essential part of the healthy function of coral reef ecosystems" dooms an aquarium?

In this article, I want to summarize the basic components of the sediment-rich habitats adjacent to coral reefs and the function that these systems play in nature and in aquaria. The fine sediments in these near-reef communities are predominantly carbonate rubble derived from (depending on the location) coral skeletons (often via grazing by parrotfishes), coralline and calcareous algae (e.g., Halimeda, Penicillus, Udotea, and Rhipocephalus), molluscs, foraminiferans (unicellular protozoans related to amoeba) and diatoms. These sand-flat areas look barren and uninteresting to most, but these areas are really hot-beds of biological activity. If one were to magically remove all the sediment from the sea-floor in one of these areas, while 'freezing' the animals that lived within them in place, you would still be able to see the form and structure of the region maintained in the mass of live animals left behind. If you were to take a small sample of sediment (say a coffee can-sized piece of the sea floor) from a sheltered sea grass bed behind a reef, and run it through a sieve (as many marine biologists have done), you would likely find 1 or 2 (if any) large animals (1 - 10 cm in length), and dozens to perhaps a hundred small animals (0.1 - 1.0 cm). Despite their abundance, these animals would still be a tiny fraction of the total number of animals in the sand, however. Thousands or perhaps tens of thousands of even smaller animals would have just passed through your sieve (primarily these animals would be nematodes, but some rotifers, and other, more obscure forms such as kinorhynchs and tardigrades could also be present). These animals are extremely common, and are almost certainly present in every reef tank, but few people have heard of these groups and even fewer have ever seen one of the animals. Even when all these groups are combined together, those animals would account for only a tiny fraction of the total number of live organisms in the sediment sample, because the protozoans and bacteria covering every sand grain and living in the water film around individual sand grains would number in the millions to billions. The biology of these unfamiliar animals is covered in greater detail in a variety of articles (e.g., Ron Shimek's monthly "Without a Backbone" column in Aquarium Frontiers and my "Reefkeepers Guide to Invertebrate Zoology" column in Aquarium.Net), or can be easily located in any introductory Invertebrate Zoology text from a library.

The action of these animals can dramatically affect the habitats in which they live, and may also positively or negatively affect one another. For example, oysters clear the water through filter-feeding, and stabilize the substrate by building "reefs" of solid shells; burrowing shrimps (such as the Thallasinid shrimp Upogebia and Callianassa) increase turbidity (a description of the suspended particle content and "cloudiness" of water) through their excavation activities, and "soften" the sediments by continually turning them over and depositing feces around their burrows (which other organisms can ingest). Obviously, the predominance of one or the other of the organisms in a given area will influence the suitability of that habitat for a variety of other benthic animals.

The importance of various components of the sediment community and the role each member plays in the chemical and biological cycling of nutrients in the sea are the focus of numerous reviews and a variety of books covering different aspects of sediment ecology in detail. It would be impossible to summarize current knowledge of nutrient cycling or even biological activities of sediment-dwelling animals in any short article. Instead, I am going to try to provide a thumb-nail sketch of the factors that should be considered, and the characteristics of various habitats where efficient nutrient processing takes place (because I think that is what is required for a hobbyist to make an informed decision regarding the utility and design of a sandbed for the aquaria). This article is based on a more detailed and more thoroughly referenced one written as a three part series for the Breeders Registry (Toonen 1998-99). Sam Gamble has also written a series of articles on sediment function (Gamble 1996-98), and other articles on the structure and function of sediment communities that I highly recommend for interested readers are the Lowrie & Borneman article mentioned above (1998) and an IRC #Reefs talk given by Ron Shimek (Shimek 1998a).

When I discuss sand-dwelling organisms or sediment communities herein, I am referring to the community as a whole, including fungi, bacteria, protozoans, blue-green algae (cyanobacteria), algae (in which I include diatoms, dinoflagellates, etc.), nematodes, polychaetes, rotifers, micro-crustaceans (such as copepods, amphipods, isopods and ostracods), molluscs, and all the other interesting and obscure organisms that inhabit marine sediments. Regardless of the taxonomic affinity of the critters in marine sediments, there are a variety of physical, chemical and biological factors that play an important role in determining the abundance and species composition of these communities, and I will try to summarize some of the most important ones here.

Physical regulatory factors:

There are a variety of physical factors that can weigh heavily in determining the composition and abundance of infaunal communities. One of the most important factors in regulating photosynthetic organisms is, of course, the availability and quality of light. The quality and quantity of light not only affects photosynthesis directly, but may affect non-photosynthetic organisms indirectly, as well. For example, oxygen production from photosynthesis depends on light penetration, and is partially responsible for determining the depth of oxygen penetration of sediments. Thus, the predominance of aerobic (occuring in oxidized environments) or anaerobic/anoxic (occurring in reduced environments) metabolism may be due, in part, to the indirect effects of light penetration into the sediments.

Aside from light, temperature also plays an important role in determining the structure of these communities and the rates at which metabolic processes occur. It is intuitive that as temperature increases, so does the rate of metabolic processes, until the thermal limit of the organism is exceeded, at which point metabolism usually shuts down (and the animals die). The temperature of sediments can impact not only the communities of organisms which are present, but can also change the chemical pathways that are most efficient for a variety of biological processes. For example, Nedwell and Floodgate (1972) showed that in the sediments they were studying, the majority of sulfide production originated from sulfate if the temperature was above 10°C, but from organic sulfur at colder temperatures. These were the same sediments with the same organisms contained within them. It is worth noting that sediments collected from sites at which temperatures are very different than those within a tank may produce unexpected or perhaps even unwanted results not simply because the animals are incapable of surviving (which is frequently the case), but also because the conditions under which those organisms are kept may favor a different chemical pathway.

Physical disturbance (whether in the form of bioturbation or actual disturbance) is also important in determining the diversity and abundance of benthic organisms. There is a bell-shaped relationship between disturbance and animal diversity in many environments - what I mean by that is in the absence of any disturbance overall diversity is reduced, as it is in the presence of high disturbance. In general, with turnover, the microbial communities are the most negatively impacted, thereby reducing the bacterial effect of the sediment compared to what it would be if undisturbed. However, bacteria take up nutrients at the highest rate when actively growing, and some minuscule amount of disturbance allowing such growth is good (Shimek 1998b). The activity of the small animals living within the sediments causes a significant amount of disturbance (Shimek 1998b), and this is beneficial to keeping the sandbed in a constant state of growth, during which the uptake of nutrients is maximized. Adding to that disturbance by incorporating large "sand-stirring" animals or otherwise disturbing the sediments tends to push the relationship "over the hump" and diversity within the sediments becomes reduced. Thus, the common recommendations to to add "substrate sifting" organisms (such as gobies, horseshoe crabs, and sea stars) to "clean" the substrate by removing excess organic matter also results in a decrease in bacterial substrate, a removal of burrowing infauna (which are the justification for the cost of live sand in the first place), and a general decrease in the effectiveness of the live sand. Likewise, siphoning, stirring or otherwise disturbing the sediments in an effort to "clean" them is misguided and ultimately counter-productive. The normal compliment of sediment infauna (I'll discuss these in more detail below) coupled with a few low-impact bioturbators, such as stichopod sea cucumbers (e.g., the "Tiger-tail" sea cucumber, Holothuria sp.), polychaete worms (e.g., spaghetti and other "bristleworms"), certain brittle stars (e.g., Ophioderma or Ophiocoma spp.), or some "mud snails" (e.g., ceriths or even a small conch) is all that is needed for the sandbed to function at full capacity.

Water flow and quality are "undoubtedly two of the most important, and yet least studied, factors that regulate the activities of microbes" (Day et al. 1989). Within the heading "water quality" are included all chemical parameters of seawater composition, including salinity, oxygen content, nutrient content, and so on (we'll cover these under the next category: chemical factors). Many of these factors may be interrelated (such as salinity, temperature and oxygen content), and these relationships often complicate our understanding of these systems.
High energy environments, such as wave-swept beaches, tend to have few species of sand-swelling organisms, whereas low energy environments, such as sea grass beds, tend to have many (Table 1). See Table 2 for a description of the actual particle sizes of the "Predominant sediment type" in Table 1.

Aside from the "energy level" of the environment in which sediments exist, the exchange of nutrients between the water column and the sediments, and the degree to which dissolved nutrients can be acted upon by sediment-dwelling organisms depends on the velocity of the ambient water and the contact time of that water mass with the sediments. In our tanks, contact time is of little concern because the systems are closed, and water recirculates over the sand bed repeatedly. However, the velocity of the water flow across the sediments will still affect the suspension of sediments, the physical forces which organisms inhabiting the sediments must deal with, and the efficiency of nutrient transfer between the water and the sand bed. The movement of particles is more-or-less linearly related to water velocity, and it is only at speeds below about 20 cm per second that fine to medium-grained sands (0.1 - 0.5 mm particle sizes) will remain unsuspended (Davis 1983). At large grain sizes, the water velocity required to move particles obviously increases, but surprisingly, the same can happen at much finer grain sizes: consolidated (packed and bacterially clumped) silts and clays may be as difficult to resuspend as are gravel and boulders (Davis 1983). In our tanks, however, we are aiming for non-consolidated sediments, and thus the flow rates required by your animals must also figure in to your calculation of ideal grain sizes for a sand bed, if you are to include one.

Finally, the texture, composition, and grain size of the sediments can have perhaps the most dramatic effect of all on the communities living within them. Of course, the distribution of particle sizes of sediments is not independent of other factors, such as the "energy" level of the system, but all things being equal, coarse-grained sediments tend to have both reduced populations and fewer species than do fine-grained sediments (Fenchel 1967, Dale 1974). There are a variety of potential explanations for this, first coarse-grained sediments tend to occur in high energy systems, which tend to have smaller reserves of organic and inorganic nutrients than do sediments rich in silts and clays, which contain more water than an equivalent volume of coarse-grained sediments (see Table 2). All of these factors are, of course, potentially interrelated, but the bottom line is that collodal sediments contain (on average) 68 times as many bacteria per unit volume as do fine sands.

These relationships have a variety of other consequences on other physical factors including decreased light and oxygen penetration in finer sediments (thereby decreasing the depth at which anaerobic activities replace aerobic ones). In coarser sediments, there is increased speed of water passage, decreased habitat stability and decreased time to desiccation (drying out when not submerged) as physical consequences of sediment particle size.

Chemical regulatory factors:

It is impossible to discuss the role of a sand bed in the aquarium without covering some basic chemistry, because much of the function we ask our sand bed to perform is involved in nutrient cycling. Most nutrient cycling is accomplished by the bacteria inhabiting the sediments, but habitats differ in both the demand for and the ability to fix nitrogen (Table 3), and the relative rates of nitrification and denitrification (Table 4).

The quantity and quality of various organic and inorganic nutrients obviously play an important role in determining the composition of microbial communities. Perhaps the most obvious and important of these is oxygen. Despite the active reworking of the sediments by a variety of biological and physical agents, the abundance of microbes and infauna, coupled with the high metabolic activity of organic particles typically leads to high oxygen consumption. Anaerobic (reducing) conditions usually exist close to the surface (in most cases less than 10 cm deep), and in regions of increasingly lower oxygen, first facultative and then obligate anaerobes dominate. Facultative bacteria can use oxygen if available, but do not require it, while obligate anaerobic bacteria cannot survive in the presence of oxygen. In typical estuarine and seagrass sediments, available oxygen drops from roughly 12 mg/l in the water column (Eh ~ 400 - 600 mV, for those of you who measure redox potentials) to about 9-10 mg/l (Eh ~ 300-325) at 1-3 cm depth, and to less than 5 mg/l (Eh ~ 200 mV) at 4-6 cm depth (Levinton 1982). Once available oxygen declines to less than about 4 mg/l (Eh ~ 150 mV), anaerobic metabolism (facultative) begins to dominate. When all available oxygen is depleted (Eh = 0) , the production of hydrogen sulfide increases rapidly with depth (Eh << 0) (Levinton 1982). At this point, there is usually an obvious change in sediment color, and the light brownish or beige oxidized layer gives way to an oxygen depleted gray layer which is, in turn, replaced by a black, anoxic reduced layer. The obvious line separating the upper oxygen-containing layers (Eh > 0) and the deeper oxygen-free layers (Eh < 0) is referred to as the redox discontinuity layer (or RDL, see Fig. 1B). This rapid reduction in available oxygen levels is often not related to diffusion of oxygen per se, but rather the available oxygen is consumed at a rate greater than it is able to diffuse within the sediments. This leads to low oxygen sediments frequently laying within a centimeter or two of the sediment surface, despite ample diffusion from highly oxygenated water above it (Day et al. 1989). It is these low oxygen sediments (in which nitrate - NO3 - is the preferred electron acceptor) that are most significant in terms of our inclusion of a sandbed in our tanks (I'll come back to this below).

So what exactly is all this "redox potential" stuff about? Well, technically, redox potential (Eh) is a measure of the energy gained in transferring 1 mole of electrons to an oxidant (such as oxygen, nitrate, sulfate or methane) from hydrogen gas. That probably doesn't help you much, does it? Well, in order for any organism to survive it needs energy, right? We get energy by "burning" (oxidizing) ATP (remember that "powerhouse of the cell" class in high school biology?). That means that we break down sugars, using oxygen (O2) , and release carbon dioxide (CO2) as a waste product of that redox (reduction/oxidation)
reaction. We use the energy of transferring an electron to fuel our bodies, but because there are really no free electrons running around in the environment, for every reduction there is a corresponding oxidation (hence the term redox). The easiest way to think of redox potential is a measure of the relative availability of electrons (and their receptors) in a particular chemical environment (just like pH is a relative measure of the availability of hydrogen ions). Redox potential is standardized using oxygen (just as pH uses pure water as the standard for its "neutral" measurement of 7.0), and is highly positive in highly oxidized environments, low but positive in oxygen poor environments, and zero or negative in environments lacking oxygen (see Fig. 1B). Available oxygen is controlled by a variety of factors including salinity and temperature, in addition to the biological processes in which we are primarily interested in our tanks (see Box 1).

Why do we care about these sort of details? Well, because "anoxia is, arguably, the most important condition of effective decomposition and denitrification" (Lowrie & Borneman 1998), and an essential component of the natural function of fine sediment communities in near-reef habitats. Although there are a variety of metabolic pathways by which organisms can satisfy their energy and carbon needs, the decomposition of organic detritus is primarily accomplished by only a couple functional groups of anaerobic bacteria. These catabolic (organic breakdown) pathways are fermentation, dissimilatory nitrogenous oxide reduction, dissimilatory sulfate reduction & methanogenesis (see Fig 1A.). Each of these pathways can be considered to be associated with a "functional" group of bacteria that collectively complete that particular biochemical process. The complex array of organic compounds available for decomposition can, in general, be used only by the fermenters and dissimilatory nitrogenous oxide reducers initially, but once the organic molecules are broken down into small molecular-weight decompositional products by these bacteria, they become available to the methanogens and sulfate reducers (Fig. 1A). Obviously this is a gross oversimplification of a very complex subject, but the topic is beyond the scope of this article - perhaps someone else will take up the torch and write a follow-up article on catabolic metabolism.

These populations of bacteria are rarely limited by the availability of organic detritus in natural sediments, but rather by the availability of suitable electron receptors (Martens & Berner 1974, Sorensen 1984). This means that the bacterial communities in these sediments should be capable of processing more detritus in the presence of increased concentrations of electron receptors (such as nitrite, nitrate and sulfate). Therefore, these groups of bacteria provide a double service to the aquarist: they consume organic detritus and perform nitrite, nitrate and sulfate reduction at the same time. What more could one ask!?

Aside from biotic assimilation, many ions may be adsorbed on the surface of marine sediments, or they may flocculate or precipitate. Direct adsorption of dissolved ions to silty sediments is particularly important for highly charged anions such as phosphate and iron. The binding of these nutrients to sediments makes it more difficult for animals and plants to assimilate, thereby reducing the need to remove these compounds from the tank through chemical means. Marine sediments tend to bind a variety of undesirable nutrients in salt water, and these include Fe and PO4 (as mentioned above), as well as Mn, Al, and Si (MacKay & Leatherland 1976).

Studies of nutrient cycling in marine sediments demonstrate that labile (readily useable) compounds may turn over in a matter of hours in the water column, but within minutes in sediments. Refractory (difficult to use) compounds, such as cellulose and lignin, on the other hand, may take months or years to turn over, even in sediments. Obviously, therefore, most compounds found within sediment cores are refractory, because the vast majority of labile compounds (such as ammonia, nitrite & nitrate) are quickly taken up by living organisms.

The breakdown of refractory compounds is complex, and it remains unclear whether benthic marine microbial communities derive most of their nutritional requirements from the small proportion of labile organic matter available or from the slow decomposition of the more abundant refractory compounds (Day et al. 1989). Included in the refractory compounds, of course, are the organic acids (humic, tannic, etc.). These refractory compounds are slow to break down, but microbial decomposition still takes place, and measurements of the uptake of these decomposition end-products released by microbes indicated that it was comparatively slow, but complete (Day et al. 1989). The nutrients released by the decomposition of these refractory compounds was immediately taken up by subsurface algae, bacteria and phytoplankton. The most common genera of bacteria involved in these reactions include E. coli, Vibrio, Desulfotomaculum, Desulfovibrio, Aeromonas, Thiobacillus, Ferrobacillus, Beggiota, Chromalium, Clostridium, Pseudomonas, Thiothrix and Chlorobium (Day et al. 1989). The uptake of these nutrients is considerable, and contrary to popular thought, the highest rates of nitrate removal are not by denitrifying bacteria deep in the sandbed, but rather by this aerobic subsurface community (Oren & Blackburn 1979).

It is clear that because only a select few prokaryotic organisms (true bacteria and cyanobacteria, or "blue-green algae" are prokaryotic because their cells lack organelles which all eukaryotic organisms possess) are capable of "fixing" N2 gas into the inorganic salt ammonium, nitrogen is often a limiting factor for growth of plants and algae. Nitrogen-fixation is an energy intensive anaerobic reaction, which is "poisoned" by the presence of oxygen (oxygen inactivates the enzyme nitrogenase which is used to convert N2 + 3H2 è 2NH3). If blue-green algae can fix nitrogen, but that reaction must be anaerobic, you may wonder how they accomplish that feat. Many blue-green algae have special packets inside them (called heterocysts) within which they can maintain anoxic conditions and allow nitrogen fixation to occur, despite the presence of oxygen in the surrounding cellular medium. In the few aquatic systems within which it has been measured, nitrogen fixation by bacteria and blue-green algae constitutes a major component of the total annual nitrogen input for the entire system, commonly contributing 30-80% of the overall annual nitrogen budget of the system (Mitchell 1978). The majority of this nitrogen-fixation occurs in the sub-surface benthos, in biofilms (coatings on submerged structures such as rocks), and attached to suspended particulate organic matter (aka "marine snow"). These blue-green algae are an integral part of the sediment community, and will (should) be found in any live sand shipment.

The presence of this important component of a functional sandbed does not imply that you are doomed to continual outbreaks of dreaded "red slime" in your tank, however. As with any other closed system, it is only when nutrients get out of control that such outbreaks tend to occur. In the absence of nitrogen limitation, blue-greens typically find themselves iron or phosphorous limited instead. Phosphorous additions (without adding any nitrogen at all) have resulted in 5-10 fold increases in the rate of N2 fixation in natural systems (Lean et al. 1978). Similarly, doubling available iron levels in a couple of large scale experiments in the early 1990's (called IronEx I and II) resulted in phytoplankton and cyanobacterial blooms in the Southern Ocean.

Thus, as an aside, if you are suffering from an algal bloom, look to the sources of excess nutrients in your tank as a first step in bringing these outbreaks under control (rather than dosing with antibiotics for red-slime or other ultimately counter-productive treatments of the symptoms rather than the causes of the problem). A test kit reading of zero for the presence of these nutrients does not mean that they are absent from the tank. You may wonder how that can be, but remember these organisms can be very efficient at removing required nutrients from the system, and if they are growing quickly, they can strip nutrients as quickly as they are added to the system (the turnover time for these nutrient in the sediments is a matter of minutes, remember). Unless you happen to catch a huge release within minutes of the event, or the uptake of the nutrients is less than the input (which is rare unless there are other problems with your aquarium), your test kit will continue to read zero, but the algae will continue to thrive.

The ability of organisms to uptake nutrients depends on something known as it's saturation coefficient (Ks). When Ks is low, relative to competitors, that organism has a "greater affinity" for the nutrient and will generally have a competitive advantage over the others when that nutrient is limiting. However, with higher Ks values, the organisms can take advantage of higher levels of nutrients because they saturate at much higher nutrient levels (Table 5). In general macrophytic (multicellular) and vascular plants tend to have much higher Ks values than do microalgae and blue-greens. This varies by group, and it is only at nutrient concentrations below the Ks value (µM concentrations) that nutrient uptake can limit primary productivity and growth. Thus, you may be repeatedly testing your water and getting zero readings, but if you are adding PO4 daily (with food, top-off water, or even detergent left on your hands), the cyanobacteria will still be able to take it up and grow.

A number of studies of natural nutrient regeneration levels during decomposition of organic detritus have shown that NH4, SiO2 and PO4 are released by decaying phytoplankton in the approximate proportions of 16:16:1 by atom (Redfield et al. 1963, D'Elia et al. 1983). Ideally, these nutrients are not released in excess, but are rather taken up by tank inhabitants in the same proportions (known as the Redfield ratio) as quickly as they are regenerated. It is the excess addition or selective removal of one nutrient or another (thereby changing the relative ratios of these nutrients) that tends to lead to problems in terms of nutrient loading. Hence if you find your aquarium suffering from the uncontrolled growth of some "pest" species, you should first look to control the sources of its nutrition (for example many supplements contain iron, which causes algal blooms when added to natural seawater) rather than try to eradicate the problem with treatments that temporarily remove the symptoms of problem rather than its cause.

In the next issue I will finish my introduction to the factors that control the decomposition of organic detritus in marine sediments, and describe the biological community (the critters) associated with these habitats. I will explain where organisms are found in marine habitats and why, and will provide some concrete suggestions for aquarists to take advantage of these systems in the home aquarium.

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