Even the tiniest sea animals, the zooplankton, apparently excrete what is termed fecal pellets (which sink). One underlying principle in aquatic nutrient recycling seems to be: minimize the liquid input, as much as possible keep edibles in a solid form. This may be a reflection of the low tolerance for liquid nutrient input in living aquatic ecosystems. It appears that fish excrete ammonia and urea in liquid form, both are forms of fixed N that are very easily available to the phytoplankton, and are therefore perhaps intended for their d the remainder of the fishes food waste is packaged in a solid. This strategy may be an important part of the overall design to preserve the balance between the many organisms living in the system. The consumption of anothers feces, (a concept most repulsive to us is a routine part of nutrient cycling in nature.
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Admiring the fish in an aquarium elegant store recently, i became very interested in the question: why do fish excrete solid feces? Not a terribly extensive survey to be sure, but all of the fish that i observed, both the freshwater and salt water species, were defecating what can only be described as formed stool, which was sinking to the bottom of the tanks. Now why would this be the case? Land animals resorb the water from their feces before excretion presumably because we are conserving water. But for the purposes of digestion in all animals, the way to efficiently absorb nutrients from ones food is to convert it to liquid form in the gut. If fish do not have a great need to conserve water, why dont they just excrete their feces in liquid form? After all, that would be the quickest burger way to send the nutrients back to the bottom of the web to be recycled. If they did, the nutrients would be better dispersed throughout the water column and more likely to be quickly taken up in the process of photosynthesis. But instead, fish excrete a solid stool that sinks. By keeping the waste product in a solid form, the nutrients remain more available for consumption by a wider range of scavengers, and relatively less available to the phytoplankton.
It seems not, considering the summary current depleted state of life on many ny will say thats only because they have been t thats only a term referring to recent fishing intensity, a more credible argument may be that all-fishing, over the recent centuries of human. Its because we are the unnatural predator, the one who takes but does not give back (anything useful).and that is part of the essence of the whole problem with disappearing marine life today. Mass coral bleaching (death of clean water corals by food starvation during warm spells) is a truly new and ominous phenomenon, and its most likely to be merely the ultimate end result of all the fishing that humans have done in the tropics. Regarding the importance of the form (solid. Liquid) in which nutrients exist in the coral reef system, and indeed in all marine ecosystems, i would like to elaborate on this point a bit further. It seems that fish assimilate on average about 10 of the energy and nutrients in the food that they consume, and the remaining 90 they excrete rather promptly back into the environment. In discussions of this point with marine biologists, one gets the impression that the 90 thats not assimilated is shunted back to the bottom of the food web for recycling, in effect its transformed into the equivalent of human sewage. But this seems to be somewhat less than accurate, because when our sewage hits the ocean its essentially liquid.
The other obvious mechanism of nutrient database loss from tropical ecosystems is fishing. The removal of fish from these systems by human fishing most likely represents the biggest net loss of nutrients. Large quantities of solid nutrients. The argument offered in the temperate zones that the food web can always replace the fish that have been removed because the ocean contains a vast pool of bioavailable nitrogen. Well, that particular line of reasoning will obviously not go far in the tropics, since it is essentially referring to some great reserve of dissolved (liquid, non-living) nutrients. It is clear that the tropical fish were a major fraction of the nutrient pool themselves. Is it likely or possible that the n fixing activity of the blue-green algae can keep pace with human fishing removals? If so, they would need to pick up the the pace very significantly, and fix n at a greater rate than they did during the pre-fishing millenia of the reefs existence. Have they done this?
As the previous discussion has (possibly) made clear, most of the normal or naturally acceptable forms of nutrient input to reef systems are solids. The most acceptable (and significant) form of terrestrial-source nutrient input to the coral reef systems arrives in the form of living, swimming fish that have ingested nutrients nearshore, grown larger and then migrated off to the reef. The fixed N contributed by the blue-green algae enters into the web to a large extent via fish feces, or by organisms eating the solid algae mats directly. The oceanic plankton captured basically also represents tiny solid forms, dissolved nutrients playing only a very minor role in the larger input picture of the reef. What about natural losses of nutrients from coral reef systems? Some nutrients are inevitably swept away in the seawater, and some will undergo denitrification in the seabed. Accurate quantification of the amount of fixed N lost by the system in these ways is really not possible with the state of todays knowledge. Therefore, the annual amount of extra fixed N/protein in the reef system - the amount that could safely be removed without diminishing the system overall - is unknown, but seems unlikely to be a very great amount.
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And the fish flesh type of wallpaper nutrient enrichment seems to be quite acceptable to the reef oligotrophic systems least tolerant of nutrient enrichment isnt precise enough, its only liquid forms of nutrient enrichment that are intolerable. In contrast, the solid, swimming fish form of nutrients has traditionally been very well tolerated by the reefs. One other natural route of net nutrient input into these tropical ecosytems may once have been significant. Large migratory animals, like the great whales, that travel long distances to the temperate zones to feed; these animals spend part of the year in the tropics. Not feeding, but subsisting on stored energy from their northern excursions, the whales are in the warm water to give birth and nurse their young. Therefore, on occasion, whales must die naturally in the tropics. If their corpses come to rest within scavenging distance of the coral reef fish, that event must be considered a significant nutrient input, and it is of a type that is natural and acceptable, non-damaging to the ecosystem.
And a key feature of the acceptability of this nutrient enrichment is its solid form. (If the dead whale came to rest beside a coral reef and was ultimately consumed bit by bit, by the organisms living there, the sea water would remain clear, and life in the community would basically go on as before, just a whole lot. On the other hand, if the dead whale had first been put through a blender and then the nutrients were poured over the reef, a rapid bloom of free-floating phytoplankton and an increase in sedimentation and a decrease in sunlight hitting the unhealthy mess,. One theme that emerges from the story is this one: Its the form of the nutrient input that is highly significant to aquatic systems. It actually comes down to the important advantages of solid vs liquid food. We are often reminded that coral reefs are intolerant of nutrient enrichment and normally nutrient poor. More accurately it should be stated that they are intolerant of liquid nutrient enrichment and normally liquid (dissolved and particulate) nutrient poor.
(Although a normal feature in many undisturbed areas, the mangrove-seagrass-coral reef type of system does not support all coral communities. Undisturbed, the atoll reefs appear able to sustain themselves very nicely without a current direct route for terrestrial source nutrients. Efficient recycling is the key.) Elsewhere, in the mangrove-seagrass-coral reef systems, the mangroves grow at the shoreline, tolerant of relatively high dissolved nutrient levels, very productive, and providing habitat for a wide range of other organisms. The mangrove environment makes use of large amounts of dissolved nutrients, also significant denitrification takes place there, so the natural effect of the mangrove growth is a significant drop in nutrient levels in the water. Seaward of the mangroves are the seagrass beds, another highly productive area that also removes a significant amount of dissolved nutrients from the water.
Seaward of the seagrass beds are the coral reefs, described as oligotrophic systems least tolerant of nutrient enrichment. Mangrove forests and seagrass beds both provide ideal feeding and shelter for a variety of juvenile reef fish, and they are well known to function as nursery areas for these. From Coral reefs, seagrass Beds and Mangroves (unesco, 1983 The sheltered nature of these areas also contribute to make them important as nurseries. Mangrove areas thus export protein to coastal areas in the form of aquatic organisms that use the mangrove areas for their early development and then migrate offshore. Well known are the massive migrations of mullets and shrimp from these areas. This is high quality protein that links mangroves directly to other coastal systems like coral reefs, seagrass beds, and ultimately to man. (bak in Ogden and Gladfelter, unesco report, 1983). The function of the mangrove and seagrass areas could be described as converting liquid nutrients to solid nutrients, prior to their export to the coral other words, organic runoff is converted to fish flesh by this system.
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How much do we know about the overall input-output patterns, or nutrient balance in the system? The capture of plankton biography from oceanic water passing over the reefs is thought to be one important input route, although this water is characteristically very low in nutrients. Another significant input is the contribution of fixed N by the blue green algae - this group of organisms does well in tropical water since they can out-compete other N-limited algae forms, having the advantage of making their own fertilizer. As described in a basic marine ecology text: Atmospheric nitrogen is also year fixed by blue-green algae such as Calothrix crustacea, which occurs on intertidal reef flats in the pacific as a thin, mono-specific film (it also occurs in other reef habitats in different growth forms). Calothrix can fix nitrogen at the rate.8 kg/ha/day - two to five times the rate achieved by fields of lucerne or alfalfa. Fixed nitrogen enters the food web through at least three routes: (1) the blue-green algae are consumed by herbivores, especially by certain fish with low assimilation efficiencies, so that the water over the reef gains nitrogen via the fish faeces; (2) in areas subject. (Barnes and Hughes, 1999, page 140). Besides the capture of oceanic plankton, and the use of N fixed by the blue-greens, many coral reefs derive significant nutrient input from terrestrial sources.
The partners have the ability to extract dissolved bio-available n from the water at low ambient levels, and also they capture and consume microscopic prey (zooplankton) as well as bacteria and particles of edible detritus that come into contact with their mucus layer. In short, corals also need to eat to live. Like many organisms, corals are adept at storing food-energy for the lean times. Under favorable feeding conditions they can become quite fat. This is important, writing since the food sources listed above may not be consistently available. Of the dissolved forms of fixed n, ammonia is by far the most easily available for uptake from the water by the symbionts, and this apparently is true for phytoplankton in general. Since living fish constantly excrete ammonia from their suggests that removing major amounts of fish from the system might ultimately deprive corals of needed nutrients. So, the coral reef ecosystem as a whole is characterized by a relatively low net exchange of nutrients with the surrounding areas.
the picture is fixed nitrogen, a critical element in the construction of all proteins. The symbiotic arrangement allows the partners to avoid the loss of fixed N to the water, a process that normally occurs in both free floating algae and solo-living marine animal lifeforms. In larger marine systems, overall fixed n is essentially passed back and forth between the plant and animal compartments of the community - corals have evolved a way to capture and complete this loop inside their own special symbiosis. Its a micro-model of what happens in the bigger picture, and the same principle applies in the more nutrient rich systems of the temperate and polar zones. Plants and animals perpetually passing the precious ball (fixed N) back and forth - its a common theme in all living systems on the planet. It is definitely an efficient plan for conserving nutrients, but the coral-algae symbiosis cannot live on sunshine, co2 and water alone. A net input of n and p is still essential to maintaining life and growth, and all corals have feeding strategies in addition to deriving energy from their plant partners.
At first glance, coral reef ecosystems seem to present something of an incongruity. A healthy coral reef is a diverse, highly productive community of marine organisms, thriving in exceptionally nutrient poor waters. Productive refers to the relatively high amount of carbon presentation fixation that takes place in these asurements and calculations have been made to show that by this measure, coral systems rank among the most productive of marine ecosystems anywhere. And this is accomplished in the relative absence of dissolved nutrients (N and P) in the clear, oligotrophic water. Rather than being a poor system, however, the coral reef is a rich living system that manages to have (and keep) its nutrients largely tied up in solid, living matter. The standing stock of reef fish represents a significant nutrient resevoir for a these systems. The secret to the success of the coral reefs is commonly believed to be the tight recycling of nutrients in the system, particularly in the corals, in which tiny plants and animals live together in a symbiosis that conserves key nutrients quite effectively. Algae (zooxanthellae living inside the tissues of the cnidarian host, harness energy from sunlight and fix carbon by photosynthesis.
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Coral reef ecosystem dynamics - how does nutrient cycling work on the reefs? how do these systems respond to changes in nutrient levels? what is the significance of the form (solid/liquid) of the nutrients? what is the ecosystem response to the removal of organic biomass by fishing? Are fish equivalent to nutrients? might their removal be equivalent to nutrient loss? can fishing negatively affect primary production?" - how do the changing trends in coral thesis reef ecosystem compare with those in other marine systems? Coral reef Ecosystem dynamics.