Carmenpura Rodríguez's List: Cadenas y redes tróficas

• “One of the universal processes inherent in all ecosystems is the recycling  of matter. Without this process life on Earth cannot be sustained, as most of  the 30-40 elements necessary for the growth and development of living organisms  are in finite supply.” (Barnes & Hughes, p 233)
  
• High production with very low available nutrients is explained by high levels of  nitrogen fixation plus very intense nutrient recycling.”
• reefs might be something like “7,000 gC/m(squared)/year.” A lot more carbon  fixation takes place on the reefs as compared to the surrounding areas. The  potential problem is when the assumption seems to be made that such a “highly  productive” system can afford to give up a lot of fish repeatedly....there seems  to be an implication that there should be no trouble in replacing them...just  “producing” more. A lot of carbon may be fixed, but a lot of it may be  reconverted to CO2 via respiration of organisms...and a lot more may become  rather permanently sequestered in the seabed. It is not at all clear that there  is any relationship between “grams of carbon” fixed on an annual basis, and the  potential for “sustainable” fisheries yield.
• “Primary production” in marine biology refers to the amount of carbon fixed by  the phytoplankton, basically the rate of conversion of CO2 to food
• Carbon fixed by photosynthesis on the reef is lost partly by offshore  transportation but most is accounted for by the metabolic activity of  intermediate consumers. The production to respiration ratio (P:R) therefore is  close to unity and there is insufficient fixed carbon left to support high,  sustainable yields of large carnivores at the end of the food chain.”  (Barnes and Hughes p 141)
  
  
• “There are plenty of high-profile examples of marine species, such as whales,  dolphins, seals, turtles and seabirds, that are considered to be endangered.  However, these organisms tend to be at the top of the food chain, and so  contribute little to the productivity of the ecosystem. Of more concern are the  abilities of human activities to affect animals from lower trophic levels, the  populations of which are considered, often over-optimistically, to be more  resilient to exploitation.” (Barnes and Hughes, p 252, 1999)
  
• “Despite all the uncertainties about the relative contributions to the  coral-reef ecosystems of different kinds of autotroph, bacteria, dissolved  organic matter, and internal vs. external inputs, it is clear that the  phenomenally high total productivity is in large measure due to the combination  of a tremendous surface area of photosynthetic tissue (either in the form of  zooxanthellae or benthic algae and higher plants), optimal light and temperature  conditions for photosynthesis, and the tight recycling of nutrients in an  otherwise nutrient-poor environment. The efficient recycling of nutrients occurs  both at the level of the coral-zooxanthellae symbiosis and at the general level  of the overall food web. Many consumers are present in reefs and although in  absolute terms primary production is high, relative to the number of consumers  food can be regarded as scarce. Hence food is consumed rapidly and utilization  is efficient. A high proportion of the environmental pool of nutrients is  therefore maintained within living tissues, so reducing opportunities for the  loss of nutrients out of the system. Any such losses are compensated by the slow  accrual of nutrients from water masses passing over the reef and by the  nitrogen-fixing activities of blue-green algal associations on the reef or  rhizomes of adjacent sea-grasses.” (Barnes and Hughes, 1999, p 141)
  

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• Evidence is quite convincing that mass coral bleaching like that which is  happening today has never occurred before in geological time. It is truly  something new. The phenomenon is most obviously somehow related to the  occurrence of warmer water temperatures...and the popular understanding of this  seems to be that the one (warm water) “causes” the other (mass bleaching). This  cause and effect is in fact far from established, there are some large  inconsistencies in the data that weaken the theory. This suggests that some  other factor or factors is playing a large role in the cause of coral bleaching.  A look at the changes in the corals from the point of view of “nutritional  health” makes an interesting study.
  
  
• it has been noted that the individual algae in bleached corals contain less  chlorophyll per unit. Another point; it has been noted that the expelled  zooxanthellae are viable (alive) in the water.
  
• There is good reason to suspect that the low lipid reserves and lowered immunity  features precede the bleaching event rather than result from it, as one might  suspect
• Mass coral bleaching events occur most often during times when water  temperatures are at their annual high point, and this is also highly correlated  to periods of calm winds, or “doldrums.” Although not always consistent, this  one emerges as a very strong pattern. In situations like this, the warm, still  water is well known to become very “stratified” and “oligotrophic” (nutrient  depleted). Lack of mixing seems to be a key, but extreme nutrient-depletion of  the upper layer of water is a well known result when these conditions develop.  -- So, the warm, calm period is naturally a time of less nutrient availability  in the water.
• The problem seems to be that corals are no longer as well prepared to weather  these annual warm “fasting” periods
• And one plausible explanation is that it’s because they have not accumulated  sufficient body fat stores to see them through it. Adding to the corals’  nutritional stress is the fact that their respiration and metabolic energy  demands increase with increasing temperature...so if they have not stored enough  extra energy, they just might not make it through. They might starve. (On a  related note, when looking for overall marine patterns, the large number of gray  whales succumbing to starvation are dying from the exact same problem - the  inability to pack on enough blubber to see them through the lean  time.)
  
  
• There is a strong correlation between bleaching and exposure to light. Although  not completely consistent, most often the bleached surfaces are those receiving  greater amounts of sunshine
• stimulates photosynthesis by the zooxanthellae...could the inability to  “perform” due to nutrient shortage inside the coral host somehow lead to the  zooxanthellae abandoning the host...or maybe it’s vice versa, maybe the light  (heat) absorption causes yet a greater temperature stress on the polyps living  on the sunlit surfaces, hiking metabolic demands higher yet...and when the algae  doesn’t provide food....but seems to be wanting to get “something for  nothing”...maybe this all results in the partners abandoning their relationship.  It has been noted that the photosynthesis/respiration ratio falls and produces  and “energetic imbalance” (Jokiel and Coles, 1990). Perhaps the ratio crosses a  threshold below which the symbiosis simply no longer works. And a “win-win”  scenario becomes transformed into at “lose-lose” one? ...with the result that  the relationship is abandoned, an emergency bailout...every organism for itself?

  • ¿Hipótesis?

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