Weekly summary of Tropic101x as posted by student Lucia_Agudelo. With minor grammatical and content edits by Tropic101x team

Transcription

1 Weekly summary of Tropic101x as posted by student Lucia_Agudelo With minor grammatical and content edits by Tropic101x team Summary of Week 3 NUTRIENT CYCLES LECTURE Elements like carbon (C) and nitrogen (N) form the building blocks of all living organisms. Different organisms acquire these elements from their environment in: Organic forms (carbohydrates, fats, proteins) Inorganic forms (CO2, NH3, NO2-, NO3-) Nutrient cycles explain the transition between different forms by different organisms living in different parts of the ecosystem. Plants acquire carbon in the inorganic form CO2 to build biomass (carbon fixation) through photosynthesis, together with light energy to drive this process. Light energy is used to oxidise water generating the energy that is required to reduce carbon dioxide to sugar. When an animal builds biomass directly from the organic carbon they ingest, this is called chemo-heterotrophy.

2 Then, sugars are oxidized to carbon dioxide using oxygen as an acceptor for electrons and releasing chemical bond energy. In ammonification, protein is broken down into its component amino acids that are deaminated to produce organic acids and ammonia. LECTURE Classic trophic pyramid associated with a coral reef ecosystem. The plants/primary producers fix carbon dioxide and all organisms, producers and consumers alike, oxidize excess biomass (organic carbon) to carbon dioxide. Carbon dioxide entering the water from the atmosphere adds an additional source of carbon to allow the system to grow and flourish. The plants assimilate inorganic nitrogen as ammonia excreted by the ammonification of protein occurring in plants and animals alike.

3 Microbes earn energy from aerobic respiration in the nitrogen cycle. Some of them substitute ammonia for sugar, this process is called nitrification because nitrite is the product. The organisms are chemotrophs. Another set of nitrifying microbes are capable of oxidizing nitrite to nitrate, these nitrifying microbes use the energy gained to fix carbon dioxide into biomass. These nitrifying microbes are therefore chemo-autotrophs. Another very important process in the nitrogen cycle is the conversion of ammonia to dinitrogen gas, N2. The ANAMMOX bacteria and chemo-heterotrophic bacteria obtain energy from the chemical bonds in ammonia but with the assistance of nitrite, it oxidizes them anaerobically all the way down to nitrogen gas.

4 LECTURE Microbes that can directly convert ammonia to N2 gas, explain how nitrogen gained by the ecosystem can be lost to the atmosphere, but they do not explain how to gain it from the atmosphere. Microbes that can introduce nitrogen gas in a form that is acceptable to other organisms in the ecosystem are: The group of organisms capable of nitrogen fixation is exemplified by autotrophic and heterotrophic cyanobacteria. An input of nitrogen from the atmosphere will allow for growth, but that input would have to be very large unless we can re-access the nutrients locked away in the carcasses and fecal matter. Microbes decompose dead bodies into inorganic nutrients and purge excess nitrogen from the system back into the atmosphere.

5 CALCIFICATION AND THE CARBONATE BALANCE LECTURE The framework of coral reefs is primarily generated by reef-building corals, whose dead skeletons build up over time and are glued together by organisms such as red coralline algae. Eroding organisms and storms are two processes on coral reefs which remove large amounts of calcium carbonate. Calcification is highest in warm sun-lit waters of the tropics and sub-tropics where the concentration of calcium carbonate ions is highest. Here a range of organisms such as reef-building corals, algae, molluscs, crustaceans and small creatures known as foraminiferans are able to deposit large amounts of calcium carbonate from the surrounding water column. The surrounding water column is supersaturated with calcium carbonate ions which means that there's an abundant source of materials to make the crystals of calcite and aragonite which are used to build the shells and skeletons of marine organisms. Ways of measuring calcification: 1. 1) Put corals in a non-toxic dye then put them back in the field (coral still alive). Cut skeleton open (this kills the coral) at a later stage. The stained layer within the skeleton allows you to calculate how much calcium carbonate has been laid down since the dye mark ) Incubate corals in seawater to which a small amount of radioisotope has been added. Over time, the radioisotope is deposited along with non-radioactive isotopes of calcium and it's used to measure the rate that calcium carbonate is being formed. 3)Buoyant weight method, in this method coral fragments or other calcifying organisms are weighed under water. Now if you know the density of calcium carbonate, you can then calculate the amount of calcium carbonate that's been added to the coral over time.

6 4) One of the most important methodologies is based on the fact that long lived corals like Porites put down a layer of calcium carbonate each year. Biologists can visualize yearly bands using techniques such as x-ray radiography to get a picture of how calcification has changed over time, and just like a tree, dark lines delineate the beginning and end of each year of calcium carbonate. LECTURE Large scale factors such as storms and the activities of specific coral reef organisms contribute to the process of removing calcium carbonate from the structure of coral reefs or decalcification. One factor that influences that tendency for calcium carbonate to remain or disappear from carbonate reefs is the ph and carbonatechemistry of seawater. The carbonate chemistry of sea water depends on the concentration of CO2 in the atmosphere. As carbon dioxide builds up in seawater, the concentration of carbonate ions declines. This affects the overall availability of carbonate ions for the deposition of minerals such as aragonite and calcite. External biological eroders: eat coral directly or chew on pieces of substrate that inadvertently are eroded from the surface of the reef.

7 Internal biological eroders: use a range of different techniques to bore into the skeletons of corals and other calcifying organisms. This directly reduces the structural integrity of coral skeletons leading to their eventual removal from the reef by storms. KAM Considering that the growth rate of a coral reef is inversely proportional to the decalcification and is directly proportional to calcification, we can expect a higher growth in these scenarios: Low - Mid ppm of CO2 Mid - High light Mid - High temperatures LECTURE The three-dimensional structure of coral reefs is extremely important to the abundance and diversity of fish species. When reefs lose coral and structure, they tend to be populated by more herbivorous fish and those fish which are specialists, in terms of feeding on coral, disappear.

8 The degradation of calcium carbonate stocks has implications for the protection of reefs. Sheltered ecosystems, such as seagrass and mangroves, may be increasingly exposed to wave energy if the calcified structures of the outer reefs disappear. These global changes, when coupled with more local disturbances such as overfishing and pollution, have the potential to produce large-scale impacts to the way that tropical coastal ecosystems function and provide services and goods for humanity. PREDATOR-PREY INTERACTIONS: LECTURE A standard food chain starts with an autotroph that's harnessing the sun's energy and is photosynthesizing at the base of the food chain. Then there's a simple relationship of energy moving up through the food chain, so from phytoplankton up to zooplankton. The transfer of energy from one level to the next is an incredibly inefficient process: typically only about 10% of the energy is actually transferred from one level to another. The number of different trophic levels is no more than five because energy transfer up the food chain is an inefficient process. Pelagic food chains tend to be fairly simple, but in a benthic system the complexity grows considerably.

9 CORRECTION: they CAN swim quickly to evade predation

10

11 LECTURE Avoiding predators is a strong selection force on all organisms that you find a coral reef, so many organisms have developed specific specializations and adaptations in order to cope with this kind of pressure. One of the characteristics of marine organisms is to have a bipartite life cycle which means they spend some of their life in one habitat and part of their life in another (ontogenic migrations). Example: Parrotfish life cycle 1. An adult parrotfish is reproduces generating eggs or fertilizing them. 2. Those eggs get swept offshore by ocean currents and they might spend a month or so developing into young fish. 3. Those fish travel to new, sheltered places (like mangroves) and colonize new habitat. 4. Once they are large enough they then come back to the reef. 5. The reef is full of predators. 6. They might go and live in branching corals that provide good refuges. 7. As they get larger they become less vulnerable to predators.

12 Fish and some invertebrates don't come back to the reef, they go over the reef and come into the lagoon, into the mangrove or seagrass. It's a great place as a nursery because the predators can t hunt very effectively in that environment and there's plenty of food. KAM Scarus guacamaia: has a complex life cycle that is accompanied by a series of color changes called polychromatism. They are sequential hermaphrodites, which mean that they start their life as females and eventually change into males.

13 LECTURE The relationship between the amount of grazing (which might be estimated by the number of grazing fish present) and the amount of seaweeds or macroalgae living on that reef is a negative relationship.

14 In Jamaica, people have been fishing intensively for many decades. In the 1970s coral cover was actually very high, something like sixty percent or more. In 1980 a very large hurricane came along and it smashed up the reef and reduced the amount living coral. In 1983 the long spined sea urchins were dying. The reef was hit a bit more by some hurricane activity that began to continue to decline and the reason for that is because the fleshy algae, the seaweeds, had increased because they were no longer being controlled by herbivores. Storms and coral bleaching events tend to push the coral cover down towards the algal sort of end of the spectrum. The problem happens when we have added too many stressors to the ecosystem. We can also add nutrients to that story. So many of the objectives of coral reef management are to try and manage the system to push that fulcrum further and further to the right, to try to maintain healthy reefs with high coral cover.

15 Keystone species: A species that has a disproportionately big impact on the ecosystem even though they're not very abundant. Indirect effects of predation: The effects of predators on prey are greater than just direct consumption. Many prey species alter their behaviour to avoid predation, perhaps reducing feeding, growth and reproduction rates. In this situation the smaller grouper, which is the one that's hiding, is hiding from the big predator and therefore the chances that this smaller predator is going to manage to get out from its hiding place and catch these prey is relatively low because there's lots of predators around. Humans came along and we took out some of those bigger predators. Smaller predator is going to feel empowered to go out and spend more time foraging looking for prey, it might be more successful and it'll take off. This can change the whole structure of the biodiversity and ecology of the ecosystem. REPRODUCTION, RECRUITMENT AND CONNECTIVITY LECTURE What's are the differences between the reproduction of marine organisms and those living on land? Water is abundant Direct release of gametes possible Reduced need for complex reproductive structures Conditions are more constant Reduced thermal variability and extremes Greater constancy of gas and salt concentrations Greater buoyancy force Larger transport distances of marine gametes and organisms

16

17 LECTURE The life cycle of the crown of thorns starfish is a good example because it is typical of many organisms associated with tropical coastal ecosystems: 1. Spawning occurs during the spring and early summer. Large numbers of egg and sperm are delivered into the water column. 2. Eggs and sperm will meet and fertilization will occur. 3. The larvae develop mouths and digestive systems, spending up to two weeks in the water column before they become negatively buoyant and recruit to the benthos (the bottom). 4. The larva turns into a juvenile starfish. 5. The juvenile starfish begin feeding and maturing until they are ready to spawn and release eggs and sperm into the water column. Types of larval life cycles Planktotrophic: Larval stages eat things (feed on plankton) Development is generally longer Lecithotrophic: Larval stages don t eat (feed on egg reserves/egg yolk) Larvae released from parent Brooding: Lecithotrophic development but larvae remain in Brood Chambers Reproductive timing:

18 Spawning tends to occur such that larval and juvenile development occurs at the most suitable time of year. Factors that can influence the production of gametes: Age and size Nutrition of spawning adults Sea temperature Light levels and periodicity Specific chemicals designed to coordinate spawning Spawning cycles: Discontinuous or opportunistic Non-phasic or continuous Lunar o semi-lunar spawning Annual spawning Mass spawning depends on: Temperature Lunar phase LECTURE Reef building corals show a broad range of reproductive strategies: Asexual reproduction Sexual reproduction Gonochorism (individuals are single-gendered, not hermaphrodites) Hermaphrodism (simultaneous and sequential) Broadcast spawners as well as brooders. Most larvae are Lecithotrophic. Acropora tend to be simultaneous hermaphrodites which broadcast spawn: 1. Release eggs and sperm in bundles. The buoyant eggs drag the bundles to the surface where they break open and fertilization occurs. 2. Planulae larvae develop. 3. The larvae may flatten out into a prawn chip shape, and after four to five days they begin to head away from the surface down to the bottom in search of suitable habitat. 4. The larvae develop into the primary polyp which then undergoes asexual reproduction dividing (cloning) into a number of other polyps forming the coral colony. Years later the colony grows and becomes mature the cycle is repeated.

19 Importance of reproduction and recruitment: Crucial determinant of dispersal, population structure and connectivity. Reduced larval life means reduced dispersal - means less connectivity between populations. Increased larval life means greater dispersal distances greater connectivity between populations. The more different subpopulations become, the greater the genetic distance between them. If dispersal distances are larger, then there's a greater exchange of individuals and genetic material between subpopulations leading to reduction in the genetic differences or structure between the eight subpopulations. The issue of population structure is important for effective conservation planning and has been taken into account in recent attempts to try and apply a system which recognizes the fact that some sub-populations, as well as some reefs, are in greater or less communication with each other via dispersal.