Module 3 Flashcards
Real-world problems facing the Great Barrier Reef
effects of sediment on coral fertilisation
scaling up fertilisation problems to the ecosystem
effects of climate change on fecundity and connectivity
Sediments and coral fertilisation
the clearing of land for agriculture affects its ability to stabilise the soil
- this is particularly concerning when it occurs near rivers and where tropical rainstorm occurs
- mangroves are the first line of defence as they trap sediment from entering bodies of water
- the removal of mangroves therefore allows sediments to flow into seagrass beds and coral reefs
Riparian corridor
a strip of intact forest along the boundaries of rivers
- necessary for the protection of rivers
Coral life cycle
Corals have a bipartite life cycle
- they spend their early larval stage in the plankton, drifting on ocean currents
- after a minimum of 4 days, corals settle on the seabed and the reef
- once on the reef, they metamorphose into their adult form and grow
- in adult form, corals reproduce through broadcast spawning
Broadcast spawning
most adult corals are hermaphroditic and create both sperm and eggs
- these are bound into bundles in which a packet of sperm is held within a cluster of eggs
- on spawning night, the corals release these egg and sperm bundles
- the eggs are rich in lipids, allowing the bundles to float to the surface and form an oily slick
- after a period of 15 minutes, the bundles break apart, releasing the sperm and eggs
- the eggs float on the surface as fertilisation begins around 15 minutes later
- once the eggs are fertilised, the process of development continues until a larva is formed and the cycle repeats
Problems caused by sediments
- suspended sediment in the water column
- deposited sediment on the seafloor
- light attenuation
- altered light quality
Suspended sediment in the water column
the suspension of sediment in the water column can interfere with the lifecycle of corals
- water circulation, often driven by waves, keeps sediment particles high in the water column
- sediments can adhere to egg-sperm bundles, preventing them from reaching the surface
- bundle ballasting: the adherence of sediment to egg-sperm bundles causes them to sink to the seabed
- the presence of sediment can also cause the stem to become entangled, lowering sperm concentration
Deposited sediment on the sea floor
sediment that settles on the seabed provides an unstable substrate that prevents coral larvae from settling and attaching
- can also smother corals, forcing them to generate large amounts of mucus in an attempt to “slough off” the sediment
Light attenuation
light is absorbed and scattered off by sediment particles
- this prevents the majority of light from reaching the seabed, disrupting the growth and health of corals
- corals have photosynthetic symbionts in their tissue called Zooxanthellae, so a reduction in light can reduce the growth potential of corals
Allee effects
positive relationship between population size or density and its fitness
- describes a wide range of mechanisms that link the population size or density to survival
Coral reefs and allee effects
coral populations are reliant on gamete concentrations
- must achieve a sufficient sperm concentration during broadcast spawning events
- the combined effects of sediment preventing egg bundles reaching the surface as well as the entangling of sperm serves to reduce the concentration of sperm in the water
- sperm limitation is more likely where sediment is a problem
- areas of high sedimentation also have relatively poor coral recruitment as well as high mortality if they do settle
Coral bleaching
caused by extreme heat temperatures over prolonged periods of time which results in the death of large areas of corals
- also causes chronic, insidious effects
- corals experiencing bleaching have reduced fecundity that occurs either as a reaction in size and/or density of eggs
- some corals can become entirely non-reproductive
- thermal stress associated with coral bleaching can also reduce the mortality of sperm by up to 50% and the volume of sperm by 50-70%
coral bleaching therefore reduces coral populations by causing mortality and then reduces the reproductive output of the surviving populationds
Allee effects and small coral populations
small coral populations release so few egg bundles that there is insufficient sperm density on the sea surface to fertilise eggs adequately
Effects of climate change on connectivity
disruptions to connectivity are caused by:
- coral bleaching – kills some corals
- reduced fecundity of the survivors
- reduced fertilisation success
- collectively fewer larvae which swim less far – causes fewer new corals which slows population recovery
Coral connectivity
coral reefs are connected by their larvae that travels on the ocean currents
- allows damaged reefs to recover
