MARI 4003: Lecture 5

Light availability

• light directly related to photosyntehsis of zooxanthellae which affects coral calcification rates

• light is responsible for the depth distribution and geographic distribution of coral reefs

  • max depth of reef formation 30-50m (in very clear water it could be up to 60-75m)

PAR

• Photosynthetically Active Radiation

  • PAR

• visible light spectrum from 400-700 nm

Depth of corals

• light is responsible for the depth distribution and geographic distribution of coral reefs

  • max depth of reef formation 30-50m (in very clear water it could be up to 60-75m)

• corals can grow deeper as we know from cold-water corals, but limestone rock reefs dont form

What affects how much light can reach a coral reef ?

• angle of the sun and atmospheric attenuation

• water clarity

  • distance from coast

  • coastal population

• depth

  • irradiance decreases exponentially with depth

Light and growth

• less light= slower growth rates (lower levels of photosynthesis)

• fastest growth occurs in 5-10m depth

  • slower growth in deeper water

• reefs may not form where growth is slower than bioerosion

• limited light explains why coral reefs living at extreme latitudes are limited to relatively shallow depths

Why does fastest growth occur in 5-10m, and not shallower?

• shallower depths would lead to too much light, and UV radiation

  • too long of an exposure to these rays could be damaging to corals DNA, resulting in mutations

  • additionally, may lead to damage of other biological molecules

    • protein enzyme inactivation

    • lipids disruption of cell membranes and membrane transport systems

Photoprotective mechanism

• production of MAAs (mycosporine-like amino acids) which act as a sunscreen, and absorbs UV

• there are also proteins that can absorb and/or redirect visible light

Corals and UV radiation

• decreased growth/rates of calcification

• transplantation experiments (deeper corals brought to surface) demonstrate corals may be UV sensitive (exhibit bleaching and increased mortality)

• coral male gametes appear to be UV sensitive (normal spawning takes place at night)

Aragonite saturation state

• aragonite saturation state has been shown to affect a coral’s ability to calcify

  • important to determine if coral is able to form CaCO3 from the calcium and carbonate available in the water, or if they do not have the adequate energy

• optimal aragonite saturation for reef formation (Ω) > 4

• increases in carbon into the ocean through burning of fossil fuels has limited the aragonite saturation state of the ocean

Causes of change in abiotic environments

• salinity

  • natural: precipitation, river outflow, evaporation

• temperature

  • anthropogenic: generally climate change increases ocean temperature

• light

  • anthropogenic: light attenuation due to coastal development

• aragonite saturation

  • anthropogenic: CO2 addition to the ocean changes the Ω

Oligotrophic waters

• bodies of water that have very low levels of nutrients, such as nitrogen and phosphorus, leading to low biological productivity

• paradox of coral reefs growing (and maybe preferring) oligotrophic waters

  • N and P (essential for the production of organic matter by photosynthesising organisms) are much lower around typical reefs than in deep water

N cycle

• transforming unavailable form of N₂ into available NH₄⁺, NO₂⁺, or NO₃^-

Chlorophyll A

• common indicator of productivity

• studies show tropical areas devoid of nutrients compared to more northerly or southernly latitudes

  • this is because there is a lack of seasonality in the tropics, and there is no upwelling of nutrient rich waters brought to the surface

Algae and corals

• algae on reefs are nutrient deficient and there is a fine balance which prevents algae from outcompeting corals and taking over the reef

• corals are able to outcompete the algae because:

  • efficient recycling and conservation of nutrients by symbiosis of various animals and phototrophs

  • algae and cyanobacteria

  • bacterial degradation of organic matter in intersitial porewater of coral heads and reef sediments

  • sediment resuspension by storms

• not 100% efficient recycling; coral reefs are net exporters of nutrients so there must be regular source of nutrient inputs

Nitrogen fixation

• largest inputs of N come from rivers, Trichodesmium (a kind of cyanobacteria), and resuspension

Human sources of nutrients to the reefs

• agricultural runoff

  • fertilizers which contain N and P lead to runoff entering the ocean and creating harmful algal blooms

• sewage

• livestock production

• coal

• mining

  • nearby phosphorus mines deposists large traces of the mineral into the ocean

Sedimentation

• long been identified as a controlling factor in tropical coral reef development

  • coral abundance and cover

  • coral species distribution

  • colony size and structure

• subject to:

  • terrestrial runoff

  • sediment resuspension from ocean bottom

• coral reef waters are not truly ‘crystal clear’

  • there is a gradient of suitable habitat

What influences the amount of sediment in the Great Barrier Reef

• amount of runoff

  • water that lands in a watershed based off precipitation and what flows off into the ocean vs. what is reabsorbed into the soil

• amount of natural vegetation

• topography

  • steeper land bases have greater volumes of runoff

• rainfall

  • amount and intensity of rain

Which source makes greatest contribution to reef sediment?

• typically sediment from rivers contributes much less to reef waters than does sediment resuspended by wave action

• fringing reef of GBR <10% of suspended sediment came from river inputs

  • likely far lower than this in high energy wave environments

River inputs

• low salinity leads to riverine inputs at surface and can block sunlight

• transport nutrients and pollutants to reefs

Increased sedimentation to coral reefs caused by human activities

• land use

• dredging

• discharges from hydrocarbon exploitation

  • offshore drilling and mining

Observed effects of corals and sedimentation

• polyp death and tissue necrosis

• changes in a coral’s energy budget including increased respiration relative to photosynthetic production by algal symbionts

• changes in metabolic costs will depend on the coral’s response to sediment influx and the required actions to clear sediment

  • if more energy is given to keeping a polyp free from sediment, then other processes such as growth and reproduction may ultimately be affected

• sediment may limit recruitment by covering hard substrata

• sediment makes it more difficult to feed heterotriphically

Methods of clearing sediement

• layer of mucus produced in excess so as to trap the sediment and slough it off the branches

• seen in Gass’ experiment

  • cillia moves sediment off coral polyp

  • ingestion and expulsion of sediment

• inflation/deflation of coral to move sediment off polyp

Active mechanisms to clear sediment

• related to coral behaviour

• tentacle movements

• inflating/deflating

• mucus production

• cilia

Passive mechanisms to clear sediment

• related to colony morphology

• fine skeletal architecture

• gravity and environmental factors (such as local currents) that allow corals to passively shed sediment

corals in well lit areas may be less impacted by an influx of sediment

Coral recruitment and sedimentation

• coral planulae prefer to settle on hard substrates

• when substrates are covered in loose sediment they are no longer suitable for settlement

• leads to lower recruitment success