reduced total reef area, diminished bleaching refuges. ⢠Linear relationship ... Reef development (Titlyanov and Latyp
Every bucket counts: New insights into the role of water clarity for the health of the GBR Katharina Fabricius, AIMS
Overview • NESP Project “Benthic light as water quality indicator”: – Methods development to estimate benthic irradiance – Biotic responses
• Why water quality management is even more important in the context of so many other disturbances
Benthic irradiance ‘101’ Benthic irradiance is determined by: 1.
2.
3. 4. 5.
The amount of solar irradiance received at the sea surface (sun angle, day length, clouds, surface roughness) Optically active materials in the water (Inherent Optical Properties, IOPs: Total suspended solids, chlorophyll, coloured dissolved organic matter, water) Water depth Benthic albedo Angle of organism surface
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Benthic irradiance ‘101’ Benthic irradiance is determined by: 1.
2.
Source: Roelfsema et. al 2010 (http://www.gpem.uq.edu.au/brg-rstoolkit/)
3. 4. 5.
The amount of solar irradiance received at the sea surface (sun angle, day length, clouds, surface roughness) Optically active materials in the water (Inherent Optical Properties, IOPs: Total suspended solids, chlorophyll, coloured dissolved organic matter, water) Water depth Benthic albedo Angle of organism surface
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Incoming Irradiance GBR • Conditions more benign in the north than south: higher winter values, lower summer values. Winter
Summer 20% seasonal diff. 10° S 25° S
July Sep Nov Jan Mar May
60% seasonal difference Kirk 2011 Mol photons m-2 d-1
Mol photons m-2 d-1
Source: NASA: Mean insolation incident on horizontal surfaces over 22 years
Open-water PAR loggers SHIS: YON: MYR: PP: LJCO:
PP LJCO
South Heron Is Yongala Myrmidon Palm Passage Lucinda Jetty
Deploy light loggers on 4 IMOS moorings in deep water away from reefs: • •
MYR
•
YON
Test region Entire GBR region SWIM IOP
SHIS
• •
WETLabs ECO PAR sensors Deployment depth: – Above surface – Above seafloor (~26 m) Sampling interval: every 15 minutes Validate RS bPAR algorithm Validate eReefs
Yongala NRS PAR loggers:
Townsville
Surface + 26 m depth
Yongala
Yongala
MODIS-Aqua True Color image (10 Feb 2016)
Yongala PAR Loggers • Lowest benthic irradiance in March-July • Relative contributions of solar insolation, clouds, water clarity? 2015
20° S
2016 July Sep Nov Jan Mar May
Yongala PAR Loggers • Lowest benthic irradiance in March-July • Relative contributions of solar insolation, clouds, Algorithm-derived bPAR water clarity? 2015
2016
bPAR (umol m-2 s-1)
Remote sensing to quantify PAR Model input parameters
bPAR validation
Eb
(MODIS) Es (𝜆)
bPARmod,i
a (𝜆), bb (𝜆)
Kd (𝜆)
𝜆 = (400-700nm)
bPARin-situ
Z (geometric depth)
IOP validation
match-ups
(SWIM IOPs)
Spatiotemporal analysis
Benthic irradiance stress map creation
Canto and McKinna (in prep)
Spatial and temporal analyses, indicator development • Benthic albedo greatly affects light availability • Develop methods for Yongala • Define PAR indicator for March – July: – Turbidity from wet season still measurable – Greatest risk for coral bleaching
• Upscale to whole GBR – 14 years of daily data at 1 km Benthic albedo
McKinna et al 2015
Acropora as PAR indicator • Acropora is important reef builder, habitat for other species • Global light threshold for Acropora established (5.2 mols photons m-2 d-1 in winter: Muir et al, Science 2015)
Acropora as PAR indicator Acropora cover, and proportion of Acropora in coral communities, are among the best and most relevant WQ indicators, highly correlated to Secchi depth (Fabricius et al. 2012)
Secchi depth (m)
•
Fabricius et al. (2012)
Effects of variable light on corals
Photo-acclimation: quick in Pachyseris, very weak in Acropora Acropora: Growth rate = sum of daily light exposure Acropora millepora Pachyseris speciosa
Acropora millepora
Growth rate
• •
High Low Var.1 Var.2 Light Treatment (Di Perna et al, in prep)
Effects of variable light on Acropora ● = constant DLI, o = variable DLI x
•
•
Acropora growth: increases linearly with daily light exposure, little photoacclimation Effects of light reduction accumulate over time – 40% light reduction (steady or variable) -> 40% growth reduction – 80% light reduction -> 70% growth reduction.
Ø Days with reduced light reduce growth, accumulate to smaller colony sizes Noonan et al (in prep)
Every bucket counts… Linear relationships: PAR → Acropora growth
Exponential relationship: In situ water clarity → Acropora cover
Secchi depth (m)
Linear relationships: Annual river discharges → in situ water clarity
Fabricius et al. (2016)
Every bucket counts… • •
Define thresholds for 5 months with greatest PAR attenuation, when turbidity from wet season still measurable = March – July E.g. improve Secchi depth from 10 to 12.5 m → increase Acropora colonisation depth from 12 m to 15 m → GBR Acropora population increases by 43 million colonies
Why WQ management is more important than ever •
Decline in coral cover mostly attributed to bleaching, storms and COTS (De’ath et al. 2012)
•
Low light, sediments and nutrients: – Don’t cause acute mass coral mortality – Do slow reef recovery, impair coral growth, recruitment, lead to macroalgal proliferation (Fabricius 2005) – Decrease the depth limit for reef development → reduced total reef area, diminished bleaching refuges.
•
Linear relationship between annual river discharges and annual mean water clarity (Fabricius et al. 2016)
•
WQ pollution is reversible - Unlike CO2 pollution: CO2 remains in atmosphere and oceans for millennia.
Why WQ management is more important than ever ‘Coastal acidification’: • Nutrient enrichment ® organic enrichment, algal/bacterial blooms • Increased respiration adds CO2 to coastal waters, complementing rising atmospheric CO2 • Inshore GBR has acidified 2.5 – 3 times faster than atmospheric CO2 (Uthicke et al. 2014). • WQ management is likely to ameliorate CO2 pressure. Atmospheric CO2
Nutrient discharges
Higher algal biomass
CO2 from benthic respiration
Reduced pH, O2 = Coastal Acidification
Why choose light as WQ indicator? • Objective: – Ecologically relevant – Daily measurable at 1 km2 scale through MODIS – Validate/refine WQ Guidelines for specific GBR regions – Validate eReefs biogeochemical model
Methods development to estimate benthic irradiance
Biotic responses
Daily PAR thresholds Corals: • Acropora colonisation depth (Muir et al. 2015): 5.2 mol photons m-2 d-2 in winter • Kleypas 1997: global limit for reef formation (net CaCO3 production): 7 - 8 mol photons m-2 d-2 • Reef development (Cooper et al, 2007, Whitsundays): ~2 mol photons m-2 d-2 • Reef development (Titlyanov and Latypov (1991): Gulf of Siam: ~2 mol photons m-2 d-2 • Mean maximum coral colonisation depth (Gattuso et al. 2006): 1.2 mol photons m-2 d-2 Seagrass guidelines (Collier et al 2016): • Zostera, Cymodocera, Thallassia, Enhalus: • Deepwater seagrass:
10 mol photons m-2 d-2 over 3 month 2.5 mol photons m-2 d-2
Macroalgae: Seabed biodiversity data (Pitcher et al. 2007, Hurrey et al 2013) • Steep decline macroalgal richness : 2 mol photons m-2 d-1 • Lobophora turfs: 3.6 mol photons m-2 d-1 • Halimeda/Bryopsidales: 4.6 mol photons m-2 d-1 Benthos general: • Medium minimum PAR required for photosynthetic organisms: 0.06 – 14.1) (Gattuso et al 2006) • Compensation irradiance for benthic communities:
5.1 mol photons m-2 d-2 (range 0.24 - 4.4 mol photons m-2 d-2
Outcomes • Integrated, easy-to-measure WQI:
– improve assessments of the impacts and risks to GBR ecosystems from river runoff and coastal development. • estimate trends • provide info on remote parts (Far North!) • inform Reef Plan report card, RWQPP
– Changes in bPAR can be related back to its drivers
• Inform future Water Quality Improvement plans: set ecologicallyrelevant targets, • Assess effectiveness of region-specific river load reductions • predict ecological consequences, • Comparing risks from river loads against those from dredging • Inform scenario models: estimate effects of land management scenarios to improve bPAR
Take-home points: WQ is key to GBR health Sediments and nutrients lost from fertilised and grazing lands … … change the ecology of inshore reefs
… increase the number of crownof-thorns starfish outbreaks
Latitudial changes in incoming irradiance • Winter: relatively even (slightly lower south of Rockhampton, Wet Tropics)
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Issues: Biological response measures Physiological responses
Life history/ Populations
Ecosystems
Photosynthesis Respiration
Survival Morphology Fecundity
Reef recovery time Depth of reef development
Bleaching susceptibility Energy reserves
Growth Rate Recruitment
Max depth of colonisation Coral competition with MA
Tissue thickness
Transition phototrophic / heterotrophic taxa Max depth of bleaching
Time Scales to respond: Physiology Life history/ Populations
Ecosystems
Hours to weeks
Weeks to months
Years
Daily integrated values, Running weekly averages and variances
Seasonal means (esp. winter!)
Seasonal or annual means
Seabed biodiversity data inform about light thresholds in inter-reefal areas Eg Seagrass Halophila spinulosa does best at > 7 mol photons m-2 d-1
