Environmental forcing of benthic community
productivity within the Kimberley’s
macrotidal reefs
WAMSI Kimberley Marine Research Program Project 2.2.3
Ryan Lowe, Renee Gruber, Jim Falter
Wave- versus tide-dominated reefs
• Historical focus on the hydrodynamics of wave-dominated reefs
(examples: southern GBR, Ningaloo, Pacific islands)
• “Tide-dominated reefs” = where the tidal range > incident wave height
(examples: NW Australia, northern GBR)
Intertidal reef
platform
channel
lagoon
reef flat
Montgomery Reef, Kimberley
Ningaloo Reef
Global distributions of tide-dominated
reef systems
•
‘Tide-dominated’ tropical
reefs are abundant
globally (~30% worldwide)
•
Several macrotidal reef
regions, with the Kimberley
as an extreme
•
Very little is known about
the function and
productivity of these
macrotidal reef systems
Global distribution of coral reefs
(Lowe and Falter, 2015, Annual Review of Marine Science)
Productivity and nutrient dynamics of
Kimberley reefs
• “Primary production” = measure of the total rate of production of new
biomass (organic carbon) via photosynthesis.
• Studies of reef productivity and biogeochemistry have concentrated on
tropical oligotrophic, wave-dominated coral reefs
• How do these distinct ocean drivers (physical and biogeochemical)
influence the productivity of these coastal Kimberley reefs?
Montgomery Reef
Terrestrial discharge
Buccaneer
Archipelago
Fitzroy
river
Versus e.g., Ningaloo Reef
Reef ecological zonation
• The Kimberley reefs tend to
have common ecological
zonations
Tallon Island
Montgomery Reef
Adele Island
(Collins et al. 2015) – Project 1.3.1
Specific project objectives
1.
Quantify the physical variability (i.e., hydrodynamics and
thermodynamics) across a representative intertidal Kimberley
platform reef → develop predictive models of these dynamics
2.
Assess the spatial variability in benthic community
production rates across different zones of a coastal
Kimberley reef system, including how these rates vary
seasonally
3.
Identify how reef productivity rates respond to local
environmental variability (i.e., physical and water quality),
and how different reef organisms contribute to these rates
Case study: Tallon Island
•
Three field experiments (spring-neap
cycle)
− 1) Dry: Oct 2013; 2) Wet: Feb 2014; 3)
Late-Wet: Mar 2014
•
Array of oxygen sensors, nutrient
sampling and hydrodynamics
•
Intensive physical study conducting in
parallel during March/Apr 2014
(comprehensive circulation, water level
and temperature measurements)
reef
crest
forereef slope
algal ridges
seagrass
meadow
Physical conditions: hydrodynamics
•
•
Highly asymmetric tide on the reef (2 hr flood vs 10 hr ebb duration)
Rapid filling and slow draining → tidal ‘ponding’ on the reef at low tide
despite the reef being ~0.2-0.3 above MSL
• Draining restricted by bottom friction and hydraulic control at the reef
edge -> simple model developed for Kimberley reefs
(Lowe et al. 2015, Journal of Geophysical Research - Oceans)
Consequences of tidal ponding:
Extremes in water quality (e.g. temperature)
• Gridded temperature data
from ~70 temperature
loggers
• Increased residence
times by ponding drives
extreme temperature
variations
• Reefs are characterised by
water quality extremes
(e.g. oxygen, pH, etc.)
Water quality: field programs
• Instrumentation used to separate
multiple environmental forcing factors
Continuous logging ~3 weeks
• Dissolved oxygen (3 stations)
• Temperature
• Light at reef surface
Ebb tide flow
Lagrangian drifters
(~daily)
• Used to confirm
flow speed and
metabolism when
water level low
Water quality: field programs
Hand sampling water quality
• Hourly at 3 sites
• Nutrients (dissolved and particulate N + P)
• Chlorophyll-a and total susp. solids (TSS)
Overnight sampling water quality
• Scaffolding on reef flat with ISCO automated
sampler
Measuring benthic fluxes
• Primary production occurs during daytime and is the assimilation of carbon
through photosynthesis (results in oxygen release)
• Respiration occurs continuously and is the conversion of organic carbon into
energy (results in oxygen uptake)
• Net production = Gross primary production - Community respiration
• Dominant communities:
Algal ridge
Seagrass
Reef oxygen variability
reef water depth (m)
During 10-hr low tides
• DO varied up to 440
µM (14 mg L-1)
• DO saturation reached
~260%
• Hypoxia (DO<60 µM or
2 mg L-1) occurred on
reef flat for several
hours each night
Productivity results: Net production
Seagrass
Algal ridge
• Photosynthesis-irradiance curves show
how productivity rates respond to light
• Net production appeared to decline at
high light BUT this was due to high rates
of respiration
• Respiration was driven by O2
concentration
Productivity results 2: Gross production
• Gross production shows
no decline even at high
irradiance (and
temperature)
• The brown algaldominated (Sargassum
spp) zone is more
productive than Thalassia
hemprichii zone
Productivity results 3: Tide-light cycles
200
Apr 2014
•
Phase between light and tide
seems to regulate daily net
community production – due to
variable hours of exposure
•
This cycle is ~15 days long
•
Both seagrass and algal
communities are balanced
between net autotrophic and net
heterotrophic (P:R = 1)
Daily NCP (mmol O2 m-2 d-1)
Oct 2013
100
0
0
90
180
270
-100
Phase difference between tide and light (degrees)
-200
Noon at high tide
Noon at low tide
360
Kimberley reefs in the global context
Daily metabolism (mmol O2 m-2 d-1)
2000
1500
GPP
1000
CR
500
0
-500
-1000
-1500
• Tallon productivity and respiration are ~mean of other reefs studied, despite
the huge variability in environmental conditions
Reef nutrient dynamics (brief overview)
• Nutrients (e.g. N and P) usually limit growth and productivity of tropical reef
communities -> controls the structure and function of reef ecosystems
• Ocean versus catchment sources? Do Kimberley river systems deliver pulses
of nutrients (dissolved and particulate) to reef communities during the Wet?
• Nutrient pools in different forms: dissolved and particulate (e.g. phytoplankton)
• Benthic fluxes of nutrient uptake and release measured in different reef zones
• Results show tight recycling of nutrient recycling across different reef zones
Tallon Island
Fitzroy River
mouth
Image courtesy Chelys
Summary
• Substantial tidal asymmetries on the reef due to
reef bottom friction and morphology
• Ebb tide duration on the reef is much longer than
the flood → substantially reduces flushing leading
to thermal and water quality extremes.
• Despite the physical extremes, this reef’s
productivity is very typical of other reefs (including
typical coral reefs and those in the GBR)
• At a reef-scale, production and respiration are
balanced, though they differ between seagrass
and algal communities
• Different zones of the reefs act as sources or
sinks of nutrients → implications for reef-scale
nutrient budgets and recycling
Publications and acknowledgements
1.
Lowe, R., Leon, A., Symonds, G., Falter, J., Gruber, R. (2015), The intertidal hydraulics of tide-dominated
reef platforms, Journal of Geophysical Research – Oceans, DOI: DOI: 10.1002/2015JC010701
2.
Lowe, R., Falter, J. (2015) Oceanic forcing of coral reefs, Annual Review of Marine Science, 7, 43-66
3.
Dandan, S., Falter, J., Lowe, R., McCulloch, M. (2015) Resilience of coral calcification to extreme
temperature variations in the Kimberley region, northwest Australia, Coral Reefs, 34 (4), 1151-1163
4.
Gruber, R., Lowe, R., Falter, J. (2016) Productivity of a tide-dominated reef platform with extreme diurnal
temperature variation, Limnology and Oceanography, submitted
5.
Lowe, R, Pivan, X., Falter, J., Symonds, G., Gruber, R. (2016) Rising sea levels will help quench extreme
temperature variations in tide-dominated reef habitats, submitted
Special thanks
• The State Government of Western Australia and WAMSI partners for partially funding this
research
• Additional funding provided by an Australian Research Council Future Fellowship and the ARC
Centre of Excellence for Coral Reef Studies
• Project links and collaboration with 2.2.1, 2.2.4, 2.2.5 and 1.3.1
• Special thanks to the Bardi Jawi Rangers and community, and the Kimberley Marine Research
Station
• Field support by Graham Symonds, Mike Cuttler, Leonardo Ruiz Montoya, and Nick Mortimer
Reef geomorphology and
ecological zonation
• Many / most reefs high above low tide elevation
• Yet productive reef ecosystems remain submerged over tidal cycles
Reef elevation distributions
Macrotidal Kimberley reefs
mean sea
level
low tide
(Lowe and Falter 2015)
(Solihuddin et al. 2016) – Project 1.3.1