Environmental forcing of benthic community productivity within the Kimberley’s macrotidal reefs

KIMBERLEY MARINE RESEARCH PROGRAM NODE

PROJECT 2.2.3 – RYAN LOWE, RENEE GRUBER, J IMFALTER, GRAHAM SYMONDS

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

(Lowe and Falter, 2015, Annual Review of Marine Science)

Global distribution of coral reefs

Oceanic forcing of reef nutrient dynamics and productivity

Montgomery Reef Versus e.g., Ningaloo ReefTerrestrial discharge

Buccaneer Archipelago

Fitzroy river

• 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?

Project objectives• Quantify the physical variability (i.e., hydrodynamics and thermodynamics) across a representative intertidal Kimberley platform reef → develop predictive models of these dynamics [project 2.2.1]

• Assess the spatial variability in benthic community production rates across different zones of a coastal Kimberley reef system, including how these rates vary seasonally

• 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 reef (Jalan)

forereef slope

algal ridges

reef crest

seagrass meadow

• 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 variability and temperature measurements)

Reef water level variability• 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 (Lowe et al. 2015, Journal of Geophysical Research, in press)

Reef temperature variability

• Gridded temperature data from ~70 temperature loggers • Increased residence time drives large temperature variability

Water quality measurements

Dissolved oxygen

• Continuously logged at 3 representative sites across the reef

Nutrients and chlorophyll a

• Hourly station sampling

• Overnight autonomous pump sampling on scaffolding

Seagrass

Mixed

Off-reef

Ebb tide flow

Algae

(Renee Gruber PhD project)

Lagrangian drifters

• Sampled oxygen, nutrients and chlorophyll a following slow-moving water masses at low tide

Benthic flux calculations

• Gradients in water quality parameters used to compute benthic fluxes

− Oxygen → Net production

− Nutrients/Chl a → Uptake and release rates

Diel extremes in temperature and dissolved oxygen

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Reef becomes hypoxic (<2 mg L-1 O2) during nighttime low tidesOxygen saturation ranges from 8% to 230% during a single day

Productivity and Respiration

•Seagrass communities are twice as productive as algal communities

•Overall, seagrass communities are net autotrophic (P:R=1.1), while the algal zone is net heterotrophic (P:R=0.6)

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Daily production and respiration (mmol O2 m-2 d-1) of Tallon Island and other Indo-Pacific reefs

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Production is similar to many systems (inclGBR) – global median 640 mmol O2 m-2 d-1

Production almost exactly balanced by respiration (P:R=1)

GBRNingalooReef

Chlorophyll-a uptake rates differ by community type (zones)

Coralline algal communities were a net consumer of chlorophyll, while seagrass communities were a net chl-a source

− Likely due to filter-feeding assemblages in the coralline algal zone

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Strong nutrient gradients develop across the reef

• During ebb tide periods, dissolved inorganic nitrogen levels increase over the algal terrace

− Likely due to filter feeding and remineralisation of PON (not flow-limited)

• Seagrass communities show gradual uptake of DIN

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Summary and future work• 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

Acknowledgments•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

•Close project links to 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

• Additional field assistance provided by Mike Cuttler and Nick Mortimer

Increasing reef width

Increasing bottom roughness

Role of reef morphology on tidal ponding

• Simple model developed to predict reef draining as a function of:

‒ Reef width, tidal properties, and bottom roughness(Lowe et al. 2015, Journal of Geophysical Research, in press)

• Keeps reefs from drying out → BUT, dramatically increases reef residence times

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