Water Motion and Sediment Grain Size Effects on Benthic Microalgae Adrian Jones, Andrew Watkinson, Joelle Prange & William Dennison Botany Department, The University of Queensland, Brisbane, QLD, Australia Marine Botany T HE U NIVERSITY O F Q UEENSLAND

Benthic Microalgae Diatoms, dinoflagellates and cyanobacteriaDiatoms, dinoflagellates and cyanobacteria Ubiquitous distributionUbiquitous distribution Can tolerate low light (<1% I o )Can tolerate low light (<1% I o ) Capable of small scale migrationCapable of small scale migration Large contribution to primary productivity in marine systemsLarge contribution to primary productivity in marine systems

Aims Determine biomass and photosynthetic capacity of benthic microalgae in relation to sediment grain size and water motionDetermine biomass and photosynthetic capacity of benthic microalgae in relation to sediment grain size and water motion Determine the changes in photosynthetic capacity of benthic microalgae under different water flow regimesDetermine the changes in photosynthetic capacity of benthic microalgae under different water flow regimes Examine the photosynthetic viability of BMA at different sediment depths within fine and coarse grain sedimentsExamine the photosynthetic viability of BMA at different sediment depths within fine and coarse grain sediments

Study Sites 10 sites on Heron Reef from areas of different water motion and sediment grain sizes10 sites on Heron Reef from areas of different water motion and sediment grain sizes Sites near the reef crest experience greater water motion than those in the lagoonSites near the reef crest experience greater water motion than those in the lagoon

Sediment Grain Size Coarse grained sediments Sediment ridges due to high water motionSediment ridges due to high water motion High water flow prevents smaller particles from accumulatingHigh water flow prevents smaller particles from accumulating Occurs near reef crestOccurs near reef crest Fine grained sediments Even, “undisturbed” sediment surface due to lack of water motionEven, “undisturbed” sediment surface due to lack of water motion Low water flow allows fine particles to settle out of suspensionLow water flow allows fine particles to settle out of suspension Occurs in middle of the lagoonOccurs in middle of the lagoon

Wentworth’s Grade Scale Granule > µmGranule > µm Very Coarse Sand µmVery Coarse Sand µm Coarse Sand µmCoarse Sand µm Medium Sand µmMedium Sand µm Fine Sand µmFine Sand µm Very Fine Sand µmVery Fine Sand µm Silt< µmSilt< µm

Sand Grain Size Very FineFineMediumCoarse

PAM Fluorescence (Pulse Amplitude Modulated) Photosynthetically Active Radiation (PAR) (µE m -2 s -1 ) Electron Transport Rate (ETR) (µmol e - m -2 s -1 )Photoinhibition Generates rapid light curvesGenerates rapid light curves Maximum ETR can be used as a measure of potential photosynthetic capacityMaximum ETR can be used as a measure of potential photosynthetic capacity Maximum ETR

Grain Size & BMA Concentration

Grain Size & Fluorescence (F o )

Grain Size & Photosynthetic Capacity

Water Motion Experiments Fine grain sand Coarse grain sand Fast water flow regime Slow water flow regime No disturbance of sediment surfaceNo disturbance of sediment surface Fast water flow regime Significant disturbance of sediment surface and exposure of lower layersSignificant disturbance of sediment surface and exposure of lower layers

Fast Slow Fast Slow Fine Grain Sand Coarse Grain Sand Water Flow & Photosynthetic Capacity

Effects of boundary layer on BMA vary between coarse and fine sediments Fine grained compacted sediment Coarse grained loose sediment Irregular flowIrregular flow Boundary layer brokenBoundary layer broken Increased gas and nutrient diffusionIncreased gas and nutrient diffusion Higher photosynthesisHigher photosynthesis Greater BMA biomassGreater BMA biomass Slow water flow regime Laminar flowLaminar flow Thick boundary layerThick boundary layer Reduced gas and nutrient exchangeReduced gas and nutrient exchange Reduced photosynthesisReduced photosynthesis Lower BMA biomassLower BMA biomass

Effects of boundary layer on BMA vary between coarse and fine sediments Coarse grained loose sediment Irregular flowIrregular flow Boundary layer brokenBoundary layer broken Increased gas and nutrient diffusionIncreased gas and nutrient diffusion Higher photosynthesisHigher photosynthesis Greater BMA biomassGreater BMA biomass Fast water flow regime Mixed flowMixed flow Reduced boundary layerReduced boundary layer Increased gas and nutrient exchangeIncreased gas and nutrient exchange Increased photosynthesisIncreased photosynthesis Fine grained compacted sediment

Vertical Distribution Experiments

Vertical distribution of BMA Fine Grain Sand Coarse Grain Sand 30 mins Initial 30 mins Initial

Conclusions BMA abundance is highest in sites of intermediate grain sizeBMA abundance is highest in sites of intermediate grain size Water motion over the coarse grain substrate does not affect BMA photosynthetic capacityWater motion over the coarse grain substrate does not affect BMA photosynthetic capacity Photosynthetic capacity within fine grained substrate is enhanced by water motionPhotosynthetic capacity within fine grained substrate is enhanced by water motion Vertical distribution of BMA is variable between coarse and fine grain sedimentsVertical distribution of BMA is variable between coarse and fine grain sediments