Abstract
Sediments are found in the epilithic algal matrix (EAM) of all coral reefs and play important roles in ecological processes. Although we have some understanding of patterns of EAM sediments across individual reefs, our knowledge of patterns across broader spatial scales is limited. We used an underwater vacuum sampler to quantify patterns in two of the most ecologically relevant factors of EAM sediments across the Great Barrier Reef: total load and grain size distribution. We compare these patterns with rates of sediment production and reworking by parrotfishes to gain insights into the potential contribution of parrotfishes to EAM sediments. Inner-shelf reef EAMs had the highest sediment loads with a mean of 864.1 g m-2, compared to 126.8 g m-2 and 287.4 g m-2 on mid- and outer-shelf reefs, respectively. High sediment loads were expected on inner-shelf reefs due to their proximity to the mainland, however, terrigenous siliceous sediments only accounted for 13–24% of total mass. On inner-shelf reef crests parrotfishes would take three months to produce the equivalent mass of sediment found in the EAM. On the outer-shelf it would take just three days, suggesting that inner-shelf EAMs are characterised by low rates of sediment turnover. By contrast, on-reef sediment production by parrotfishes is high on outer-shelf crests. However, exposure to oceanic swells means that much of this production is likely to be lost. Hydrodynamic activity also appears to structure sediment patterns at within-reef scales, with coarser sediments (> 250 μm) typifying exposed reef crest EAMs, and finer sediments (< 250 μm) typifying sheltered back-reef EAMs. As both the load and grain size of EAM sediments mediate a number of important ecological processes on coral reefs, the observed sediment gradients are likely to play a key role in the structure and function of the associated coral reef communities.
Highlights
Sediments can be found on all coral reefs and are derived from a number of sources ranging from on-reef production via bioerosion to inputs from terrestrial systems [1,2,3]
Clear differences in epilithic algal matrix (EAM) sediment loads were recorded among shelf positions (Fig 2)
On the inner-shelf reef crest EAM sediments were composed, on average (± SE), of 13.13 ± 1.42% silicates, while on inner-shelf back reefs EAM sediments contained an average of 23.79 ± 3.24% silicates
Summary
Sediments can be found on all coral reefs and are derived from a number of sources ranging from on-reef production via bioerosion to inputs from terrestrial systems [1,2,3]. Almost every hard surface on the reef is covered with some sediment, and it is these sediments, especially those within the epilithic algal matrix (EAM), that may have the most long-lasting effect on coral reef organisms [14,25]. Sediment can become trapped among the algal filaments of the EAM for long periods [25] as the complex structure of the algae reduces surface water flow increasing deposition [29,30] Both the total load of sediment and the specific grain size of sediments trapped in the EAM can affect a variety of coral reef organisms. By affecting vital processes such as coral recruitment and herbivory, the total load and size of inorganic sediments can directly affect benthic communities and the resilience of coral reefs to further anthropogenic disturbances
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