Effects of algal turfs and sediment on coral settlement

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Abstract

Successful settlement and recruitment of corals is critical to the resilience of coral reefs. Given that many degraded reefs are dominated by benthic algae, recovery of coral populations after bleaching and other disturbances requires successful settlement amidst benthic algae. Algal turfs often accumulate sediments, sediments are known to inhibit coral settlement, and reefs with high inputs of terrestrial sediments are often dominated by turfs. We investigated the impacts of two algal turf assemblages, and of sediment deposits, on settlement of the coral Acropora millepora (Ehrenberg). Adding sediment reduced coral settlement, but the effects of different algal turfs varied. In one case, algal turfs inhibited coral settlement, whereas the other turf only inhibited settlement when combined with sediments. These results provide the first direct, experimental evidence of effects of filamentous algal turfs on coral settlement, the variability in those effects, and the potential combined effects of algal turfs and trapped sediments.

Introduction

Degradation or even simple disturbance of coral reefs, whether caused by terrestrial run-off, crown-of-thorns starfish outbreaks, over-fishing, or mass-bleaching events, generally involves increased dominance by benthic algae (Hughes, 1994; McCook, 1999; McClanahan et al., 2001; Diaz-Pulido and McCook, 2002; Wilkinson, 2002). Increasingly it is becoming recognized that the process of degradation involves a failure to recover from acute disturbances (Hughes, 1994; Connell, 1997; Done, 1999; McCook, 1999; Hughes et al., 2003). The successful settlement and recruitment of corals is critical to the recovery of coral populations after disturbance, and hence a fundamental aspect of overall reef resilience (Hughes, 1996; Hughes and Tanner, 2000). However, settlement and recruitment of corals occurs amidst algal assemblages, and this will be increasingly so as climate change leads to increasingly frequent mass bleaching (Hoegh-Guldberg, 1999; Hughes et al., 2003), followed by algal colonisation (Diaz-Pulido and McCook, 2002). It is therefore surprising that very little is known about the effects of benthic algae on coral settlement and recruitment (McCook et al., 2001).

Filamentous algal turfs in particular are likely to have critical effects on coral settlement and recruitment. A recent survey of coral recruits on a range of inshore reefs on the Australian Great Barrier Reef found that coral recruits were more often found in close proximity to filamentous algal turfs than any other major benthic group, more so than predicted by overall abundance in the habitats surveyed (Birrell, 2003). Turf algae rapidly colonise the surfaces of dead corals, and may persist for several years (Price, 1975; Diaz-Pulido and McCook, 2002), particularly when herbivores maintain low biomass of large, upright macroalgae (McCook, 1999). Benthic algae, predominantly algal turfs, comprised up to 90% of benthic cover across large areas of the Great Barrier Reef, particularly the inshore regions, after disturbances by mass-bleaching events and regional outbreaks of crown-of-thorns starfish (Sweatman et al., 2000, Sweatman et al., 2001). Similarly, algal turfs dominated reefs in Kenya on the East coast of Africa, following the 1998 mass-bleaching event (McClanahan et al., 2001).

Terrestrial run-off has been shown to have numerous impacts on reefs, including serious impacts on coral reproduction, settlement and recruitment (Rogers, 1990; Gilmour, 1999; Fabricius et al., 2003). In particular, sediments deposited on the reef substratum have been shown to inhibit the settlement of coral larvae and to smother newly settled juveniles (Babcock and Davies, 1991; Babcock and Mundy, 1996). However, previous studies have not placed these effects in the context of community dominance by benthic algae, especially algal turfs, although sediment trapping and accumulation by algae, and algal turfs in particular, is widely observed (Eckman et al., 1989; Steneck, 1997; Airoldi, 1998; Purcell, 2000). Algal assemblages reduce the flow of water in the boundary layer, enhancing sediment deposition and reducing resuspension (Eckman et al., 1989; Carpenter and Williams, 1993; Vogel, 1994), and can also physically trap sediment, preventing resuspension (Purcell, 2000). Sediment accumulation in algal turfs has been shown to enhance the ability of these assemblages to smother and overgrow other benthic biota, such as crustose coralline algae (Steneck, 1997). Clearly, there is potential for algal turfs and sediments to act in combination to hinder coral settlement, especially on degraded or disturbed inshore reefs subject to high inputs of sediments.

This study provides a preliminary exploration of the effects of filamentous algal turfs, and algal turfs combined with sediments, on the settlement of coral larvae. We tested whether the presence of algal turfs and/or sediments on dead coral substrata can reduce larval settlement of the hard coral, Acropora millepora (Ehrenberg).

Section snippets

Experimental design

To test the hypotheses that algal turfs hinder coral settlement and that sediments accumulated in algal turfs enhance this effect, we manipulated the presence of algal turfs and sediments on dead coral substrata, and compared the number of corals that settled in six different treatments. Importantly, to explore potential variability in the impact of the turfs, we used two different algal turf assemblages. Thus, the experimental design included three main factors: Algal turf type (two levels:

Results

There were considerable differences in the effects of the two algal turf assemblages on coral settlement, resulting in a significant 3-way interaction between algal turf type, algal turf presence and sediment presence (Fig. 1, Table 1). In particular, the turfs differed in effects on larval settlement, depending on whether or not sediment had been added to the experimental treatment.

For Turf 1, the presence of either, or both, of ungrazed, newly established algal turfs (SNK, p < 0.05), or

Discussion

The results of this study provide several useful pointers in terms of coral settlement in the context of recovery of reefs from degradation and disturbance. Firstly, they provide the first direct, experimental evidence that some algal turfs reduce coral settlement in their own right (McCook et al., 2001). When coral settlement surfaces colonised by ungrazed, newly established (<6 week old) algal turfs were presented (Turf 1 treatment), coral larvae settled almost exclusively on surfaces that

Acknowledgments

We thank M. Abe, K. Louen, C. Gralton, and the staff of the Orpheus Island Research Station for field assistance. K. Anthony, G. Diaz-Pulido and T. Hughes provided valuable advice. We acknowledge the support of the CRC Reef Research Centre to CB and LM and the Australian Research Council to BW.

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