Anne W. Barrett1
1University of Maine, School of Marine Sciences
Abstract
Juvenile coral (< 40mm diameter) densities and macroalgal abundance were quantified in Bonaire in 2007 at six monitored reefs and compared to past studies in 2005. Since 2005 both juvenile coral density and macroalgal abundance increased throughout the monitored reefs. Among sites there is an inverse relationship between algal abundance and juvenile coral density. Although algal abundance in Bonaire remains low, if abundances continue to increase, coral recruitment may decline thereby reducing the resilience of Bonaire’s reefs.
Introduction
In recent decades, many coral reefs shifted from coral to algal dominance (Hughes 1994, Hughes et al. 2007). The reefs of Bonaire are an exception in that they are still dominated by corals, and macroalgae are relatively rare (Kramer 2003). Studies have shown that the long term survival of reef systems rely greatly on successful coral recruitment (Kojis and Quinn 2001). However coral recruitment can be jeopardized by macroalgae. Increased macroalgae on coral reefs can kill corals by smothering them and blocking available sunlight (Lewis 1986, McClanahan et al. 1999, Kramer 2003). Significantly, it also reduces the amount of available substrate conducive to coral settlement (Birkeland 1997, Hughes 1994, Lirman 2001, Steneck in prep.).
The objective of this study was to quantify and monitor juvenile coral recruitment and demography relative to the macroalgal abundance in the habitat in which they live. For this, data were gathered in the spring of 2007 and compared to previous studies using the same methods during the same month in 2003 and 2005 (Slingsby and Steneck 2003, Brown and Arnold 2005). Specifically, I seek to determine if coral recruitment and algal biomass is increasing, decreasing or holding constant.
Methods and Materials
The methods followed by this study were from Brown and Arnold (2005). The study was conducted at six monitoring sites in Bonaire; Forest, Plaza, Windsock, Barkadera, Reef Scientifico and Karpata, where a 25cm X 25cm quadrat was placed every two and a half meters along a ten meter transect located at the monitored locations marked by settlement plates at a depth of 10 meters. Four such transects were surveyed at each dive site.
Quadrats were placed on substrate amiable to coral recruitment few adult corals (< 25%)
and little sand or holes. I recorded species and size of juvenile corals (those < 40 mm in diameter (Rogers et al. 1984)) as well as the percent cover of macroalgae, turf algae, coralline algae, and live coral present within the quadrat. Canopy height measurements were recorded for macroalgae. A proxy for algal biomass, called algal index (Kramer 2003), was calculated by multiplying percent cover by canopy height (mm).
Results
Overall average juvenile coral abundance, for Bonaire, was 39.2 individuals per m2 (+ 2.3 SE). This is greater than the density of juvenile corals per m2 recorded in 2005 (20.0 + SE of 1.9 individuals per m2) and 2003 (23.2 individuals per m2) (Figure 1). Note, however, that unlike the 2005 and 2007 data, the 2003 data is not from the same marked transects.
Among sites, “Reef Scientifico” and “Forest” had the highest and lowest density of juvenile corals per m2 respectively (Figure 2). Algal biomass approximated using an algal index found four times greater algal abundance in 2007 then was found in 2005 (Figure 3).
Agaricia agaricites and Porites astreoides were the most abundant juvenile corals in 2007 and in 2005 (Figure 4). A slight inverse relationship between macroalgal abundance and juvenile coral abundance (R2=0.2) was recorded in 2007 (Figure 5). In 2005, macroalgae were uniformly rare, preventing a similar regression analysis (Figure 5).
Figure 1. Comparison of the overall average juvenile coral densities (< 40 mm diameter) from the spring of 2003 (data from Slingsby and Steneck 2003), 2005 (data from Brown and Arnold 2005) and 2007.
Figure 2. Comparison of juvenile coral densities by site between the spring of 2005 (data from Brown and Arnold 2005) and the spring of 2007.
Figure 3. Overall average macroalgal indices (%cover of macroalgae* macroalgae canopy height in mm) of all sites in the spring of 2007 compared with data from the spring of 2005 (data from Brown and Arnold 2005).
Figure 4. Coral abundance by species in 2007 compared with 2005 (data from Brown and Arnold 2005).
Figure 5. Comparison of the relationships among macroalgal abundance and juvenile coral abundance between the spring of 2005 (data from Brown and Arnold 2005) and the spring of 2007.
Discussion
No clear trend in juvenile coral densities are apparent since 2003. In contrast,
macroalgae seem to be increasing significantly at several sites in Bonaire. The increase of macroalgae may be cause for concern. Similar increases have been observed
throughout the Caribbean over the past two decades and are most likely due to a decrease in grazing herbivores on the reefs (Hughes 1994, Williams and Polunin 2004). Declines in herbivorous reef fishes (Williams and Polunin 2001) and the mass mortalities of the urchin Diadema antillarum (Hughes 1994) resulted from overfishing of herbivorous fish and the disease of urchins (Hughes and Tanner 2000). Given the strong negative relationship between macroalgal biomass and coral recruits (Birkland 1997, Brown and Arnold 2005) the preservation of herbivores may be critical to re-establishment of corals following disturbance.
This study suggests an increase in juvenile coral densities since 2005. This is a positive result for the status of Bonaire’s reefs. However, if macroalgal abundance increases on a long-term basis, effort towards the restoration of herbivore population densities should be a priority of managers in Bonaire. Increases in juvenile coral density also increase coral diversity which could increase the stability of the reef, making it more resistant to and resilient from disturbances (Bellwood et al. 2004). Given the troubling results of this study further monitoring of coral and macroalgal abundance is advised.
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Chapter 8: The impact of traditional fishing practices on the abundance