There is a threat that coral reefs will shift to an algal dominated state due to natural and human related disturbances. This could cause reef-wide coral mortality, and is of great concern in coral reef conservation. Knowledge of the mechanism driving coral reef macroalgae dynamics, especially changes in spatial cover over time, is important in understanding such shifts. Previous studies on coral reef macroalgae dynamics have focused on the growth dynamics of macroalgae and its driving factors (e.g. nutrients, herbivore grazing) within individual species without considering the spatial dynamics involved. The knowledge built from these studies is mainly on the temporal change of coral reef macroalgae. This however, fails to determine whether the observed temporal change is spatially homogenous throughout a given reef or even within a specific reef zone. Such spatial perspective is important especially if the mechanism behind algal phase shifts, which are a reef-wide occurrence, is to be fully understood.
The overall objective of this study was to characterise the spatial configuration of processes influencing macroalgal growth and to understand how these influenced the dynamics of macroalgal cover in a reef flat area covering 210,000 m2 located at Shark Bay, Heron Reef from February 22, 2009 to February 2, 2010.
This objective was divided into three components:
1) Determine the temporal dynamics of macroalgal cover between the five sampling periods (February 2009, May 2009, September 2009, November 2009 and January 2010) and relate these to the associated dynamics in macroalgal assemblage species composition, in-water temperature and in-water light conditions.
2) Determine whether sites with similar environmental characteristics exhibited a common type of macroalgal cover change (either increase or decrease) in order to understand the underlying mechanism of change. This involved assessment and analysis of the spatial variation of the mechanism driving macroalgal cover change based on 18 sites using a classification procedure. This classification is based on three environmental characteristics (macroalgal assemblage species composition, benthic cover composition and physical environment), and resulted in sites classified into “group types”, each representing unique mechanisms of change.
3) Assess how high spatial resolution multispectral remote sensing imagery can provide a unique, spatially explicit perspective of macroalgal cover change. This was done by linking the measured change with those based on field data and by enabling a spatially explicit analysis of change and the associated benthic cover transitions.
The key findings of this study were:
First, between the five sampling periods, macroalgal cover change was different in terms of type (increase or decrease) and magnitude. The largest change occurred in the February – May 2009 period and the May – September 2009 period. These changes were associated with distinct changes in macroalgal species composition, light and temperature conditions.
Second, temporal change in macroalgal species composition alluded to an annual cycle and revealed two time periods with distinct species composition: February – May and September – November.
Third, the type and magnitude of the temporal change in macroalgae varied spatially between the sites for the four consecutive sampling periods, which indicated spatial variation in the underlying mechanism.
Fourth, spatial variation in influential environment characteristics (macroalgal species, benthic cover type and physical environment variables) was detected. Using these in classifying the sites into “group types” representing unique mechanisms of macroalgal cover change, revealed a high degree of spatial heterogeneity in the mechanism of macroalgal cover change as the number of group types ranged from six to nine.
Fifth, macroalgal cover changes observed from field data were linked to changes observed from high spatial resolution multispectral remote sensing imagery derived benthic cover maps and enabled a means to check whether the field data observations held true throughout the entire study area.
Lastly, new features in the spatial and temporal dynamics of macroalgal cover change were revealed from benthic cover transition information based on the time series of high spatial resolution multispectral remote sensing imagery derived benthic cover maps.
There are several limitations of this study. The dynamics of macroalgae were measured on a reef flat. Macroalgal establishment, macroalgal mortality and nutrient dynamics were excluded. Light data from Great Keppel Island, Southern Great Barrier Reef was used to complement in-situ light data. Spatial variation analysis was based on 18 sites. Observation was limited to five sampling periods within a 12 month period. Comparison of macroalgal cover change between benthic cover maps derived from high spatial resolution multispectral remote sensing image data and field monitoring data involved monthly comparisons despite differences in acquisition year. Benthic cover maps produced from the five high spatial resolution multispectral remote sensing images, used field data sets acquired at a date different from the image acquisition date. Lastly, benthic cover maps derived from high spatial resolution multispectral remote sensing imagery did not quantify different levels of macroalgal cover but only distinct presence or absence.
In summary, this study has revealed the spatial aspect of macroalgal cover dynamics which is an important step towards a spatially explicit reefscape-level knowledge. This should help improve the current assessment and modelling of the vulnerability of coral reefs to algal phase shifts