No abstract

Coral-algal phase shifts in which coral cover declines to low levels and is replaced by algae have often been documented on coral reefs worldwide. This has motivated coral reef management responses that include restriction and regulation of fishing, e.g. herbivorous fish species. However, there is evidence that eutrophication and sedimentation can be at least as important as a reduction in herbivory in causing phase shifts. These threats arise from coastal development leading to increased nutrient and sediment loads, which stimulate algal growth and negatively impact corals respectively. Here, we first present results of a dynamic process-based model demonstrating that in addition to overharvesting of herbivorous fish, bottom-up processes have the potential to precipitate coral-algal phase shifts on Mesoamerican reefs. We then provide an empirical example that exemplifies this on coral reefs off Mahahual in Mexico, where a shift from coral to algal dominance occurred over 14 years, during which there was little change in herbivore biomass but considerable development of tourist infrastructure. Our results indicate that coastal development can compromise the resilience of coral reefs and that watershed and coastal zone management together with the maintenance of functional levels of fish herbivory are critical for the persistence of coral reefs in Mesoamerica.

The complex relationship between coral reef fish assemblages and reef habitats at Davies Reef, a middle-shelf reef in the Great Barrier Reef, Australia, was approached from a reefscape perspective. A reefscape merges the spatial patterns found in a reef with 3 ecological components: structure, function and change. The main objective of this study was to explore the relationship between function (biomass and trophic structure of fish assemblages) and structure (reef habitat) within a reef. Three reef fish groups were defined using multi-dimensional scaling, and their similarity was evaluated using SIMPER analysis. From the benthic surveys conducted at the same sites we defined 3 reefscapes: A, 'encrusting non-Acropora', B, 'Acropora' and C, 'low density massive non-Acropora'. Reefscape A was characterised by maximum fish species richness and biomass, while maximum fish diversity was found in Reefscape B. Reefscape C had the minimum values of fish biomass. The relationship between fish groups (using biomass as a proxy) and habitat was explored using redundancy analysis, which was also used to identify significant fish species within each reefscape. The relationship between the trophic structure (guilds) and habitat was explored using the 4th corner analysis, and this analysis revealed that substratum types differed in terms of their utility as proxies for associated fish species and trophic guilds. The present study stresses the importance of within-reef variability as a determinant of composition and relative abundance of local reef fish assemblages. It also suggests that, while loss or change of habitat structure (i.e. coral cover or change in the dominant coral type) could reduce species richness and biomass, some of the habitat features to which fishes cue are probably coarser geomorphological and environmental zones.

Spatial prediction of coral reef habitats and coral reef community components was approached on the basis of the 'predict first, classify later' paradigm. Individual community components (biotic and geomorphologic bottom features) were first predicted and then classified into composite habitats. This approach differs from widely applied methods of direct classification based on remote sensing only. In situ coral reef community-condition assessment was first used to measure a response variable (percentage cover of habitat). Reef bottom features (topographic complexity, sandsediment, rock-calcareous pavement and rubble) were then predicted using generalized additive models (GAMs) applied to continuous environmental maps, high-resolution Ikonos satellite images and a reef digital topographic model (DTM). Next, using GAMs on newly created bottom maps, models were fitted to predict coral community components (hard coral, sea-grass, algae, octocorals). At this stage, high-resolution maps of the geomorphologic and biotic components of the coral reef community at an experimental site (Akumal Reef in the Mexican Caribbean) were produced. Coral reef habitat maps were derived using GIS following a hierarchical classification procedure, and the resulting merged map depicting 8 habitats was compared against thematic maps created by traditional supervised classification. This general approach sets a baseline for future studies involving more complex spatial and ecological predictions on coral reefs.

This study investigated the influence of geochemical and environmental factors on seasonal variation in metals in Yucatan's Chelem lagoon. Anthropogenic activities discharge non-treated wastewater directly into it with detrimental environmental consequences. Accordingly, this study established the spatial and temporal patterns of fine grain sediments and concentrations of heavy metals. Multivariate analyses showed fine grain facies deposition, transition sites dominated by fine grain transport, and fine grain erosion sites. Spatial and temporal variations of heavy metals concentration were significant for Cd, Cu, Cr, and Pb. As, Cd, and Sn were as much as 12 times higher than SQuiRTs standards (Buchman 2008). The results indicate that aquifer water is bringing metals from relatively far inland and releasing them into the lagoon. Thus, it appears that the contamination of this lagoon is highly complex and must take into account systemic connections with inland anthropogenic activates and pollution, as well as local factors.