Search

We report here an interannual survey (2006-2012) of coral cover in the northwestern lagoon of New Caledonia, to assess the impact of an important dredging operation (August 2008-February 2010) associated with the construction of the largest nickel mining site in the Pacific. A BACI (Before-After Control-Impact) analysis failed to detect any significant interaction between period (before, during, and after dredging) and the category of the stations (impact vs. control). Among the 31 stations surveyed, only seven showed decreasing coral cover during the study period, mainly due to a decline in Acroporidae. However, the relationship between the dredging and this decrease was highly plausible only for one station, situated 0.9 km from the dredging site. High hydrodynamism in the study area, the abundance of resistant corals and efficient protective measures during the dredging operation might explain these localised and limited impacts.

Extensive biological field surveys are costly and time consuming. To optimize sampling and ensure regular monitoring on the long term, identifying informative indicators of anthropogenic disturbances is a priority. In this study, we used 1800 candidate indicators by combining metrics measured from coral, fish, and macro-invertebrate assemblages surveyed from 2006 to 2012 in the vicinity of an ongoing mining project in the Voh-Kone-Pouembout lagoon, New Caledonia. We performed a power analysis to identify a subset of indicators which would best discriminate temporal changes due to a simulated chronic anthropogenic impact. Only 4% of tested indicators were likely to detect a 10% annual decrease of values with sufficient power (>0.80). Corals generally exerted higher statistical power than macro-invertebrates and fishes because of lower natural variability and higher occurrence. For the same reasons, higher taxonomic ranks provided higher power than lower taxonomic ranks. Nevertheless, a number of families of common sedentary or sessile macro-invertebrates and fishes also performed well in detecting changes: Echinometridae, Isognomidae, Muricidae, Tridacninae, Arcidae, and Turbinidae for macro-invertebrates and Pomacentridae, Labridae, and Chaetodontidae for fishes. Interestingly, these families did not provide high power in all geomorphological strata, suggesting that the ability of indicators in detecting anthropogenic impacts was closely linked to reef geomorphology. This study provides a first operational step toward identifying statistically relevant indicators of anthropogenic disturbances in New Caledonia's coral reefs, which can be useful in similar tropical reef ecosystems where little information is available regarding the responses of ecological indicators to anthropogenic disturbances.

Here we identify ecological and structural characteristics of a novel plaque-like growth anomaly (GA) at outbreak levels in a population of the staghorn coral, Acropora muricata, on the Great Barrier Reef. The smooth appearance of the plaques results from thickening of skeletal structures comprising the coenosteum, leading to infilling of spaces between corallites, and was associated with hyperplasia and hypertrophy of calicodermal cells. This resulted in a two-fold reduction in corallite height, a 1.6-fold increase in corallite width, and a 2.3-fold increase in the thickness of the calicodermal layer compared to healthy corallites. Plaque-like GAs affected ~67% of corals surveyed, and on average, encased 50% of the surface area of diseased branches. Progression rates along branches averaged 0.22 mm day−1 over a 2.5-month period. GAs spread throughout colonies but their presence did not affect the linear extension rates of branches. Reproductive products were absent in 55% of GA tissues, and when present, mean oocyte and spermary numbers were reduced by 50%. However, when present, mean sizes of oocytes and spermaries did not differ between healthy and GA tissues. Symbiodinium densities were also reduced by 50% in polyps within GA tissues, which were characterized by an absence of polyp structure and chaotic arrangement of gastrovascular canals, compromising host nutrition. A three-fold increase in stores of the immune-related precursor, prophenoloxidase, within GA tissues compared to healthy tissue suggests a primed immune response. Concomitantly, only 35% of prophenoloxidase was converted to the active enzyme phenoloxidase compared to 81% in healthy tissues, consistent with inhibition of immune-related enzymatic reactions by an unknown causative agent. The increasing frequency of emerging disease hotspots highlights the importance of understanding sublethal effects of diseases that have important implications for the fitness and long-term resilience of coral populations.

Coral health data are poorly documented in New Caledonia, particularly from reefs chronically subject to anthropogenic and natural runoff. We investigated patterns of coral disease and non-disease conditions on reefs situated downstream of mining sites off the coast of New Caledonia. Surveys were conducted in March 2013 at 2 locations along the west coast and 2 locations along the east coast of the main island. Only 2 coral diseases were detected: growth anomalies and white syndrome. The most prevalent signs of compromised health at each location were sediment damage and algal overgrowth. These results support earlier findings that sedimentation and turbidity are major threats to in-shore reefs in New Caledonia. The Poritidae-dominated west coast locations were more subject to sediment damage, algal overgrowth and growth anomalies compared to the Acroporidae-dominated east coast locations. If growth form and resistance of coral hosts influence these results, differences in environmental conditions including hydro-dynamism between locations may also contribute to these outputs. Our results highlight the importance of combining coral health surveys with measurements of coral cover when assessing the health status of a reef, as reefs with high coral cover may have a high prevalence of corals demonstrating signs of compromised health.