Fluorescent and skeletal density banding in Porites lutea from Papua New Guinea and Indonesia

Abstract

Shallow water Porites lutea corals were collected along two transects normal to mainland shorelines, parallel to gradients in water quality: one, 7 km long, near Motupore Island in South Papua New Guinea, the other, 70 km long, from Jakarta Bay along the Pulau Seribu chain in the Java Sea. The corals were slabbed and studies were made of skeletal density bands as revealed by X-ray photography and fluorescent bands as revealed by ultraviolet light. Water quality measurements and rain-fall data were assembled for the two areas and related to skeletal banding patterns. For both areas, with increasing distance form mainland there is a decrease in overall brightness of fluorescence in corals and an increase in the contrast between bright and dull fluorescent bands. Fluorescence is bright, but seasonal banding is obscure in corals within about 2 km of stream mouths at Motopure and about 5 km of the coast in Jakarta Bay; this suggests that, despite low freshwater run-off during dry seasons, there are sufficient organic compounds which cause fluorescence in coral skeletons, to swamp seasonal effects. During the wet seasons, deluges of freshwater consequent on mainland rainfall of greater than about 150 mm/ month extend at least 7 km offshore in the Motupore area and perhaps tens of kilometres into Java Sea, giving distinctive bright and dull fluorescent banding in off-shore corals. The fluorescent banding pattern within corals on the Motupore reefs is similar in most corals along the transect and it correlates well with the Port Moresby monthly rainfall data. This relationship suggests that the same body (or bodies) of freshwater affect all reefs of the area during the wet season. The fluorescent banding in Java Sea corals does not show a precise correlation with either mainland or island monthly rainfall data; indeed the pattern of fluorescent banding on Pulau Seribu can only be matched in corals from reefs less than about 25 km apart. This suggests that in this area discrete water bodies carrying the relevant organic acids for coral fluorescence affect the fringing reefs on the chain of islands. Comparisons of fluorescent and density banding have revealed that for these areas, in general, periods of high freshwater run-off are times of deposition of less dense skeleton in Porites lutea corals.