Foraminiferal distribution and diversity, Madang Reef and Lagoon, Papua New Guinea

Abstract

In the Madang Lagoon, on the northern coast of Papua New Guinea (PNG), distinct groups of foraminifera, defined by numerical Q-mode cluster analysis of foraminiferal species occurrences, occupy four major environments and sedimentary regimes, generally aligned parallel to the coast: (1) the harbor and bay inlets, which have large fresh-water runoff and organic detrital inputs; (2) the fringing reefs along the west side of the lagoon which are influenced by coastal factors such as overhanging mangroves or fresh-water runoff; (3) the central lagoon floor which is over 50 m deep and covered with fine sand and patch reefs rising from it; and (4) the reef barrier with adjacent live coral-covered fore-reef slope and generally sandy back-reef slope. The four clusters are also mirrored in both species richness and Fisher alpha diversity analysis. Cluster 4 includes 79 species of large, thick-shelled miliolids, robust agglutinated species, calcarinids, and amphisteginids (Fisher α ≥20) that occur on the coral-rich barrier reef and back-reef. Cluster 3 has 50 species (Fisher α=8-20) and occupies the central lagoon floor. Cluster 2 has 25 or fewer species (Fisher α=2-6) and occurs on the shallow fringing reefs. Cluster 1 is the least diverse (≤ 7 species, Fisher α ≤ 2) and occurs in the harbors and bays in the mouths of larger rivers and streams. The larger, endosymbiont-bearing foraminifera (alveolinellids, soritids, amphisteginids, nummulitids, and calcarinids) generally live on the back- and fore-reef slopes and in the lagoon, avoid the organic-rich coastal and harbor habitats, and preferentially dwell in well-lit environments to the bottom of the lagoon. The river mouths and bays are unusual for reef systems because of their high organic content, which creates low-oxygen and nutrient-rich conditions. Here the foraminiferal fauna is dominated by only a few and, for the most part, particularly thin-shelled and highly fragile species. Each faunal group contains a number of numerically abundant indicator species that do not occur in other faunal clusters. This implies low horizontal transport rates within the reef and lagoon complex and signifies that faunal mixing among the cluster groups is limited. Foraminiferal death assemblages may thus be autochthonous and retain information regarding the original community structure. They may also preserve environmental information useful in paleoecological studies and they are good ecological indicators of reef and lagoon habitats.