Effect of shallow-water hydrothermal venting on the biota of Matupi Harbour (Rabaul Caldera, New Britain Island, Papua New Guinea)

Coastal hydrothermal vents in the depth range 0–27 m were studied in Matupi Harbour, a marine bight that is partly isolated from the open sea on the north–east coast of New Britain, Papua New Guinea, where volcanoes are active. Planktonic and benthic communities (including bacteria) in the Harbour were compared with adjacent fully marine areas. The environmental parameters assessed included temperature, salinity, O2, H2, hydrocarbons, metals, some species of nitrogen and sulphur, inorganic phosphate, silicate and chlorophyll a. There was pronounced stratification of the waters in the Harbour as a result of inflow of heated volcanic fluids, most evident in the surface 0–3 m layer. The volcanic fluids are rich in phytoplankton nutrients and reduced compounds which stimulate growth of bacterial plankton, bacterial production and enhance primary production. The highest values of photosynthetic fixation (almost 100 mg C m3 d) and bacterial production (216 mg C m3 d) found in the Harbour are much greater than previously reported for marine ecosystems. The chemosynthetic fraction of bacterial production varied from 15 to 60%, with highest values in the surface and the near-bottom layers. The zooplankton in the Harbour was dominated by cyclopoid copepods and was low in diversity. Bacterial mats were very evident at the areas of venting, dominated by Thiodendron-like species, but both epifauna and infauna were sparse at the vents. However, areas adjacent to the hydrothermal vents showed the richest benthic communities, with epifauna dominated by corals and sponges and infauna by nematodes. It appeared that benthic community development was inhibited by the hot sediment, rapid sedimentation, lack of hard substratum and large numbers of sponge spicules in the sediment. From dating of the last major volcanic eruption (1943), the Harbour provides an excellent model for studying processes of succession and adaptation in marine communities stressed by shallow-water gasohydrothermal activity.