Recent benthic foraminifera from Twofold Bay, Eden NSW: community structure, biotopes and distribution controls

Abstract

Twofold Bay lies at the confluence of two oceanic boundaries, the near end of the East Australian Current and waters from the Southern Ocean. The Bay is NSW's largest commercial fishing port and has been home to much anthropogenic activity, ranging from whaling, mussel farming, timber export, large boat traffic, Australian Navy activities and recreational fishing. We investigated the sediments and associated foraminifer fauna in the bay and identified four different sedimentological associations linked to local currents and water depths and three foraminiferal associations, with live to dead specimen ratios established for each sample site. Three principal biotopes have been identified using cluster analysis of foraminiferal assemblages, with each biotope characterised by a specific faunal population and substrate type: (1) Inner Bay Biotope, characterised by Elphidium aff. advenum and predominantly very fine–fine sand substrates (63–250 μm); (2) Outer Bay Biotope characterised by Discorbis dimidiatus and medium–coarse sand substrates (250–1000 μm); (3) Harbour Biotope characterised by the widespread species Elphidium advenum macelliforme and bimodal sediments with very fine–coarse silts (2–63 μm) and medium sands (250–500 μm). Substrate type and energy intensity are proposed as limiting factors to foraminiferal distribution and composition. In addition, a short (21 cm long) core obtained from near the entrance of Eden Harbour has been examined in terms of foraminifer assemblages established for the bay. Microfaunal composition shifts clearly correlate with sediment colour changes, while foraminiferal and sedimentary data indicate strong concordant physical and ecological patterns. However, radiocarbon dates from various levels in the core point to sediment disturbance, contradicting this hypothesis. Hence, the dating of the sequence remains problematic.