Variability in benthic biomass and activity beneath the Subtropical Front, Chatham Rise, SW Pacific Ocean

Spatial variability of benthic biomass and activity was investigated along two depth gradients (350–2600 m) across Chatham Rise on 178°30′E between 41°S and 47°S, Southwest Pacific Ocean. The sampling transect lies beneath and extends either side of the highly productive Subtropical Front (STF), east of New Zealand, in subtropical and sub-Antarctic waters. Stations were occupied in austral autumn (April–May) 1997, spring (October) 1997 and summer (January–February) 2000. At each station, macrobenthic (autumn only) and meiofaunal and sediment bacterial biomass was determined with box and multi-corers, respectively. Shipboard sediment community oxygen consumption (SCOC) and sediment bacterial production (3H-thymidine incorporation) incubations were also undertaken, and the physical characteristics and organic matter content of the sediments determined at each site. Benthic biomass was elevated on the crest and upper southern flank of the rise, compared to similar depths to the north, despite similarities in sediment properties (silty sands, moderate calcium carbonate and organic content). SCOC rates were also elevated on the crest and shallow southern flank stations with seasonal averages of 120–190 μmol O2 m−2 h−1, compared with 90–150 to the north and ∼60 to the south. Sediment bacterial production rates exhibited similar, though seasonally inconsistent, spatial patterns (0.3–0.5 mgC m−2 d−1 at shallow sites compared with 0.3–0.7 to the north and 0.06–0.4 to the south). These observations are attributed to the flux of more labile organic material to the sea-floor on the southern upper flank from the STF (<1200 m), as also reflected in heightened average sediment particulate organic carbon (POC) values (0.7–0.8%, molar C:N∼14–15) at the shallowest sites (350 m on the crest and 450 m on the southern flank). The greatest contrasts were observed between the deepest stations: organic-rich (∼1% POC, C:N∼15), muddy sediments, with anomalously high macrobenthic and low meiofaunal biomass, were found on the northern flank of Chatham Rise at 2300 m, compared with organic-poor (0.2–0.3% POC, C:N ∼21–22), carbonate-dominated, muddy sands at 2300 and 2600 m on the southern flank of the rise, characterised by low benthic biomass and activity. These differences are related to the influence of meso-scale eddies, bottom currents and submarine slope mass wasting processes that operate north of Chatham Rise and the dominance of carbonate sedimentation to the south. A simple carbon budget model suggests that at times, especially autumn and summer, benthic communities on southern Chatham Rise do not receive sufficient energy from sinking organic particles (as collected by moored sediment traps) to compensate for benthic carbon remineralisation rates, resulting in >80% shortfalls in carbon demand. Despite the proximity of the productive STF, the benthic community here must derive organic carbon from lateral sources, such as the Southland Front and Wairarapa Eddy, or have life cycle strategies to survive extended periods of low food supply.