Enhanced benthic response to upwelling of the Indonesian Throughflow onto the southern shelf of Timor‐Leste, Timor Sea

Abstract

Benthic microbial metabolism and bacterial diagenetic pathways were measured along the southern shelf of Timor‐Leste during an upwelling event in the winter SE monsoon season. Vertical profiles of water properties and bottom water nutrient concentrations, and operational ocean modeling showed subsurface upwelling from the Indonesian Throughflow (ITF) along the southern shelf west of longitude 126°25′E and surface upwelling at the far eastern end of the shelf. Warm surface waters above the halocline had salinities of 33.6 to 33.9 overlying cooler ITF water with salinities of 34.4 to 34.6. Beneath the zone of subsurface upwelling and stratification, sediment chlorophyll a (range: 2.8–4.4 µg g−1) and phaeopigment (range: 4.5–7.0 µg g−1) concentrations were sufficient to fuel very rapid rates of benthic oxygen consumption (range: 89.9–142.3 mmol m−2 day−1) and dissolved inorganic carbon (DIC) release (range: 108.1–148.9 mmol m−2 day−1) across the sediment‐water interface, and DIC (range: 94.7–142.5 mmol m−2 day−1) and NH4+ (range: 13.3–19.9 mmol m−2 day−1) production from incubated surface (0–10 cm) sediments. Molar ratios of DIC/NH4+ production were lower (range: 6.6–7.7) in fine‐grained sediments under the subsurface upwelling regime than in sandy, possibly scoured sediments under surface upwelling (range: 11.9–21.2) where there was no evidence of benthic enrichment. It is proposed that subsurface upwelling along the widest portions of the shelf stimulates phytoplankton production, leading to deposition of fresh phytodetritus that is rapidly decomposed on the seafloor. These zones of high biological activity may attract and support large populations of pelagic fish and cetaceans that have been caught for centuries along the south coast.