The observations of physical, geological and biological impacts of seasonal internal tides in a deep embayment of the oceanic island system: Insights from outer Ambon Bay, eastern Indonesia

Abstract

The effects of seasonal internal tides at highly supercritical slope (hereafter, the slope) of outer Ambon Bay, eastern Indonesia on water stratification, sediment resuspension and the vertical chlorophyll-a (Chl-a) displacement were investigated. This study employed (i) longitudinal CTD casts starting from an offshore site with the absence of internal tides to the slope where internal tides likely exist, and (ii) 24-h CTD yoyo measurements at a point in the slope to observe hourly isothermal displacements linked to internal tides. The CTD measurements were coupled with Chl-a and turbidity profilers and were conducted during spring tide condition to consider the peak of isothermal excursion and under seasonal-varying thermocline during easterly and westerly monsoons. Both seasons under the prevailing spring tide condition showed the signatures of the phenomenon indicated by vertical isothermal excursion with the internal tide amplitude and the associated vertical velocity larger during easterly monsoon (140 m; 9 × 10–3 m/s) than during westerly monsoon (50 m; 4 × 10–3 m/s). This corresponded to the weaker stratification during easterly monsoon than westerly monsoon. The effects of seasonal internal tides to water column, sediment resuspension and Chl-a distribution are as follows. The signature of vertical mixing was detected during the measurements when comparing Brunt-Vaisala frequency (N²) in both seasons between an offshore station without internal tides and stations at the slope with internal tides. Here, the reduced stratification was found at stations located at the slope, indicating that vertical mixing occurred due to internal tide dissipation over the slope. The internal tide dissipation along the slope created bottom sediment resuspension, indicated by the increased bottom turbidity, over a larger area during easterly monsoon (1.5 km across the slope) than westerly monsoon (0.4 km across the slope). The formation of a bottom nepheloid layer (turbidity ≥3 NTU) was only observed in easterly monsoon. For the two seasons, maximum of Chl-a was observed at different depth (near surface for easterly and mid-depth for westerly monsoon) and only the mid-depth maximum was affected by the vertical displacement of the internal tides.