Microbial respiration of organic matter (OM) is a key driver of deoxygenation and hypoxia. In fjord-like estuaries with established aquaculture industries understanding drivers of oxygen demand, and the relative importance of different drivers, is crucial for improving fish farming management in those systems. We designed a study to examine patterns of pelagic oxygen demand (POD) in a fjord-like estuary on the west coast of Tasmania, Macquarie Harbour, and relate those observations to physical forcings and major OM sources. Monthly water column sampling and bottle incubation experiments were conducted from June to November 2017. Water was collected throughout the harbour including river and oceanic endmembers as well as transects leading away from fish farms. Water was incubated from 4 different depths spanning the surface water to the seabed. Regression modelling was used to examine the relationship between POD, riverine OM loading, proximity to fish farms and the major system endmembers, depth, harbour region, concentration of dissolved oxygen, and month. POD reached rates as high as 0.108 mg L⁻¹ hour⁻¹ with the greatest POD observed above the halocline and during high river flow/OM loading months. Regression modelling showed that important drivers of POD are spatially specific along vertical and longitudinal gradients. The importance of riverine OM loading waned with depth primarily due to mixing dynamics of dissolved organic carbon across the halocline. Proximity to fish farms was an important but localized explanatory variable for POD in the halocline and basin waters, but not a significant driver of POD compared to the Gordon River. Based on the POD rates observed in this study, hypoxia can be established in less than 9 days in the basin waters and is primarily driven by pelagic oxygen sinks (95%–98%), not sediment sinks. It is crucial that aquaculture management accounts for natural, and/or preexisting, variation in endmember OM loading and its effect on DO dynamics, in these systems.

有机物（OM）的微生物呼吸作用是脱氧和缺氧的关键驱动因素。在拥有类似水产养殖业的峡湾式河口中，了解氧气需求的驱动因素以及不同驱动因素的相对重要性，对于改善这些系统中的养鱼管理至关重要。我们设计了一项研究，以检查麦格理港塔斯马尼亚岛西海岸峡湾状河口中上层氧气需求（POD）的模式，并将这些观测值与物理强迫和主要OM来源相关联。2017年6月至2017年11月进行了每月水柱采样和瓶子孵化实验。整个港口都收集了水，包括河流和海洋末端以及从养鱼场引出的样带。从跨越地表水到海床的4个不同深度温育水。回归模型用于检验POD，河道OM负荷，与养鱼场的距离以及主要系统末端成员，深度，港口区域，溶解氧浓度和月份之间的关系。POD含量高达0.108 mg L^-1小时^-1在盐湖上方和高河流量/ OM加载月份中观察到最大的POD。回归模型表明，POD的重要驱动因素沿垂直和纵向梯度在空间上是特定的。河流有机物负荷的重要性随着深度的增加而减弱，这主要是由于整个盐湖中溶解的有机碳的混合动力学。靠近鱼类养殖场是卤水和流域水域POD的重要但局部的解释性变量，但与戈登河相比，POD并不是重要的驱动因素。根据本研究中观察到的POD率，缺氧可以在不到9天的时间内建立在盆地水域中，并且主要由上层氧气汇（95％–98％）而非沉积物汇驱动。水产养殖管理必须考虑自然和/或已存在的，