Primary productivity driven by escalating nutrient fluxes?

过去二十年来，北冰洋季节性海冰覆盖面积的减少是极地气候变化的一个主要指标。在同一时期，卫星观测表明，净初级生产力至少增加了30%。然而，观察到的净初级生产力的增加大于对海冰减少和无冰季节随之延长的预测反应。这意味着，硝酸盐有限的北极海洋生态系统也可能正在经历越来越多的养分供应。虽然河流营养物的影响仅限于沿海地区，但在大陆架断裂区观察到最大的净初级生产量增加。在这些地区，营养物质的主要来源是入侵的太平洋和大西洋水。然而，这些水可以存在于100米或更深的深度，因此需要物理混合过程将营养物质输送到营养物质充足的真光层。这使我们假设，所观察到的增加，净初级生产力的货架休息区是由不断升级的营养盐通量从深沃茨（大西洋，太平洋）到真光层作为一个结果，增强垂直混合率。然而，由于海冰的形成和白昼长度的变化-从冬季的永久黑暗到午夜的阳光-北极的光照具有很强的季节性，使得冬季靠近表层的营养物质积累，因此意味着营养物质流向表层的季节性循环很强。此外，我们自己的湍流测量表明，北极的混合在短至一小时的时间尺度上是高度不稳定的（因此通量变化高达3个数量级）。这些事实意味着，为了量化从中间深度到海面的营养物质通量，需要进行测量，从每小时到每季度都要进行时间尺度的测量。本项目的目的是检验这样一个假设，即增加的初级生产是通过增加养分通量而增加养分的可利用性来促进的。将收集数据，采用新的声学多普勒技术开发的班戈尔大学进行湍流混合率和营养通量估计停泊在北极大陆架断裂周围的对比位置，从每小时到整个季节周期的时间尺度上，以测试这一假设。然后将这些数据与我们和其他人在最近的2007/8国际极地年期间在这些地点进行的基线测量进行比较。新的测量结果将与同步荧光时间序列测量相结合，在一个地球化学模型的框架内，量化所观察到的营养环境变化对净初级生产力的影响，并推断季节内生态系统对特定通量事件的反应。

The decline of Arctic Ocean seasonal sea ice cover over the past two decades is a major indicator of polar climate change. Over the same period satellite observations have implied that net primary productivity (NPP) has increased by at least 30%. However, the observed increase in net primary productivity is greater than the predicted response to the declining sea ice, and the consequent lengthening of the ice-free season. This implies that the nitrate-limited Arctic marine ecosystems may also be experiencing increasing nutrient availability. Whilst the impact of riverine nutrients is limited to coastal areas the greatest net primary production increases are observed over the shelf break regions. In these regions the primary source of nutrients is intruding Pacific and Atlantic Water. However this water can reside at depths of 100 or more metres and so physical mixing processes are required to transport nutrients up to the nutrient replete euphotic zone. This leads us to hypothesize that the observed increases in net primary production in the shelf break regions are driven by escalating nutrient fluxes from the deep waters (Atlantic, Pacific) into the euphotic zone as a result of enhanced vertical mixing rates. However, there is a very strong seasonality in the availability of light in the Arctic, due to both the formation of sea ice and also changing day length - from the perpetual darkness of winter to the mid-night sun - enabling accumulation of nutrients close to the surface in winter and so implying a strong seasonal cycle in nutrient fluxes to the surface layer. Furthermore our own turbulence measurements have shown mixing in the Arctic to be highly intermittant (and in consequence fluxes varying by up to 3 orders of magnitude) on timescales as short as an hour. These facts imply that in order to quantify the flux of nutrients from intermediate depths towards the sea surface measurements are required which resolve timescales from hourly to seasonally. The aim of this project is to test the hypothesis that increased primary production is promoted by increased availability of nutrients resulting from increased nutrient fluxes. Data will be collected to test this hypothesis by employing novel acoustic Doppler techniques developed at Bangor University to make turbulent mixing rate and nutrient flux estimates from moorings at contrasting locations around the Arctic shelf break, on timescales from hourly to the full seasonal cycle. These will then be compared to baseline measurements made at these locations by ourselves and others during the recent 2007/8 International Polar Year. The new measurements will be integrated with coincident fluorescence timeseries measurement, within the framework of a biogeochemical model, to quantify the impact of the observed changes in the nutrient environment, on net primary productivity, and to deduce intra-seasonal ecosystem responses to specific flux events.