Primary productivity driven by escalating nutrient fluxes?

过去二十年来，北冰洋季节性海冰覆盖率的下降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(NPP)至少增加了30%。然而，观测到的净初级生产力的增加大于对海冰减少以及随之而来的无冰季延长的预测反应。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营养可获得性的增加。虽然河流营养物质的影响仅限于沿海地区，但最大的净初级生产量增长出现在陆架断裂区域。在这些地区，营养物质的主要来源是侵入太平洋和大西洋的水。然而，这些水可以存在于100米或更深的地方，因此需要物理混合过程来将营养物质输送到营养充足的真光区。这导致我们假设，陆架断裂区域观测到的初级净生产量的增加是由于垂直混合速率增强而导致从深水(大西洋、太平洋)到真光区的营养通量不断增加。然而，北极地区的光照具有很强的季节性，这既是因为海冰的形成，也是因为日长的变化--从冬季的永远黑暗到午夜的太阳--使冬季营养物质能够在靠近地表的地方积累，因此意味着营养物质流向表层有一个很强的季节性循环。此外，我们自己的湍流测量显示，北极的混合是高度间歇性的(因此，通量变化高达3个数量级)，时间尺度短至一小时。这些事实意味着，为了量化从中等深度到海面的营养物质通量，需要进行测量，以确定从小时到季节的时间尺度。这个项目的目的是检验这样一种假设，即营养通量的增加导致营养物质的可获得性增加，从而促进初级生产的增加。将收集数据来验证这一假设，包括使用班戈大学开发的新的声学多普勒技术，在从每小时到整个季节周期的时间尺度上，从北极大陆架断裂和内部的不同位置的系泊地估计湍流混合率和营养物质通量。然后将这些数据与我们和其他人在最近的2007/8国际极地年在这些地点进行的基线测量进行比较。新的测量将在生物地球化学模型的框架内与一致的荧光时间序列测量相结合，以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, including employing novel acoustic Doppler techniques developed at Bangor University, to make turbulent mixing rate and nutrient flux estimates from moorngs at contrasting locations around the Arctic shelf break and interior, 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.