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。同期的卫星观测表明，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的预测响应，以及由此导致的无冰季节的延长。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养分供应的增加。虽然河流营养物质的影响仅限于沿海地区，但在大陆架断裂区观察到的净初级产量增幅最大。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的水。然而，这些水可能存在于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丰富的光区。这使我们假设，在陆架断裂区观测到的净初级产量的增加是由于垂直混合率增强，从深水（大西洋、太平洋）进入光带的养分通量不断增加所驱动的。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），北极光照的可得性具有很强的季节性，这使得营养物在冬季在接近地表的地方积累起来，因此意味着营养物向表层的流动具有很强的季节性循环。此外，我们自己的湍流测量表明，在短至一小时的时间尺度上，北极的混合具有高度间歇性（因此通量变化可达3个数量级）。这些事实表明，为了量化从中深流向海面的营养物通量，需要进行从每小时到每季节的时间尺度的测量。这个项目的目的是检验这样一种假设，即由于养分流动增加而增加了养分供应，从而促进了初级生产的增加。为了验证这一假设，将收集数据，采用班戈大学开发的新型声学多普勒技术，在从每小时到整个季节周期的时间尺度上，从北极大陆架断裂周围不同位置的系泊处估算湍流混合率和营养通量。然后将这些数据与我们和其他人在最近的2007/8国际极地年期间在这些地点进行的基线测量进行比较。新的测量将与生物地球化学模型框架内的同步荧光时间序列测量相结合，量化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.