Photosynthesis versus irradiance relationships were used to estimate primary production (PP) at a number of locations across the seasonally stratified Celtic Sea during summer in 2003 and 2005. A subsurface chlorophyll maximum (SCM) was ubiquitously located at the base of the seasonal thermocline on the nitracline. Average PP for the stratified waters was 170 to 390 mg C m−2 d−1, of which 40 to 50% occurred within the thermocline, the range reflecting cloudy or sunny conditions. Phytoplankton photosynthesis was light-limited at the SCM, resulting in PP in the thermocline being more sensitive to surface irradiance conditions than that in the surface mixed layer. During the stratified period, production fuelled by nitrate from below the thermocline was estimated to be ~10 to 19 g C m−2, around half that determined for the spring bloom. Comparison of carbon (14C) fixation and nitrate (15NO3–) uptake versus irradiance experiments revealed that the proportion of 15NO3– uptake compared to 14C fixed was higher at the SCM than in the upper part of the thermocline where nitrate was depleted. The proportion also decreased with increasing irradiance. Estimates of 15NO3– and 14C uptake, dissolved oxygen production and the potential vertical diffusive nitrate flux from below suggested that ‘excess’ nitrate is assimilated relative to photosynthetic carbon fixation in the thermocline, particularly under cloudy conditions.
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机译:利用光合作用与辐照度的关系来估算2003年和2005年夏季在季节性分层凯尔特海中多个地点的初级生产力(PP)。地下叶绿素最大值(SCM)无处不在地位于季节温跃层的底部。硝苯胺。分层水的平均PP为170至390 mg C m-2 d-1,其中40至50%发生在温跃层中,该范围反映了多云或晴天。浮游植物的光合作用在SCM处受光限制,导致温跃层中的PP对表面辐照条件的敏感性高于表面混合层中的PP。在分层期间,估计从温跃层以下以硝酸盐为燃料的产量约为10至19 g C m-2,约为春季开花的一半。碳(14C)固定和硝酸盐(15NO3-)吸收与辐照度实验的比较显示,与14C固定相比,SCM中15NO3-吸收的比例高于硝酸盐被耗尽的温跃层上部。比例也随着辐照度的增加而降低。从下面估算的15NO3和14C吸收量,溶解氧产生量和潜在的垂直扩散硝酸盐通量表明,“过量”的硝酸盐相对于热跃层中的光合作用碳固着是同化的,尤其是在多云条件下。
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