Briggs Nathan, Guðmundsson Kristinn, Cetinić Ivona, D'Asaro Eric, Rehm Eric, Lee Craig, Perry Mary Jane
National Oceanography Centre, Southampton SO14 3ZH, UK.
Marine Research Institute, PO Box 1390, 121 Reykjavík, Iceland.
Biogeosciences. 2018 Jul;15(14):4515-4532. doi: 10.5194/bg-15-4515-2018. Epub 2018 Jul 25.
Fixation of organic carbon by phytoplankton is the foundation of nearly all open-ocean ecosystems and a critical part of the global carbon cycle. But quantification and validation of ocean primary productivity at large scale remains a major challenge, due to limited coverage of ship-based measurements and the difficulty of validating diverse measurement techniques. Accurate primary productivity measurements from autonomous platforms would be highly desirable, due to much greater potential coverage. In pursuit of this goal we estimate gross primary productivity over two months in the springtime North Atlantic from an autonomous Lagrangian float using diel cycles of particulate organic carbon derived from optical beam attenuation. We test method precision and accuracy by comparison against entirely independent estimates from a locally parameterized model based on chlorophyll and light measurements from the same float. During nutrient replete conditions (80% of the study period), we obtain strong relative agreement between the independent methods across an order of magnitude of productivities (r=0.97), with slight under-estimation by the diel cycles method (-19±5 %). At the end of the diatom bloom, this relative difference increases to -58 % for a six-day period, likely a response to SiO4 limitation, which is not included in the model. In addition, we estimate gross oxygen productivity from O diel cycles and find strong correlation with diel cycles-based gross primary productivity over the entire deployment, providing further qualitative support to both methods. Finally, simultaneous estimates of net community productivity, carbon export and particle size suggest that bloom growth is halted by a combination of reduced productivity due to SiO limitation and increased export efficiency due to rapid aggregation. After the diatom bloom, high chlorophyll normalized productivity indicates that low net growth during this period is due to increased heterotrophic respiration and not nutrient limitation. These findings represent a significant advance in the accuracy and completeness of upper ocean carbon cycle measurements from an autonomous platform.
浮游植物对有机碳的固定是几乎所有开阔海洋生态系统的基础，也是全球碳循环的关键部分。但是，由于船基测量覆盖范围有限以及验证多种测量技术存在困难，大规模海洋初级生产力的量化和验证仍然是一项重大挑战。由于自主平台具有更大的潜在覆盖范围，因此非常需要从自主平台获得准确的初级生产力测量值。为了实现这一目标，我们利用基于光束衰减的颗粒有机碳的日变化周期，从一个自主拉格朗日浮标估计了春季北大西洋两个月内的总初级生产力。我们通过与基于同一浮标叶绿素和光测量的局部参数化模型的完全独立估计值进行比较，来测试方法的精度和准确性。在营养丰富的条件下（占研究期的80%），我们在一个数量级的生产力范围内获得了两种独立方法之间的强相对一致性（r=0.97），日变化周期法略有低估（-19±5%）。在硅藻大量繁殖末期，这一相对差异在六天内增加到-58%，这可能是对模型中未包括的SiO4限制的响应。此外，我们从O日变化周期估计了总氧生产力，并发现与整个部署期间基于日变化周期的总初级生产力有很强的相关性，为这两种方法提供了进一步的定性支持。最后，对净群落生产力、碳输出和颗粒大小的同时估计表明，硅藻大量繁殖的增长因SiO限制导致的生产力下降和快速聚集导致的输出效率提高而停止。在硅藻大量繁殖之后，高叶绿素标准化生产力表明，这一时期的低净生长是由于异养呼吸增加而非营养限制。这些发现代表了自主平台在海洋上层碳循环测量的准确性和完整性方面的重大进展。
