Net community production, Thorium-234 activity and particle flux during the Southern Ocean iron fertilization experiment LOHAFEX@en

A closed eddy core in the Subantarctic Atlantic Ocean was fertilized twice with two tons of iron (as FeSO4), and the 300 km**2 fertilized patch was studied for 39 days to test whether fertilization enhances downward particle flux into the deep ocean. Chlorophyll a and primary productivity doubled after fertilization, and photosynthetic quantum yield (FV/FM) increased from 0.33 to >0.40. Silicic acid (<2 µmol/L) limited diatoms, which contributed <10% of phytoplankton biomass. Copepods exerted high grazing pressure. This is the first study of particle flux out of an artificially fertilized bloom with very low diatom biomass. Net community production (NCP) inside the patch, estimated from O2:Ar ratios, averaged 21 mmol POC/m**2/d, probably ±20%. 234Th profiles implied constant export of ~6.3 mmol POC/m**2/d in the patch, similar to unfertilized waters. The difference between NCP and 234Th-derived export partly accumulated in the mixed layer and was partly remineralized between the mixed layer and 100 m. Neutrally buoyant sediment traps at 200 and 450 m inside and outside the patch caught mostly <1.1 mmol POC/m**2/d, predominantly of fecal origin; flux did not increase upon fertilization. Our data thus indicate intense flux attenuation between 100 and 200 m, and probably between the mixed layer and 100 m. We attribute the lack of fertilization-induced export to silicon limitation of diatoms and reprocessing of sinking particles by detritus feeders. Our data are consistent with the view that nitrate-rich but silicate-deficient waters are not poised for enhanced particle export upon iron addition.在南大西洋亚极地海域的一处闭合涡旋核心（closed eddy core）区域，研究团队以2吨硫酸亚铁（FeSO₄）的形式分两次施加铁肥，并对面积达300平方千米的施肥斑块开展了为期39天的研究，旨在验证铁施肥是否可提升向深海输送的颗粒通量（particle flux）。施肥后，叶绿素a（Chlorophyll a）浓度与初级生产力（primary productivity）均提升一倍，光合量子产率（photosynthetic quantum yield, F_V/F_M）从0.33上升至0.40以上。硅酸（silicic acid，浓度低于2微摩尔/升）限制了硅藻（diatoms）的生长，而硅藻仅占浮游植物生物量（phytoplankton biomass）的10%以下。桡足类（Copepods）对浮游植物施加了极强的牧食压力。本研究是首个针对硅藻生物量极低的人工施肥水华体系的颗粒通量输出开展的调查研究。基于氧氩比（O₂:Ar ratios）估算得到的施肥斑块内群落净生产（Net community production, NCP）平均值为21毫摩尔颗粒有机碳（Particulate Organic Carbon, POC）·平方米⁻¹·天⁻¹，误差范围约为±20%。钍-234（²³⁴Th）的垂直剖面显示，施肥斑块内的颗粒输出通量稳定在约6.3毫摩尔颗粒有机碳·平方米⁻¹·天⁻¹，与未施肥海域的水平相当。群落净生产与钍-234推导得到的输出通量之间的差值，一部分在混合层（mixed layer）内积累，另一部分在混合层至100米水深的区间内发生再矿化作用。放置在斑块内外200米和450米水深的中性浮式沉积物捕集器（Neutrally buoyant sediment traps）所捕获的颗粒通量大多低于1.1毫摩尔颗粒有机碳·平方米⁻¹·天⁻¹，且捕获的颗粒主要来源于粪粒；施肥后通量并未出现上升。因此本研究数据表明，在100米至200米水深区间内存在强烈的通量衰减现象，且该衰减可能同样存在于混合层至100米的水深范围内。我们将施肥未引发输出通量上升的原因归因于两点：一是硅藻受硅元素限制，二是碎屑食性动物（detritus feeders）对沉降颗粒（sinking particles）的再加工作用。本研究数据与下述观点一致：当海域富含硝酸盐（nitrate）但硅酸盐（silicate）匮乏时，施加铁肥并不会提升颗粒输出通量。