Effects of Enhanced UV-B on Sea Ice Algae

From the abstract of some of the papers:It has been suggested that increased springtime UVB radiation caused by stratospheric ozone depletion is likely to reduce primary production and induce changes in the species composition of Antarctic marine phytoplankton. Experiments conducted at Arthur Harbour in the Antarctic Peninsula revealed a reduction in primary productivity at both ambient and increased levels of UVB. Laboratory studies have shown that most species in culture are sensitive to high UVB levels, although the level at which either growth or photosynthesis is inhibited is variable. Stratospheric ozone depletion, with resultant increased springtime UVB irradiance, has been occurring with increasing severity since the late 1970's. Thus the phytoplankton community has already experienced about 20 years' exposure to increasing levels of UVB radiation. Here we present analyses of diatom assemblages from high-resolution stratigraphic sequences from anoxic basins in fjords of the Vestfold HIlls, Antarctica. We find that compositional changes in the diatom component of the phytoplankton community over the past 20 years cannot be distinguished from long-term natural variability, although there is some indication of a decline in the production of some sea-ice diatoms. We anticipate that our results are applicable to other Antarctic coastal regions, where thick ice cover and the timing of the phytoplankton bloom protect the phytoplankton from the effects of increased UVB radiation.Growth rate, survival, and stimulation of the production of UV-B (280 to 320 nm) absorbing compounds were investigated in cultures of five commonly occurring Antarctic marine diatoms exposed to a range of UV-B irradiances. Experimental UV-B exposures ranged from 20 to 650% of the measured peak surface irradiance at an Antarctic coastal site (0.533 J per square metre per second). The five diatom species (Nitzschia lecointei, Proboscia alata, P. inermis, Thalassiosira tumida and Stellarima microtrias) appear capable of surviving two to four times this irradiance. In contrast to Phaeocystis cf. pouchetti, another major component of the Antarctic phytoplankton, the concentrations of pigments with discrete UV absorption peaks in diatoms were low and did not change significantly under increasing UV-B irradiance. Absorbance of UV-B by cells from which pigments had been extracted commonly exceeded that of the pigments themselves. Most of this absorbance was due to oxidisable cell contents, with the frustule providing the remainder. Survival of diatoms did not correlate with absorption by either pigments, frustules or oxidisable cell contents, indicating that their survival under elevated UV-B irradiances results from processes other than screening mechanisms.Springtime UV-B levels have been increasing in Antarctic marine ecosystems since the 1970's. Effects on natural phytoplankton and sea-ice algal communities, however, remain unresolved. At the Marginal Ice Edge Zone, enhanced springtime UV-B levels coincide with a shallow, stratified water column and a major phytoplankton bloom. In these areas it is possible that phytoplankton growth and survival is adversely impacted by enhanced UV-B. In coastal areas, however, the sea ice, which attenuates most of the UV-B before it reaches the water column, remains until December/January, by which time UV-B levels have returned to long-term seasonal averages. Phytoplankton from these areas are unlikely to show long-term changes resulting from the hole in the ozone layer. Fjords of the Vestfold Hills, eastern Antarctica, have anoxic basins which contain high-resolution, unbioturbated sedimentary sequences. Diatom assemblages from these sequences reflect the diatom component of the phytoplankton and sea-ice algal assemblages at the time of deposition. Twenty-year records from these sequences show no consistent record of change in species composition, diversity or species richness. Six-hundred-year records from the same area also show changes in species abundance greater than those seen in the last 20 years. From these records it can be seen that recent changes in diatom abundances generally fall within the limits of natural variability and there is little evidence of recent changes that might be associated with UV-B-induced change.

部分相关论文摘要显示：已有研究表明，由平流层臭氧耗竭引发的春季紫外线B（UVB）辐射增强，可能会降低南极海洋浮游植物的初级生产力，并改变其物种组成。在南极半岛亚瑟港开展的实验显示，无论在自然背景还是增强的UVB辐射水平下，初级生产力均出现下降。实验室研究表明，培养体系中的多数物种对高剂量UVB辐射敏感，但抑制其生长或光合作用的UVB阈值存在物种差异。自20世纪70年代末以来，平流层臭氧耗竭及其导致的春季UVB辐照度增强问题日益严峻，南极浮游植物群落已处于UVB辐射水平持续升高的环境中达约20年之久。
本研究针对南极维斯特福尔丘陵峡湾缺氧盆地的高分辨率地层序列开展硅藻组合分析。研究发现，过去20年间浮游植物群落中硅藻组分的组成变化，无法与长期自然变异区分开来，但有迹象表明部分海冰硅藻的生物量有所下降。本研究结果推测可推广至其他南极沿海区域——这些区域的厚冰盖与浮游植物水华的发生时序，可保护浮游植物免受UVB辐射增强的影响。
针对5种常见南极海洋硅藻的培养体系，本研究探究了其在不同UV-B（280~320 nm）辐照度下的生长速率、存活情况，以及UV-B吸收化合物的合成诱导效应。实验设置的UV-B暴露剂量范围为南极沿海站点实测峰值表面辐照度（0.533 J·m⁻²·s⁻¹）的20%~650%。这5种硅藻（*Nitzschia lecointei*、*Proboscia alata*、*P. inermis*、*Thalassiosira tumida*及*Stellarima microtrias*）可在该辐照度的2~4倍条件下存活。与南极浮游植物的另一主要类群疑似鲍氏囊泡藻（*Phaeocystis cf. pouchetti*）不同，硅藻中具有明确UV吸收峰的色素浓度较低，且随UV-B辐照度升高未发生显著变化。去除色素后的硅藻细胞对UV-B的吸收率通常高于色素本身的吸收率。该吸收率主要来自可氧化细胞内含物，剩余部分则由硅质壳（frustule）贡献。硅藻的存活率与色素、硅质壳或可氧化细胞内含物的吸收能力均无相关性，表明其在高剂量UV-B辐照度下的存活机制并非依靠屏蔽作用。
自20世纪70年代以来，南极海洋生态系统的春季UV-B水平持续升高，但该辐射对自然浮游植物及海冰藻类群落的影响仍未明确。在海冰边缘区，春季UV-B水平升高的时段恰好对应浅水层化水体与大规模浮游植物水华的发生期，因此该区域的浮游植物生长与存活可能受到UV-B增强的负面影响。但在沿海区域，海冰会在UV-B抵达水层前削弱大部分辐射，且海冰消融时间为12月至次年1月，此时UV-B水平已回落至长期季节平均水平。因此该区域的浮游植物不太可能出现由臭氧洞引发的长期变化。
南极东部维斯特福尔丘陵的峡湾内存在缺氧盆地，其中保存有高分辨率、未受生物扰动的沉积序列。该序列中的硅藻组合可反映沉积时期浮游植物与海冰藻类群落的硅藻组分特征。基于该序列得到的20年记录显示，硅藻的物种组成、多样性及丰富度均未出现一致性变化。同一区域的600年记录显示，物种丰度的变化幅度甚至大于过去20年的观测结果。综上可见，近期硅藻丰度的变化通常处于自然变异的范围之内，几乎没有证据表明存在与UV-B辐射诱导相关的近期变化。