Landeira José M, Ferron Bruno, Lunven Michel, Morin Pascal, Marié Louis, Sourisseau Marc
Département Dynamiques de l'Environnement Côtier/Pelagos, IFREMER/Centre de Brest, Plouzané, France.
Laboratoire de Physique des Océans, UMR CNRS/IFREMER/IRD/UBO 6523, IFREMER/Centre de Brest, Plouzané, France.
PLoS One. 2014 Feb 28;9(2):e90507. doi: 10.1371/journal.pone.0090507. eCollection 2014.
Phytoplankton blooms are usually dominated by chain-forming diatom species that can alter food pathways from primary producers to predators by reducing the interactions between intermediate trophic levels. The food-web modifications are determined by the length of the chains; however, the estimation is biased because traditional sampling strategies damage the chains and, therefore, change the phytoplankton size structure. Sedimentological studies around oceanic fronts have shown high concentrations of giant diatom mats (>1 cm in length), suggesting that the size of diatom chains is underestimated in the pelagic realm. Here, we investigate the variability in size and abundance of phytoplankton chains at the Ushant tidal front (NW France) using the Video Fluorescence Analyzer (VFA), a novel and non-invasive system. CTD and Scanfish profiling characterized a strong temperature and chlorophyll front, separating mixed coastal waters from the oceanic-stratified domain. In order to elucidate spring-neap variations in the front, vertical microstructure profiler was used to estimate the turbulence and vertical nitrate flux. Key findings were: (1) the VFA system recorded large diatom chains up to 10.7 mm in length; (2) chains were mainly distributed in the frontal region, with maximum values above the pycnocline in coincidence with the maximum chlorophyll; (3) the diapycnal fluxes of nitrate enabled the maintenance of the bloom in the frontal area throughout the spring-neap tidal cycle; (4) from spring to neap tide the chains length was significantly reduced; (5) during neap tide, the less intense vertical diffusion of nutrients, as well as the lower turbulence around the chains, intensified nutrient-depleted conditions and, thus, very large chains became disadvantageous. To explain this pattern, we suggest that size plasticity is an important ecological trait driving phytoplankton species competition. Although this plasticity behavior is well known from experiments in the laboratory, it has never been reported from observations in the field.
浮游植物水华通常由形成链状的硅藻物种主导，这些物种可通过减少中间营养级之间的相互作用来改变从初级生产者到捕食者的食物路径。食物网的改变由链的长度决定；然而，这种估计存在偏差，因为传统的采样策略会破坏链，从而改变浮游植物的大小结构。大洋锋面周围的沉积学研究表明存在高浓度的巨型硅藻垫（长度>1厘米），这表明浮游生物领域中硅藻链的大小被低估了。在此，我们使用视频荧光分析仪（VFA）这一新型非侵入性系统，研究了法国西北部乌桑特潮汐锋面浮游植物链的大小和丰度变化。温盐深仪（CTD）和扫描鱼剖面分析描绘出一个强烈的温度和叶绿素锋面，将混合的沿海水域与大洋分层区域分隔开来。为了阐明锋面的大潮-小潮变化，使用垂直微结构剖面仪来估计湍流和垂直硝酸盐通量。主要发现如下：（1）VFA系统记录到长度达10.7毫米的大型硅藻链；（2）链主要分布在锋面区域，在密度跃层上方的最大值与最大叶绿素值重合；（3）硝酸盐的垂向通量使得整个大潮-小潮潮汐周期内锋面区域的水华得以维持；（4）从大潮到小潮，链的长度显著缩短；（5）在小潮期间，营养物质较弱的垂向扩散以及链周围较低的湍流加剧了营养物质耗尽的状况，因此，非常大的链变得不利。为了解释这种模式，我们认为大小可塑性是驱动浮游植物物种竞争的一个重要生态特征。尽管这种可塑性行为在实验室实验中已为人熟知，但从未在野外观察中报道过。
