Abstract Phytoplankton size is a significant functional trait affecting many ecological processes and influencing the carbon fate and the trophic regimes of the ecosystems. In this study, we examined the effect of the lateral advection of nutrient-rich Messina Mixed Waters on the phytoplankton size structure and community composition in the euphotic layer of the Gulf of Augusta (Western Ionian Sea) in early autumn. The chlorophyll a concentration ranged from 0.08 to 0.8 mg m−3 and the phytoplankton abundances ranged from 21 × 103 to 245 × 103 cells L−1. The size structure of phytoplankton biomass, obtained by sequential filtration of water samples, suggested the dominance of micro-planktonic diatoms in an active Deep Chlorophyll Maximum (DCM) whereas smaller forms were mainly represented by phytoflagellates. A short-term tidal-induced variability of the assemblage structure was observed at a “sentinel” station located at the Gulf shelf-break where lateral advection due to meso-scale dynamics sustained most of the phytoplankton biomass. Our research shows that, in stratified conditions, nutrient enrichment by lateral advection at DCM modifies the size-structure of phytoplankton with respect to that expected from the reference baseline oceanic model validated in open ocean waters. In particular, while smaller fraction incidences reveal a surface oligotrophic system, an increase of the micro-sized fraction is triggered at DCM: this indicates the prevalence of “new production” processes which is a quite unusual condition for the Mediterranean DCM in early autumn, when trophic web is typically sustained by “regenerate production”. Our observations suggest that both “microbial” and “herbivore” pathways could coexist and regulate the carbon flow in the Gulf in this season.

中文翻译：

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侧向平流对地中海沿岸浮游植物大小结构和组成的影响

摘要 浮游植物大小是一个重要的功能性状，影响许多生态过程，影响生态系统的碳归宿和营养体系。在这项研究中，我们研究了营养丰富的墨西拿混合水域的横向平流对早秋奥古斯塔湾（西爱奥尼亚海）的透光层中浮游植物大小结构和群落组成的影响。叶绿素 a 浓度范围为 0.08 至 0.8 mg m-3，浮游植物丰度范围为 21 × 103 至 245 × 103 个细胞 L-1。通过水样的顺序过滤获得的浮游植物生物量的大小结构表明微浮游硅藻在活跃的深叶绿素最大值 (DCM) 中占主导地位，而较小的形式主要由植物鞭毛虫代表。在位于海湾大陆架断裂处的“哨兵”站观察到由潮汐引起的组合结构的短期变化，其中由于中尺度动力学引起的横向平流维持了大部分浮游植物生物量。我们的研究表明，在分层条件下，DCM 侧向平流的养分富集改变了浮游植物的大小结构，这与在公海水域中验证的参考基线海洋模型的预期结果相比。特别是，虽然较小的部分发生率揭示了表面贫营养系统，但在 DCM 处触发了微尺寸部分的增加：这表明“新生产”过程的盛行，这对于早秋的地中海 DCM 来说是一种非常不寻常的情况，当营养网通常由“再生生产”维持时。