Abstract Oceanographic and biogeochemical observations collected in Pemba Channel, a deep-water (800 m) channel separating Pemba Island from mainland Tanzania, during the South East monsoon indicate the presence of active upwelling along the western edge of Pemba Island. Surface salinity values, nutrient concentrations and the presence of coccolithophore species previously reported from the mid to lower euphotic zone all suggest upwelling from at least 80–100 m depth. The surface waters of the channel were characterised with low NO3−:PO43− (0.68:1) and NO3−:Si (0.04:1) ratios far below the Brzezinski-Redfield ratio indicating the presence of N-limitation and the possibility that these waters may be susceptible to anthropogenic N inputs. Surface NO3− concentrations averaged 0.09 ± 0.10 μmol L−1 but increased to 0.5 μmol L−1 in the centre of upwelling where coincidentally both integrated nutrient concentrations and surface POC/PON pools were approximately 2-fold higher than the channel average. Despite its significance for local productivity upwelling is tentatively estimated, via stoichiometric assumptions, to enhance local productivity by only 20%. The modest productivity response to upwelling may be explained by picoplankton (0.2–2 μm) dominance of the phytoplankton community with this size-class representing ~80% of total chlorophyll-a. Nevertheless, important spatial variability was identified in larger size fractions and supported by taxonomic analyses with indications that the distribution of Chaetoceros spp. alone may be particularly relevant for understanding the variability in larger (>20 μm) chlorophyll-a size fractions. The location of upwelling has previously been shown to host large concentrations of small pelagic fish thus management of this regionally important resource would benefit from additional investigation of the underlying physical mechanism driving upwelling and subsequently how trophic interactions and ecosystem productivity are influenced.

中文翻译：

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东南季风期间奔巴海峡（坦桑尼亚）局部上升流的证据

摘要 在东南季风期间，在奔巴海峡（一条将奔巴岛与坦桑尼亚大陆分隔开的深水（800 m）通道）收集的海洋学和生物地球化学观测表明，奔巴岛西部边缘存在活跃的上升流。表面盐度值、养分浓度和先前报道的中至低透光带中存在的颗石藻物种都表明从至少 80-100 m 深度上涌。通道地表水的特征是低 NO3-:PO43- (0.68:1) 和 NO3-:Si (0.04:1) 比率远低于 Brzezinski-Redfield 比率，表明存在氮限制以及这些水域可能容易受到人为氮输入的影响。表面 NO3− 浓度平均为 0.09 ± 0.10 μmol L−1，但增加到 0。5 μmol L-1 在上升流中心，巧合的是，综合养分浓度和表面 POC/PON 池比通道平均值高约 2 倍。尽管它对当地生产力上升具有重要意义，但通过化学计量假设，初步估计只能将当地生产力提高 20%。对上升流的适度生产力反应可以通过浮游植物群落的微型浮游生物（0.2-2μm）优势来解释，该大小等级占总叶绿素 a 的约 80%。然而，重要的空间变异性在较大的尺寸部分中被识别出来，并得到了分类学分析的支持，表明角角藻的分布。单独使用可能与理解较大（> 20 μm）叶绿素-a 大小分数的变异性特别相关。