The Red Sea depicts a north–south gradient of positively correlated temperature and nutrient concentration. Despite its overall oligotrophic characteristics, primary production rates in the Red Sea vary considerably. In this study, based on five cruises and a 2-year time series (2016–2018) sampling in the Central Red Sea, we determined phytoplankton photosynthetic rates (PP) by using ¹³C as a tracer and derived phytoplankton net growth rates (μ) and chlorophyll a (Chl a)-normalized photosynthesis (P^B). Our results indicate a ninefold variation (14–125 mgC m⁻² h⁻¹) in depth-integrated primary production and reveal a marked seasonality in PP, P^B, and μ. Depth-integrated PP remained <30 mg C m⁻² h⁻¹ during spring and summer, and peaked in autumn–winter, particularly in the southernmost stations (~17°N). In surface waters, phytoplankton grew at a slow rate (0.2 ± 0.02 d⁻¹), with the population doubling every 3.5 days, on average. However, during the autumn–winter period, when Chl a concentrations peaked in the central and southern regions, μ increased to values between 0.60 and 0.84 d⁻¹, while P^B reached its maximum rate (7.8 mgC [mg Chl a]⁻¹ h⁻¹). We used path analysis to resolve direct vs. indirect components between correlations. Our results show that nutrient availability modulates the photosynthetic performance and growth of phytoplankton communities and that P^B and μ fluctuations are not directly associated with temperature changes. Our study suggests that similarly to other oligotrophic warm seas, phosphorus concentration exerts a key role in defining photosynthetic rates and the biomass levels of phytoplankton communities in the region.

中文翻译：

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热带生态系统初级生产和浮游植物净生长的营养和温度限制

红海描绘了温度和营养浓度呈正相关的南北梯度。尽管具有整体贫营养特征，但红海的初级生产力差异很大。在这项研究中，基于红海中部的五次航行和 2 年时间序列（2016-2018）采样，我们通过使用¹³ C 作为示踪剂来确定浮游植物光合速率（PP），并得出浮游植物净增长率（μ ) 和叶绿素a (Chl a )-归一化光合作用 (P ^B )。我们的结果表明深度整合的初级生产有九倍的变化（14-125 mgC m ^-2 h ^-1），并揭示了 PP、P ^B的显着季节性, 和μ。深度综合 PP在春季和夏季保持 <30 mg C m ^-2 h ^-1，并在秋季-冬季达到峰值，特别是在最南端的站点（~17°N）。在地表水中，浮游植物以缓慢的速度 (0.2 ± 0.02 d ^-1 )生长，平均每 3.5 天数量就会翻一番。然而，在秋冬季，当 Chl a浓度在中部和南部地区达到峰值时，μ增加到 0.60 和 0.84 d ^-1之间的值，而 P ^B达到最大值（7.8 mgC [mg Chl a ] ^-1 h ^-1）。我们使用路径分析来解决相关性之间的直接与间接分量。我们的研究结果表明，养分有效性调节浮游植物群落的光合性能和生长，并且 P ^B和μ波动与温度变化没有直接关系。我们的研究表明，与其他贫营养暖海类似，磷浓度在确定该地区浮游植物群落的光合速率和生物量水平方面发挥着关键作用。