Marine phytoplankton play a central role in supporting life in the oceans and profoundly affect global biogeochemical cycles. Previous studies have revealed positive effects of sea-surface temperature (SST) on phytoplankton in terms of chlorophyll a concentrations (Chla) in high latitude oceans, while negative effects prevail in tropical and midlatitude oceans as well as under stratified summer conditions at higher latitudes. Based on a global analysis of 20 years of ocean Chla and SST data, we first investigated how interannual variability in SST is associated with Chla for each month of the season for every ocean province. We then quantified how the SST-Chla relationships varied with the long-term average (baseline) SST. We found significant season-dependent SST effects on Chla in most ocean provinces. The signs and magnitudes of these effects were consistently associated with the baseline SST, with a shift from positive to negative effects of SST on Chla around 14°C. Based on field observations and literature data, we also estimated the interaction between nitrate limitation and temperature on the SST-Chla relationship. Our findings suggest that the ocean warming effects on Chla depend consistently on the baseline temperature, both with regard to seasonal effects within regions and regional effects between high and low latitude provinces. Our analysis further suggests that the monthly 14°C isotherms can be used as a first approximation to separate areas and seasons where warming has opposite signed effects.

中文翻译：

---

浮游植物对海洋变暖的非线性响应的阈值海面温度在14°C

海洋浮游植物在支持海洋生物方面发挥着核心作用，并深刻影响全球生物地球化学循环。先前的研究已经揭示了海面温度（SST）对叶绿素a对浮游植物的积极影响高纬度海洋中的浓度（Chla），而负面影响普遍存在于热带和中纬度海洋中以及在较高纬度的分层夏季条件下。基于对20年海洋Chla和SST数据的全球分析，我们首先调查了每个海洋省每个季节每个月SST的年际变化与Chla的关系。然后，我们量化了SST-Chla关系如何随长期平均（基准）SST的变化。在大多数海洋省份中，我们发现对Chla的季节依赖性SST效应显着。这些影响的迹象和严重程度与基线SST一致，在14°C左右，Sla对Chla的影响从正向转移到负向。根据实地观察和文献数据，我们还估计了硝酸盐限制和温度之间在SST-Chla关系上的相互作用。我们的研究结果表明，海洋变暖对Chla的影响始终取决于基线温度，无论是区域内的季节性影响还是高低纬度省份之间的区域影响。我们的分析进一步表明，每月14°C的等温线可以用作对变暖影响相反的单独区域和季节的第一近似值。