DataSheet_4_Using convective mixing in mesocosms to study climate-driven shifts in phytoplankton community distributions.csv

With climate change predicted to alter water column stability and mixing across the world’s oceans, a mesocosm experiment was designed to ascertain how a natural phytoplankton community would respond to these changes. As a departure from other mesocosm experiments, we used heating and cooling to produce four different climate-inspired mixing scenarios ranging from well-mixed water columns representative of typical open turbulence (ϵ = 3 x 10-8 m2/s3) through to a quiescent water column with stable stratification (ϵ = 5 x 10-10 m2/s3). This method of turbulence generation is an improvement on previous techniques (e.g., grid, shaker, and aeration) which tend to produce excessive dissipation rates inconsistent with oceanic turbulence observations. Profiles of classical physical parameters used to describe turbulence and mixing (turbulent dissipation rate, buoyancy frequency, turbulent eddy diffusivity, Ozmidov scale) were representative of the profiles found in natural waters under similar mixing conditions. Chlorophyll-a profiles and cell enumeration showed a clear biological response to the different turbulence scenarios. However, the responses of specific phytoplankton groups (diatoms and dinoflagellates) did not conform to the usual expectations: diatoms are generally expected to thrive under convective, turbulent regimes, while dinoflagellates are expected to thrive in converse conditions, i.e., in stable, stratified conditions. Our results suggest that responses to mixing regimes are taxon-specific, with no overwhelming physical effect of the turbulence regime. Rather, each taxon seemed to very quickly reach a given vertical distribution that it managed to hold, whether actively or passively, with a high degree of success. Future studies on the effects of climate change on phytoplankton vertical distribution should thus focus on the factors and mechanisms that combine to determine the specific distribution of species within taxa. Our convection-based mesocosm approach, because it uses a primary physical force that generates turbulence in open waters, should prove a valuable tool in this endeavor.

鉴于气候变化预计将改变全球海洋的水柱稳定性与混合过程，本研究设计了一项中宇宙实验（mesocosm experiment），以探明自然浮游植物群落（phytoplankton community）对这些变化的响应。与其他中宇宙实验不同的是，本研究通过加热与冷却手段构建了四种不同的、基于气候场景的混合情景，涵盖了代表典型开放水域湍流的充分混合水柱（ϵ = 3 × 10^-8 m²/s³）至具有稳定层化结构的静止水柱（ϵ = 5 × 10^-10 m²/s³）。该湍流生成方法相较于以往的格栅法、搅拌法与曝气法等技术更具优势——传统方法往往会产生与海洋湍流观测结果不符的过高耗散率。实验中用于描述湍流与混合过程的经典物理参数，包括湍流耗散率（turbulent dissipation rate）、浮力频率（buoyancy frequency）、湍流涡动扩散率（turbulent eddy diffusivity）与奥兹米多夫尺度（Ozmidov scale），其剖面特征与相似混合条件下自然水体中的剖面特征一致。叶绿素a（Chlorophyll-a）剖面与细胞计数结果显示，不同湍流情景下产生了显著的生物学响应。然而，特定浮游植物类群——硅藻（diatoms）与甲藻（dinoflagellates）——的响应却与常规预期不符：通常认为硅藻会在对流湍流环境中大量繁殖，而甲藻则更适宜在稳定层化的环境中生长。本研究结果表明，浮游植物对混合模式的响应具有类群特异性，湍流模式并未产生压倒性的物理影响。相反，每个类群似乎都能快速达到并维持特定的垂直分布，无论这种分布是主动还是被动形成的，且均取得了较高的维持成功率。因此，未来开展气候变化对浮游植物垂直分布影响的研究时，应聚焦于共同决定类群内物种特定分布的各类因素与机制。本研究采用的基于对流的中宇宙实验方法，通过模拟开放水域中产生湍流的主要物理驱动力，有望成为该研究方向中的一项宝贵工具。