Human activities, increasing precipitation, and changes in land run-off deliver a large input of allochthonous nutrients into coastal waters including terrigenous dissolved organic matter (tDOM). Increased subsidies of tDOM into the coastal water are expected to reduce light availability and thus might be one of the factors causing coastal ocean darkening. To investigate the effect of increased tDOM input and thus limitation in light availability on primary production as well as the transfer to higher trophic levels (zooplankton), we conducted a large-scale indoor mesocosm “Planktotrons” experiment with natural (pelagic and benthic) plankton communities from the North Sea. We simulated a coastal ocean system with daily light and tidal cycles for 35 days. The experimental treatments included a light gradient consisting of three levels of tDOM addition (i.e., low tDOM, medium tDOM, and high tDOM) and a control without tDOM addition. Results showed that tDOM addition reduced the light availability by 27% (low tDOM addition), 62% (medium tDOM addition) and 86% (high tDOM addition). Light reduction through tDOM addition negatively influenced phytoplankton biomass during the first half of the experiment (<18 days) mainly in the “medium tDOM” and “high tDOM” treatments. The tDOM addition changed the phytoplankton community composition, potentially due to adaptations to different light conditions. Neither phytobenthos biomass nor composition was significantly affected by tDOM addition, probably because the tidal cycle assured sufficient light availability during low tide. Overall, our results indicate that the tDOM addition negatively influenced phytoplankton biomass and composition via light availability and tDOM effects were also observed on the zooplankton level (biomass and C:N ratio). Our experiment demonstrates possible implications of coastal darkening under climate-driven environmental changes on primary producers and their interactions in the aquatic food web.

中文翻译：

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通过陆源 DOM 添加对浮游生物的沿海海洋变暗影响：室内中世界实验

人类活动、降水增加和陆地径流变化将大量外来养分输入沿海水域，包括陆源溶解有机物质 (tDOM)。预计增加对沿海水域的 tDOM 补贴会减少光照，因此可能是导致沿海海洋变黑的因素之一。为了研究增加 tDOM 输入的影响，从而限制光的可用性对初级生产以及向更高营养级（浮游动物）的转移，我们对天然（远洋和底栖）浮游生物进行了大规模的室内中层“浮游生物”实验来自北海的社区。我们模拟了一个具有 35 天每日光和潮汐循环的沿海海洋系统。实验处理包括由三个水平的 tDOM 添加（即低 tDOM、中 tDOM 和高 tDOM）和不添加 tDOM 的对照组成的光梯度。结果表明，tDOM 的添加使光可用性降低了 27%（低 tDOM 添加）、62%（中等 tDOM 添加）和 86%（高 tDOM 添加）。在实验的前半段（<18 天），tDOM 添加导致的光照减少对浮游植物生物量产生了负面影响，主要是在“中等 tDOM”和“高 tDOM”处理中。tDOM 的添加改变了浮游植物群落组成，这可能是由于对不同光照条件的适应。底栖植物的生物量和组成都不受 tDOM 添加的显着影响，这可能是因为潮汐周期确保了低潮期间有足够的光照。全面的，我们的结果表明，tDOM 添加对浮游植物生物量和组成产生负面影响，通过光可用性和 tDOM 效应也在浮游动物水平（生物量和 C:N 比）上观察到。我们的实验证明了在气候驱动的环境变化下沿海变暗对初级生产者及其在水生食物网中的相互作用可能产生的影响。