The underwater light regime is fundamental to the ecological health of aquatic systems because it is a limiting factor for photosynthesis in marine plants such as seagrasses. Although seagrass meadows are a key component of coastal systems, their survival has been threatened by increased turbidity levels, both from resuspension of marine sediments and input of terrestrial material. The objective of this study was to investigate how marine (typically grey/white in colour) and terrestrial (typically more yellow-orange in colour with finer texture) sediments affect underwater light quality. Two experimental systems were used: (1) a large outdoor tank and (2) laboratory controlled small sampling container, using natural terrestrial and marine sediment samples (with different colours and grain sizes) from New Zealand. In the tank experiments, high concentrations of sediment reduced transmittance considerably, particularly below 450 nm. Since seagrasses absorb light optimally at wavelengths < 500 nm, as well as between 650 nm and 700 nm, the photosynthetic capacity will be less efficient with pigment absorption occurring mainly at the 650–700 nm wavebands. The difference in colour (white and grey) between marine sediments with the same grain sizes was tested in the laboratory. White sediment resulted in lower transmittance at the same concentration compared with grey sediments; concentration differences had more impact on the spectral distribution of light for white sediments. Within the ranges tested, sediment concentration contributed most to changes in overall light transmittance, with grain size being slightly less important. Sediment colour was important in changing the distribution of light, with orange and white sediments increasing attenuation of shorter wavelengths, which are most needed for seagrass photosynthesis. Our results emphasise the importance of quantifying the spectral changes to underwater light regimes in managing estuaries that are subjected to regular catchment runoff.

中文翻译：

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新西兰河口沉积物类型引起的水下光照条件的光谱差异：对海草光合作用的影响

水下光照条件对水生系统的生态健康至关重要，因为它是海草等海洋植物光合作用的限制因素。尽管海草草甸是沿海系统的关键组成部分，但由于海洋沉积物的再悬浮和陆地物质的输入，它们的生存受到了浊度水平增加的威胁。本研究的目的是调查海洋（通常为灰色/白色）和陆地（通常为更黄橙色且质地更细）的沉积物如何影响水下光质量。使用了两个实验系统：（1）一个大型室外水箱和（2）实验室控制的小型采样容器，使用来自新西兰的天然陆地和海洋沉积物样本（具有不同的颜色和粒度）。在坦克实验中，高浓度的沉积物大大降低了透射率，尤其是在 450 纳米以下。由于海草在波长 < 500 nm 以及 650 nm 和 700 nm 之间的最佳吸收光，光合作用能力将降低效率，色素吸收主要发生在 650-700 nm 波段。在实验室中测试了具有相同粒度的海洋沉积物之间的颜色差异（白色和灰色）。与灰色沉积物相比，白色沉积物在相同浓度下导致较低的透光率；浓度差异对白色沉积物的光谱分布影响更大。在测试的范围内，沉积物浓度对整体透光率的变化贡献最大，颗粒大小的重要性略低。沉积物颜色对于改变光的分布很重要，橙色和白色沉积物增加了较短波长的衰减，这是海草光合作用最需要的。我们的结果强调了量化水下光照状态的光谱变化在管理受定期集水径流影响的河口中的重要性。