Spectral differences in the underwater light regime caused by sediment types in New Zealand estuaries: implications for seagrass photosynthesis

Abstract

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.