Lichtanpassung in Seegräsern des Gezeitenbereichs : räumliche und diurnale Veränderungen und die Rolle von Sedimentfrachten

Seagrasses are the prevailing macrophytes along soft sediment coasts. One of the most important anthropogenic stressors for benthic macrophytes is increased suspended sediment loads and the attenuation of underwater light. Along many temperate coasts seagrasses occur predominantly in the intertidal but knowledge on the photophysiology of intertidal seagrasses is scarce. Studies focussing on the photoecophysiology of intertidal seagrasses are therefore urgently needed and the aim of this thesis was to fill this gap and provide some in-depth information on light acclimation mechanisms in intertidal seagrasses over diurnal and spatial scales. In a second step the impacts of suspended sediments on the underwater light regime and thus seagrass photosynthesis were examined. The first study assessed light adaptive strategies in a comparative analysis of the congeneric seagrass species Zostera muelleri and Zostera marina at two case study areas in New Zealand and Germany. The results showed marked fluctuations of photophysiology (maximum and effective quantum yield, non-photochemical quenching, cycling of xanthophyll cycle (XC) pigments) over daily and tidal cycles with a full xanthophyll cycle at both locations. At the New Zealand site we also observed significantly larger XC-pigment pool sizes in seagrass leaves sampled in a week when low tide coincided with noon. This very dynamic adjustment of xanthophyll pool sizes has not been previously reported for intertidal seagrasses and highlights their ability to adjust to strongly fluctuating irradiances in the intertidal. The high physiological plasticity of Zostera in light-saturated environments was also illustrated by the second study, which evaluated spatial differences in seagrass cover related to environmental conditions (light, temperature, sediment grain size distribution, and porewater nutrients) and differences in seagrass photosynthetic pigment composition and morphometry (above and below ground biomass, shoot length and leaf width, and percentage cover) at four sites in the meso-tidal estuarine lagoon of Tauranga Harbour, North Island, New Zealand. There were marked differences in pigment content and composition (as a marker of physiological plasticity), seagrass metrics as well as in environmental conditions. Our findings emphasize the high physiological plasticity and revealed also morphological plasticity of Zostera muelleri. Both studies underlined the general high light adaptation of intertidal seagrasses and their ability to thrive in fluctuating light environments. However, strong reductions in light conditions, e.g., through increased sediment loads entering coastal systems, may negatively affect seagrasses. Hence, the third study elucidated to what extent different sediment types (marine vs terrestrial) in different concentrations changed the underwater light regime in terms of overall photosynthetically active radiation (PAR) and the attenuation coefficient of downward irradiance of PAR (Kd) and quality (spectral composition). In a tank experiment the influence of different sediment types and concentrations were determined. In situ measurements of light quality and quantity, and suspended sediments (total suspended solids, TSS) at 12 sites in and outside of Tauranga Harbour, demonstrated that the lowest Kd and TSS values were found outside, and the highest within the harbour. Different scenarios for sediment loads, how they influence the underwater light regime and the potential for the vertical distribution of seagrass meadows are discussed. Relatively low Kd values of ~1 (as observed e.g., at the inner site of the lagoon) may already lead to maximum colonization depths of less than ~2 m, i.e., a limitation of seagrasses to intertidal habitats. This shows that current sediment loads and the resulting light conditions impair the growth of seagrasses beyond the intertidal at most sites. The (re)colonization of subtidal habitats may require significantly lower sediment loads entering the coastal zone. Therefore, authorities should consider an effective management on land and at the coast to keep anthropogenic sediment inputs at a minimum.