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Sedimentation impacts on photosynthetic performance and competitive success of marine macrophytes

State of the Art
Increased sedimentation is one of multiple anthropogenic stressors in the coastal zone. Suspended sediments originate from terrestrial run-off through rivers but also from construction activities and the dumping of dredged material. As a result light attenuation will be enhanced by more suspended material in the water column and might therefore inhibit growth of benthic macrophytes at the lower limit of their vertical distribution (Ralph et al. 2007, Inglis 2003). Characterising physiological responses of marine macrophytes towards decreased light regimes caused by increased sediment loads were the initial aim of this project. Since seagrasses are the dominant native macrophytes on soft-sediment coasts (Hemminga and Duarte 2000) and as congener seagrass species populate the coasts of New Zealand and Germany it was decided to focus on the ecophysiology of seagrasses.
The most important abiotic factor controlling growth, distribution and abundance of photosynthetic organisms is the availability of light. On the one hand, light availability in the habitat of a plant must be sufficient to sustain growth (Falkowski and Raven 2007, Ralph et al. 2007, Dubinsky and Schofield 2010). On the other hand, excess light and/or UV radiation may inhibit photosynthesis or even damage the photosynthetic apparatus (Bischof et al. 2007, Falkowski and Raven 2007, Dubinsky and Schofield 2010). Ralph et al. (2007) reviewed the impacts on light limitation on seagrasses and pointed out that suspended sediment is an important factor.
The following three studies were conducted and will lead into one publication each:

Study 1: Vertical gradients and daily turnover of seagrass photosynthesis
In this study the photosynthetic properties of the seagrasses Zostera muelleri (at Tauranga Harbour, New Zealand) and Zostera marina (at Sylt, Germany) were investigated as they are modulated by varying light and tidal conditions. The photosynthetic characteristics, pigment contents and light climate were expected to be different along vertical gradients, at different sites and in the course of diurnal and tidal cycles. It was hypothesized that seagrasses from different tidal levels have different photosynthetic properties in order to meet the respective light conditions. How these acclimation mechanisms allow seagrasses to grow in fluctuating light conditions was assessed in order to obtain information about productivity.
Results show that there are no significant differences between tidal levels in photosynthetic activity. Also only little inhibition or down-regulation of photosynthesis could be measured even at really high irradiances (when the seagrass was exposed when low tide was at noon). This is well explained by the pigment data, which show a high activity of the xanthophyll cycle. There are only very few publications on the operation of the xanthophyll cycle in seagrasses (i.e. Flanigan and Critchley 1996, Ralph et al. 2002, Collier et al. 2008). The results of this study contribute to the understanding of this very important photophysiological mechanism in seagrasses. The findings also emphasize the high light adaptation of the two seagrass species Zostera marina (GER) and Zostera muelleri (NZ) and indicate their vulnerability to decreased light conditions caused by increased sedimentation.

Study 2: Spatial gradients and seasonal changes of seagrass photosynthesis and sedimentation patterns
The aim of this study was an assessment of photosynthesis at a spatial level, i.e. at different sites in Tauranga Harbour and to correlate these differences to the abiotic conditions, especially sediment type/characteristics and changes in light quality and quantity. The hypothesis was, that photosynthetic characteristics of seagrasses will differ between different sites and that there will be a correlation with the sediment present at the sites.
Results showed marked differences in environmental conditions, seagrass metrics as well as in pigment content and composition. In particular, one site (the sheltered site in the north of Tauranga Harbour) was distinctly different from the other sites, having the lowest light regime and highest mud and phosphate content. Seagrass at this site had the highest overall biomass, with especially high above ground biomass as well as elevated overall pigment content and both light harvesting and protective pigments present in high concentrations. Our findings show that Zostera muelleri exhibits a high physiological and morphological plasticity (i.e. being able to change its pigment composition depending on the prevailing light conditions). However, some of the prevailing environmental factors (e.g. heavy metals, organic pollutants, physical disturbances or biological interactions) seem to compromise its vitality leading to the observed loss of this seagrass in Tauranga Harbour.
Study 3: Experimental determination of physiological short term responses of marine macrophytes towards different sediment loads: a mechanistic approach
In this experiment different amounts and types of sediments were added to a tank set-up and kept in suspension and the light climate (both PAR and UV) was measured. The results will be used to discuss the effects of different sediment loads on underwater light climate and - based on the results of own previous work of study 1 and 2 – seagrass photophysiology.

References
Bischof, K., Gómez, I., Molis, M., Hanelt, D., Karsten, U., Lüder, U., Roleda, M., Zacher, K., Wiencke, C., 2007, Ultraviolet radiation shapes seaweed communities. In: Amils, R., Ellis-Evans, C., Hinghofer-Szalkay, H. (Eds.), Life in Extreme Environments. Springer Netherlands, pp. 187-212.

Collier, C., Lavery, P., Ralph, P., Masini, R., 2008. Physiological characteristics of the seagrass Posidonia sinuosa along a depth-related gradient of light availability. Mar. Ecol. Prog. Ser. 353, 65-79.

Dubinsky, Z., Schofield, O., 2010. From the light to the darkness: thriving at the light extremes in the oceans. Hydrobiologia 639, 153-171.

Falkowski, P.G., Raven, J.A., 2007, Aquatic Photosynthesis. Princeton University Press, Princeton.

Flanigan, Y.S., Critchley, C., 1996. Light response of D1 turnover and photosystem II efficiency in the seagrass Zostera capricorni. Planta 198, 319-323.

Hemminga, M., Duarte, C., 2000, Seagrass ecology. Cambridge University Press, Cambridge, UK.

Inglis, G.J., 2003, Seagrasses of New Zealand. In: Green, E.P., Short, F.T. (Eds.), World Atlas of Seagrasses: Present Status and Future Conservation. University of California Press, pp. 148-157

Ralph, P.J., Durako, M.J., Enríquez, S., Collier, C.J., Doblin, M.A., 2007. Impact of light limitation on seagrasses. J. Exp. Mar. Biol. Ecol. 350, 176-193.

Ralph, P.J., Polk, S.M., Moore, K.A., Orth, R.J., Smith Jr, W.O., 2002. Operation of the xanthophyll cycle in the seagrass Zostera marina in response to variable irradiance. J. Exp. Mar. Biol. Ecol. 271, 189-207.

Members

Proponents:Prof. Dr. Kai BischofUniversity of Bremen
:Prof. Dr. Conrad PilditchUniversity of Waikato
PhD Candidate:Dorothea KohlmeierUniversity of Bremen

Publications

Under review:
Kohlmeier, D., C.A. Pilditch, J.F. Bornman and K. Bischof, (in rev.), Site specific differences in morphometry and photophysiology in intertidal Zostera muelleri meadows, Aquatic Botany.

Miscellaneous

Research stays at the University of Waikato, Hamilton: 11.11.2010 - 02.05.2011 and 06.02.2010 - 27.03.2010