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Zostera marina root demography in an intertidal estuarine environment measured using minirhizotron technology
Johnson, M., C. Andersen, D. Phillips, AND Jim Kaldy. Zostera marina root demography in an intertidal estuarine environment measured using minirhizotron technology. MARINE ECOLOGY PROGRESS SERIES. Inter-Research, Luhe, Germany, 557:123-132, (2016).
This research describe the novel application of minirhizotrons, a formerly terrestrial ecosystem root observation technique, to an intertidal seagrass bed and to non-destructively quantify Zostera marina root dynamics. Minirhizotrons are clear plastic tubes which can be placed in the sediment rooting zone, and periodically sampled using a small video camera to observe roots through time. We monitored root production, growth and turnover with 18 consecutive monthly measurements. We found that roots deeper in the sediment lived almost twice as long as those near the sediment surface, with median lifespans of 75 days and 48 days respectfully. We calculated that root biomass turnover in the Z. marina beds was approximately 57 g m-2 yr-1, while root C mass turnover was estimated to be 16.6 g C m-2 yr-1, which is dramatic when used to estimate worldwide C cycling in seagrasses. These are the first Z. marina root lifespan and turnover data from direct observation. We have demonstrated the capability of minirhizotrons for making root observations in intertidal beds of Z. marina for the purpose of monitoring root production, growth and lifespan. This technique can also have application for observing similar roots dynamics in freshwater systems and wetlands for root-zone research including studies on root dynamics, root-rhizosphere interactions, and the effects of environmental stressors such as climate change or pollution.
Over the last four decades there have been major advances in our understanding of the biology, ecology and physiology of seagrasses and their interaction with the environment. Despite these advances, there has been relatively little advancement in our understanding of the belowground dynamics of seagrasses. Minirhizotron tubes are a novel terrestrial ecology method used to visualize root birth, growth and death in order to evaluate root deployment, development and demography. Minirhizotrons are clear plastic tubes which can be placed in the sediment rooting zone, and periodically sampled using a small video camera to observe roots through time. Our objective was to apply the minirhizotron method in an intertidal seagrass bed and to non-destructively quantify Zostera marina root dynamics. A total of 204 individual roots were observed and tracked over 18 monthly sampling periods in the 12 minirhizotron tubes we installed to a depth of 30 cm. The occurrence of roots within the sediment exhibited marked vertical distribution in the sediment with peak root numbers at 9 cm sediment depth which constituted about 20% of the total number of roots and then again at 25 cm sediment depth accounting for about 7% of the total. The median life span for roots deeper than 10 cm in the sediment was more than 75 days and only 48 days for roots in the top 10 cm of sediment. Root biomass turnover was estimated at 56.9 g m-2 yr-1, while root C mass turnover estimate was 16.6 g C m-2 yr-1. Although significant logistical obstacles remain (e.g., minirhizotron use in subtidal beds) the development of a non-destructive sampling technique for seagrass belowground root dynamics will provide better insight to seagrass root-sediment interactions that cannot be captured using traditional destructive sampling methods.