An assessment of seagrass survival and functioning in response to manipulations in sediment redox at Nyali Lagoon, Kenya
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Thalassodendron ciliatum was studied during two monsoon cycles of the year 2001/2002 to determine the interactive effects of experimentally elevated redox potentials on in situ plant structure and shoot-root (S/R) biomass allocation patterns. Thalassodendron plants from shallow back-reef tidal pools were substrate manipulated under ambient field conditions for 90 days on each monsoon cycle. Plant sampling was done from quadrats impacted with organic substrates at three different concentrations (proxies to induce differential redox potentials). Our data show clear differences (at ANOVA, p < 0.05) in Eh and plant traits from 2 treatment effects and from monsoon seasonality effects. Thalassodendron had relatively higher canopy cover, shoot density, shoot blade density, and absolute shoot and root biomass at control sites than at two substrate-impacted sites. Between the monsoons, the same plant traits were higher during the North-East Monsoons (NEM) than they were during the South- East Monsoons (SEM). Thalassodendron beds experienced negative carbon balances within 3 weeks of impaction and were dead by 2 months, which coincided with periods when daytime redox potential values consistently lower than –70mV. S/R ratios were relatively higher at high substrate impacted sites and during the South-East Monsoons (SEM), both conditions also coinciding with higher reducing conditions. Final S/R ratios were 0.32 and 0.34 during NEM and SEM periods respectively at control sites, and 0.33, 0.42, and 0.55, and 0.34, 0.38 and 0.60 at zero, mid and high substrate impacted sited during NEM and SEM depth respectively. About 60% and 95% of root biomass were distributed in the top 10-cm and top 20-cm of soil respectively. Our findings support our hypothesis that reduced redox concentrations would increase plant stress and, consequently, decrease investments in growth capacity.
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