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| Mahakam Seagrass systems under nutrient loads and grazing; Interactive effects and feedbacks |
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Periode: Oktober 2007 tot Oktober 2012 Status: Afgelopen Thesaurustermen Begrazing; Nutriënten; Zeegras |
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| Abstract |
| Rapidly growing economic development in SE Asia has led to increased human activities. In coastal areas this often results in strongly increased nutrient and sediment loads, which threaten the economically and ecologically important seagrass beds. The seagrass beds often disappear rather suddenly, and restoration often had limited success. Generally, non-linear feedbacks may give rise to complex dynamics, and in particular to sudden and irreversible shifts between different states of the ecosystem. Based on existing knowledge and pilot studies, our main overriding hypothesis is that nutrient load, sediment load, hydrodynamics and grazing on the seagrass are the most important
drivers that push the seagrass ecosystem into different functional states Within the present proposal, we focus on the interaction between nutrient loads, changes in water transparency and grazing by large grazers for the productivity and health status of seagrass beds. More specifically, we propose to test the hypotheses that (1) direct toxicity of nitrogen to seagrass occurs at environmentally relevant concentrations in tropical seagrass beds; a positive feedback may arise because increased toxicity decreases growth rate, and thus increases vulnerability to nitrogen loads.; (2) strong grazing may protect seagrass meadows from overgrowth by epiphytes, increasing critical loads; (3) under increased organic loading of the sediment, iron deficiency may push the system in a state of increased sulphate reduction, a positive feedback may arise through sulphide toxicity effects and decreased aeration of the rhizosphere. In East-Kalimantan, at 4 seagrass sites representing different conditions of the system, we will perform a field survey to measure indicators of the actual seagrass states and relevant environmental variables (used in all 3 hypotheses). Controlled mesocosm experiments will be performed in the Netherlands with different concentrations of nitrogen under varying growth-limiting conditions (hypothesis 1). In East-Kalimantan, at low and intermediately eutrophicated sites we will manipulate grazing and nutrient supply (hypothesis 2), and vary organic loading and iron supply (hypothesis 3). At sites where seagrasses have disappeared we will perform a transplantation experiment and test the effect of iron additions (hypothesis 3). Additional to the valuation of our hypotheses, our results will provide nutrient thresholds for shifts that can be used in modelling of project 8, and an assessment of the restoration possibilities in heavily disturbed areas (i.e., Mahakam). |
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