Seagrass systems under nutrient loads and grazing. Interactive effects and feedbacks. 5 year PhD project within the NWO-WOTRO project WT 84-645

MJA (Marjolijn) Christianen MSc.

Description of the project
Rapidly growing economic development in SE Asia has led to increased human activities, particularly in coastal areas (L. Kamp-Nielsen et al. 2002). Among others, deforestation, fish and shrimp culturing and increasing inhabitation results in strongly increased nutrient and sediment loads to coastal waters, which threaten the economically and ecologically important seagrass beds, and have caused their almost complete disappearance in many areas (FT Short et al. 1996, R Constanza et al. 1997, A Balmford et al 2002). We hypothesise that seagrass systems are characterised by strong positive and negative feedbacks, which make them liable to sudden and irreversible state transitions. 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 (figure 1). Our research is directed towards testing this hypothesis, and understanding the basic feedbacks. This is of great scientific importance, and has important implications for practical management. Within the present proposal, we focus on the interaction between nutrient loads, changes in transparency and grazing for the productivity and health status of seagrass beds.

Figure 1. Schematic representation on how seagrass meadows may degrade in response to land-use, and which shifts are easy/difficult to reverse.

Results in the literature show that in relatively oligotrophic waters subject to increased nutrient and sediment loads, complex interactions between (stoichiometry of) nutrient additions and sediment enrichment govern the growth and dynamics of seagrass.
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, and an assessment of the restoration possibilities in heavily disturbed areas

Figure 2. Locations of field research within the Berau Delta, East-Kalimantan, Indonesia.

Research locations

The research in The Netherlands will be carried out at the Radboud University Nijmegen at the Institute for Wetland and Wetland Research and at the Netherlands institute for Ecology in Yrseke at the department of Spatial Ecology, in close coorperation with the Research Centre for Oceanography of the Indonesian Istitute of Sciences (LIPI). The fieldwork will be carried out in Indonesia, where studies will be conducted at locations with varying degrees of nutrient and sediment loading (Figure 2). The fieldwork is focussed on the Berau delta in East Kalimantan cluster. The Berau delta forms a watershed with very low logging intensity in the upper watershed, good forest cover and a very low level of mining operations. Mangroves in the Berau delta are still largely intact.

Other participants
Dr. M.M. van Katwijk Dept. of Environmental Studies RU Nijmegen
Prof.Dr. J.G.M. Roelofs* Dept. of Environmental Biology RU Nijmegen
Dr. L.P.M. Lamers Dept. of Environmental Biology RU Nijmegen
Dr. S. Wouthuyzen Res. Center for Oceanography LIPI
Dr. M. Hutomo Res. Center for Oceanography LIPI
Dr. T.J. Bouma Dept. of Spatial Ecology NIOO-CEME – Yerseke
Prof. Dr. P. Herman* Dept. of Spatial Ecology NIOO-CEME – Yerseke

Here you can find the complete proposal

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