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Marine Ecology Progress Series

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MEPS 231:75-83 (2002)  -  doi:10.3354/meps231075

Groundwater effects on diversity and abundance of lagoonal seagrasses in Kenya and on Zanzibar Island (East Africa)

Pauline Kamermans1,*, Marten A. Hemminga1, Jurgen F. Tack2, Miguel A. Mateo1, Núria Marbà1, Matern Mtolera3, Johan Stapel1, Anouk Verheyden2, Toon Van Daele4

1Netherlands Institute of Ecology, Centre for Estuarine and Coastal Ecology, PO Box 140, 4400 AC Yerseke, The Netherlands
2Fund for Scientific Research Flanders, c/o Free University, Institute of Environmental Research, Pleinlaan 2, 1050 Brussels, Belgium
3Institute of Marine Sciences, University of Dar es Salaam, PO Box 668, Zanzibar, Tanzania
4Free University, Hydrology Department, Pleinlaan 2, 1050 Brussels, Belgium
*Present address: Netherlands Institute for Fisheries Research (RIVO), Centre for Shellfish Research (CSO), PO Box 77, 4400 AB Yerseke, The Netherlands. E-mail: p.kamermans@

ABSTRACT: Seagrass species diversity and abundance were studied in East African back-reef lagoons with contrasting groundwater-outflow rates. The selection of the lagoons was based on a groundwater flow model. A total of 10 seagrass species was observed at all sites together. Sites with a higher groundwater outflow displayed a lower species diversity than sites with a lower groundwater outflow. Thalassodendron ciliatum dominated at sites with high groundwater outflow rates, while Thalassia hemprichii showed higher coverage at sites with low groundwater outflow. Porewater salinities were up to 5 psu lower at locations with predicted high groundwater-outflow rates indicating supply of freshwater. The reduction in porewater salinity at groundwater outflow sites is relatively low, which makes it unlikely that a difference in optimum salinity for growth is the main factor causing reduced diversity at these sites. Nitrogen-stable isotope signatures of seagrass leaves showed a significant increase with increased groundwater outflow rates. This suggests that the nitrogen source for these plants was, at least in part, groundwater. Differences in competition for nitrogen may explain the observed pattern in species diversity and abundance. To establish a substantive link between the observed reduced seagrass diversity or enhanced δ15N values of T. ciliatum leaves on the one hand and increased groundwater outflow rates on the other, further exploration through detailed measurements of groundwater outflow rates and groundwater nitrogen isotopic composition are needed.

KEY WORDS: Groundwater model · Tropical seagrasses · Abundance · Diversity · Thalassia hemprichii · Thalassodendron ciliatum · Nitrogen · Salinity

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