Differences in macrofaunal and seagrass assemblages in seagrass beds with and without seaweed farms J.S. Eklo¨f a, ) , M. de la Torre Castro a , L. Adelsko¨ld a , N.S. Jiddawi b , N. Kautsky a a Department of Systems Ecology, Stockholm University, S-106 91 Stockholm, Sweden b Institute of Marine Science, Dar Es Salaam University, P.O. Box 668, Zanzibar, Tanzania Received 1 January 2004; accepted 23 November 2004 Abstract Since it was introduced to Zanzibar (Tanzania), seaweed farming has significantly contributed to local, socio-economic development. However, several investigations have shown impacts on the coastal environment near where the farms are located. As many seaweed farms are located on seagrass beds, there is a risk that seaweed farming could affect seagrass beds, and thereby disturb important ecosystem functions and the flow of ecological goods and services. This study compares characteristics of macrophytes (focusing on seagrasses), benthic macrofauna and sediment in seagrass beds, with and without seaweed farms, and a sand bank without vegetation in Chwaka Bay, Zanzibar. The results showed that seagrass beds underneath seaweed farms generally had less seagrass and macroalgae, finer sediment, lower sediment organic matter content and a reduced abundance and biomass of macrofauna, than seagrass beds without seaweed farms. Further, the macrofaunal community structure in seaweed farms showed more similarities to that on the sand bank than in the unfarmed seagrass beds. Most of the dissimilarity was attributable to Lucinidae (suspension-feeding bivalves), which were almost absent in the seaweed farms, resulting in the large difference in biomass between the seaweed farms and the unfarmed seagrass beds. When interpreted together with information from farmers, the observed pattern is believed to be caused by the seaweed farming activities. This indicates that more research is needed to establish the effects of seaweed farming on seagrass beds, and that more attention should be given to the location of farms and the choice of farming methods. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: seaweed farming; seagrass bed; macrofauna; sediment; aquaculture; Eucheuma denticulatum; Zanzibar; East Africa 1. Introduction Open-water seaweed farming was introduced to Unguja Island (Zanzibar, Tanzania) around 1990 and mainly two species of red algae, Eucheuma denticulatum (formerly E. spinosum) and Kappaphycus alvarezii (formerly E. cottonii), are grown and harvested for extraction of hydrocolloid carrageenans (Petterson- Lo¨fquist, 1995). Unlike other, more destructive and resource-inefficient aquaculture methods (e.g. semi-in- tensive shrimp and salmon farming), seaweed farming does not require any inputs of fertilisers or pesticides, and is considered not to alter the physical environment in any major way (Johnstone and O ´ lafsson, 1995; Bryceson, 2002). Further, living standards have in- creased in many villages following the introduction of seaweed farming (Msuya, 1993; Petterson-Lo¨ fquist, 1995; Jiddawi and Ngazy, 2000) This has lead to a common perception that seaweed farming is a highly sustainable aquaculture practice (e.g. Saleh, 1998; ) Corresponding author. E-mail address: johane@ecology.su.se (J.S. Eklo¨f). 0272-7714/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2004.11.014 Estuarine, Coastal and Shelf Science 63 (2005) 385e396 www.elsevier.com/locate/ECSS