Introduction Directory UMM :Data Elmu:jurnal:J-a:Journal of Experimental Marine Biology and Ecology:Vol256.Issue1.Jan2001:

Journal of Experimental Marine Biology and Ecology 256 2001 99–121 www.elsevier.nl locate jembe Effects of leaf litter addition on meiofaunal colonization of azoic sediments in a subtropical mangrove in Hong Kong Hong Zhou The Swire Institute of Marine Science and Department of Ecology and Biodiversity , The University of Hong Kong , Hong Kong, China Received 20 July 2000; received in revised form 17 October 2000; accepted 18 October 2000 Abstract The responses of major meiofaunal taxa and nematode species assemblage to the decaying leaf litter of the mangrove Kandelia candel were investigated through a field colonization experiment in subtropical Hong Kong. Sixty-four replicate azoic and organic-free sediment cores were treated with leaf litter additions of 0 3 , 0.5 3 , 1 3 and 2 3 natural sediment organic concentration, respectively, and retrieved 1, 10, 30 and 60 days post-placement. Replicate cores of ambient sediment were also taken at each sampling date to provide baseline information. Results of ANOVAs suggested that either different meiofaunal taxa responded to the leaf litter in different ways or the response of the same taxon changed over decomposition time. Multivariate ordination performed on nematodes revealed an alteration in community structure after 10, 30 and 60 days between controls and treatments. This alteration was attributed to some deposit feeding nematodes, particularly a bacterivorous species, Diplolaimella sp., which bloomed in all the cores treated with leaf litter, testifying to the important role such meiofauna plays in the process of detritus decomposition.  2001 Elsevier Science B.V. All rights reserved. Keywords : Mangrove; Kandelia candel; Leaf litter; Decomposition; Meiofauna; Colonization

1. Introduction

Detritus is a major energy source for most marine benthic systems Findlay and Tenore, 1982. Detritally-enriched marine habitats support a high abundance and diversity of meiofauna. The role meiofauna, particularly free-living nematodes, plays in the detritus-based food chain is, however, still not well understood. Tel.: 1852-2809-2179; fax: 1852-2809-2197. E-mail address : hzhouhkusua.hku.hk H. Zhou. 0022-0981 01 – see front matter  2001 Elsevier Science B.V. All rights reserved. P I I : S 0 0 2 2 - 0 9 8 1 0 0 0 0 3 1 0 - 5 100 H . Zhou J. Exp. Mar. Biol. Ecol. 256 2001 99 –121 Earlier studies concerning the meiofauna–microbe–detritus relationship were mostly conducted in laboratory cultures Tietjen and Lee, 1975; Tenore et al., 1977; Milton, 1981; Rieper and Flotow, 1981; Findlay, 1982; Findlay and Tenore, 1982; Pamatmat and Findlay, 1983 and have been reviewed by Warwick 1987. These studies generally suggested that meiofauna, particularly nematodes, could maintain bacterial and diatom assemblages in log phase growth and play an important role in detritus decomposition and even make it available to macro-consumers. In temperate areas, two closely related species of the nematode family Monhysteridae, namely Diplolaimella dievengatensis and Diplolaimelloides bruciei, are prevalent in salt marshes and their role in stimulating detritus mineralization has been demonstrated Hopper, 1970; Alkemade et al., 1992a,b, 1993, 1994. Compared with temperate salt marshes, subtropical and tropical mangroves, however, have received relatively less attention and there exists a question about the significance of meiofauna in these detrital systems. For example, Tietjen and Alongi 1990 investigated the relationship between the population growth of two nematode species Monhystera sp. and Chromadorina sp., bacterial abundance and the age chemical composition of Rhizophora and Avicennia leaf litter. Their results suggested that the nematode populations were more affected by the chemical composition of the leaves than by the bacteria. They concluded that the low field densities , 5 per leaf, Alongi, 1987a and the inability of nematodes to influence bacterial abundance indicates that meiofauna may not play a major role in the cycling of organic matter in tropical mangrove forests. Gee and Somerfield 1997, however, studied the effects of mangrove diversity and the process of leaf litter decay on the diversity and community structure of meiofauna using a field experiment. Results from this experiment in conjunction with a field investigation on the same mangrove Somerfield et al., 1998 indicated that the meiofaunal communities on mangrove leaf litter are distinctly different from those in the mud. Nematodes are characterized by a succession of mainly bacterial and deposit-feeding species over the whole decay process while copepods are principally influenced by the life cycle of members of the Darcythompsoniidae. Although the nitrogen content of detritus appears to be the best measure of its nutritional quality Findlay, 1982, mangrove leaf litter is usually rich in tannins and other polyphenols, and thus complex relationships may exist between tannin and nitrogen content and age which will determine the utilization of mangrove litter by nematodes Tietjen and Alongi, 1990. The mesocosm experiments carried out by Gee et al. 1985 and Austen and Warwick 1995 showed that the community structure of two major meiofaunal components, i.e. nematodes and copepods, responded to organic enrichment and detritus food supply in different ways. More evidence is needed, therefore, from mangrove habitats to evaluate the importance of detritus as a meiofaunal food source as well as the role the meiofauna plays in the mineralization of such leaf litter. The present study thus aimed at investigating the responses of major meiofaunal taxa and nematode species assemblage to decaying mangrove leaf litter and meiofaunal community succession associated with different stages of the decomposition process. To assess these, a field colonization experiment was conducted in a Hong Kong mangrove from July to September 1999. Colonization experiments have been used widely to study the impacts of disturbance on benthic community structure Alongi et al., 1983; Decker and Fleeger, 1984; Colangelo et al., 1996. In this study, tubes containing azoic and H . Zhou J. Exp. Mar. Biol. Ecol. 256 2001 99 –121 101 organic-free sediment with different levels of added leaf litter were inserted into the mangrove sediment and retrieved at different times to determine rates of meiofaunal colonization at varying levels of leaf litter enrichment. Hypotheses to be tested were: • H Meiofaunal colonization of sediment within the mangrove study area is unaffected by the presence of decaying leaves; • H Meiofaunal colonization and community structure within the mangrove study area is unaffected by the quantity of decaying organic matter in the sediment; • H All meiofaunal taxa within the mangrove study area respond in the same way to the presence of decaying organic matter in the sediment.

2. Materials and methods