Ž . Journal of Marine Systems 26 2000 135–143 www.elsevier.nlrlocaterjmarsys Abundance and size distribution of sublittoral meiobenthos along estuarine salinity gradients Masumi Yamamuro ) Marine Geology Department, Geological SurÕey of Japan, 1-1-3 Higashi, Tsukuba, Ibaraki 3058567, Japan Received 26 March 1999; accepted 8 July 1999 Abstract Abundance and size distribution of the sublittoral meiobenthos in the upper 3 cm of the sediment were surveyed in the eutrophic, estuarine lagoons, Lakes Nakaumi and Shinji, Japan. Samples were taken in late spring from 10 stations located along salinity gradients. The density of meiobenthos was at a minimum in the intermediate salinity zone. Nematodes in L. Shinji were significantly larger in size than in L. Nakaumi, due to the size reduction of marine species in the latter. The biomass of co-existing bivalves, which would contribute more food supply to the meiobenthic fauna, did not affect the abundance and size of meiobenthos. It was suggested that the salinity gradient was the most influential factor on the abundance and size structure of meiobenthos in estuarine lagoons where salinity is fairly constant. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Lake Shinji; Lake Nakaumi; brackish; water; lagoon 1. Introduction Organic matter is abundantly supplied to estuaries from a variety of sources, e.g. terrestrial riverborne and marine tidally advected allochthonous compo- nents, as well as autochthonous biomass derived from planktonic and benthic primary production. The production of zoobenthos in temperate estuaries is generally high compared to other aquatic systems, because of the relative abundance of organic matter Ž . supply Wolff, 1983 . Among the macrobenthic or- ) Tel.: q 81-298-61-3766; fax: q 81-298-61-3766. Ž . E-mail address: yamamuro@gsj.go.jp M. Yamamuro . ganisms, filter-feeding bivalves often predominate over other groups in both the production and biomass Ž . in estuaries Wolff, 1983 . They enhance the sedi- mentation of suspended organic materials including phytoplankton, which promotes meiofaunal abun- dance through increasing available trophic resources Ž . Castel et al., 1989 . Ž . In the review of Thiel 1975 on the deep-sea zoobenthos including meiobenthos, he proposed the hypothesis that the nutrient-limited deep-sea environ- ment offers ecological advantages to those animals with relatively small body sizes. Coexistence of fil- ter-feeding macrobenthos may supply more food to the sediment and thus support the relatively larger size of meiobenthos in those areas. 0924-7963r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0924-7963 00 00050-6