03290nas a2200241   4500008004100000245014000041210006900181260008400250300001200334490000700346520232800353653002602681653002302707653001402730653003402744653003702778100001702815700001902832700002002851700002102871700001902892856013702911       2009                        eng d00aSpatial and seasonal variations in the ecological characteristics of the free-living nematode assemblages in a large microtidal estuary0 aSpatial and seasonal variations in the ecological characteristic  a24-28 OVAL RD, LONDON NW1 7DX, ENGLANDbACADEMIC PRESS LTD ELSEVIER SCIENCE LTD  a309-3220 v823 aThis study has determined the ways in which the density, number of species, species composition and trophic structure of free-living nematode assemblages in the subtidal waters of a large southern hemisphere microtidal estuary change spatially and seasonally, and has explored whether those four biotic characteristics are related to certain environmental factors. Based on data derived from samples collected seasonally at 12 sites throughout the estuary, the densities and number of species of nematodes decreased progressively with distance from estuary mouth, to reach a minimum at sites where salinities were most variable. and then increased slightly in the uppermost part of the estuary where salinities were least. Densities were also generally greatest in spring, due largely to increases in the abundance of epistrate-grazing species at this time and thus when the amount of primary food (microphytobenthos) peaked. The spatial distribution of the composition of the nematode assemblages was closely correlated with salinity and, to a lesser extent, grain-size composition and amount of particulate organic material (%POM) in the sediment. Although species composition changed sequentially along the estuary, the change was particularly pronounced between sites above and below the area where salinities started to decline markedly and become more variable and %POM increased markedly. This reflected, in particular, far greater abundances of Spirinia parasitifera at the six downstream sites and of Theristus sp. 1 at the six sites further upstream. Species composition underwent pronounced seasonal cyclical changes at all sites, presumably reflecting interspecific differences in the timing of peak reproduction and thus of recruitment. The trophic structure of the nematode assemblages changed both spatially and temporally in relation to the relative abundance of different food sources. Thus, for example, non-selective deposit feeders, such as Theristus sp. 1, dominated samples in the upper estuary, where %POM was by far the greatest, and was rare or absent at downstream sites. Conversely, epistrate grazers, such as species of the Chromadoridae, were most abundant at downstream sites in spring, when the density of the microphytobenthos reached its maximum. (C) 2009 Elsevier Ltd. All rights reserved.10aenvironmental factors10amicrotidal estuary10anematodes10anumber of species and density10aspecies and trophic compositions1 aHourston, M.1 aPotter, I., C.1 aWarwick, R., M.1 aValesini, F., J.1 aClarke, K., R.  uhttps://nematoda.myspecies.info/content/spatial-and-seasonal-variations-ecological-characteristics-free-living-nematode-assemblages-