Relationships between nematode size and water depth were studied using 309 values of mean nematode individual weight (MNW) from 120 worldwide locations in the depth range from 0 to 8260 m. Neither the type of sampling device (tubes, box corer or multiple corer) nor sample size influence MNW, while the weight evaluation technique does: MNW values obtained by gravimetric measurements were twice as high as those obtained from comparable depth ranges by a volumetric method. For the whole depth range, MNW was negatively correlated with depth, mainly due to the significant (on average 3-fold) drop in nematode size across the top of the continental rise (usually at 500+ m) and deeper. This overall depth-related trend was clearly defined only in oligotrophic waters, in eutrophic regions the regressions were not significant. Multiple regression analysis showed that sediment composition and trophic conditions (pigment concentration or estimated POM flux) explained together from 40% to 60% of overall variations in MNW, while the effect of depth was not significant. Mean particle size determined a considerable part of total MNW variation in relatively shallow zones (continental shelf and upper slope) only. At depths of 200 m and deeper, where the heterogeneity of the sediments drops sharply, the influence of trophic factors increased 10-fold, yet the effect of sediments remained significant up to 2000 m. Under shallow waters down to 1000 m (except for the highly productive region of Peruvian upwelling), neither trophic factors nor depth had any effect, while the sediment properties were the only significant ones. Deeper, the effect of trophic factor is much more prominent in oligotrophic areas.It is suggested that the depth as an environmental variable does not directly affect the size of meiobenthic organisms but marks and modifies the effects of the other factors such as trophic conditions and sediment properties. Clear depth-related patterns are observed because many of these key factors turn out to be depth-correlated, but disappear if the factor is uncorrelated.
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