Abstract
The high local-scale species diversity of marine meiofauna, and of nematodes in particular, has puzzled ecologists for decades. Both pronounced niche differentiation and neutral dynamics have been suggested as mechanisms underlying that high diversity. Differential resource use is the most plausible basis for niche differentiation, yet the vast majority of studies demonstrating that this is prominent in marine nematodes are based on laboratory experiments on single species or highly simplified assemblages. Only a small number of studies have investigated resource differentiation under natural conditions. Here we use natural stable-isotope ratios of carbon and nitrogen, as well as fatty-acid profiles, to assess differential resource use and trophic structure in nine abundant estuarine tidal flat nematode species, comprising different presumed feeding modes (deposit feeders, epistratum feeders, predators) and resource guilds (herbivores, carnivores) based on buccal cavity morphology. Nematodes comprise up to three different trophic levels (from primary to tertiary consumers), yet with the exception of some herbivores, omnivory is prominent. Bivariate isotopic niche spaces were of similar size among most species, irrespective of their trophic level. Herbivory not only contributed importantly to the nutrition of suspected herbivores, but also to that of species that were previously considered carnivores based on the morphology of their buccal cavity. Herbivory mainly targets diatoms in some nematode species, yet includes dinoflagellates in others. Bacteria, in contrast, appear to be of limited nutritional importance. Odontophora setosus is identified as a predator/omnivore (possibly of heterotrophic protists) with a trophic level in between that of secondary and tertiary consumers. Our study thus demonstrates that resource differentiation is pronounced among as well as within nematode feeding modes and resource guilds. However, this study included only the most abundant species of the in situ community, hence it remains to be established whether and to what extent its conclusions can be extrapolated to entire, often highly species-rich communities.
Highlights
Estuarine tidal flat sediments are highly productive ecosystems, the productivity of which can be driven by a broad range of organic matter inputs, including deposited phytoplankton and particulate detritus of both terrestrial and marine origin, as well as of macroalgae, seagrasses, mangrove and/or salt marsh vegetation (Heip et al, 1995; Herman, Middelburg & Heip, 2001; Middelburg et al, 1996; Riera & Hubas, 2003; Riera, 2007)
It is noteworthy that the pigment pyropheophytin, which is commonly used as an indicator of zooplankton faecal pellets (Wright & Jeffrey, 1997), was an important driver of total nematode abundance and nematode genus composition at the Paulina intertidal flat (Wu et al, 2019), suggesting that the potential of zooplankton-related inputs as a resource to estuarine nematodes deserves further investigation
Our results suggest that bacteria are unlikely to be that intermediate, mainly because bacterial marker Fatty acid (FA)’s were present in limited abundances in all nematode species
Summary
Estuarine tidal flat sediments are highly productive ecosystems, the productivity of which can be driven by a broad range of organic matter inputs, including deposited phytoplankton and particulate detritus of both terrestrial and marine origin, as well as of macroalgae, seagrasses, mangrove and/or salt marsh vegetation (Heip et al, 1995; Herman, Middelburg & Heip, 2001; Middelburg et al, 1996; Riera & Hubas, 2003; Riera, 2007). Several studies have provided compelling evidence that MPB is the main basal resource fueling both a part of the macro- (Herman et al, 1999; Herman, Middelburg & Heip, 2001; Lebreton et al, 2011) and the majority of the meiofauna (mainly nematodes and harpacticoid copepods) (Carman & Fry, 2002; Cnudde et al, 2015; Moens, Bouillon & Gallucci, 2005; Moens et al, 2002; Moens et al, 2014; Rzeznik-Orignac et al, 2008) on estuarine intertidal flats. There are typically at least some species which appear influenced by deposited phytoplankton or detritus, more so when tidal flats are more sheltered or have features that enhance deposition of suspended particulate organic matter, such as the presence of vegetation (Cnudde et al, 2015; Moens, Bouillon & Gallucci, 2005)
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