Meiofauna are an ecologically significant component of marine benthic communities worldwide. Nematodes, in particular, are arguably the most abundant metazoans on Earth and often form highly diverse communities in marine sediments. The study of meiofauna, however, often lags behind that of macrofauna, probably due to the small size of meiofaunal organisms and the paucity of suitably-trained taxonomists or para-taxonomists. Our understanding of basic ecological questions, such as the role of meiofauna in the energetics of benthic communities, as well as the nature and extent of possible relationships between meiofauna and macrofauna in detritus-based communities, is still limited. The present study aimed to address these questions in the context of intertidal seagrass (Zostera muelleri) meadows and adjoining unvegetated sediments in Papanui Inlet, Otago Peninsula. The near absence of taxonomical and ecological studies on free-living marine nematode species in New Zealand also prompted the description of several new species and as well as a comparison of species assemblages from the study sites.
Meiofaunal abundance (range: 1325-4979 ind. 10cm�) in the top 5cm of sediments was not significantly different inside and outside the seagrass meadow. Meiofaunal biomass and production estimates were small relative to macrofauna (approx. 1 and 5% of macrofaunal values, respectively), with little variation in both meiofaunal and macrofaunal values between vegetated and unvegetated habitats. Analysis of the isotopic and fatty acid composition of primary producers, sediment organic matter, and consumers at the study sites suggest that a significant proportion of secondary productivity in seagrass meadows is fuelled by macrophyte detritus. These data suggest that secondary productivity in the seagrass meadow was limited by the low palatability of macrophyte detritus compared with microalgal food sources, resulting in lower trophic transfer efficiency in vegetated sediments. These results contrast with previous findings showing a large contribution of meiofauna to the secondary productivity of vegetated sediments, as well as greater secondary productivity inside than outside seagrass meadows. The nature of the sediments (coarse particle size with low organic matter content) is likely to have led to low meiofauna biomass whilst favouring large suspension feeders. The low availability of high quality food sources (i.e. seston, benthic microalgae, and epiphytes) in vegetated sediments may have contributed to the lower than expected secondary productivity inside the seagrass meadow.
Most of the dominant nematode species found at the study sites were new to science, and 6 new species were described. Significant differences were found in the nematode assemblages between sites and sediment depths. There was a trend of greater nematode diversity and abundance in vegetated sediments, but this was restricted to the uppermost 2cm of sediments. The effect of seagrass on nematode assemblages is likely to be mediated by the effect of blades on sediment characteristics and the presence of roots. The present study suggests that the presence of Zostera muelleri results in greater biodiversity of sheltered soft-shore communities on a local scale.
The addition of ��C-labelled macroalgal detritus (Enteromorpha) to microcosms in a feeding experiment resulted in minimal labelling of harpacticoid copepods and nematodes. Lack of change in the fatty acid composition of the latter also suggests limited assimilation of detritus during the experiment. It is likely that consumers choose benthic microalgae over detritus when both food sources are available in the field. High content of highly unsaturated fatty acids (HUFAs) in nematodes from the field suggests, for the first time, that they are a high quality food source to higher trophic levels.
A feeding experiment investigating the effect of bacterivorous nematodes on detritus assimilation rates by amphipods did not support the hypothesis that nematodes increase the trophic transfer efficiency between detritus and macrofaunal detritivores. It is unclear whether amphipods preferentially fed on detritus or whether nematodes escaped predation by burrowing deeper in the sediment. Data obtained in the present study, however, shows that marine bacterivorous nematodes can biosynthesise/accumulate HUFAs in high concentrations despite their low concentrations in their diet. Nematodes could, therefore, provide an important source of these essential nutrients to bulk deposit feeders living in otherwise HUFA-poor sediments.
| Identifer | oai:union.ndltd.org:ADTP/266224 |
| Date | January 2008 |
| Creators | Leduc, Daniel, n/a |
| Publisher | University of Otago. Department of Marine Science |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://policy01.otago.ac.nz/policies/FMPro?-db=policies.fm&-format=viewpolicy.html&-lay=viewpolicy&-sortfield=Title&Type=Academic&-recid=33025&-find), Copyright Daniel Leduc |
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