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Spatial and temporal effects of organic enrichment on meiobenthos


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Moverley, J 1995 , 'Spatial and temporal effects of organic enrichment on meiobenthos', PhD thesis, University of Tasmania.

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Meiofauna assemblages were studied along putative organic
enrichment gradients at two marine salmon farms: Loch Ewe,
Scotland, U.K. and Nubeena, SE Tasmania, Australia; and a sewage
outfall: Sandy Bay, SE Tasmania. The broad objective was to identify
any general meiofauna responses to organic enrichment.

1. No evidence of parallel meiobenthic communities (the same genera
occurring at different geographical locations in environments
with similar environmental conditions) was found in areas of
high organic input.

2. Nubeena was the only site where a "classical" pattern of
community response to organic enrichment was found. Severe
enrichment (sediments are organically rich and anoxic zones are
near the surface; nematodes and annelids can be found; other
taxonomic groups , e.g. harpacticoids, are rare) existed under
the cage centre. Moderate enrichment (sediments have slightly
elevated organic content and anoxic zones may, or may not, be
close to the surface; annelids, nematodes, and harpacticoids are
present, though numbers of taxa are reduced and densities may
vary from depaup erate to population blooms, i.e. densities one
or two orders of magnitude above normal) occurred from under
the cage periphery out to 5 m. Slight enrichment (sediment
changes are difficult to detect, densities a n d species richness
are elevated and there are a b normal species abundance
patterns) occurred from 10 to 20 m.

3. At Loch Ewe, severe enrichment occurred under the cage
periphery. The region 10 to 25 m from the cage appeared to be
moderately enriched, with slight enrichment out to 100 m. This
contradicts the results of macrobenthic and physico-chemical
surveys that have never indicated effects extending more that
50 m from Scottish s almon cages.

4. The Loch Ewe meiofauna as semblages were markedly different to
those from Nubeena because no population blooms were detected
and no harpacticoid genera were limited to the area of highest organic enrichment. Population blooms may have been missed
because they are a seasonal phenomenon, or because the
moderately enriched zone occurred b etween two sampling sites.
Differences in harpacticoid distributions at the two locations are
probably because of the background organic content which was
high at Loch Ewe and low at Nubeena.

5. At Sandy Bay, there was a sharp demarcation in sediment
structure and meiofauna a ssemblages 20 m from the sewage
outfall. It is reasonable to assume that the sewage effluent was
responsible for the sediment modification and therefore the
faunal changes. Compared to sites > 20 m from the outfall, sites
close to the outfall had a different nematode species
composition; fewer nematode taxa; and a depauperate
harpacticoid assemblage. Nematode densities were similar at all

6. At a Nubeena cage site, recovery changes in the meiofauna were
monitored for six months after the fish had been removed.
Neither nematodes nor harpacticoids showed a full recovery
over the period. No change in the organic content of the
sediment was detected. Benthic oxygen uptake measurements
indicated it would take more than 18 months for the physicochemical
environment to return to normal.

7. Nematode population changes were gradual and there was a trend
for assemblages at different enriched sites to converge.
Changes in morphological types were apparent. When the cage
was stocked, Type l a nematodes (buccal cavity small, no
armature) made up the greatest proportion of the assemblage.
After cessation of organic input, these decreased and Type 2a
(buccal cavity medium sized, armature as small teeth) increased;
while later in the recovery Type 2a decreased and Type Ib
(buccal cavity medium sized, no armature) increased. This
pattern could b e detected along temporal and spatial gradients
from the organic input source.

8. Harpacticoids experienced rapid population fluctuations with
numerically dominant taxa changing in the space of weeks. When the cage was stocked the harpacticoid assemblages at the
enriched sites were similar and consisted mostly of epibenthic
taxa. In later samples the interstitial taxa increased and the
assemblages tended to diverge. This is probably caused by
spatial differences in the rate of recovery of subsurface

9. Among nematode and harpacticoid a ssemblages, density and
number of genera were more sensitive to organic enrichment
than Evenness or Shannon-Wiener diversity. Contrary to many
other studies described in the literature, Evenness tended to be
high at severely enriched sites decreasing to minimum but
extremely variable values in the moderately enriched zone.

10. Harpacticoids were not very sensitive when comparing sites in
highly enriched zones, due to high b etween-replicate variability.
Neither harpacticoids nor nematodes were very sensitive when
comparing sites in areas of mild disturbance and thus were not
useful for delineating the precise area affected by enrichment.
However, by combining data for the two taxocenes with that for
other components of the fauna e.g. macrofauna, greater
sensitivity could be obtained.

11 . If monitoring is restricted to a single taxocene, a multivariate
method which determines associations of taxa and their
distributions is more sen sitive for detecting changes in
environmental conditions than site classification. This method
gave similar results to those obtained by combining all data in
a site classification.

Item Type: Thesis - PhD
Authors/Creators:Moverley, J
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