Spatial and temporal effects of organic enrichment on meiobenthos

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 sites. 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 sediments. 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."