Constraints

Coull and Chandler ( 1992 ) reviewed twenty papers reporting field abundances of meiofauna around sewage and sludge discharges. These included tidal, intertidal and some freshwater discharges and point and non-point source discharges. Eight of these papers

reported increases of meiofauna density in polluted areas, ten . density decreases, one a summer decrease and winter increase, and

the other no detectable change. Even when the list was reduced to studies of estuarine and marine subtidal point source discharges there was no consistent pattern. The way the waste is released as well as topographical and hydrographical conditions at the discharge point will be responsible for primary distribution of sewage in the receivin g water ( F</>yn, 197 1 ) . Consequently, each location may be unique and t he literature appears to support this.

The analysis of sediment, nematode assemblages and harpacticoid assemblages all indicate that there was no clear gradient of organic enrichment in the direction of the transect I sampled. Instead there was a relatively sharp demarcation of polluted and unpolluted

sediments at approximately 20 m from the outfall. This is particularly well illustrated by the ordination of nematode assemblages where one of the Site 20 replicates was classified with cores from < 20 m and

It is possible, though unlikely, that typical enriched communities existed, and azoic communities existed but were missed because of the sampling scale used. The site classification indicates that nematode assemblages from Sites 50 to 300 are similar, and that

5 0

Site 2 0 was the ecotone for this assemblage and that found closer to the sewage outfall. When the site 20 replicates were combined into a single site sample the nematode assemblage was similar to that for Sites collected > 20 m from the outfall. Harpacticoid assemblages from Sites 20 to 300 were similar. It will be shown in Section 1.B that these assemblages were similar to others from the Nubeena "clean" sites. Therefore, it is unlikely that any one or more of the sites from � 20 m is inside the zone of enrichment.

I also believe it unlikely that different zones may have been missed at distances < 20 m from the outfall. The boat used as a sampling platform when anchored swung over several metres while the cores were being collected. Consequently, the twelve cores collected < 20 m from the outfall covered a relatively large area. Under these

conditions, it is unlikely any substantially large area with different assemblages would have remained un sampled.

A survey of the macrobenthos around the Sandy Bay sewage outfall was conducted in February, 1994. This survey sampled different

sites to those used in the meiofauna study ( Fig. 1 . 1 ) . One of the sites in this set of samples contained approximately 5 times the number of individuals compared to the other sites and approximately 25% more taxa. This suggests that the site was located in an

enriched area. Because the macrofauna survey was conducted by divers, samples could not be collected close to the outfall. Therefore, the fine very silty sediments found in the meiofauna study were not sampled.

There are three possible reasons why an enriched community was uncovered in the macrofauna survey and not the meiofauna survey;

1. Conditions may have changed between the two dates i.e. October, 1989 and February, 1994.

2. Zonation patterns may vary with direction from the outfall. A transect at a different angle may have encountered enriched communities.

3. Meiofauna zonation around sources of organic enrichment may be different than macrofauna. It will be shown in Chapter 4 that around the Nubeena fish farm cage the two fauna groups showed similar zonation patterns. So I believe that this is unlikely.

5 1

Association of nematode taxa analysis revealed there was more than one assemblage at sites � 20 m from the outfall and that Site 150 was different from t he others. Because the nematode assemblages in cores from Site 300 were similar to those in cores from Sites 20 and 50, it is unlikely that changes at Site 150 related to organic input. No such differences were detected in harpacticoid assemblages.

Marcotte and Coull ( 1974) conducted a study of harpacticoids at a subtidal point source sewage outfall in which they found a

"classical" organic enrichment pattern. The depth ( 13 to 16 m was similar to that at Sandy Bay. Like Sandy Bay, effluent originated from residential areas ( 5,000 people vs 14,000 at S andy Bay) and did not contain industrial wastes. The sediments surrounding the sewage outfall Marcotte and Coull studied were well sorted silt. Unlike Sandy Bay, where there was a vast difference in sediment structure close to the outfall, they did not detect any sediment particle size changes related to the sewage outfall. Presumably this is because the normal

sediment was of the same size range as any deposited silt or the sediments were being rapidly transported and mixed. They measured "labile" organic contents rather than total contents as I did. This peaked at 400 m from the outfall.

In Marcotte and Coull's study hydrographical, microbiological and nutrient chemistry data indicated conditions returned to normal 800 to 1,000 m from the outfall. From field observations, they believed that particle matter was swept some distance along the transect by the turbulent mixing of the sea. " Turbulent" con ditions were not observed at S an d y Bay, and I located my transect perpendicular to the shore to minimise depth variations ( the outfall being located on a submerged ridge that ran out from the shore), and to possibly

foreshorten changes with distance, presuming that most material would be carried up and down the estuary with the tides.

52

Marcotte and Coull ( 1974) found detectable differences in

harpacticoid density and diversity up to 800 m from the outfall. In contrast at Sandy Bay I found effects only extended 20 m from the outfall. There are two possible reasons for these differences.

1. The transect was directed perpendicularly to the main channel to minimise depth changes, and foreshorten changes with distance from the outfall. It is possible that instead of foreshortening the changes, the transect intersected a well defined plume at the outfall. A transect parallel to the channel may have resulted in a different pattern.

2. It is possible the turbulent mixing referred to by Marcotte and Coull ( 1974) ensured an efficient dispersion of material for some distance from the outfall. Also enhancing the idea of mixing is the fact Marcotte & Coull ( 1974) describe the sediment as b elonging to the belt of detritus-ooze which dominates the mobile substrate of the North Adriatic. With turbulent mixing and mobile substrata it is possible material that settles out from the sewage tends to be more widely dispersed than in the more stable areas.

Possible differences between sediment mixing at my site and that of Marcotte and Coull, highlight the differences that can occur at different locations and may explain the variability of meiofauna to sewage discharge that has been published. These differences also

highlight several deficiencies in limited studies such as mine. It is apparent that an ideal study should not be restricted to a simple one dimensional transect but should examine the sea bottom in two dimensions around the outfall to ensure that there are no spatial differences. Marcotte and Coull's results also indicate that such a study should be conducted with a radius scaled to distances related to current speeds and turbulent mixing, possibly exceeding 1,000 m. S uch a study should also include hydrographic and sediment

transportation and mixing components. As well, a detailed analysis of the sediment physical/chemical environment needs to be undertaken. The qualitative description Marcotte and Coull give of their sediment, a soft detritus ooze covered with a flocculent layer, conveys a

different impression to my greasy mud. Obviously, more detailed sediment analysis needs to be carried out so that qualitative

53

comparisons are possible. S uch a study is beyond the resources of a single person study and highlights the need for a multi-disciplinary approach to studies of pollution discharge effects.

The outstanding difference b etween my results and those of Marcotte and Coull, is the sharply defined change in Sandy Bay sediment that occurred at approximately 20 m. A marked feature of this change was that at sites < 20 m from the outfall the silt/clay fraction was > 30% at sites � 20 m it was found to be < 5%.

Theoretically it has been shown that interstitial spaces in the sediment should cease to exist when the silt/clay content is 7%

( Crisp & Williams 197 1 ) . In the field it has been found that a change in the silt/clay fraction from 4 to 10% leads to a loss of interstitial harpacticoids (Moore, 1979c).

Therefore, coinciding with the sediment changes that occurred at 20 m, it would be expected that there would be a loss of the interstitial meiofauna. Interstitial harpacticoids can be easily

distinguished from epi- and endobenthic ones b y their body shape. Two Ectinosomatidae were the only harpacticoids that increased in density at sites < 20 m. Ectinosomatidae are recognised as "Jack-of­ all-habitats", and representatives appear in most marine habitats. Consequently, the absence of interstitial harpacticoids from sites < 20 m from the outfall was one of the key differences in the Sandy Bay meiofauna. Interstitial nematodes are less easily recognised. Chromadorids are recognised as b eing interstitial; two that were common at S andy Bay were Chromadora and Neochromadora, neither of these were absent from samples < 20 m from the outfall. However, densities were much reduced, and most individuals were collected in the same core, suggesting that this may have been collected from j ust outside the patch of silt.

It would appear that any changes in the meiofauna close to the sewage outfall, due to organic enrichment, are being masked by

changes due to loss of interstitial space due to the siltation from the sewage.

5 4

CHAPTER 4 NUBEENA

The aim of this study was to describe nematode and harpacticoid assemblages along a putative organic enrichment gradient at a Tasmanian fish farm. The intense cultivation of fish in sea cages deposits organic waste from uneaten food, faeces, scales and slime into the sediment (Woodward, 1989 ) , Up to 10 Kg . m-2 . yr,l can be deposited directly beneath the cages and 3 Kg . m,2 . yr,l in the immediate vicinity (Brown et ai., 1987 ) . In Europe the wastes can include therapeutics (Gowen & Bradbury 1987) used to kill ecto­ parasitic crustaceans. Free-living marine crustaceans may be highly sensitive to these. However, such therapeutics are not used in Tasmania. Therefore, unlike sewage, Tasmanian fish farm waste should not contain chemicals that are directly toxic to meiofauna.

Nine sites were sampled along a transect starting under the cage centre and extending 100 m from the edge. Cores are designated b y a number representing the distance i n metres they were from the cage edge (-5, 0, 5, 10, 20, 30, 45, 60 and 100), and an A, B or C for

the different replicates. Location details and collection methods are given in Chap ter 2.2.

4.1 SEDIMENT

4.1.1 Results

There was only a small p ercentage of silt/clay in the Nubeena

sediments (0.8 to 2.2%) with no indication of silt build up associated

with the cage (Fig. 4.1).

FIGURE 4 . 1 w C!J

�

1 .5 w

�

w a. -5 0 5 1 0 20 30 45 60 100 SITES

Percentag e , by weight, of s i l t/clay in Nubeena sediments .

5 5 Site 4 5 had a higher p roportion of p articles < 125 11m, and twice the silt/clay content of the other sites ( Table 4 . 1 ), The other sites had similar p roportions of particles < 250 pm. Proportions of 500 and 250 11m particles were lower in sediment from Sites 30 to 100 than

sites :5 20 m from the cage. At Sites 30, 60 and 100 there was a

higher proportion of p articles > 1000 11m compared to sites :5 20 m

from the cage. These changes are indicated by the decrease in inclusive graphic skewness with distance from the cage (Table 4.2), However, these changes tended to cancel each other out so that the

sediment p�rtic1e size graphic means were similar at all sites. The graphic mean was approximately 250 11m, which on the Wentworth scale is fine sand. At all sites there was a preponderance of p articles below the median grain size as shown by the negative inclusive graphic skewness.

Under the cage total organic content was 4.6%. This is 2.3 times the average value from sites > 5 m from the cage (Table 4.3 ) . At the cage edge and 5 m from the cage values were slightly higher ( 1 . 2 and 1 . 3 times respectively) than the average from sites > 5 m from

the cage. Under the cage there was elevated organic content in the silt fraction and particles greater than 125 ]..lm.

TABLE 4 . 1 Nubeena spatial s tudy sediment fractions percentage of dry weight (� units ) .

SITE -I 0 + 1 + 2 + 3 + 4 > 4 -5 0 . 7 4 . 5 9 . 6 1 9 . 6 3 7 . 5 28 . 2 1 . 1 0 4 . 2 6 . 0 1 1 . 7 1 6 . 5 3 5 . 4 2 5 . 3 0 . 8 5 1 . 8 1 1 . 7 1 3 . 0 1 4 . 6 3 1 . 9 2 5 . 7 1 . 2 10 2 . 5 10 . 7 1 2 . 7 10 . 4 3 3 . 5 2 9 . 0 1 . 3 20 2 . 1 8 . 1 I I . 3 1 0 . 5 4 0 . 9 2 6 . 3 0 . 8 3 0 13 . 6 1 1 . 6 6 . 8 8 . 1 3 5 . 1 2 3 . 8 0 . 9 4 5 3 . 0 6 . 4 6 . 6 6 . 1 3 4 . 4 4 1 . 4 2 . 2 7 . 7 1 2 . 0 8 . 4 6 . 3 3 2 . 2 3 2 . 2 1 . 3 100 12 . 4 3 . 1 4 . 1 5 . 0 3 9 . 4 3 4 . 7 1 . 4

TABLE 4 . 2 S i t e - 5 0 5 1 0 2 0 3_D 45 6 0 1 _DO TABLE 4 . 3 -5 0 5 1 0 20 30 45 60 1 00 5 6 Nubeena spatial study sediment characteristics , units are

¢'

GM graphic mean, QDI inclusive graphic quartile deviation , SKI inclusive graphic skewnes s .

Percenti _{1 es} GM 5 1 6 84 95 QOI SKI 2 . 3 3 _{O . DO} 1 . 1 0 3 . 3 0 3 . 85 1 . 1 3 - 0 . 3 6 2 . 1 3 - 0 . 85 0 . 6 0 3 . 35 3 . 8 0 1 . 39 - 0 . 3 8 1 . 95 - 0 . 75 0 . 1 5 3 . 30 3 . 8 0 1 . 48 - 0 . 4 1 1 . 8 8 - 0 . 75 0 . 2 0 3 . 3 0 3 . 8 0 1 . 4 6 - 0 . 2 7 2 . 1 8 - 0 . 5 5 0 . 6 0 3 . 3 0 3 . 75 1 . 3 3 - 0 . 5 0 1 . 6 0 - 2 . 0 0 - 0 . 80 3 . 2 5 3 . 7 0 1 . 88 - 0 . 5 4 2 . 45 - 0 . 7 0 1 . _DO 3 . 45 3 . 85 1 . 3_D - 0 . 5 7 1 . 92 -1 . 3 0 - 0 . 35 3 . 45 3 . 75 1 . 7 2 - 0 . 57 2 . 1 5 - 2 . _DO 0 . 3 0 3 . 40 3 . 75 1 . 65 - 0 . 6 2

Percentage weight loss on ignition for Nubeena spatial study sediments retained on different size sieves

(¢

units ) . Total is the calculated percentage weight loss on ignition for the whole samp l e .

-1 0 ._, .2 .3 _" >4 8 . 7 1 5 . 2 1 3 . 6 5 . 2 2 . 2 1 . 9 1 0 . 9 4 . 6 4 . 5 4 . 0 4 . 9 1 . 8 1 . 4 2 . 0 5 . 2 2 . 3 3 . 2 4 . 0 4 . 2 2 . 0 2 . 0 4 . 1 2 . 5 3 . 1 2 . 7 3 . 5 1 . 8 0 . 9 1 . 7 2 . 8 1 . 8 2 . 7 3 . 2 3 . 5 2 . 3 0 . 5 0 . 9 2 . 2 1 . 4 4 . 0 3 . 6 3 . 7 1 . 8 1 . 1 0 . 9 4 . 9 2 . 0 2 . 7 3 . 7 4 . 7 2 . 0 1 . 1 1 . 6 5 . 8 1 . 9 2 . 8 3 . 6 2 . 5 2 . 8 1 . 7 4 . 4 2 . 2 1 . 9 3 . 0 3 . 9 3 . 0 1 . 7 1 . 7 4 . 2 2 . 0

Compared to sites ;::: 1 0 m from the cage, Sites 0 and 5 did not have elevated organic content in the silt/clay fraction ( Table 4.3 ) , The increase in total organic content for these sites was due to increased organic content in fractions ;::: SOD !lID fraction.

5 7

4.1.2 Discussion

There was a gradient of increasing proportions of larger particles with increasing distance from the fish farm cage. This gradient is probably a natural pattern. Because these large particles were only a small fraction of the total sediment, this gradient did not impose a pattern on the sediment particle size graphic means. As these larger particles only formed a small proportion of the sample, and meiofauna are most sensitive to changes in the smaller grain sizes ( Marcotte & Coull, 1974 ) , these changes probably had little influence on the meiofauna.

Site 45 had a higher p roportion of finer sediments and twice the percentage of silt/clay compared to other sites. As only one sample was collected per site the level of b etween-replicate variability is unknown. Therefore, it is not possible to determine if this is a real difference. However, it will be shown that there were a number of community variables different for Site 45 compared to a dj acent sites. Generally, more diverse sediment structures have higher meiofauna diversity ( McIntyre & Murison, 1973; Heip & Decraemer, 1974; Bodin & Le Guellec, 1992 ) . It is possible that a different sediment structure at this site resulted in localised changes to the meiofauna.

When diving under the cage it was necessary to be careful not to stir up a cloud of fine particles. Therefore, it is surprising the proportion of the silt/clay fraction under the cage was similar to that at other sites. In January and April 1989, Li-Xun et al. ( 1991)

examined sediments along a transect in the vicinity of the one used in the present study. They also found no indication of an increase in the silt/clay fraction near the cage. Analysis of temporal changes after the cage was removed ( Chapter 8 ) suggests that the cage

structure was responsible for a reduction in the silt/clay fraction in the sediment under the cage and its periphery. Presumably the fine material, disturbed when diving, was organic material that, when dried, contributed little to the weight of the silt/clay fraction.

The absence of any increase in silt/clay content of the sediment with increased organic content, means that the interstitial spaces would remain in the sediment even where there was high organic input. This is an important difference between the Nubeena and Sandy Bay

5 8 transects and many o f the field studies o n organic enrichment cited in the literature.

The sediment organic content was elevated under the cage centre, and decreased rapidly so that at the cage periphery the value was near normal and no increase could be detected 10 m from the cage. Li-Xun et ai. ( 1991) also found total organic content elevated under the cage ( 9 % in January and 4.7% in April) and approximately 2% at sites 10 to 150 m from the cage. They did not sample between the cage centre and 10 m. Their values from clean sites are similar to mine and the value they found under the cage centre in April is similar to the value I found in August. They postulated that the higher January value was due to the fish being smaller, and therefore, feeding rates b eing different.

Elevated organic content under the cage was found in the silt/clay fraction and in particles retained on the 250 ).1m and larger sieves. The highest percentage organic content was found in the fraction

retained on the 1000 ).1m sieve. O ne source of the larger particles was " filler" material added to the fish food to soak up excess oil. Finely chopped up plant material that made up this " filler" material was visible in the sediment. There was also a greater amount of organic material with a terrestrial origin ( twigs, bark, seeds, etc. ) in sediment under the cage compared to sediments collected away from the cage. Terrestrial material may have accumulated because the cage alters the hydrology so that these particles settle, or it may be

because the high organic content increases the "stickiness " of the sediment so that particles normally moving across the bottom

accumulate.

Li-Xun, et ai. ( 1991) found the organic content of the sediment indicated by weight loss on ignition appeared normal 10 m from the cage. However, for up to 60 m from the cage there was a linear decrease in the proportion of the organic content that originated from fish food with increasing distance. Under the cage 75% of the organic content was derived from fish food and 40% at 60 m. They

concluded that increased organic content detected by weight loss on ignition was a poor indicator for the extent of organic input from

4.2 NEMATODE ASSEMBLAGES

4.2.1 Populations of Abundant Taxa

Four taxa each contained at least 5% of the total nematodes:

Lep tolaimU5 (46%), Chromadora (28%), Daptonema (6%) and

Paramonhystera ( 5 % ) .

4.2.1.1 Results

59

Lep tolaimU5 was collected from all sites. Its distribution was

characterised b y a population bloom of approximately 8,000 · 10 cm-2 at Site 0, and slightly elevated densities at Site 5 (approximately 150 · 1 0 cm-2) compared to the other sites (2 to 65 · 1 0 cm-2) ( Fig. 4.2).

Densities of Leptolaimu5 under the cage centre were similar to those

at sites > 5 m from the cage ( Fig. 4.2).

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