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A.J. Underwood . M.G. Chapman
missioned; sewage was, instead, discharged from diffusers over 5-800 m of seabed at
60-80 m depth, 3-4 km offshore. This major plan of changed management was long
overdue and largely driven by public concerns about human health and environmental issues for coastal fauna and flora.
As one aspect of such concerns, Fairweather (1990) had reported that assemblages
of algae and invertebrates on rocky intertidal shores near some outfalls were quite different from those elsewhere. This observation was explained by increases in nutrients
around outfalls (e.g. Borowitzka 1972; Littler and Murray 1975). Generally, assemblages
close to outfalls were dominated by early-stage colonising species, primarily green algae and had fewer filter-feeders (barnacles and mussels). Such effects could, however,
have been due to the freshwater discharged at the outfalls - which often favours growth
of green algae where salinity is reduced. Either way, the managerial hypothesis was that
cl~sing the outfalls would result in changed assemblages, to become more like those
where there was no disturbance due to sewage.
To test this hypothesis in the Sydney region, Banwell (1996) completed a properly controlled study, with appropriate replication before and after the outfalls closed. She sampled intertidal assemblages on rocky shores at North Head and Malabar, which had outfalls that were to be closed. She also sampled at Potter Point and Bellambi, which had outfalls that were to continue to discharge sewage. At each outfall, she sampled near the outfall
and at two reference locations about 1 km away - a distance chosen because Fairweather
(1990) had demonstrated no effect of sewage at that distance from an outfall.
The number of algal species is used here to illustrate the outcome of changes of management of sewage. Before the outfalls were closed, there were significantly fewer species of algae at every outfall compared with reference areas unaffected by sewage (Fig. 6.3).
After 2 outfalls were closed, numbers of algal species remained depressed at the 2 control
outfalls that continued to operate (Fig. 6.3). As predicted in the hypothesis, removal of
sewage led to increases in numbers of species of algae at North Head, to match the situation in areas previously unaffected by sewage. So, for North Head, the predicted changes
occurred and can be clearly associated with the changed management. There was a general increase in algal diversity during the period of the study. Increased algal diversity following closure of the outfalls was greater than where outfalls were not closed (Fig. 6.3).
There was a smaller increase in numbers of species at Malabar after the outfall closed.
In fact, fishermen kept one replicate site clear of algae to increase their safety while
standing there. Such damage was sufficient to prevent average numbers of species increasing to match reference areas. Banwell (1996) demonstrated experimentally that
algal diversity increased to numbers in reference areas, provided fishermen did not
destroy them.
In this case, using normal procedures of experimental design, the changed management was shown to be entirely effective. This was, however, not the case for subtidal
rocky regions. Concern had been raised that inshore subtidal outfalls were deleteriously affecting assemblages of invertebrates and algae. Sampling at several depth around
the North Head subtidal outfall after it was closed showed no major changes in assemblages (Chapman et al. 1995; Underwo09 and Chapman 1997). The explanation was that
the outfalls were, in fact, having very little effect on these subtidal assemblages dominated by filter-feeders and red algae. Assemblages were not really different from those
in similar, reference habitats where there was no sewage. In this case, the data collected
from a controlled and replicated experimental d,esign were useful for guiding decisions
A.J. Underwood . M.G. Chapman
missioned; sewage was, instead, discharged from diffusers over 5-800 m of seabed at
60-80 m depth, 3-4 km offshore. This major plan of changed management was long
overdue and largely driven by public concerns about human health and environmental issues for coastal fauna and flora.
As one aspect of such concerns, Fairweather (1990) had reported that assemblages
of algae and invertebrates on rocky intertidal shores near some outfalls were quite different from those elsewhere. This observation was explained by increases in nutrients
around outfalls (e.g. Borowitzka 1972; Littler and Murray 1975). Generally, assemblages
close to outfalls were dominated by early-stage colonising species, primarily green algae and had fewer filter-feeders (barnacles and mussels). Such effects could, however,
have been due to the freshwater discharged at the outfalls - which often favours growth
of green algae where salinity is reduced. Either way, the managerial hypothesis was that
cl~sing the outfalls would result in changed assemblages, to become more like those
where there was no disturbance due to sewage.
To test this hypothesis in the Sydney region, Banwell (1996) completed a properly controlled study, with appropriate replication before and after the outfalls closed. She sampled intertidal assemblages on rocky shores at North Head and Malabar, which had outfalls that were to be closed. She also sampled at Potter Point and Bellambi, which had outfalls that were to continue to discharge sewage. At each outfall, she sampled near the outfall
and at two reference locations about 1 km away - a distance chosen because Fairweather
(1990) had demonstrated no effect of sewage at that distance from an outfall.
The number of algal species is used here to illustrate the outcome of changes of management of sewage. Before the outfalls were closed, there were significantly fewer species of algae at every outfall compared with reference areas unaffected by sewage (Fig. 6.3).
After 2 outfalls were closed, numbers of algal species remained depressed at the 2 control
outfalls that continued to operate (Fig. 6.3). As predicted in the hypothesis, removal of
sewage led to increases in numbers of species of algae at North Head, to match the situation in areas previously unaffected by sewage. So, for North Head, the predicted changes
occurred and can be clearly associated with the changed management. There was a general increase in algal diversity during the period of the study. Increased algal diversity following closure of the outfalls was greater than where outfalls were not closed (Fig. 6.3).
There was a smaller increase in numbers of species at Malabar after the outfall closed.
In fact, fishermen kept one replicate site clear of algae to increase their safety while
standing there. Such damage was sufficient to prevent average numbers of species increasing to match reference areas. Banwell (1996) demonstrated experimentally that
algal diversity increased to numbers in reference areas, provided fishermen did not
destroy them.
In this case, using normal procedures of experimental design, the changed management was shown to be entirely effective. This was, however, not the case for subtidal
rocky regions. Concern had been raised that inshore subtidal outfalls were deleteriously affecting assemblages of invertebrates and algae. Sampling at several depth around
the North Head subtidal outfall after it was closed showed no major changes in assemblages (Chapman et al. 1995; Underwo09 and Chapman 1997). The explanation was that
the outfalls were, in fact, having very little effect on these subtidal assemblages dominated by filter-feeders and red algae. Assemblages were not really different from those
in similar, reference habitats where there was no sewage. In this case, the data collected
from a controlled and replicated experimental d,esign were useful for guiding decisions