LOS PRECIOS DE MONOPOLIO

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�contrast, many similar studies have found greater overall densities at unshaded sites when

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.�_{�ompared with forested, shaded sites (Behmer &Hawkins, 1986� Dudgeon, 1989� Tuchman &}

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( ': _bg;.l993� Quinn·�tal., 1 997b; Townsend et al., 1 997). However, as in this study, other

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/�tudies focusing on differences between willowed reaches and unshaded reaches have been

' ·,�strikingly inconsistent in their findings. Lester et al. (1994a) found lower densities under

vegetation in autumn, spring and summer while Glova & Sagar (1994) found higher

ildensities in moderately willow-lined reaches, but only in swnmer. Latta (1974) found no

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�consistent trends in willowed and unshaded reaches in the rivers he examined.

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."In contrast, the patterns of taxon richness and diversity found in this study are generally

consistent with other studies comparing willow-lined reaches and unshaded reaches. Glova &

Sagar (1994) found greater species richness and diversity in willowed reaches only in swruner and Besley (1992) found higher species richness under willow tree roots than under bare bank habitats in autumn and winter. Lower species richness (Hawkins et al. , 1982; Lenat &

Crawford, 1994; Boulton et al., 1997; Townsend et al. , 1997) and lower taxon diversity (Newbold et al., 1980; Quinn et al. , 1992b; Clenaghan et al., 1998) have also been reported for unshaded reaches in comparison to forested reaches in more general studies. In this study, the higher diversity found in autumn in willowed reaches results from increased numbers of shredder taxa in this season.

In a series of studies investigating relationships between riparian shading, land use and

benthic community composition, Quinn et at. ( 1992 b) consistently measured lower taxon

richness in unshaded conditions in pasture reaches. They suggested that lower taxon richness in the unshaded reaches they examined reflected a general reduction in organic matter

retention leading to lower habitat diversity; they further suggested that, in forested

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catchments, litter inputs from riparian trees would be expected to increase the quantity of

allochthonous organic matter and hence the diversity of food resources available, and that this, in tum, would be reflected in an increased diversity of macroinvertebrates. Other ecologists

have attributed low taxon richness or diversity in unshaded reaches to unsuitable habitat

conditions either through a degradation in water quality in swnmer (Boulton et al., 1997) or

excessive growth of filamentous algae in unshaded reaches, which would compromise

harvesting efficiency of scraping and filtering taxa (Towns, 1979; Quinn et al., _{1992b; Glova}

'"& _{Sagar, 1 994). The precise mechanism responsible for the reduction of taxon number and}

· -�versity in this study are unclear as no differences were detected in the density of scrapers in

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ason, and higp.er densities of filterers were detected in removal reaches in autumn.

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. �n terms of functional feeding groups and community composition, the patterns were clearer.

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Higher densities of shredders were found in will owed reaches in autwnn and swnmer, and, from the multivariate analyses, willowed reaches were characterised by a core groqp of shredders in all seasons. This was also ret1ected by the higher densities of the shredding

family Eusiridae (Amphipoda) in willowed-lined reaches in swnmer. The close association

between shredding taxa and POM standing stocks has been documented in numerous studies (e.g. Hawkins & Sedell, 1 98 1 ; Dudgeon, 1 98 9; Reed et a/., 1 994; Townsend et al., 1 997)

ranging forested reaches to unshaded reaches, although studies examining willowed reaches specifically have generally not found the tight relationships between shredders and available leaf detritus as proposed by Cwrunins et al.(1989). Lester et a/.(1 994a) found no relationship between shredder densities and elevated POM standing stocks will owed reaches in any

season. Latta's (1 974) findings were equivocal given conflicting patterns between rivers he

studied, although he did find higher densities of shredding amphipods in willow-lined reaches,

and this association has also been found for other forested reaches (Towns, 1 979). The

relationship between higher shredder densities in willowed reaches in swruner and CPOM

was unclear as CPOM was not in significantly higher quantities in willowed reaches although CPOM standing stocks were on average, higher in willowed reaches than removal reaches. Higher shredder densities may be related to generally higher CPOM standing stocks in

willowed reaches or alternatively, shredders may be facultative in their feeding mode as found by Hawkins & Sedell (1981).

Scraping taxa were also associated with willow-lined reaches in shallow rivers, which

contrasted with my prior expectation that this group would be more numerous in removal sites where epilithon biomass was higher. Minshall eta/.(1 983) predicted that grazers should

follow shifts in the abundance of primary producers, and many ecologists have found higher densities of scraping taxa in unshaded reaches (Dudgeon, 1989; Tuchman & King, 1993; Reed et al., 1 994; Scott et al., 1994). In this swvey although different scraping taxa were associated

with both willow-lined and removal reaches, they were more commonly associated with leaf detritus in willowed reaches. Scraping taxa (Elmidae, Psephenidae, Glossosomatidae,

·· Hydrobiidae), in association with shredders, were commonly associated with willow lined 82

\reaches and the gastropod hydrobiids were denser in willow lined reaches in autumn. These findings may be attributable to a combination of food resource availability at willowed reaches and an oversupply of food in removal reaches compromising harvesting efficiency. Many studies investigating willow leaf colonisation and feeding preference trials with different leaf species have found strong associations between scraping molluscs �d willow leaves (Hanlon, 1 98 1 ; Collier & Winterboum, 1 986; Chergui & Patee, 1 993; Yeates, 1 994), and Latta (1 974) found high numbers of the the hydrobiid Potamopyrgus antipodarum in willowed reaches in autumn in his study. In addition, Townsend et al. (1997) found grazers were associated with shredders, and both feeding groups were typical of sites with high percentages of leaf material, while Lester et a!. (1 995) found that willow leaves were widely used either directly or indirectly as a food source by members of all functional feeding groups.

One reason for the close association of scrapers with willows was proposed by Collier & Winterboum (1 986). It is likely that in autumn, grazer or scraping taxa use willow leaves as substrate for harvesting fungal hyphae and bacteria that have colonised willow leaf surfaces. Many ecologists have found that scrapers, filterers and collectors do not respond predictably to changes in the relative contribution of allochthonous and autochthonous resources

(Hawkins & Sedell, 1 98 1 ; Glova & Sagar, 1 994) and suggest that scrapers are better adapted for harvesting thin organic films on substrate surfaces rather than algal species with

filamentous or matlike morphologies. Furthermore, not all species of algae, and presumably other components of periphyton, are equally palatable. These factors, combined with spatial and temporal shifts in algal community composition could therefore affect the capacity of streams to support scraper populations (Hawkins et al. , 1982). Thus, although production and standing stocks may be highest in open streams, much of the epilithon may not be available to the scrapers or grazers as filamentous green algae dominated most coarse benthic substrates in shallow removal reaches although in the finer substrates in the North Esk and Liffey River, the macrophyte species Isolepis jluitans was more common.

Higher densities of filterers were found in summer in removal reaches and, for some removal sites, filterers showed a strong association for unshaded conditions. This was also supported by the higher densities Simuliidae in removal reaches in autumn. The preference for

ak,' l997a} was· nqt found in this study. Experimental studies have attributed this to the

requirement for Austrosimulium to have clean attachment sites as unshaded sites were

characterised by thick algal mats or filamentous algal blooms (Towns, 198 1 ; Downes & Lake,

1991; Quinn et al., 1997a). However, in this study the epilithon was patchily distributed, and there were sufficient bare areas on rocks to allow Simulliids to attach; their high�r densities may be attributable to a combination of patches of microhabitat with higher velocities than in willowed reaches (Collier et at. , 1998) and perhaps higher quality food entrained from the surrounding epilithon.

In spring, differences in habitat conditions and macroinvertebrate community composition were not as apparent as in autumn or summer, although the scraper-shredder assemblage was

still evident in willowed reaches. Chironomidae were more dense in removal reaches in this

season and this has been common in many other studies (Newbold et at. , 1980; Behmer &

Hawkins, 1986; Tuchman &King, 1993; Tait et al. , 1994; Quinn et al. , 1997b). Differences in

food resources were still apparent in spring, although macroinvertebrate communities did not seem to respond to food availability and it is possible that higher flows in spring ameliorate

the marked differences observed in summer and autumn. As a result, macroinvertebrate

communities in willow-lined and removal reaches were more similar to one another. Another factor related to the influence of discharge variability on food resources during spring was put

forward by Quinn et al. (1992b ). They suggested that the abundance of algal mats and

associated detritus during summer low flows may be susceptible to sloughing during flood events in later seasons. Similarly, variability in discharge may influence CPOM and FPOM retention in willow-lined reaches.

In all seasons, individual removal reaches are associated to varying degrees with a wide diversity of collecting filtering and scraping taxa (Caenidae, Baetidae, Notonemueridae, Simuliidae, Hydropsychidae Chironomiidae, Leptophlebidae, Grypopteridgidae) and these taxa have been commonly associated with unshaded reaches in many other studies (Newbold

et al. , 1 980; Reed et al. , 1994; Tait et al. , 1994; Quinn et al. , 1997b). Other studies have shown collectors to have no correlation with preferred food resources (e.g. Chessman, 1986; Barmuta, 1 988), and Chessman (1986) found that many of the above taxa have diverse diets. For example caenid mayflies could both process ultrafine detritus and skeletonise willow

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,, while othe� taxa may not only feed on filtered materials but graze as well

(Hydropschycidae). Rounick eta/. (1982), Lester et a/.(1995) and Winterbourn et a/.(1984) found variable utilisation of allochthonous and autochthonous materials shown by

macroinvertebrate species in grass reaches. These studies emphasise difficulties in studies seeking simple correlations between standing stocks of food resources and the relatively coarse generalisations inherent in assigning taxa to functional feeding groups. For families of invertebrates, there may not only be a variety of feeding groups within the taxon, but

component taxa may change their feeding mode as they grow or may even be opportunistic. Likewise coarse measures of standing stocks do not account for differences in productivity or differences in nutritional value and palatability within the resource categories.

Deep reaches

The differences in macroinvertebrate community composition between degraded willow reaches and removal reaches were more consistent than for the shallow reaches. Will owed reaches were characterised by a shredder-dominated fauna in all seasons while scrapers and collectors were consistently more typical of removal reaches. Two reaches were heavily infested with willows with dense shade, negligible flow (due to blockage by fallen limbs), very high organic matter deposition , and low dissolved oxygen values of 4-5 mg.L-1• These reaches had high numbers of Oligochaeta and "bloodworms" (Chironomus spp.) which are

taxa, along withHeterias (lsopoda), and Physa (Gastropoda) that Suter(1990) has found to be

typical of highly enriched environments in Australia. Similarly, Bobbi et al. (1996)

documented chronically depressed dissolved oxygen levels (6 mg.L-1) in a survey of degraded willow reaches in northern Tasmania; they suggested that these low dissolved oxygen levels were due the increased shading, reduced flow, and elevated biological oxygen demand required for the breakdown of the prevailing high organic loads.

Fish

All shallow removal reaches bar one had higher population estimates for total mean fish

densities than willowed reaches although no clear patterns were evident for size distributions between reaches for any fish species. Higher densities of fish in unshaded reaches has been a

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finding in studies comparing fish populations in canopied with unshaded reaches

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(Hortle & Lake 1983· Dolloff, 1986; Elliott, 1 986; Tait et al., 1994; Jowett & Richardson,

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, 1996). Elevated fish numbers in unshaded reaches are usually attributed to higher autotrophic

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production , and consequently higher densities of prey (Newbold et al., 1980; Gregory et al.,

1991; Glova & Sagar, 1994; Tait et al., 1994); however, in this study, the similari� in total macroinvertebrate density between willowed and removal reaches suggests that other factors may be responsible for higher fish densities in removal reaches.

The ordination patterns indicate that individual fish abundance was more strongly related to differences in the prevailing habitats between rivers than it was to differences in of riparian type within rivers. The limited size-frequency distribution comparisons of brown trout showed weak support of an effect due to riparian vegetation with smaller sized S.trutta cohorts being less abundant in some willowed reaches. This is consistent with the more common finding that in-stream cover is more important to smaller fish, and that high riparian cover is more

important to larger sized fish (Penczak, 1995; Collares-Pereira et al., 1997). Only smaller fish

can use the smaller elements of in-stream cover or interstitial spaces in stream beds as cover within reaches once overhanging vegetation has been removed or if a reach is unshaded

(Latta, 197 4 ).

This pattern has been found in other studies investigating differences in biomass and

population density between willowed reaches and unshaded reaches where willows have been removed (Latta, _1974; Glova & Sagar, _1994). Glova & Sagar ₍₁₉₉₄₎ found the abundance and biomass of both brown trout and eels to be greater in will owed reaches compared to unshaded reaches and that size distributions of eels were larger in willowed reaches compared to

unshaded sites. Although mean fork length of trout was greater in willowed reaches this was not significant. Latta ₍₁₉₇₄₎ found that removal of willows int1uences both the density and mean length of trout. Both these workers concluded that willows were beneficial to brown trout populations although Glova & Sagar (1994) found low densities offish in densely willowed sections as in this study. Other studies investigating the impacts of riparian logging on fish populations (Dolloff, 1986; Elliott, 1986) and for reaches with low habitat

heterogeneity that have been channelised or cleaned of woody debris (Hortle & Lake, 1 983) . have found similar patterns of smaller size cohorts in unshaded environments and larger sized

' In-this survey, in-stream elements of cover in removal reaches were expected to become more important for fish after the removal of willows. Similarly, the increased light penetration may be .expected to enhance some in-stream cover elements such as submerged and emergent macrophyte vegetation and algae beds. In this study extensive macrophyte beds were observed in removal reaches and eels and redfm were locally abundant in these habitat types. Slow flowing weedy areas have been identified as the preferred habitat of these species

(Cadwallader, 1975; Lake & Bennison, 1977; Sloane, 1984; Koehn & or Connor, 1990) and macrophyte cover has also been found to provide important habitat for smaller sized

individual of other fish species (Cunjak, 1987; Collares-Pereira et al., 1 997). High

macrophyte cover appears to be an important cover element in this study for both willowed and removal reaches particularly for eels and redfin perch.

The strong association of blackfish with

L WD

habitat is well documented (Koehn, 1987;

Davies, 1 989). These species use

LWD

for egg deposition and cover and both reaches in the

Liffey River with comparatively high L

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standing stocks showed a strong association with

blackfish. The localised abundance ofG.marmoratus in the Liffey River was expected given

the species historic range for the northern coastal rivers although the presence of

G. marmoratus in southern Tasmanian rivers surveyed is largely the result of tmnslocation.

Densely willowe'd sites had the lowest numbers offish as was found by Glova & Sagar (1994) who found lower fish abundance in densely willowed reaches than in moderately willowed or unshaded reacheS. They suggested that the dense willow canopy lowers primary production which results in reduced benthic invertebrate abundance and fish abundance. However, I

suggest that the low oxygen levels in very densely willowed reaches might be a more

plausible explanation. Dean & Richardson (1999), for example, detected fish mortality at 3 mg.L·' and various water quality standards suggest many fish species are susceptible to stress

at4 -6 mg.L·1 (Dallas &Day, 1993) which are comparable to the dissolved oxygen levels

found in most degraded reaches in this survey ( 4 - 5 mg.L"1).

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and abundance of the fish fauna at greater _. stream order and proximity to the coast as

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a: result, in part, of the addition of migratory species to the fauna of Tasmanian rivers (Davies,

f, 1989). The Liffey and North Esk Rivers are at higher altitudes than the remaining rivers

surveyed, and only trout and blackfish were found in these reaches. In addition, red fin perch

(P. j/ttviatalis) is a known warm water species and prefers slower flowing habitats; Faragher

& Lintermans (1997) have suggested that these factors may determine upstream limits for this species. However, the majority of reaches were located in lowland areas under 200m altitude and the above factors would have minimal effects on the broad patterns found.

In addition, most native fish species, such as R. tasmanica (smelt), G. maculatus (jollytail

galaxiid) and _N. australis (pygmy perch), were rare. Although the effect of overshading of

willows may have reduced the preferred habitat of these species for both emergent and submerged macrophytes in willowed reaches as described by Collier (1993), it is likely that

predation and niche overlap by competitive species such as S. trutta and A. australis in

removal reaches may have cont ibuted to reduced galaxiid and smelt populations in open reaches as documented elsewhere (Tilzey, 1 976; Jackson & Williams, 1980; Jackson, 1981;

McDowall, 1990). These relationships have also been documented for N australis and

P.

j/uviatalis (Faragher &Lintermans, 1997).

Summary

Shallow willowed reaches are characterised by dense canopies and decreased sunlight for

much of the year, and this directly influenced the standing stock of epilithon. They are also characterised by high organic input in autumn. This is used by shredders and probably scrapers, and these taxa dominated the fauna at willowed sites in all seasons. In contrast,

removal reaches a e unshaded and increased sunlight elevates epilithic standing stocks _in all

seasons surveyed. Removal reaches have a depressed organic input and generally collector and filtering taxa dominate the fauna. Macroinvertebrate taxa at these sites may be

opportunistic, feeding on a variety of food materials and particles sizes to varying degrees at

each site. Generally, removal reaches supported greater densities of fish, but it appears _that

these removal reaches all appeared to have abundant elements of in-stream cover. _In reaches

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. in-s�eam cover population density decreased. There was some evidence that

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removal

of canopied vegetation does however appeared to have some adverse impacts as ; some willowed reaches in shallow rivers support larger cohorts of S. trutta. although as the

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number of comparisons were limited, consistent broadscale patterns cannot be determined in

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