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Examine the influence of habitat diversity and substratum on the composition of macroinvertebrate communities from riverine systems

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Examine the influence of habitat diversity and substratum on the composition of macroinvertebrate communities from riverine systems

Introduction

There have been and continues to be attempts to explain spatial and temporal patterns of diversity in riverine systems (Death and Winterbourne, 1995) and there have been numerous studies aimed at investigating macroinvertebrate community variations.

Macroinvertebrate distributions in streams are determined by variations of many factors such as habitat and food availability, substratum, hydraulic structure and channel morphology, which create distinct morphological units within streams. (Church 1996; Giller and Malmqvist, 1998). Three main habitat types have been identified: erosional, depositional and with macrophytes, habitats which are more commonly referred to as riffles, pools and runs. (Jenkins et al., 1984; Buffagni et al., 2000)

The majority of academic study has concluded that the run-riffle habitat supports the highest number of taxa while the lowest is that of the backwater habitats and that riffles are more diverse than pools (Brown and Brussock, 1991[2]; Logan and Brooker, 1983; Buffagni et al., 2000; Hynes, 1970: Scullion et al., 1982; Armitage et al., 1974; Death and Winterbourne, 1995; Costa et al., 2008).

Although some studies in the mountain stream channel form reported equal or more species abundance and diversity in depositional pool areas (Mcculoch, 1986; Egglishaw and Mackay, 1966; Armitage et al., 1974; Hynes et al., 1976).

Species distribution has been found to vary considerably with specific species displaying preferences to certain habitats and the conditions they provide. For example a difference is seen in the dominant abundance of chironomids in pools and Simuliids in riffles were discovered, along with more riffle and run taxa such as Ephemeroptera, Trichoptera, Plecoptera, and Oligochaeta. (Scullion)

Most Baetids, Simuliidae and all Plecoptera were found to be more abundant in riffles whereas Chironomidae, Dytiscidae and Leptophlebiidae apparently always more common in pools (Armitage et al., 1974). Plecoptera, which are sensitive to low oxygen concentrations due to a lack of extensive gills indicate favourable conditions in riffles, where oxygen concentration is higher. (Scullion et al., 1982).

Many species display exclusivity and adaptations to certain habitats, such as Hydropshyche in riffles or coleopteran, Odonata and Hirudinae in pools. (McCulloch, 1986) found more taxa restricted to pools, however the number of taxa restricted to riffles and faster flowing conditions is generally greater than that of pools (Logan & Brooker 1983, Brown and Brussock, 1991[2]),

At local scales, the spatial variation of organisms is driven often by their behaviour as the search for and the distribution of resources, which is clearly influenced by substrate characteristics (Boyero, 2003).

Substrate is one of the prime factors governing macroinvertebrate compositions with two main variable features, stability and heterogeneity, along with coarse and fine substrates which prove suitable for different taxa (Brown n Brss 11). Substrate diversity or heterogeneity along with high hydraulic variability provides high numbers of feeding niches and micro habitats, resulting in positive relations to abundance, diversity and evenness. (Pedersen and Friberg, 2007; Beisel et al.; Boyero, 2003; Rice et al., 2001)

Substrate stability or consolidation is another key factor. It is proposed unstable unconsolidated riffles had higher abundance and species richness but lower evenness than compact riffles (Pederson and Friberg, 2007), alternatively Rice suggests structured substrates provides a more attractive habitat to benthic fauna as it represents areas of least disturbance during flood events (Rice et al., 2001).

Pools often lack coarse gravel substrate and this can mean fewer interstitial refugia are available resulting in higher predation vulnerability than riffles (Rice et al., 2001). Williams & Mundie (1978) suggest medium size gravel supports the greatest number and biomass although scullion believes otherwise with little difference in particle size compostion of bed materials in pools and riffles.

McCulloch’s (1986) contrasting study of sandy bed rivers found that the instability of sandy substrate in riffles limited colonization by benthos and depressing the numbers of certain species. Grain roughness is another contributing factor as it defines near bed hydraulic conditions such as turbulence and shear stress and also influences the retention of detritus, a key factor affecting abundance according to Fahy (1975), which effect organisms’ ability to move position and feed effectively. (Rice et al., 2001)

As most benthic animals are functionally sessile, they depend on flow to supply physiological needs other of their requirements such as food and oxygen (Death and W). Highest water velocities are found in riffles and lowest in pools and subsequent oxygen concentrations are highest in riffles and lowest in the pool, temperature is also related to flow velocity and the slower flow in pools results in higher temperatures which limit species abundance (Scullion et al., 1982; Hynes, 1970). A higher intra-gravel oxygen supply rate in riffle substratum also favours burrowing forms (Scullion et al., 1982). Flow rate increases further over riffles and decreases through pools during summer flows making riffles more attractive to organisms that need flow to meet their physiological needs and provide silt-free substrate (Brown and Brussock, 1991[2]).

All natural streams are subject to disturbance events most commonly fluctuations of discharge which vary in severity, intensity and frequency and impact significantly on community development and structure. (Resh et al., 1988)

Substrate is an important factor determining the impact of disturbances on macroinvertebrate communities, for example McCulloch (1986) in a study of sandy substrates in riffles suggested instability limited colonization with disturbances influencing the finer, less consolidation riffles more than pools, however in gravel bed streams more like that examined here, relative disturbance intensities between riffles and pools may be the reverse (Hynes 1970). Hynes (1970) also proposes that many of the organisms in the pools may only be there as a direct result of having been moved by disturbing currents from the preferred riffle habitats.

However these are often individual studies and McCulloch (1986) summarises that the preference of a riffle habitat is a “response to the physical characteristics of the river channel as determined by the geomorphology of the watershed.”

Method

The study was conducted in the Wood Brook stream in Loughborough on March 13th 2008 (Fig.1, 2, 3). Kick sampling was used (Fig.3) to gather 4 samples from 8 areas of this section of Wood Brook.

image00.jpgimage01.jpgimage02.jpg

Results

Invertebrates were identified using an identification key and data was recorded ready for investigation (Appendix 1). A total of 3419 individuals from 28 species were collected from the four different riverine habitats, Pool, Run, Riffle and one other unique area which varied from man made rock or concrete steps (Fig. 2) to a small weir and tributary confluences (Appendix 1).

The Shannon or Shannon-Weiner Index is a measurement of diversity which considers species evenness and richness where higher values indicate higher species diversity. The riffle habitat displayed the highest mean standard error of species diversity, followed by runs and then pools, with the ‘other’ habitat showing least diversity (Fig.4).

Species diversity appears to correspond with the distribution of unique or rare species which increase the index value substantially, for example, of the 7 species found only once, 3 were found in riffles, 3 in runs and only 1 in pools, and of the 3 species with only 2 members collected, 2 were in riffles, 3 in runs and only 1 in pool habitats (Appendix 1). The data supports the suggestion that Trichoptera show exclusivity to faster flowing habitats, as 4 species of those found to be unique to riffles and runs were from the insect order Trichoptera (Polycentropidae, Rhycophilidae, Psycomyidae, Philopotamidae) (Brown and Brussock, 1991 [2]; Wang et al., 2006). Around 85% of the order Plecoptera were also found in riffle and run habitats, with Hemiptera comprising the only pool exclusive species (Appendix 1). The slower flowing pools would favor these insects (Hemiptera) as they skate or swim in the stream and need to surface for air.

The Berger-Parker Index measures species dominance by calculating how much of the sample population the most abundant species comprises. All habitats display relatively high scores, ‘other’ habitats display the highest value, followed by the run and then the pool, and the riffle the lowest.

These relatively high values may be attributed to dominant abundances of Chironomidae, along with high abundances of Gammaridae and Oligochaetas throughout the entire sample. A negative correlation with the Shannon-Weiner Index can be observed, where as dominance increases, diversity decreases, with the correlation increasing in strength at lower abundances and higher dominance values (Fig.10).

Riffle habitats proved the most abundant habitats (126.125) whereas pools produced the least abundant habitats (80) (Fig.6). Run habitats produced the highest number of taxa just exceeding and average of 8, with riffles second and the ‘other’ habitat third both showing relatively equal average numbers of taxa of around 7. The pool represented the habitat with the lowest number of taxa, just over 6 (Fig.7). These findings corroborate much previous evidence (Brown and Brussock, 1991[1]; Logan and Brooker, 1983; Buffagni et al., 2000; Hynes, 1970: Scullion et al., 1982; Armitage et al., 1974; Death and Winterbourne, 1995; Shirley, 2008).

The mean average Biological Monitoring Working Party (BMWP) scores are all below 40 with the lowest around 27 which suggest that all the habitats and this area of Wood Brook are positioned in the poor quality category where the water is expected to be polluted or impacted (Bournestreampartnership.org,WWW).

The BMWP scores for each habitat (Fig.8) are positively correlated to the number of taxa (Fig.7) which may indicate that the average species BMWP score was not excessively low. This Average Score per Taxon (ASPT) is derived by dividing the BMWP score by the number of taxa in that sample. It was found that the ‘other’ habitats achieved the highest ASPT scores followed then by run and pool habitats respectively; with riffle habitats produced the lowest ASPT scores.

Riffles displayed the highest abundance (Fig.6) and diversity (Fig.4) yet the lowest mean ASPT indicating a macroinvertebrate community not comprised of high organic pollution sensitivity and relatively high levels of organic pollution.

Better biological quality is indicated by an ASPT value exceeding 5.5 (ehsni.gov, WWW) and with ASPT values for the four habitats varying from as low as around 4.4 in riffles to approximately 5 in the other habitat (Fig.9) it is concluded that Wood Brook has a relatively poor biological quality. (Wang et al., 2006)

The data from one the 8 samples collected shows convergence with the overall findings, with the run habitat displaying the highest number of taxa (12) and BMWP score (60), however the riffle habitat provided taxa and a lower BMWP score than the average (Fig.11, 12). The fourth habitat in this case was a rock step (Fig. 2) which produced similar abundance and taxa to the pool.

The ‘other’ habitat displayed the lowest diversity and highest dominance, producing more taxa than pools and higher abundances than both pool and run habitats. Due to the ambiguity of locations, the relatively middling results are as expected (Fig.4, 6, 7).

Discussion and Conclusion

The four riverine habitats run, riffle, pool and other, show distinct differences in the composition of macroinvertebrate communities with organisms displaying preference for faster flowing riffle and run habitats. Riffles produced the most abundant habitats reflecting many previous studies such as Buffagni et al. (2000) and Pedersen and Friberg (2007), however contradicting the study of McCulloch (1986) although that study was performed on sandy bed systems. Riffles, followed by runs displayed the most diverse habitats and the run habitats also displayed the highest number of taxa with riffle habitats second with this mirroring many previous academic studies. Pools displayed the lowest number of taxa, lowest abundance and the second lowest species diversity.

Species variation in accordance with previous studies was apparent; chironomids for example were by far the most abundant species in all habitats and did not appear to show any habitat preference, this is reflected by their low BMWP which indicates they are highly tolerant to pollution (Scullion et al., 1982; Brown and Brussock, 1991 [2]). The order Plectoptera are alternatively markedly sensitive to low oxygen concentrations due to their lack of extensive gills and again mirroring previous findings they were found in greatest abundance in runs (20) and riffles (8) with pools and the other habitat with lowest abundances. However there was little evidence to support the claim that some species are pool-adapted, with only one species, Hemiptera being exclusive to pool habitats. (McCulloch, 1986)

The Biological Monitoring Working Party (BMWP) score and the subsequent Average Score per Taxon (ASPT) used are both measures of organic water pollution. It is suggested that the BMWP scoring system may be more accurate in identifying gross disturbance than more subtle changes such as those between microhabitats within Wood Brook investigated in this study. The Average Score per Taxon (ASPT) is also far less sensitive to seasonal variations but is relatively insensitive to the number of BMWP families found. (Mustow, 2002).

One possible reason for recordings representing poor water quality is that much of the Wood Brook experiences anthropogenic impacts, with much of its length culverts

Although not directly measured, substrate and hydraulic characteristics may be estimated, for example it may be possible to conclude that high substrate diversity high hydraulic variability provides high number of feeding niches and micro habitats (Buffagni et al., 2000) reflect the riffle and run habitats of Wood Brook which provide highest abundance and diversity and number of taxa.

It may also be proposed that the pool habitats provide something of the opposite, supporting the lowest number of taxa and abundance and lower species diversity. The fourth ‘other’ habitat is difficult to summarise as it consisted of 3 diverse areas and further studies into these individual areas would be required to fully asses them in terms of their macroinvertebrate community compositions.

Trends have been identified amongst the habitats examined and the results conform in general to what is expected in regards to numerous studies performed prior to this, it can be concluded that in-stream macroinvertebrate distributions are governed by the availability of different habitats and food resources and by biotic interactions (Giller and Malmqvist, 1998).

References

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