Relationships between drifting and benthic ...

New Zealand Journal of Marine and Freshwater Research

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Relationships between drifting and benthic invertebrates in three New Zealand rivers: Implications for drift‐feeding fish Karen A. Shearer , John D. Stark , John W. Hayes & Roger G. Young To cite this article: Karen A. Shearer , John D. Stark , John W. Hayes & Roger G. Young (2003) Relationships between drifting and benthic invertebrates in three New Zealand rivers: Implications for drift‐feeding fish, New Zealand Journal of Marine and Freshwater Research, 37:4, 809-820, DOI: 10.1080/00288330.2003.9517210 To link to this article: http://dx.doi.org/10.1080/00288330.2003.9517210

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Relationships between drifting and benthic invertebrates in three New Zealand rivers: implications for drift-feeding fish

KAREN A. SHEARER JOHN D. STARK JOHN W. HAYES ROGER G. YOUNG Cawthron Institute Private Bag 2 Nelson, New Zealand email: [email protected] Abstract We assessed whether taxonomic structure and density of aquatic drift could be predicted from the benthos in three New Zealand rivers. The three main orders contributing to both the benthos and drift were Ephemeroptera, Diptera, and Trichoptera. Drift and benthic densities for all taxa and all rivers combined were not significantly correlated (adults inclusive and exclusive). There were significant positive correlations between benthic and drift densities for the three main drifting orders—Ephemeroptera, Diptera, and Trichoptera when data from all rivers were combined. However, these relationships were not always detected in individual rivers. The propensity for Deleatidium to drift was negatively related to chlorophyll a concentration; suggesting density-dependent drift mediated by food limitation. Drift was reduced when periphyton chlorophyll a concentration was high in relation to benthic Deleatidium density. This highlights an unexpected effect of periphyton proliferation on invertebrate drift and drift-feeding fishes. Despite finding some correlations between benthic and drifting communities, defining general relationships between benthic and drifting communities is challenging given the complexity of density-dependent and densityindependent mechanisms that influence invertebrate drift.

M02079; Online publication date 31 October 2003 Received 3 October 2002; accepted 15 July 2003

Keywords aquatic invertebrate; benthos, densitydependent; drift, drift-feeding; fish; periphyton

INTRODUCTION Drift—the downstream transport of invertebrates in the water column—is a feature of rivers worldwide. Reviews by Waters (1972), Statzner et al. (1984), Brittain & Eikeland (1988), and more recent studies (e.g., Ramírez & Pringle 1998), have cited many biotic and abiotic variables that influence drift. Moreover, drift density can be dependent on (McLay 1968; Lehmkul & Anderson 1972; Hildebrand 1974; Statzner et al. 1987; Sagar & Glova 1992; Siler et al. 2001) or independent of (Waters 1972; Graesser 1988; Rutledge et al. 1992) benthic densities. Density-independent mechanisms for invertebrate drift include accidental dislodgment from the substratum, changes in the physical environment (e.g., variations in velocity, flow, water chemistry, oxygen, sedimentation, seasonality, or the light regime), and behavioural drift (e.g., emergence of adult aquatic insects) (Elliott 1967b; Reisen & Prins 1972). Density-dependent mechanisms include interactions with other organisms such as predation, and competition for food or space (Statzner et al. 1987; Ramírez & Pringle 1998). Drifting invertebrates are of interest because of their importance as food for fish (Elliott 1967a, 1970; Waters 1969; Sagar & Glova 1988; Dedual & Collier 1995; Hayes et al. 2000; McIntosh 2000), their role in benthic invertebrate community recolonisation and production (Waters 1972; Sagar 1983), and potential as a useful measure of water quality (Chutter 1975; Pringle & Ramirez 1998). Invertebrate drift productivity of a river is particularly important for modelling net rate of energy intake (NREI) of drift-feeding fish to predict growth and habitat selection (Hayes et al. 2000; Guensch et al. 2001)—models that are playing an increasingly important role in fish ecology (Hughes et al. in press). These models require estimates of drift density (or biomass) to predict fish growth,


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New Zealand Journal of Marine and Freshwater Research, 2003, Vol. 37

maximum size, and abundance. Pre-existing benthic data are more readily available than drift for many rivers. Furthermore, collection and processing of drift samples can be time-consuming and expensive compared with benthic sampling. This led us to consider whether aquatic invertebrate drift estimates could be extrapolated from benthic data for assessing the productivity of rivers for drift-feeding fish and for obtaining the inputs required by fish foraging and NREI models. We looked for taxonomic and density relationships between benthos and drift, and the effect of food supply (chlorophyll a concentration) on drift. Daytime (or diurnal) drift has been referred to as "constant" or "background" drift (Brittain & Eikeland 1988) and, therefore, is more likely to be correlated with benthos than 24-h drift. We confined our comparisons to daytime drift to reduce the influence of behavioural/life history-related drift (i.e., diel drift periodicity) that may occur during dawn or dusk peaks. Furthermore, measures of daytime drift are relevant to drift-feeding fish since they are primarily visual feeders and therefore forage most efficiently on drift during the daytime.

METHODS Study area Drift and benthic invertebrate sampling was undertaken in three rivers in the South Island of New Zealand. The Maruia River is a tributary of the Buller River towards the north of the South Island. The catchment is predominately beech forest (Nothofagus sp.), and in the 0.8 km study reach (42°S 172°E, altitude 300 m) the river flows through stands of beech, manuka scrub (Leptospermum scoparium), and pastoral farmland. The river has a mean discharge of 60 m3 s–1, gradient of 0.0050, with cobbles and boulders dominating the substrate. The Pomahaka (45^6°S 169°N) and Waikaia Rivers are both located in the south of the South Island in the Otago province. The headwaters are in close proximity to each other with the Waikaia River draining the western side of the Umbrella Range into the Mataura catchment and the Pomahaka River draining the eastern side into the Clutha catchment. Native tussock grasses (Chionochloa spp.) dominate the upper catchment of the Pomahaka River (altitude 240-370 m, gradient 0.0044) with some light to medium intensity pastoral farming. The catchment of the lower reaches of this river (altitude 20-160 m, gradient 0.0035) is intensive mixed stock

and pastoral farmland (Harding et al. 1999). Fragments of native forest and exotic plantations occur in some subcatchments. Riparian zones are dominated by tussock and pasture in the upper catchment, and willows (Salix fragilis) occur intermittently amongst pasture in the lower reaches. Sampling was undertaken over 109 km of river covering an altitude range from 38 to 380 m above mean sea level. Mean discharge ranged from 4.13 to 27.51 m3 s–1 at the sampling sites. The catchment of the Waikaia River comprises both native tussock and beech forest (Nothofagus sp.), with the riparian vegetation of the 0.8 km study reach at Piano Flat dominated by beech forest with some pastoral grasses (45°S 169°E, altitude 220 m, gradient 0.0053, mean discharge 12.9 m3 s–1). Invertebrate sampling Drift and benthic samples were collected from three locations within the 0.8 km reaches in the Maruia and Waikaia Rivers, and at 10 sites longitudinally in the Pomahaka River (headwater to lower river). Samples were collected on six occasions in the Maruia River (December 1994; January, February, April, May, and October 1995) and on three occasions in the Waikaia and Pomahaka Rivers (February and December 1996; February 1997) during baseflow conditions. At each site in the Maruia River, three 0.0052 m2 (0.5 mm mesh) drift samplers (modified from FieldDodgson 1985) were stacked vertically in the water column at the water surface, 0.1 m above the bed, and at 0.4 x depth. In the Pomahaka and Waikaia Rivers, two nets were used: one at the water surface and the other at 0.4 x depth. Drift samplers were located in run habitat in typical adult trout feeding locations, i.e., within a depth range of 0.6-1.0 m with a mean column velocity of 0.26-0.33 m s–1 (Hayes & Jowett 1994). Drift nets were exchanged regularly and cleared throughout the sampling period to prevent blockages. The sampling period ran from c. 0.5-1 h after dawn and before dusk. Water velocity was measured at the mouth of each sampler with a Gurley pygmy current meter at the beginning and end of each net exchange in order to calculate the volume of water sampled. Five Surber (0.5 mm mesh, area 0.1 m2) samples were collected from riffles and fast shallow runs in the Maruia, Pomahaka, and Waikaia Rivers on each drift-sampling occasion. Aquatic invertebrates from the drift and benthic samples were counted and identified to the lowest practical taxonomic level. Each drift sample

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Data analysis The relationships between benthos and drift were investigated by calculating: (1) percentage composition of the major taxonomic orders; (2) Pearson's correlation coefficients on log10-transformed count data and arcsinVx-transformed percentage data; and (3) the percentage of benthos in the drift (P) (or propensity for invertebrates to drift) using a modified version of Keups (1988) equation:

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Algal sampling To provide a measure of periphyton biomass for benthic browsing invertebrates, epilithic chlorophyll a was assessed by collecting 15 cm2 scrapings from each of 10 randomly selected stones in the vicinity of the Surber sample locations for all rivers on each sampling occasion. Chlorophyll a concentrations were determined using the method of Biggs (1995). The concentration of periphyton chlorophyll a was used as a measure of invertebrate food availability.

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comprised the animals collected in the two (or three) vertical nets. Emergent adult insects—which comprised primarily chironomids—were included since these originate from the benthos and can be important in the diets of drift-feeding fish (especially juvenile salmonids). Drifting terrestrial invertebrates were not included, as they do not originate from the benthos. Densities were calculated for benthic (no. m–2) and drift (no. m–3) samples. The inclusion or exclusion of emergent adult aquatic insects in drift data affects the taxonomic and percentage composition of the drift significantly (Fig. 1). Initially we included emergent adult aquatic insects because they can be important in the diets of drift-feeding fish, but to explore potential densitydependent relationships between the benthos and drift, adults were removed from the data because their entry into the drift is likely to be a densityindependent life-history phenomenon. Drift and benthic densities and percentage taxonomic composition were averaged over the vertically stacked drift samples, and over the five Surber samples, from each site on each occasion, respectively.

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Benthos Drift A+ Drift A• • • s • a •

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Fig. 1 Percentage composition of benthic and drift communities in the Maruia, Waikaia, and Pomahaka Rivers, New Zealand. (A+, emergent adults of aquatic insects included; A-, adults excluded.)

= emigration). Adjustment of drift densities by the average depth is required since densities can be diluted or concentrated with changes in flow, i.e., water volume (Keup 1988). Depth measurements were taken at c. 1 m intervals along a cross-section of the river at each drift sampling site. Mean depth (D) was calculated by dividing the cross-sectional area by wetted width. We calculated P for benthic-browsing Deleatidium larvae, which were abundant in benthic and drift samples from all three rivers, and using regression analysis, compared P in relation to the availability of food (chlorophyll a).

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We calculated the ratio of invertebrate benthos:chlorophyll a for larval Deleatidium and all taxa (excluding emergent adults), and compared rivers using one-way Analysis of Variance (ANOVA).


RESULTS Community composition of drift and benthos Data from 290 Surber samples and 103 samples of daytime drift were used in our analyses. A total of 172 aquatic invertebrate taxa were recorded in combined benthos and drift samples from the three rivers, with 160 of these in drift samples, and 102 in benthic samples (Table 1). Only 90 taxa (52% of the total) were found in both benthic and drift samples. Nearly 60% more taxa were recorded in drift samples than in benthic samples indicating that drift samples are more effective for estimating biodiversity at a site. This pattern of higher taxa richness in drift samples was consistent across almost all taxonomic groups (except Mollusca) and particularly apparent for Coleoptera and Trichoptera (Table 1). It supports, also, the view that drifting invertebrates may be derived not only from benthic invertebrate communities in habitat immediately upstream of drift sampling locations but also from habitat in the river channel and tributaries considerable distances upstream. Trichoptera, Diptera, and Plecoptera had the greatest variety of taxa in benthic communities (Table 1). However, Ephemeroptera (mainly Deleatidium), Trichoptera, and Diptera were dominant numerically (Fig. 1). If adults were included, Diptera (52% by numbers), comprising mainly chironomids, dominated the drift percentage composition, with mayflies (20%) and Trichoptera

(12%) also well represented (Fig. 1). If adult aquatic insects were excluded, however, Ephemeroptera and Trichoptera (both 24%) became the numerically dominant groups in the drift (as they were in the benthos), with Diptera (20%) and Coleoptera (19%) subdominant (Fig. 1). The percentage of Ephemeroptera in the drift was lower than in the benthos (Fig. 2A). Conversely, Diptera were more prominent in the drift than the benthos (Fig. 2B). As the proportions of these groups were calculated from the total number of animals present in each sample, an increase in the proportion of one group results in the decrease in another group, which would account for the under/over-representation of Ephemeroptera and Diptera respectively. Trichoptera were equally represented in the drift and benthos (Fig. 2C). This group contained a mixture of drifting (e.g., the free living Hydrobiosis) and nondrifting (e.g., the cased caddis Olinga) invertebrates, which probably accounts for the variation around the 1:1 line in Fig. 2C. Over all rivers, the correlations between the percentage of benthos and drift for Ephemeroptera, Diptera, and Trichoptera were significant (Table 2). The percentages of Ephemeroptera and Diptera in the benthos versus the drift were significantly correlated for the lower Pomahaka River. We also found significant relationships between the percentage benthos versus drift for Diptera and Trichoptera in the upper Pomahaka River (Table 2). Comparison of benthic and drift density Using data from all rivers and for all taxa, the densities of benthic and drifting invertebrates were not significantly correlated (Fig. 3A, Table 3). However, there was a significant positive relationship in the Waikaia River (Table 3). For all rivers, significant

Table 1 Numbers of aquatic invertebrate taxa in benthic and drift samples from the Maruia, Pomahaka, and Waikaia Rivers, New Zealand.

Benthos Ephemeroptera Trichoptera Diptera Coleoptera Plecoptera Mollusca Other taxa Total number of taxa

7 38 22 4 17 3 11 102

Drift Including adult Excluding adult aquatic insects aquatic insects 11 57 33 15 19 3 22 160

10 52 26 15 19 3 22 147

Shearer et al.—Drifting and benthic invertebrate relationships Fig. 2 Percentage contribution to the benthos and drift (emergent adult insects included) for A, Ephemeroptera; B, Diptera; and C, Trichoptera in the Maruia River (C), Waikaia River (B), Pomahaka River (upper reaches) (O), and Pomahaka River (lower reaches) (Ø), New Zealand. Dotted line represents a 1:1 relationship.

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% Trichoptera in benthos Table 2 Pearson correlation coefficients (R) for the percentage of Ephemeroptera, Diptera, and Trichoptera in the benthos versus drift (without adults) in the Maruia, Waikaia, and Pomahaka Rivers, New Zealand. Correlations performed on arcsinVx-transformed data. Correlations not significant unless otherwise indicated. (d.f., degrees of freedom.) River

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When data from the three rivers were combined the Deleatidium drift:benthos abundance ratio was significantly related to periphyton chlorophyll a concentration (r 2 = 0.48, P < 0.001) (Fig. 5). In the Maruia River, the low periphyton chlorophyll a concentrations were associated with the highest proportion of Deleatidium in the drift (i.e., for every 10 Deleatidium in the benthos, there was one in the drift). Conversely, the lower reaches of the Pomahaka River had the highest concentration of periphyton chlorophyll a, and the lowest proportion of Deleatidium in the drift (one Deleatidium drifting for every 1000 in the benthos). The outlying site in the lower Pomahaka River was associated with highly variable benthic samples. The ratio of benthos:chlorophyll a is a measure of food limitation for invertebrates. This ratio was highest in the Maruia River and lowest in the Pomahaka River (Table 4). The ratio for the Maruia River was significantly greater than for the other rivers (larval Deleatidium (F = 9.7, d.f. = 3, P < 0.001); all taxa excluding emergent adults (F = 17.2, d.f. = 3, P < 0.001)).

DISCUSSION Is a general benthos:drift relationship attainable? Despite the obvious importance of understanding relationships between the benthos and drift, we are some way from being able to use one as a predictor of the other. Relationships between benthic and drift densities and taxonomic composition appear to be complex, variable, and river/reach or taxondependent since invertebrates enter the drift through density-dependent (e.g., Waters 1961 productioncompensation model) and independent mechanisms (e.g., Müllers 1954 colonisation cycle). This suggests that simple, generally applicable relationships between benthic and drifting invertebrate communities are likely to be elusive. Consequently, benthic data are unlikely to be a useful surrogate for drift unless some of the major density-dependent and independent factors affecting the propensity of invertebrates to drift can be factored into the analyses such as we have attempted to address in this study. Comparison of taxonomic composition The most prominent taxonomic orders in the benthos and daytime drift in all rivers were Ephemeroptera, Diptera, and Trichoptera. These groups also dominate drift communities in other New Zealand rivers (McLay 1968; Collier & Wakelin 1992; Sagar


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Table 4 Means (+ SE) of benthos:chlorophyll a ratios (X 10–4) in the Maruia, Waikaia, and Pomahaka Rivers, New Zealand for larval Deleatidium and all taxa (excluding emergent adults). River: Deleatidium All taxa

Maruia

Waikaia

Pomahaka, upper

Pomahaka, lower

287.1 (148.5) 450.4 (175)

4.8 (2.5) 7.6 (3.9)

0.04 (0.01) 0.09 (0.03)

0.005 (0.003) 0.02 (0.01)

& Glova 1992), and species in these orders are important in the diets of drift-feeding fish (Dedual & Collier 1995; Sagar & Glova 1995). Similarly, these groups have been shown to be important drifting orders in international studies (Elliott 1967b; Radford & Hartland-Rowe 1971; Brittain & Eikeland 1988; Mathooko & Mavuti 1992; Pringle & Ramirez 1998). Drift samples included a greater variety of taxa than benthic samples (including large numbers of emergent adult insects), presumably because they integrate over a much greater spatial scale and also over time. Once emergent adults were excluded from the data the taxonomic composition of benthos and drift were relatively similar for all rivers combined. The main difference was the presence of Mollusca in the benthos that were rarely seen in the drift (Fig.

1). In contrast, Coleoptera appeared to be more common in the drift than would be expected based on the benthos (Fig. 1). Benthic: drift relationships— density-dependent or independent drift? There has been considerable debate whether drift densities are dependent or independent of benthic densities. Waters (1972) claimed that there was seldom a direct relationship between drift and benthic densities. Elliott (1967b) and Reisen & Prins (1972) found that most invertebrate drift was density-independent and related to the growth and life histories of aquatic invertebrates with highest drift rates associated with pupation and emergence. The marked influence of including or excluding emergent adult insects from the analyses in our study

Shearer et al.—Drifting and benthic invertebrate relationships

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Fig. 5 Relationship between the proportion of larval Deleatidium in the drift and food in the Maruia River (C), Waikaia River (B), Pomahaka River (upper reaches) (O), and Pomahaka River (lower reaches) (Ø), New Zealand. Note: axes are a log10 scale.

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Chlorophyll a (mg. m ) indicates the importance of life history-related drift (i.e., emergence). Emergence is not directly influenced by benthic densities, although the number of emerging adults at a site probably is related to the density on the bed. If we included dawn and, particularly, dusk drift estimates in this analysis the importance of life history-related drift would be even more pronounced (cf. Elliott 1967b; Reisen & Prins 1972). With data from all rivers combined we found significant correlations between benthic and drift densities for the three main orders in the drift (Ephemeroptera, Diptera, and Trichoptera). However, significant correlations were not always detected for these groups in individual rivers. Although the existence of significant correlations is consistent with drift being primarily density-dependent, density-independent mechanisms may be exerting a greater influence in instances where significant correlations were absent. Direct benthic versus drift relationships have been observed previously, sometimes for all invertebrates and more often for some of the most abundant taxa, both internationally (Dimond 1967; Pearson & Franklin 1968; Lehmkul & Anderson 1972; Pearson & Kramer 1972; Hildebrand 1974; Statzneretal. 1987; Siler et al. 2001) and in New Zealand (McLay 1968; Sagar & Glova 1992). A proponent of the densitydependent theory, Müller (1954), suggested that competition for space as small invertebrate larvae grow results in increased drift and colonisation of downstream habitats. Waters (1966) extended this hypothesis by suggesting that increased drift was a mechanism of density-dependent population control that occurred once benthic densities approached carrying capacity. Hughes (1970) and Hildebrand

( 1974) both found a direct inverse relationship between drift and food supply suggesting that, at a given benthic density, increased activity searching for food may result in increased drift. Previous researchers have also found that when food resources are abundant the proportion of benthos in the drift usually is lower (Hildebrand 1974; Richardson 1991; HinterleitnerAnderson et al. 1992; Siler et al. 2001). Benthic food supply and drift We found a significant negative relationship between propensity for Deleatidium to drift and chlorophyll a concentration. The ratio of benthic density:chlorophyll a concentration indicated this was related to densitydependent food limitation, i.e., the river in which Deleatidium had the highest propensity to drift had the highest benthos:chlorophyll a ratio. Relationships such as this could be developed for other benthic browsing taxa that show density-dependent drift in relation to food limitation. The presence of chemical cues from trout can inhibit drifting behaviour (McIntosh & Peckarsky 1996; Tikkanen et al. 1996), but is unlikely to have contributed to drift variation in our study as all the rivers and reaches had significant trout populations. McIntosh et al. (2002) found no significant relationship between fish biomass and the propensity for invertebrates to drift. The importance of understanding and quantifying density-dependent/independent relationships between the benthos and drift was highlighted to us when we noticed high benthic densities but low drift in the lower Pomahaka River. Agricultural development of catchments often is associated with nutrient enrichment leading to a progressive downstream increase in benthic algae, a decrease in Ephemeroptera,


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Plecoptera, and Trichoptera (EPT) scores and consequent changes in invertebrate community structure (Quinn & Hickey 1990; Harding et al. 1999). Periphyton proliferations that could be considered of nuisance value were observed only at some of the lower Pomahaka River sites. Although our data are limited, the criteria provided by Dodds et al. (1998) would suggest that chlorophyll a levels recorded from the Maruia, Waikaia, and upper Pomahaka Rivers were typical of oligotrophic conditions, with the lower Pomahaka River mesotrophic. Quinn & Hickey (1990) suggested that in moderate-highly developed catchments, the replacement of larger drifting invertebrates by smaller ones and/or taxa with less propensity to drift, could reduce the food supply for trout, and hence trout biomass. However, Harding et al. ( 1999) observed that densities of EPT taxa increased downstream in the Pomahaka River even though EPT taxa comprised a diminishing proportion of community composition. This suggests that feeding opportunities for drift-feeding fishes (e.g., trout) should improve downstream, but instead the opposite occurred, i.e., despite high benthic densities, the animals rarely were drifting. Our analysis of the proportion of Deleatidium in the drift in relation to chlorophyll a concentration suggests a food mediated drift inhibition mechanism operating in this situation. This highlights an unexpected effect of periphyton proliferation (as occurs with agricultural enrichment) on invertebrate drift and drift-feeding fishes such as trout.

CONCLUSIONS A general relationship between benthos and drift is unlikely to be found unless the density-dependent and independent factors that cause invertebrates to leave the benthos and enter the drift are accounted for. We have shown that the chlorophyll a concentrations from benthic periphyton is a significant predictor of drift by Deleatidium; a density-dependent response. However, this is not the complete story for Deleatidium (nor for other taxa) since emergent adults also enter the drift via a density-independent life history-related mechanism, and elevated (behavioural) drift densities during dawn and dusk peaks must be accounted for also. Further research is required if we are to define the conditions under which density-dependent drift may occur, or to understand why drift is density dependent in some rivers, or at some times, and not others. We expect that information on environmental

conditions in rivers and reaches (including extent of catchment development and hydraulics), food availability, and species traits of invertebrates will all be required if functional relationships between benthic and drifting invertebrate communities are to be developed. Finally, our observation that the propensity for Deleatidium to drift is negatively related to chlorophyll a concentration has management implications for lowland river catchments and their trout fisheries. Inhibition of invertebrate drift, in addition to altered benthic invertebrate community competition (Quinn & Hickey 1990), will impair the profitability of drift feeding by trout. This may be one of the mechanisms that have contributed to the decline in New Zealand lowland trout fisheries (Jellyman et al. 2000,2002) and requires further research.

ACKNOWLEDGMENTS We thank Aaron Quarterman, Jenelle Strickland, Yvonne Stark, Susan Hallas, Ross Dungey, Otago Regional Council, and Environment Southland for assistance in the field, and Yvonne Stark and Krystyna Ponikla for processing the drift samples. We also thank two anonymous referees for their constructive comments on the manuscript. Flow information on the rivers was provided by Tony Hewitt, Otago Regional Council; Environment Southland; and NIWA. David and Carol Sanders kindly allowed us access to the Maruia River. Tasman District Council leant us their survey equipment for which we are grateful. This study was funded by the Foundation for Research, Science and Technology (contract numbers CAW404, CAW502, CAW802, CAWX0007), and by Fish and Game New Zealand.

REFERENCES Biggs, B. J. F. 1995: The contribution of flood disturbance, catchment geology and land use to the habitat template of periphyton in stream ecosystems. Freshwater Biology 33: 419-438. Brittain, J. E; Eikeland, T. J. 1988: Invertebrate drift—a review. Hydrobiologia 166: 77-93. Chutter, F. M. 1975: Variation in the day-time drift of a Natal river. Verhandlungen der Internationale Vereinigung für theoretische und angewandte Limnologie 19: 1728-1735. Collier, K. J.; Wakelin, M. D. 1992: Drift of aquatic macroinvertebrate larvae in Manganuiateao River, Central North Island, New Zealand. New Zealand Natural Sciences 19: 15-26.

Shearer et al.—Drifting and benthic invertebrate relationships Dedual, M.; Collier, K. J. 1995: Aspects of juvenile rainbow trout (Oncorhynchus mykiss) diet in relation to food supply during summer in the lower Tongariro River, New Zealand. New Zealand Journal of Marine and Freshwater Research 29: 381-391. Dimond, J. B. 1967: Evidence that drift of stream benthos is density related. Ecology 48: 855-857.


Dodds, W. K.; Jones, J. R.; Welch, E. B. 1998: Suggested classification of stream trophic state: distribution of temperate stream types by chlorophyll, total nitrogen, and phosphorus. Water Research 32: 1455-1462. Elliott, J. M. 1967a: The food of trout (Salmo trutta) in a Dartmoor stream. Journal of Applied Ecology 4: 59-71. Elliott, J. M. 1967b: Invertebrate drift in a Dartmoor stream. Archiv für Hydrobiologie 63: 202-237. Elliott, J. M. 1970: Diel changes in invertebrate drift and the food of trout Salmo trutta L. Journal of Fish Biology 2: 161-165. Field-Dodgson, M. S. 1985: A simple and efficient drift sampler. New Zealand Journal of Freshwater Research 19: 167-172. Graesser, A. K. 1988: Invertebrate drift densities in three flood prone streams in South Westland, New Zealand. Verhandlungen der Internationale Vereinigung für theoretische und angewandte Limnologie 23: 1422-1426. Guensch, G. R.; Hardy, T. B.; Addley, R. C. 2001: Examining feeding strategies and position choice of drift-feeding salmonids using an individualbased, mechanistic foraging model. Canadian Journal of Fisheries and Aquatic Sciences 58: 446-457. Harding, J. S.; Young, R. G.; Hayes, J. W.; Shearer, K. A.; Stark, J. D. 1999: Changes in agricultural intensity and river health along a river continuum. Freshwater Biology 42: 345-357. Hayes, J. W.; Jowett, I. G. 1994: Microhabitat use by large brown trout in three New Zealand rivers. North American Journal of Fisheries Management 14: 710-725. Hayes, J. W.; Stark, J. D.; Shearer, K. A. 2000: Development and test of a whole-lifetime foraging and bioenergetics growth model for drift-feeding brown trout. Transactions of the American Fisheries Society 129: 315-332. Hildebrand, S. G. 1974: The relation of drift to benthos density and food level in an artificial stream. Limnology and Oceanography 19: 951-957.

819

Hinterleitner-Anderson, D.; Hershey, A. E.; Schuldt, J. A. 1992: The effects of river fertilization on mayfly (Baetis sp.) drift patterns and population density in an arctic river. Hydrobiologia 240: 247-258. Hughes, D. A. 1970: Some factors affecting drift and upstream movements of Gammarus pulex. Ecology 51: 301-305. Hughes, N. F.; Hayes, J. W.; Shearer, K. A.; Young, R. G. in press: Testing a model of drift-feeding using 3-D videography of wild brown trout in a New Zealand river. Canadian Journal of Fisheries and Aquatic Sciences. Jellyman, D. J.; Glova, G. J.; Bonnett, M. L.; McKerchar, A. I.; Allen, K. R. 2000: The Horokiwi Stream 50 years on: a study of the loss of a productive trout fishery. NIWA Technical Report 83. 50 p. Jellyman, D. J.; Unwin, M. J.; James, G. D. 2002: Anglers' perceptions of the status of lowland rivers and their trout fisheries throughout New Zealand. NIWA Client Report CHC2002-046. 48 p. Keup, L. E. 1988: Invertebrate fish food resources of lotic environments. Instream Flow Information Paper No. 24. United States Fish and Wildlife Service Biological Report 88(13). 96 p. Lehmkuhl, D. N.; Anderson, N. 1972: Microdistribution and density as factors affecting downstream drift of mayflies. Ecology 53: 661-667. McIntosh, A. R. 2000: Aquatic predator-prey interactions. In: Collier, K. J; Winterbourn, M. J. ed. New Zealand stream invertebrates: ecology and implications for management. Christchurch, Caxton Press. Pp. 125-156. McIntosh, A. R.; Peckarsky, B. L. 1996: Differential responses of mayflies from rocky mountain streams with and without fish to trout odour. Freshwater Biology 35: 141-148. McIntosh, A. R.; Peckarsky, B. L.; Taylor, B. W. 2002: The influence of predatory fish on mayfly drift: extrapolating from experiments to nature. Freshwater Biology 47: 1-17. McLay, C. L. 1968: A study of drift in the Kakanui River, New Zealand. Australian Journal of Marine and Freshwater Research 19: 139-148. Mathooko, J. M.; Mavuti, K. M. 1992: Composition and seasonality of benthic invertebrates, and drift in the Naro Moru River, Kenya. Hydrobiologia 232: 47-56. Müller, K. 1954: Investigations on the organic drift in North Swedish streams. Report of the Institute of Freshwater Research, Drottningholm 35: 133148. Pearson, W. D.; Franklin, D. R. 1968: Some factors affecting the drift rates of Baetis and Simuliidae in a large river. Ecology 49: 75-81.


820


Pearson, W. D.; Kramer, R. H. 1972: Drift and production of two aquatic insects in a mountain stream. Ecological Monographs 42: 365-385. Pringle, C. M.; Ramirez, A. 1998: Use of both benthic and drift sampling techniques to assess tropical stream invertebrate communities along an altitudinal gradient, Costa Rica. Freshwater Biology 39: 359-373. Quinn, J. M.; Hickey, C. W. 1990: Magnitude of effects of substrate particle size, recent flooding, and catchment development on benthic invertebrates in 88 New Zealand rivers. New Zealand Journal of Marine and Freshwater Research 24: 411-427.

Sagar, P. M.; Glova, G. J. 1992: Invertebrate drift in a large, braided New Zealand river. Freshwater Biology 27: 405-416.

Radford, D. S.; Hartland-Rowe, R. 1971: A preliminary investigation on bottom fauna and invertebrate drift in a unregulated and regulated stream in Alberta. Journal of Applied Ecology 8: 583-603. Ramirez, A.; Pringle, C. M. 1998: Invertebrate drift and benthic community dynamics in a lowland neotropical stream, Costa Rica. Hydrobiologia 386: 19-26.

Statzner, B.; Elouard, J-M.; Dejoux, C. 1984: Field experiments on the relationship between drift and benthic densities of aquatic insects in tropical stream (Ivory Coast). I. Introduction: review of existing literature, methods, and experimental conditions. Revue d'Hydrobiologie Tropicale 14: 319-334.

Reisen, W. K.; Prins, R. 1972: Some ecological relationships of the invertebrate drift in Praters Creek, Pickens County, South Carolina. Ecology 53: 866-884. Richardson, J. S. 1991: Seasonal food limitation of detritivores in a montane stream: an experimental test. Ecology 72: 873-887. Rutledge, M. J.; Graybill, J. P.; Kilpatrick, J. F.; Kelso, J. R. M. 1992: Effect of artificial freshes in experimental channels on benthic macroinvertebrate density and drift and on quinnat salmon growth. New Zealand Freshwater Research Report 1. Pp. 1-25. Sagar, P. M. 1983: Invertebrate recolonisation of previously dry channels in the Rakaia River. New Zealand Journal of Marine and Freshwater Research 17: 377-386. Sagar, P. M.; Glova, G. J. 1988: Diel feeding periodicity, daily ration and prey selection of a riverine population of juvenile Chinook salmon, Oncorhynchus tshawtsycha (Walbaum). Journal of Fish Biology 33: 643-653.

Sagar, P. M.; Glova, G. J. 1995: Prey availability and diet of juvenile brown trout (Salmo trutta) in relation to riparian willows (Salix spp.) in three New Zealand streams. New Zealand Journal of Marine and Freshwater Research 29: 527-537. Siler, E. R.; Wallace, J. B.; Eggert, S. L. 2001: Long-term effects of resource limitation on stream invertebrate drift. Canadian Journal of Fisheries and Aquatic Sciences 58: 1624-1637.

Statzner, B.; Elouard, J-M.; Dejoux, C. 1987: Field experiments on the relationship between drift and benthic densities of aquatic insects in tropical stream (Ivory Coast). III. Trichoptera. Freshwater Biology 17: 391-404. Tikkanen, P.; Muotka, T.; Huhta, A. 1996: Fishless-stream mayflies express behavioural flexibility in response to predatory fish. Animal Behaviour 51: 1391-1399. Waters, T. F. 1961: Standing crop and drift of stream bottom organisms. Ecology 42: 352-357. Waters, T. F. 1966: Production rate, population density, and drift of a stream invertebrate. Ecology 47: 595-604. Waters, T F. 1969: Invertebrate drift—ecology and significance to stream fishes. In: Northcote, T. G. ed. Symposium on salmon and trout in streams. Vancouver, University of British Columbia. Pp. 121-134. Waters, T. F. 1972: The drift of stream insects. Annual Review of Entomology 17: 253-272.