Timo Muotka

Long-term recovery of stream habitat structure and benthic invertebrate communities from in-stream restoration

Headwater streams channelized for water transport of timber in Finland are being restored to their pre-channelization state. The primary motivation is the enhancement of sport fisheries, but restoration probably has profound impacts also on other stream organisms. We assessed how such ‘‘single-goal’’ restorations affect benthic macroinvertebrate communities. We revisited the streams sampled by Laasonen et al. [Aquatic Conservation: Marine and Freshwater Ecosystems 8 (1998)] in the early 1990s when the streams had been recently restored. In 1997, the recovery period of these streams ranged from 4 to 8 years. Habitat structure among the stream types represented a distinct recovery gradient, with streams restored 1 month before sampling and natural streams being the endpoints of a gradient in moss cover (highest in natural, lowest in recently restored streams). Channelized streams supported a characteristic set of indicator species, whereas shifts in species composition between restored and natural streams were more gradual. Macroinvertebrate communities in unmodified streams changed little between the two surveys, whereas communities in restored streams had undergone considerable changes. In-stream restoration is an unpredictable disturbance, to which stream biota cannot have any evolved responses. Therefore, the relatively rapid recovery of habitat structure and macroinvertebrate communities in restored streams is encouraging. However, long-term monitoring of benthic communities in both restored and natural streams is needed to assess whether restoring rivers by these techniques will enhance the recovery of benthic biodiversity in boreal streams. # 2002 Elsevier Science Ltd. All rights reserved.

Correspondence between regional delineations and spatial patterns in macroinvertebrate assemblages of boreal headwater streams

Geographical stratification may provide a useful framework for stream management programs, yet most studies testing the utility of such stratifications have been conducted in temperate regions. We studied the correspondence between regional delineations (5 ecoregions, 11 subecoregions), environmental characteristics, and benthic macroinvertebrate assemblages in 156 boreal headwater streams in Finland, using a combination of principal components analysis, nonmetric multidimensional scaling, and discriminant function analysis (DFA). Both stream characteristics and macroinvertebrate assemblage structure showed a closer correspondence to ecoregions than to subecoregions, a pattern partly attributable to the smaller number of classes at the ecoregion level. Based on environmental variables, DFA classified correctly 84.0% of the streams by ecoregion and 74.4% by subecoregion. For macroinvertebrate assemblages, 64.7% of the streams were correctly classified at the ecoregion, and 51.9% at the subecoregion level. Arctic-alpine and north boreal ecoregions supported the most distinct benthic assemblages. There were significant differences among ecoregions in mean species number, with lowest species richness in the middle boreal and highest in the south boreal streams. However, no clear latitudinal gradients in local species richness were detected. Relative abundances of functional-feeding groups also exhibited regional differences. Shredders dominated the middle boreal brown-water streams, whereas scrapers attained their highest abundances in the arctic-alpine and north boreal clear-water streams. Despite some congruence among ecoregions and macroinvertebrate assemblages, regional delineations alone do not provide a suitable background for stream biomonitoring programs. A combination of regional delineations and local characteristics probably provides a better framework for the classification of boreal headwater streams, and for the prediction of their benthic fauna.

Searching for biodiversity indicators in running waters: do bryophytes, macroinvertebrates, and fish show congruent diversity patterns?

The degree to which different taxonomic groups show congruence in diversity patterns has attracted increased attention, yet such studies on stream biota are lacking. We examined environmental correlates of and congruence in the species richness patterns of bryophytes, macroinvertebrates, and fish in 101 boreal streams in Finland. Congruence in species richness among the taxonomic groups was generally low, mainly because of their differing responses to major environmental gradients. Bryophytes and macroinvertebrates showed the strongest degree of congruence, but even this relationship had a relatively weak predictive power. Bryophyte diversity showed the strongest relationship with water colour, followed by habitat stability, and stream size. Macroinvertebrate diversity increased with stream size, and further variation was accounted for by water colour and acidity. Fish species richness showed a weak and complex relationship with geographical location, stream size, and in-stream habitat characteristics. The regression models explained 23, 45, and 26% of the variation in species richness of bryophytes, macroinvertebrates, and fish, respectively. Our results suggest that indicator taxa may be of limited value in stream biodiversity inventories. Habitat-based approaches are suggested as an alternative surrogate measure in the conservation evaluation of lotic biodiversity.

DEFINING MACROINVERTEBRATE ASSEMBLAGE TYPES OF HEADWATER STREAMS: IMPLICATIONS FOR BIOASSESSMENT AND CONSERVATION

We used data from 235 boreal headwater streams in Finland to examine whether macroinvertebrate assemblages constitute clearly definable types, and how well biologically defined assemblage types can be predicted using environmental variables. Two- way indicator species analysis produced 10 assemblage types, which differed significantly from each other (multiresponse permutation procedure, MRPP). However, based on MRPP and nonmetric multidimensional scaling, there was wide variation among sites within each assemblage type, and high degrees of overlap among assemblage types. Such continuous variation was also evidenced by the low number of effective indicator taxa (indicator value method) for each assemblage type. Furthermore, discriminant function analysis based on environmental variables showed a moderate yet variable prediction success (59.6% of sites correctly predicted; range 0-96% per group). Canonical correspondence analysis indicated that variation in assemblage structure was primarily related to latitude and water chemistry, especially pH and water color. The absence of discrete macroinvertebrate assemblage types in boreal headwater streams may stem from a number of reasons: (1) macroinvertebrate taxa inhabiting boreal streams tend to exhibit individualistic, taxon-specific responses to environmental gradients, (2) they have wide environmental tolerances and geographic dis- tributions, and (3) boreal streams are characterized by frequent, unpredictable disturbances. Our results suggest that local filters in headwater streams are relatively weak, resulting in poorly distinguishable assemblage types. By contrast, the major latitudinal gradients in macroinvertebrate assemblage structure suggest that regional-scale filters, e.g., temperature, exert strong control over taxon distributions in headwater streams. We suggest that a tiered approach combining regional stratification and subsequent prediction of assemblage struc- ture could provide a suitable framework for the bioassessment and conservation of boreal headwater streams.

Twenty years of stream restoration in Finland: little response by benthic macroinvertebrate communities

The primary focus of many in-stream restoration projects is to enhance habitat diversity for salmonid fishes, yet the lack of properly designed monitoring studies, particularly ones with pre-restoration data, limits any attempts to assess whether restoration has succeeded in improving salmonid habitat. Even less is known about the impacts of fisheries-related restoration on other, non-target biota. We examined how restoration aiming at the enhancement of juvenile brown trout (Salmo trutta L.) affects benthic macroinvertebrates, using two separate data sets: (1) a before-after-control-impact (BACI) design with three years before and three after restoration in differently restored and control reaches of six streams; and (2) a space-time substitution design including channelized, restored, and near-natural streams with an almost 20-year perspective on the recovery of invertebrate communities. In the BACI design, total macroinvertebrate density differed significantly from before to after restoration. Following restoration, densities decreased in all treatments, but less so in the controls than in restored sections. Taxonomic richness also decreased from before to after restoration, but this happened similarly in all treatments. In the long-term comparative study, macroinvertebrate species richness showed no difference between the channel types. Community composition differed significantly between the restored and natural streams, but not between restored and channelized streams. Overall, the in-stream restoration measures used increased stream habitat diversity but did not enhance benthic biodiversity. While many macroinvertebrates may be dispersal limited, our study sites should not have been too distant to reach within almost two decades. A key explanation for the weak responses by macroinvertebrate communities may have been historical. When Fennoscandian streams were channelized for log floating, the loss of habitat heterogeneity was only partial. Therefore, habitat may not have been limiting the macroinvertebrate communities to begin with. Stream restoration to support trout fisheries has strong public acceptance in Finland and will likely continue to increase in the near future. Therefore, more effort should be placed on assessing restoration success from a biodiversity perspective using multiple organism groups in both stream and riparian ecosystems.

Recovery of macroinvertebrate communities from stream habitat restoration

1. Many streams channelized for timber floating in Finland are now being restored to their original condition. The most frequently used restoration structures are boulder dams, flow deflectors, excavations and channel enlargements. By increasing substrate heterogeneity and leaf litter retention, restoration may enhance the formation of detritivore-dominated macroinvertebrate assemblages. In this study, macroinvertebrate communities in streams with differing recovery periods (from 0 to 16 years) from restoration, were compared with those in channelized and near-pristine streams. 2. Water depth and current velocity were lower, and relative bed roughness higher in restored than in dredged channels. Moss cover was negligibly low in recently restored streams, but mosses had recovered well within three years from restoration. The standing stock of leaf litter was lower than in natural streams, but in most cases higher than in channelized streams. 3. Abundances of all invertebrates were highest in natural streams and lowest in streams restored 1 month before sampling. All other restored streams had abundances comparable to, or slightly lower than, those in channelized streams. There was a tendency toward higher abundances of shredders with a long recovery period, but streams restored 8 or 16 years ago still contained relatively sparse shredder populations. 4. Canonical Correspondence Analysis of the October data could be attributed to among-site variation in habitat hydraulics, moss cover and leaf litter. Dredged channels with high velocities and low bed profiles, and natural streams with high retention efficiency were the end points of this gradient. There was little indication of macroinvertebrate assemblages approaching pristine conditions with a longer recovery period. 5. Enhanced litter retention increases the capacity of restored streams to support high abundances of detritivorous invertebrates. Indirectly, restoration may also benefit animals that are not dependent on detrital food. Clearly, macroinvertebrates and other non-vertebrate components of the stream biota should be given a higher priority in the design and execution of stream restoration programmes.

Breakdown and macroinvertebrate and fungal colonization of alder, birch, and willow leaves in a boreal forest stream

We examined the decomposition of 3 riparian leaf species (alder, birch, and willow) in a boreal woodland stream during winter. Based on initial leaf quality, we expected willow leaves to decompose at a slower rate than alder and birch leaves. We also expected alder and birch leaves to be colonized by fungi and macroinvertebrates more rapidly and in higher numbers and/or biomass than willow leaves. We also assessed how closely shredder life cycles are related to leaf litter availability in this stream. Alder and birch had fast processing rates (k = 0.019 and 0.011/d, respectively), whereas willow leaves were processed slower (k = 0.005/d). Ergosterol concentrations increased rapidly in all leaf species during the first 4 wk, after which concentrations remained nearly constant in alder and birch, but increased slowly in willow. Birch supported the highest overall fungal biomass, whereas fungal biomass in alder and willow did not differ on most sample dates. Macroinvertebrate abundances increased rapidly in the early phases of the experiment for all leaf species (maximum values of 297 and 198 individuals/mesh bag in November for alder and willow, respectively, and 84 ind./mesh bag in January for birch). Alder supported significantly more invertebrates than did birch and willow. Shredder abundance for each leaf species was maximum in November, with a secondary peak in numbers (all leaf species) and biomass (only birch) at the end of the experiment in April. The 3 leaf species studied here formed a range of processing rates, translating into a continuum of availability to shredding invertebrates. In late spring, when alder leaves had practically disappeared, the shredding stonefly Amphinemura borealis entered an active growth period prior to emergence. Late spring clearly represents a potential bottleneck for shredder populations in boreal streams. Spring–summer shredders may survive this bottleneck by shifting to seasonally abundant food sources, especially fine particulate detritus.

Characterizing macroinvertebrate assemblage structure in relation to stream size and tributary position

We examined the variability of macroinvertebrate assemblage structure, species identities, and functional feeding group composition in relation to stream size, tributary position, and in-stream factors in a boreal watershed in Finland. Our study included three riffle sites in each of three stream sections in each of three stream size classes. Multi-response permutation procedure, indicator value method, and canonical correspondence analysis revealed clear differences in assemblage structure among the stream size classes, with a gradual increase of species richness as the stream size increased. Significant differences in assemblage structure were also found among the tributary river systems. The functional feeding group composition broadly followed the river continuum concept, i.e., headwaters were dominated by shredders, gatherers, or filterers, whereas scrapers increased in relative abundance with stream size. There was, however, considerable variation in the functional feeding group composition both among and within the headwater stream sections. Our findings refer to a strong influence of stream size on macroinvertebrate assemblages, but also factors prevailing at the scale of individual riffles should be considered in biodiversity conservation of lotic ecosystems.

Predator detection and avoidance by lotic mayfly nymphs of different size

We studied antipredatory responses of lotic mayfly (Baetis) nymphs in a factorial experiment with four levels of fish presence: (1) a freely foraging fish (the European minnow,Phoxinus phoxinus), (2) a constrained fish, (3) water from a fish stream, (4) water from a fishless stream. LargeBaetis nymphs drifted mainly during night-time in treatments involving either the chemical or actual presence of fish, whereas no diel periodicity was observed when the water was not conditioned with fish odour. The response was strongest when the fish was uncaged, which suggests that visual or hydrodynamic cues are needed in addition to chemical ones for an accurate assessment of predation risk. Fish presence had no effect on the drift rates of small nymphs. Instead, they increased their refuge use in the presence of a live fish. Chemical cues alone did not have any effect on the refuge use of any of theBaetis size classes. Our results indicate active drift entry by mayfly nymphs. Because predation pressure is spatially and temporally variable, nymphs must sample the environment in order to locate predator-free areas or areas with low predation risk. Drifting should be the most energy-saving way to do this. To avoid the risk from visually feeding fish, large individuals can sample safely (i.e. enter drift) only at night-time, while the small ones can also do this safely during the day. We suggest that, contrary to some earlier assumptions, mayfly drift is not a fixed prey response. Instead,Baetis nymphs are able to assess the prevailing predation pressure, and they adjust their foraging behaviour accordingly.

The effect of sample duration on the efficiency of kick-sampling in two streams with contrasting substratum heterogeneity

Representative sampling is a common problem in studies comparing taxonomic rielmess or composition of ecologieal assemblages. In broad-scale surveys and biomonitoring programs using lotic macroinvertebrates, kick-sampling has been widely used as a standard sampling teclmique. Kick-sampling can be used in a variety ofhabitats (BRADLEY & ÜRMEROD 2002) and is considered more cost-effective than alternative, more quantitative sampling methods (STOREY et al. 1991 ). The efficiency ofkieksampling may be affeeted, however, by substratum structure (STOREY et al. 1991) or other in-stream habitat eharaeteristies (PARSONS & NORRIS 1996). These factors often vary between sites, thus potentially obscuring among-site eomparisons. Such variation is generally controlled for by stratifying sampling to riffie habitats where several subsamples are then taken to maximize the diversity o f microhabitats sampled (RESH & McELRAVY 1993). Riffies, however, vary in habitat eomplexity, and eomplex habitats often harbour more speeies than do strueturally simpler ones (DOWNES et al. 1998, 2000). Therefore, differenees in maeoinvertebrate community composition among streams with differing habitat characteristies may be underestimated i f the effieiency o f sampling is affeeted by substratum heterogeneity, or if the area sampled is too small to adequately cover variation in microhabitats within a site. Sample size also bears a close relationship to the detection o f rare and endangered taxa, which are often the primary targets of broad-seale biodiversity surveys. Although several studies have eompared kiek-sampling to other stream invertebrate sampling methods, few have directly assessed the effeets of sample size and/or substratum structure on the effieieney of kiek-sampling (but see BRADLEY & ÜRMEROD 2002).