1. Abiotic disturbances strongly modify spatial and temporal patterns of lotic ecosystem community structure and function. Such effects are produced because disturbances alter organic matter, nutrient and contaminant dynamics and the distribution and abundance of bacterial, algal, macroinvertebrate and fish communities, 2. River ice break-up is a seasonal disturbance in rivers at high altitudes and latitudes world-wide and is characterized, in part, by large increases in current velocity, stage, water temperature, concentrations of suspended materials and substrate scouring. 3. These abiotic factors are likely to have important effects on primary producers, consumers, and food-web dynamics of river biota. Despite the potential importance of river ice break-up on community structure and function, detailed information describing the magnitude of their effects and underlying causal mechanisms is scarce. 4. The objective of this paper is to provide a hydrological and ecological review and perspective on the potential effects of ice break-up on lotic ecosystems. Specifically, the potential importance of break-up on water temperature, river sediments and geomorphology, riverine energy sources, contaminants, and its effects on river biota and food-web dynamics are evaluated.

Temporal changes in epilithon biomass and benthic macroinvertebrate density were investigated in the Ashley River, a flood-prone river with an unpredictable discharge regime. Biomass, primary production and respiration of the epilithic community were highest in spring when filamentous algae were present and lowest following two large floods that occurred in close succession. Sixty invertebrate species were taken in benthic samples including 24 species of Trichoptera, 15 Diptera and 4 Ephemeroptera. Larvae of the mayfly Deleatidiurn (Leptophlebiidae) were numerically dominant and comprised up to 83 % of the fauna in any one month. Mean benthic invertebrate density was highest (9170-18 580 m-2, following long periods of low stable flow (< 30 m-s -' ) and lowest (230 m-2, after a major flood (454 m -s -I).Reductions in benthic density occurred when flow exceeded about 30 m-s -' , the minimum discharge at which small cobbles are moved. Refuge seeking behaviours, flexible life histories and effective recolonization mechanisms enable the benthos of the Ashley River to persist and recover from frequent, temporally unpredictable disturbances.

Benthic macroinvertebrates and small stones were collected from a riffle in the Ashley River, North Canterbury, New Zealand, on 12 occasions within a 132-day period following a particularly large flood in 1986. Despite the occurrence of smaller floods during this period, benthic macroinvertebrate communities and stone surface organic layers recovered rapidly. Mean concentration of stone surface organic carbon increased from 0.23 g/m 2 to 1.01 g/m 2 stone surface in the first 23 days after the flood and chlorophyll a concentration increased from 0.13 mg/m 2 (day 3) to 9.2 mg/m 2 by day 132. Minor floods during the recovery period had little effect on organic layer biomass. Mean faunal density increased from 230/m 2 to 7920/m 2 during this time and taxon richness from 7 to 21 per 5 benthic samples. Immediately after the flood and throughout the study period the fauna was dominated by larvae of Deleatidium (Ephemeroptera: Leptophlebiidae), Hydora (Coleoptera: Elmidae), and Chironomidae. Re-establishment of Deleatidium populations in previously denuded brands was effected by oviposition, egg-hatching, and larval immigration. Minor braids are likely to represent important epicentres from which recolonisation proceeds.

Stable isotope analysis has emerged as an important tool in aquatic ecology. For fish, dorsal muscle from sacrificed individuals has traditionally been used in stable isotope studies; however, there are many instances when lethal sampling is undesirable. We evaluated the feasibility of using adipose and caudal fin clips as alternatives to muscle in stable isotope studies for five species of salmonids. Because fish size and water temperature can affect stable isotope ratios, we also determined whether fish length and sampling date affected the difference in isotope signatures between fins and muscle. Biopsied muscle plugs and fin clips were collected from rainbow trout Oncorhynchus mykiss, brook trout Salvelinus fontinalis, and lake trout S. namaycush as well as lake whitefish Coregonus clupeaformis and pygmy whitefish Prosopium coulterii and analyzed for stable isotopes of carbon and nitrogen. The isotope signatures of both adipose and caudal fins were significantly correlated (0.33 < R2 < 0.97) with those of dorsal muscle from the five salmonid species studied, and in some cases the signatures of fins did not differ from those of muscle. Fish length and sampling date occasionally had a small effect (0.042 < R2 < 0.49) on the relationship between the isotope signatures of fin and muscle. Although muscle biopsy provides a viable, nonlethal method of collecting muscle tissue from suitably sized fish, the strong relationships between the isotope signatures of fin and muscle demonstrate that fin clips should be considered good surrogates for muscle in stable isotope studies of salmonids.