No abstract

Freshwater Biology (1982) 12, 1-15. Growth and recession of aquatic macrophytes on a shaded section of the River Lamhourn, England, from 1971England, from to 1980 SUMMARY. The growth and recession of macrophytes on a shaded section of the R. Lambourn were documented by a mapping procedure. With the exception of Ranunculus spp.. the changes in total cover did not indicate directly the pattern of growth and recession of the macrophyles. Analysis of gross changes, expressed as gains and losses tn cover, indicated that colonization of gravel and silt by the dominant macrophyte, Bertda erecta, did not vary seasonally. Colonization was at a constant rate of about 8% of the site each month throughout the year and this accounted for 50% of the total number of gains by Berula. Gains of Berula from Ranunculus showed an annual cycle with a maximum during the summer when Ranunculus was in recession. Gains of Berula from Callitriche spp. also varied annually but the maximum was during the autumn. Total losses of Berula were at a constant rate throughout the year but were to gravel and silt during the winter, to Ranunculus in spring and early summer and to Callitriche in late summer and autumn. Analysis of loss of Berula with time indicated that the position of the Berula carpet was constantly changing. The growth and recession of Berula could not be linked in a meaningful way to environmental variables. Callitriche and Rammculus both showed an annual pattern of growth and recession. TTiere was temporal separation of the two macrophytes with Ranunculus growing mainly in spring and early summer and Callitriche showing maximum growth in late summer and autumn, and some evidence of spatial separation. The observed differences between years in the growth of Callitriche could not be attributed to any of the environmental variables measured.Discharge was thought to be an important variable controlling the growth of Ranunculus since increase of Ranunculus in the spring was positively correlated with the mean discharge at that time. In years when discharge was low, the growth of Runtoicultis appeared to be restricted by shading from epiphytic algae which accumulated on the plant surfaces under these conditions. Correspondence; A. D. Berrit. Freshwaier Biological Association, River Laboratory, East Stoke. Wareham. Dorset BH206BB, England. (X)46-5070/S2/()2()()-()(X)l $02.00© 1982 Blackwel! Scientihc Publications .V. F. Ham eta].

I. Orthodadius (Euorthodadius) calvus Pinder. similar to Orthocladius (Euorthodadius) Ihienemanni Kieffer. colonized a new gravel substratum in two recirculating stream channels. A maximum population density of 68.621 m"' was attained after only 16 days. This had fallen to a negligible density by the thirty-third day.2. Some recruitment occurred over most of the study period (AprilMay 19K1) and no single sharply defined cohort was evident. There was a large range of body lengths within each instar. and considerable overlap between instars. The population density estimates for instars I and II were low compared with instars III and IV.3. The relationship between instantaneous growth rate (g) and geometric mean length indicated that growth was best described by a Gompertz curve. Growth rate decreased with increasing length from a value of about 4U% length day^' at 2 mm body length to about 5% length day"' at 9 mm. 4. Growth rates for individual larvae, kept in culture, were very variable with maximum rates close to the values determined from the field data. Mean duration of larval life was 16 days. 5. Estimates of production for the study period ranged from 13.5 g dry wt m'^ (Channel III, size-frequency method) to 34.2 g dry wt m"( Channel II, Allen's graphical method, values corrected for non-linear growth).6. Gut contents were estimated to represent about 55% of the total weight therefore production values should be reduced by this amount. Introductionwater and to assess their impact on algal populations, particularly during the spring The present study was part of a project which bloom and decline of epilithic diatoms, was designed to examine the growth, produc-The work was carried out in two 50 m long tion and feeding rates of the invertebrates experimental recirculating stream systems. which colonize gravel substrata in flowing sited at Waterston. Dorset, England (National Corrcspondetice: Dr M. LHdle, Freshwater Biolo^ ^^^^ Reference SY 742953), These systems gical Association, River Laboratory. East Stoke, possessed several features which were advanWarcham, Dorset.tageous for quantitative studies on inverte-

Aquatic habitats are severely threatened by human activities. For anadromous species, managing freshwater habitats to maximize production of more, larger juveniles could improve resilience to threats in marine habitats and enhance population viability. In some juvenile salmonid habitats, complexity created by large substrates provides resources and reduces competitive interactions, thereby promoting juvenile production. In lowland rivers, which lack large substrates, aquatic plants might provide similar complexity and enhance fish productivity. To test the influence of aquatic plants on juvenile Atlantic salmon and sympatric brown trout in a lowland river, we directly manipulated the cover of the dominant macrophyte, Ranunculus, in nine sites during summer and autumn for two years. We quantified the abundance, site retention and growth of salmon and trout under high, medium or low Ranunculus cover. To investigate the effects of Ranunculus cover on feeding opportunities and interspecific competition, we quantified available prey biomass and body size, fish diet composition and compared dietary niche overlap. Experimentally increased Ranunculus cover supported higher salmon abundance in summer and autumn, and higher site retention and growth of salmon in summer. Trout abundance and growth were not influenced by Ranunculus cover, but trout site retention doubled in high, relative to low, cover sites. Despite the weak effects of Ranunculus cover on prey availability, salmon and trout inhabiting high cover sites consumed larger prey and a higher biomass of prey. Furthermore, dietary niche overlap was lower in high, relative to low, cover sites, suggesting that abundant Ranunculus reduced interspecific competition. This field experiment shows that high Ranunculus cover can support more and better growing juvenile salmon, and facilitate foraging and co‐existence of sympatric salmonid species. Maintaining or enhancing natural macrophyte cover can be achieved through sympathetic in‐river and riparian vegetation management and mitigating pressures on them, such as sediment inputs and low flows, or through planting. Further research should test whether macrophyte cover benefits propagate to subsequent life stages, particularly juvenile overwintering associated with high mortality. This knowledge, in combination with our findings, would further clarify whether beneficial juvenile habitat can improve the viability of at‐risk salmonid populations. Overall, our findings suggest that the aims of river restoration might be achieved through promotion of in‐stream aquatic vegetation.