1. This paper describes a continuous-flow system that we have used extensively for measurement of oxygen exchange and nitrogen transformations in different communities of lowland streams, i.e. the water, the sediment, and the macrophyte-biofilm community. The system, which is set up on the stream bank, consists of several macrophyte and sediment chambers equipped with a double-flow system that ensures an internal water velocity close to that in the stream and which, by continuously renewing the water, mimics diel fluctuation in stream temperature and water chemistry. Water temperature and dissolved oxygen are measured continuously and the data stored on magnetic tape. Water samples are collected in a refrigerator and analysed using standard chemical procedures.2. The application of the system is illustrated using results obtained during a 2-day summer experiment in a shallow macrophyte-rich stream. The biological processes in the stream were mainly associated with the macrophyte-biofilm community and the sediment, those in the water being negligible. Oxygen release was confined to the macrophyte-biofilm community, the sediment consuming oxygen both by day and by night. Whole-system gross production and dark respiration occurred at similar rates (6-7g O2 m~" day"'), net balance being about zero. Inorganic nitrogen was consumed both by the sediment and to a greater extent by the macrophytes, the diel average consumption being 1 g N m~^ day^\ 3. The sum of the activity in the macrophyte and sediment chambers corresponded to the overall activity of the stream section as determined by upstream/downstream mass balance. This indicates that the results obtained with the continuous-flow chambers realistically describe the oxygen and the nitrogen metabolism of the stream. " Present address: Chr. Hanscn's Laboratorium Danmark A/S. 10-12,
SUMMARY. 1. A hypothetical leech population with known initial density, initial weight, final weight and cohort production interval (CPI) was established. Production estimated by the size‐frequency method for various growth patterns, mortalities, number of samples per CPI and number of size classes was compared with actual production estimated from daily growth and mortality by the increment‐summation method. The population had either perfectly continuous reproduction or a perfectly synchronous cohort. 2. When size‐classes were delimited in order to equalize the time spent in each size class, the deviations from actual production increased with decreasing number of size‐classes and increasing mortality. For a population with perfectly continuous reproduction, production was only overestimated by 32% with an extreme mortality of 2.0% day−1 and three size‐classes. For a perfectly synchronous cohort, production was either underestimated or overestimated, depending on the first day of sampling. The deviations from actual production increased considerably with decreasing number of size‐classes, increasing mortality and decreasing number of samples per CPI. 3. Differences between actual and assumed growth patterns may give underestimates or overestimates of more than one order of magnitude at high mortalities and few size‐classes. It is concluded that knowing the actual growth pattern, the size frequency method will give realistic estimates of production in cases when normal cohort methods cannot be used. The estimate can be improved significantly by increasing the number of size classes and the number of samples per CPI.
1, Populations of Stalls lutaria L. were sampled quantitatively at about monthly intervals during 3 years at two localities in the macrophyte rich River Sus&. Denmark.2. The life cycle was univoltine. The larvae hatched in June-July, and the initial S. lutaria population densities varied between lU and 370 ind. m~^. These differences were probably due to extreme variations in stream discharge during the period of recruitment, high velocities preventing the small swimming larvae from reaching the sediment.3. The S. lutaria rate of elimination varied between U. 11 and 1.47% d~' at the two localities. The high rate of elimination was possibly due to fish predation, 4. The growth rates varied between 0.54 and 15% d~'. Growth rates differed between localities and years.5. Numbers of potential prey organisms were not higher in sample units with S. lutaria than in sample units without S. lutaria. Numbers of leeches, which were potential competitors, did not differ significantly in sample units with and without 5. lutaria.6. Production of 5. lutaria varied between 0.16 and 2.49 g ash free dry weight m"~ y~', and the P/B ratios varied between 3.0 and 7.3 y"'. The P/B ratios increased significantly with increasing rate of elimination. • 1980-81 J 1 1 1 1 I 1 1 1 J J A 0 N D 0 N D J M A FIG, 1, Average mean weight in mg DW ind,"' ±95% CL of Sialis lutaria in Ihe Susa at Pindso and al Vettersiev. When numbers £3 minimum and maximum weights are indicated withDut vertical bars.
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