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SPIXIANA Fig. 1. Lake Mälaren with sampling stations. northern basins (Blacken, Granfjärden, Ekoln and Sigtunafjärden in Fig. 1). C. neocorax Wülker & Butler occurred in western Lake Mälaren. Micropsectra

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SPIXIANA Fig. 1. Lake Mälaren with sampling stations. northern basins (Blacken, Granfjärden, Ekoln and Sigtunafjärden in Fig. 1). C. neocorax Wülker & Butler occurred in western Lake Mälaren. Micropsectra spp., Tanytarsus spp. and Prodadius spp. dominated in the central part of the lake, where Chironomus was absent. Sergentia coracina and Stictochironomus spp. were also part of the fauna of the central basins but in lower numbers than the other taxa. It has not been possible to identify all larval material to species. Imagines of Micropsectra caught by hand-net at the central basins were identified as M. insignüobus Kieffer by Säwedal (1976). Several species of Tanytarsus and Prodadius have been found and at least two species oi Stictochironomus, one of which was S. rosenschoeldi (Zett.), have been reported to occur in hand-netted material (Wiederholm 1974). Chironomus larvae of the salinarius type from western Lake Mälaren were described as C. neocorax by Wülker & Butler (1983) and none of the other species with this larval type are likely to occur in the lake. The identity of C. plumosus and C. anthracinus has not been established from chromosome identifications, but these species are the most likely ones to occur in great numbers at the depth that has been sampled here, viz 15 m (cf. Lindeberg & Wiederholm 1979). Considerable temporal Variation in numbers have been noted for most groups of benthic organisms during the study period. Oligochaeta and Chironomidae decreased in numbers at most stations throughout the period. Chaoborus flavicans became more abundant during later years (Station Blacken in Fig. 2). Crustaceans occurred in high numbers in the central part of the lake during some few years (Station N. Prästfjärden in Fig. 2). Chironomus anthracinus C. anthracinus occurred in particularly high numbers in Blacken and Sigtunafjärden during the first part of the period (Fig. 3). The species then decreased at all stations and population density was rather low throughout the 1980's. Considerable Variation occurred between individual years, and the corre- 5000 year lation coefficients between the various data sets for single years were low, except between Blacken and Sigtunafjärden. The general pattern of development was quite similar at all stations, however, as appears from the figures and the correlation coefficients for running means (Fig. 3, Table 1). C. plumoshs C. plumoshs fluctuated in numbers at most stations without any clear general trend or correlation between stations (Fig. 3, Table 1). High numbers occurred in Ekoln during the first part of the study period, but the population density decreased strongly at the end of the 1970's and remained low throughout the remainder of the study period. C. neocorax This species has a rather uneven distribution within Lake Mälaren. Similar to the other Chironomus species it is absent from the central basins, and it occurs in high numbers only in some of the moderately eutrophic western basins. Of the stations dealt with here, only Granfjärden had high population densities of C. neocorax, and there was no clear temporal pattern or correlation between stations (Fig. 3, Table 1). Micropsectra spp. Large numbers of Micropsectra were found during only a few years (Fig. 3). Distinct peaks occurred at all stations between 1973 and 1976 and the running means of the 30 and 50 m depths in both N. Prästfjärden and S. Björkfjärden were strongly correlated (Table 1). The Overall occurrence of Micropsectra was low throughout the 1980's. TanytarsHS spp. The occurrence of Tanytarsus was similar to that of Micropsectra (Fig. 3). Peaks occurred at all stations in 1974 and 1976, and very low numbers were found after The data sets from the various stations were strongly correlated (Table 1). Sergentia coracina This species was rare or absent most years, but distinct peaks occurred in 1976 or 1977 in three out of four stations in the central part of Lake Mälaren (Fig. 3). Stictochironomus spp. Stictochironomus (not shown in figure or table) had its main occurrence in , i. e. somewhat after the peaks of Micropsectra and Tanytarsus. Significant numbers were found only in N. Prästfjärden. Procladius spp. Two peaks occurred at the 30 m stations in central Lake Mälaren one around 1976 and the other one in (Fig. 3). The running means from these stations were strongly correlated (Table 1). At 50 m depth in N. Prästfjärden the second peak was much less pronounced, but the general pattern was similar. The 50 m Station in S. Björkfjärden showed much less Variation, but the highest numbers occurred in the first part of the study period. In the western and northern basins temporal Variation was seemingly greater, without much similarity between stations (Fig. 3, Table 1). A general tendency to decreasing abundances may be noted, however. 10 The occurrence of Procladius was similar to that of Micropsectra and Tanytarsus. Running means of Procladius were strongly correlated to those of Micropsectra (r = ) (not shown in table) at the 30 m stations, thoughless so at the 50 m stations (r = ). There was also a positive correlation with running means of Tanytarsus or Tanytarsus plus Micropsectra (r = ) at the 30 m stations, though again this trend was weaker at the greater depth. Discussion Considerable Variation occurred between years. However, for most taxa trends or regulär patterns of Variation are clearly discernible. In many cases the similarity between stations, as indicated by high correlation coefficients between the data sets, indicates a common causal mechanism for the observed Variation. For example, the high numbers of Micropsectra, Tanytarsus, S. coracina and Stictochironomus at most stations in central Lake Mälaren during the mid 1970's would seem to have a common reason. Decreasing numbers of C. anthracinus were common to the stations of western and northern Lake Mälaren and one may suspect a common structuring factor here also. The numbers oi Procladius were strongly correlated at stations in the central part of the lake, but not at the western and northern stations, and this may indicate that different factors are predominating. No pattern could be seen in the numbers of C. plumosus and C. neocorax, except for the marked decrease of the former species at Station Ekoln. Food, weather conditions and biotic factors are important determinants acting independently or collectively in governing population density. Improved sewage treatment in all major communities around the lake has brought about a 50 % reduction of the phosphorus loading on Lake Mälaren since the middle of the 1960's. The occurrence of a series of dry years up to 1976 added to this and the total phosphorus loading decreased to less than half of its maximum during the last 20 years. Evidently as a result of this, the excessive blooms of bluegreen algae in the western and northern parts of the lake decreased in intensity and duration (Willen 1987). Small species of algae (e. g. flagellates) became more common and the number of species increased. The average total biomass of algae did not decrease to any great extent, however, except for the most eutrophic parts of the lake. The significance of these changes to the bottom fauna is not immediately clear. Both quantitative and qualitative changes in profundal food supply may have resulted. Because many of the small, noncolonial species of algae are more effective producers than large, colonial species, there is reason to believe that the total planktonic primary production per unit area is about the same as before or even higher. Measurements to Support this conclusion are lacking, however. One might assume that a greater Proportion of the phytoplankton is metabohzed in the water column when small and mobile forms are more predominant. Bluegreens such as Anabaena are also metabolized in the water phase (Fallon & Brock 1980), but others sink and decompose on the lake bottom. This may result in increased Sediment oxygen demand, but the increased microbial production favours many invertebrates that tolerate low oxygen levels. Among these are several species of Oligochaeta. Hence the reduced numbers of Ohgochaeta in many parts of Lake Mälaren (cf. Fig. 2, Station Blacken) may therefore be understood as reflecting a reduced nutrient flow through bacteria and Sediments, resulting from the reduced occurrence of bluegreen algae. Little Information is available on food utilization of aquatic insects certain types of ingested matter are probably of little value, whereas small amounts of other material may be critical (Lambertini & Moore 1984). C. anthracinus and C. plumosus are both detrivores, but algae sometimes make up a considerable part of the gut content (Jönasson 1972, Lambertini & Moore 1974). Johnson (1985) concluded that C. anthracinus is more of a deposit feeder, ingesting particulate matter scraped from the recently deposited surface Sediments, whereas C. plumosus is a filter feeder with the nutritional quality of ingested matter depending primarily on pelagic inputs. This would explain the decreasing num- 11 C. anthracinus C. plumosus C. neocorax Prociadius spp 4000- bers of C. anthracinus if, as argued above, one accepts that the changes in phytoplankton species composition has increased the relative importance of the pelagic metabohsm relative to that of the benthic one. The decrease of C. plumosus at Station Ekoln is probably more of a local change than part of a general pattern. Hypereutrophic conditions occurred at this Station before effective sewage treatment was installed in the nearby city of Uppsala, and C. plumosus endured this better than C. anthracinus. It is surprising, however, that both species have decreased to virtual extinction during later years. Contrary to the other species dealt with here, Procladius spp. are predominantly carnivorous, although algae and detritus may also be found in their gut content (Tarwid 1969, Baker & McLachlan 1979). Oligochaetes, other chironomid larvae and small benthic crustaceans are the preferred prey. Data on the ocurrence of crustaceans are not available. There was a positive correlation between the running means of Procladius spp. and those of Oligochaeta at two of the four stations in western and northern Lake Mälaren. The correlation was negative and very weak at the central stations, where Micropsectra and Tanytarsus were better correlated with Procladius. This may indicate that the fluctuations in numbers o( Procladius depended on fluctuations of their prey organisms, but it may also indicate that some of these taxa responded to some other factor that led to a common temporal pattern (cf below). Micropsectra, Tanytarsus, Sergentia coracina and Stictochironomus had their maximum occurrence during the mid 1970's. All are presumably browsers or filter feeders and typical inhabitants of the mesotrophic or moderately oligotrophic lake types as characterized by Brundin (1956). They were characteristic members of the profundal chironomid communities in the central part of Lake Mälaren throughout the study period dealt with here, but high population densities occurred only during a few years. It seems unhkely that this pattern would have been due to a directional change toward more mesotrophic or oligotrophic conditions that would have presented optimal conditions during these years only. Thus underlying factors with a more stochastic Variation should be sought to explain the occurrence of these taxa. Weather conditions coincident with emergence may be one such factor. Calm and warm weather should be favourable to swarming, egg-laying, and hence the recruitment of young larvae, particularly in species with a short and well synchronized flight period. Phenological data is scarce, however, and the available information indicates rather long or even several flight periods. Imagines of M. insignilobus are common in May and June at the central basins of Lake Mälaren (Wiederholm 1974). Stictochironomus rosenschoeldi and another species oi Stictochironomus occur in June and July. Records of adult Sergentia coracina from southern and central Sweden include late June and September (Brundin 1 949) ; no adults have been found from Lake Mälaren. If one assumes that the 4th instar larvae found in September October were born in the early summer the same year, weather conditions in May and June should be important. Weather records show that these months were particularly warm in 1976, with 230 day degrees above the average for and nearly 350 day degrees above the minimum during the period, which occurred in Micropsectra, Tanytarsus and Sergentia coracina had population peaks in 1976 at some stations. Peaks did occur in other years also, however, when temperatures were below average (e. g. 1985). Nevertheless it is still possible that the peaks in larval abundance that occurred during these years were related to weather conditions, but more detailed life history information is needed to support this conjecture. Between 5000 and ind. m~^, occasionally as much as ind. m^^, oi Pontoporiea affinis may be found at 50 m depth at Station S. Björkfjärden (Wiederholm unpubl.). The physical disturbance by such numbers of crustaceans should have a negative influence on the sessile, tube-living and rather fragile larvae of Tanytarsus and Micropsectra, and a strong negative correlation did occur between running means of P. affinis and the two chironomid taxa (r = 0.85 and 0.91, respectively). Hence, the absence of large numbers of P. affinis might be a prerequisite for Tanytarsus and Micropsectra to occur in reasonably large populations densities conditions and other circumstances were favourable to recruitment of young.. even during years when weather 13 Blacken Granfj. Sigtunafj Ekoln Blacken ^^-^^_^ C. anthracinus Granfj. Sigtunafj. Ekoln the role of biotic interactions, more Information is needed, in particular, on life histories of Chironomidae and how these are linked to seasonal Variation of phytoplankton production and deposition of organic matter on lake bottoms. Acknowledgements Lars Eriksson identified and counted the Chironomidae. Richard Johnson read the manuscript. References Baker, A. S. & McLachlan, A. J Food preferences of Tanypodinae larvae (Diptera: Chironomidae). Hydrobiologia62: Brundin, L Chironomiden und andere Bodentiere der südschwedischen Urgebirgsseen. Ein Beitrag zur Kenntnis der bodenfaunistischen Charakterzüge schwedischer oligotropher Seen. Rep. Inst. Freshwater Res. Drottningholm 30: Die bodenfaunistischen Seetypen und ihre Anwendbarkeit auf die Südhalbkugel. Zugleich eine Theorie der produktionsbiologischen Bedeutung der glazialen Erosion. Rep. Inst. Freshwater Res. Drottningholm 37: Fallon, R. D. & Brock, T. D Planktonic blue-green algae: Production, Sedimentation, and decomposition in Lake Mendota, Wisconsin. Limnol. Oceanogr. 25: HOLOPAINEN, I. J. & JÖNASSON, P. M Long-term population dynamics and producdon of Pisidium (Bivalvia) in the profundal of Lake Esrom, Denmark. Oikos 41: Johnson, R. K Feeding efficiencies of Chironomus plumosus (L.) and C. anthracinus Zett. (Diptera: Chironomidae) in mesotrophic Lake Erken. Freshwater Biology 15: JÖNASSON, P. M Ecology and production of the profundal benthos in relation to phytoplankton in Lake Esrom. Oikos Suppl. 14: Lambertini, G. A. & Moore, J. W Aquatic insects as primary consumers. In: Resh, V. H. & RosENBERG, D. M. (Eds.) The ecology of Aquatic Insects. Praeger Publishers, New York. Lindeberg, B. & Wiederholm, T Notes on the taxonomy of European species of Chironomus (Diptera: Chironomidae). Ent. scand. Suppl. 10: Maitland, P. S A coded checklist of animals occurring in fresh water in the British Isles. Institute of Terrestrial Ecology, Edinburgh, 76 p. SäweDAL, L Revision of the notescens-grou^ of the genus Micropsectra Kieffer, 1909 (Diptera: Chironomidae). - Ent. scand. 7: Tarwid, M Analysis of the contents of the alimentary tract of predatory Pelopiinae larvae (Chironomidae). - Ekol. Pol. Ser. A 17: Wiederholm, T Studies of the bottom fauna of Lake Mälaren. The National Environmental Protection Board Report 415. (In Swedish with summary in English.) Long-term changes in the profundal benthos of Lake Mälaren. Verh. Internat. Verein. Limnol. 20: Willen, E The large lakes of Sweden: Vänern, Vättern, Mälaren and Hjälmaren. In: Taub, F. B. (Ed.) Lakes and Reservoirs. Elsevier Science Publishers, Amsterdam Phytoplankton and reversed eutrophication in Lake Mälaren, Central Sweden, Br. phycol.j. 22: Wlilker, W. f. & Butler, M. G Karyosystematics and morphology of Northern Chironomus (Diptera: Chironomidae): Freshwater species with larvae of the salmarius-type. Ent. scand. 14: Dr. Torgny Wiederholm National Environmental Protection Board Environmental Quality Laboratory Box Uppsala, Sweden 15
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