Fish predation and reduction in body size in a Cladoceran population: palaeoecological evidence

SUMMARY. 1 We studied the recent history (1852-1982) of Lake Pyhä-järvi, south-west Finland, using both cladoceran microfossils and independent historical data. The objective of the study was to assess the impact of introduced planktivorous whitefish Coregonus lavaretus s.1. on zooplankton, especially on the main prey species Bosmina coregoni Baird s.str.
2. A size-frequency analysis of carapace remains preserved in the sediments clearly shows a size shift in a Bosmina coregoni population. The carapace length of B. coregoni decreased by 11.0% after the introduction of the size-selective planktivorous whitefish, indicating a parallel body-size reduction.
3.During the study period no changes could be found in the carapace length of Chydorus sphaericus O. F. Müller, which was not preyed upon.     

Taxon-specific variation in the stable isotopic signatures (δ13C and δ15N) of lake phytoplankton

1. The variability in the stable isotope signatures of carbon and nitrogen (δ¹³C and δ¹⁵N) in different phytoplankton taxa was studied in one mesotrophic and three eutrophic lakes in south‐west Finland. The lakes were sampled on nine to 16 occasions over 2–4 years and most of the time were dominated by cyanobacteria and diatoms. A total of 151 taxon‐specific subsamples covering 18 different phytoplankton taxa could be isolated by filtration through a series of sieves and by flotation/sedimentation, followed by microscopical identification and screening for purity. 2. Substantial and systematic differences between phytoplankton taxa, seasons and lakes were observed for both δ¹³C and δ¹⁵N. The values of δ¹³C ranged from ?34.4‰ to ?5.9‰ and were lowest in chrysophytes (?34.4‰ to ?31.3‰) and diatoms (?30.6‰ to ?26.6‰). Cyanobacteria were most variable (?32.4‰ to ?5.9‰), including particularly high values in the nostocalean cyanobacterium Gloeotrichia echinulata (?14.4‰ to ?5.9‰). For δ¹³C, the taxon‐specific amplitude of temporal changes within a lake was usually <1–8‰ (<1–4‰ for microalgae alone and <1–8‰ for cyanobacteria alone), whereas the amplitude among taxa within a water sample was up to 31‰. 3. The values of δ¹⁵N ranged from ?2.1‰ to 12.8‰ and were high in chrysophytes, dinophytes and diatoms, but low in the nitrogen‐fixing cyanobacteria Anabaena spp., Aphanizomenon spp. and G. echinulata (?2.1‰ to 1.6‰). Chroococcalean cyanobacteria ranged from ?1.4‰ to 8.9‰. For δ¹⁵N, the taxon‐specific amplitude of temporal changes within a lake was 2–6‰, (2–6‰ for microalgae alone and 2–4‰ for cyanobacteria alone) and the amplitude among taxa within a water sample was up to 11‰. 4. The isotopic signatures of phytoplankton changed systematically with their physical and chemical environment, most notably with the concentrations of nutrients, but correlations were non‐systematic and site‐specific. 5. The substantial variability in the isotopic signatures of phytoplankton among taxa, seasons and lakes complicates the interpretation of isotopic signatures in lacustrine food webs. However, taxon‐specific values and seasonal patterns showed some consistency among years and may eventually be predictable.     

Nutrient release and resuspension generated by ruffe (Gymnocephalus cernuus) and chironomids

1. Benthivorous fish may play an important role in internal nutrient loading. Ruffe are highly specialised, feeding exclusively on bottom animals; thus all nutrients released via their feeding are derived from the bottom and are new to the water column. The fish can also release nutrients from the sediment through resuspension while searching for food. 2. The aim of this study was to estimate experimentally in the laboratory the effect on water quality of resuspension and nutrient release by ruffe and bottom animals (chironomids). 3. Ruffe released nutrients during 8 h experiments as follows: total P 1.4, dissolved PO₄ 0.6, total N 24.0 and NH₄‐N 15.9 μg g^?1 WW h^?1. A decreasing trend in mass‐specific release was observed over time, probably because of starvation. The mass‐specific release of total N and NH₄‐N decreased as the mean weight of fish increased. The mean ratio of excreted N : P was 32. 4. In 26 h experiments with sediment and both ruffe and chironomids, ruffe increased nutrient concentrations and turbidity values significantly but chironomids had an effect only on turbidity. Neither ruffe nor chironomids affected the ratio of inorganic N : P concentrations. An interaction between ruffe and chironomids was found for turbidity. 5. According to these results, benthivorous fish may increase nutrient concentrations in the water column and need to be taken into account when estimating internal loading.     

Long-term ozone effects on vegetation, microbial community and methane dynamics of boreal peatland microcosms in open-field conditions

To study the effects of elevated ozone concentration on methane dynamics and a sedge species, Eriophorum vaginatum, we exposed peatland microcosms, isolated by coring from an oligotrophic pine fen, to double ambient ozone concentration in an open‐air ozone exposure field for four growing seasons. The field consists of eight circular plots of which four were fumigated with elevated ozone concentration and four were ambient controls. At the latter part of the first growing season (week 33, 2003), the methane emission was 159±14 mg CH₄ m^?2 day^?1 (mean±SE) in the ozone treatment and 214±8 mg CH₄ m^?2 day^?1 under the ambient control. However, towards the end of the experiment the ozone treatment slightly, but consistently, enhanced the methane emission. At the end of the third growing season (2005), microbial biomass (estimated by phospholipid fatty acid biomarkers) was higher in peat exposed to ozone (1975±108 nmol g^?1 dw) than in peat of the control microcosms (1589±115 nmol g^?1 dw). The concentrations of organic acids in peat pore water showed a similar trend. Elevated ozone did not affect the shoot length or the structure of the sedge E. vaginatum leaves but it slightly increased the total number of sedge leaves towards the end of the experiment. Our results indicate that elevated ozone concentration enhances the general growth conditions of microbes in peat by increasing their substrate availability. However, the methane production did not reflect the increase in the concentration of organic acids, probably because hydrogenotrophic methane production dominated in the peat studied. Although, we used isolated peatland microcosms with limited size as study material, we did not find experimental factors that could have hampered the basic conclusions on the effects of ozone.     

Spatial patterns of photobiont diversity in some Nostoc-containing lichens

Patterns of photobiont diversity were examined in some Nostoc -containing lichens using the nucleotide sequence of the cyanobacterial tRNA^Leu (UAA) intron. Lichen specimens collected in northwestern USA were analysed and the sequence data were compared with tRNA^Leu(UAA) intron sequences previously obtained from lichens in northern Europe. Generally, it is the species identity of a lichen rather than the geographical origin of the specimen that determines the identity of the cyanobiont. Identical intron sequences were found in Peltigera membranacea specimens collected in Oregon (USA) and in Sweden, and very similar sequences were also found in Nephroma resupinatum thalli collected in Oregon and Finland. Furthermore, in mixed assemblages where two Peltigera species grew in physical contact with each other, the different lichen species housed different photobiont strains. There is however not a one-to-one relation between mycobiont and photobiont as some intron sequences were found in more than one lichen species, and different intron sequences were found in different samples of some lichen taxa. Peltigera venosa exhibited a higher level of photobiont diversity than any other lichen species studied, and several intron sequences could for the first time be obtained from a single thallus. It is not clear whether this is evidence of lower cyanobiont specificity, or reflects an ability to exhibit different degrees of lichenization with different Nostoc strains. In one specimen of P. venosa , which contained bipartite cyanosymbiodemes and tripartite, cephalodiate thalli, both thallus types contained the same intron sequence.     

Spatial patterns of litter decomposition in the littoral zone of boreal lakes

1. We studied the patterns of litter decomposition in lake littoral habitats and investigated whether decay rates, as an integrating proxy for environmental conditions in the sediment, would co‐vary with net carbon dioxide (CO₂) exchange and methane (CH₄) efflux. These gas fluxes are known to be sensitive to environmental conditions. Losses in the mass of cellulose, root, rhizome and moss litter were measured during 2 years in boreal littoral wetlands in Finland and compared with published data on concurrently measured gas fluxes. Four study sites covered a range of sediment types and hydrological conditions. 2. Decomposition was not linearly related to the duration of flooding but depended on sediment type. Readily decomposable litter fractions, such as cellulose and rhizome litter, lost mass at a faster rate in marshes with a longer period of flooding but wide water level fluctuations that hinder establishment of a Sphagnum cover, than in peat‐forming fens. In marshes, the mean first‐year mass losses were 83–99% and 19–62% for cellulose and rhizomes, respectively. In fens, the respective losses were 40–53% and 33%. In the first year, the loss in the mass of the more recalcitrant root litter did not differ between sites (mean 19–30%) and moss litter lost no mass. 3. The estimated first‐year carbon loss from belowground litter was about 0.1–0.3 times ecosystem respiration and roughly similar to net carbon gas (CO₂, CH₄) efflux, suggesting that vascular plants and recent plant residues contribute substantially to ecosystem release of carbon gases. On the other hand, at least 40% of the mass of the belowground litter remained on a littoral site after the first 2 years of decomposition. Slow decomposition may indicate the accumulation of organic‐rich sediments. The accumulated carbon could explain the excess CO₂ release found in most littoral sites. In continuously inundated sites decomposition rates were similar to those in periodically flooded sites, but ecosystem‐atmosphere CO₂ exchange fell to close to zero. This discrepancy implies that the released CO₂ is dissolved in water and may be exported into the pelagic zone of the lake.     

Effects of temperature and sediment properties on benthic CO2 production in an oligotrophic boreal lake

1. Temperature and many other physical and chemical factors affecting CO₂ production in lake sediments vary significantly both seasonally and spatially. The effects of temperature and sediment properties on benthic CO₂ production were studied in in situ and in vitro experiments in the boreal oligotrophic Lake Pääjärvi, southern Finland. 2. In in situ experiments, temperature of the water overlying the shallow littoral sediment varied seasonally between 0.5 and 15.7 °C, but in deep water (≥20 m) the range was only 1.1–6.6 °C. The same exponential model (r² = 0.70) described the temperature dependence at 1.2, 10 and 20 m depths. At 2.5 and 5 m depths, however, the slopes of the two regression models (r² = 0.94) were identical but the intercept values were different. Sediment properties (wet, dry, mineral and organic mass) varied seasonally and with depth, but they did not explain a significantly larger proportion of variation in the CO₂ output rate than temperature. 3. In in vitro experiments, there was a clear and uniform exponential dependence of CO₂ production on temperature, with a 2.7‐fold increase per 10 °C temperature rise. The temperature response (slope of regression) was always the same, but the basic value of CO₂ production (intercept) varied, indicating that other factors also contributed to the benthic CO₂ output rate. 4. The annual CO₂ production of the sediment in Lake Pääjärvi averaged 62 g CO₂ m^?2, the shallow littoral at 0–3 m depth releasing 114 g CO₂ m^?2 and deep profundal (>15 m) 30 g CO₂ m^?2. On the whole lake basis, the shallow littoral at 0–3 m depth accounted for 53% and the sediment area in contact with the summer epilimnion (down to a depth c. 10 m) 75% of the estimated total annual CO₂ output of the lake sediment, respectively. Of the annual production, 83% was released during the spring and summer. 5. Using the temperature‐CO₂ production equations and climate change scenarios we estimated that climatic warming might increase littoral benthic CO₂ production in summer by nearly 30% from the period 1961–90 to the period 2071–2100.     

Complex and flexible life history of a freshwater benthic harpacticoid species

SUMMARY. 1. The life history of the benthic harpacticoid paracamptus schmeili (Crustacea: Copepoda) in the oligotrophic lake Pääjärvi, South Finland, was described from quantitative field data of all developmental stages. The generation time of P. schmeili was exceptionally long for a meiobenthic animal and varied with depth from 1 year in the littoral (depths of 1 and 2m) to 2 years in the upper profundal (13 and 14 m). and to 3 years in the lower profundal (40 m).
2. In the littoral, egg-bearing females were found from April to October, with peaks in spring and late summer, while in the profundal a single summer egg peak was present.
3. In the littoral and in the lower profundal there were three, in the upper profundal) probably four, simultaneous parallel cohorts over-wintering as adults, medium-sized (at 13 m also large) copepodids and medium-sized nauplii. Reproductive isolation of the parallel cohorts was possible at all depths.
4. The between-depth variation in the life history of P. schmeili was probably mainly a phenotypic response to the changing level and seasonal course of temperature and food supply. Growth and reproduction were confined to summer months, and the lengthening generation times with depth accompanied decreasing summer temperatures.
5. The cessation of growth in winter was probably due to a shortage of food, which may also have accentuated the size differences of the overwintering groups through intraspecific competition.     

Sediment respiration and lake trophic state are important predictors of large CO2 evasion from small boreal lakes

We show that sediment respiration is one of the key factors contributing to the high CO₂ supersaturation in and evasion from Finnish lakes, and evidently also over large areas in the boreal landscape, where the majority of the lakes are small and shallow. A subpopulation of 177 randomly selected lakes (<100 km²) and 32 lakes with the highest total phosphorus (P_tot) concentrations in the Nordic Lake Survey (NLS) data base were sampled during four seasons and at four depths. Patterns of CO₂ concentrations plotted against depth and time demonstrate strong CO₂ accumulation in hypolimnetic waters during the stratification periods. The relationship between O₂ departure from the saturation and CO₂ departure from the saturation was strong in the entire data set (r²=0.79, n=2 740, P<0.0001). CO₂ concentrations were positively associated with lake trophic state and the proportion of agricultural land in the catchment. In contrast, CO₂ concentrations negatively correlated with the peatland percentage indicating that either input of easily degraded organic matter and/or nutrient load from agricultural land enhance degradation. The average lake‐area‐weighted annual CO₂ evasion based on our 177 randomly selected lakes and all Finnish lakes >100 km² ( Rantakari & Kortelainen, 2005 ) was 42 g C m^?2 LA (lake area), approximately 20% of the average annual C accumulation in Finnish forest soils and tree biomass (covering 51% of the total area of Finland) in the 1990s. Extrapolating our estimate from Finland to all lakes of the boreal region suggests a total annual CO₂ evasion of about 50 TgC, a value upto 40% of current estimates for lakes of the entire globe, emphasizing the role of small boreal lakes as conduits for transferring terrestrially fixed C into the atmosphere.     

Cyanobiont specificity in some Nostoc-containing lichens and in a Peltigera aphthosa photosymbiodeme

The cyanobacterial symbionts in some Nostoc -containing lichens were investigated using the nucleotide sequence of the highly variable cyanobacterial tRNA^Leu (UAA) intron. When comparing different Nostoc -containing lichens, identical intron sequences were found in different samples of the same lichen species collected from two remote areas. This was true for all species where this comparison was made ( Peltigera aphthosa (L.) Willd., P. canina (L.) Willd. and Nephroma arcticum (L.) Torss.). With one exception, a specific intron sequence was never found in more than one lichen species. However, for two of the species, Peltigera aphthosa and Nephroma arcticum , two different cyanobionts were found in different samples. By examining a P. aphthosa photosymbiodeme it could be shown that the same Nostoc is present in both bipartite and tripartite lobes of this lichen. It is thus possible for one cyanobiont/ Nostoc to form the physiologically different symbioses that are found in bipartite and tripartite lichens. The connection between photobiont identity and secondary chemistry is discussed, as a correlation between differences in secondary chemistry and different cyanobionts/ Nostoc s in the species Peltigera neopolydactyla (Gyeln.) Gyeln. was observed. It is concluded that more knowledge concerning the photobiont will give us valuable information on many aspects of lichen biology.