Midsummer spatial variation in methane efflux from stands of littoral vegetation in a boreal meso-eutrophic lake

1. Spatial variation of methane (CH₄) efflux from the littoral zone of a meso‐eutrophic boreal lake was studied with a closed‐chamber technique for three summer days in 22 vegetation stands, consisting of three emergent and three floating‐leaved species. 2. Between‐species differences in CH₄ emission were significant. The highest emissions were measured from the emergent Phragmites australis stands (0.5–1.7 mmol m^?2 h^?1), followed by Schoenoplectus lacustris > Equisetum fluviatile > Nuphar lutea > Sparganium gramineum > Potamogeton natans. Within‐species differences between stands were not significant. 3. In P. australis stands, the stand‐specific mean CH₄ emission was significantly correlated with solar radiation, probably indicating the role of effective pressurised ventilation on CH₄ fluxes. The proportion of net primary production emitted as CH₄ was significantly higher in P. australis stands (7.4%) than in stands of S. lacustris and E. fluviatile (both 0.5%). 4. In N. lutea stands, CH₄ efflux was negatively correlated with the mean fetch and positively with the percentage cover of leaves on the water surface. There were no differences in CH₄ efflux between intact N. lutea leaves and those grazed by coleopteran Galerucella nymphaeae. In S. graminaeum and P. natans stands, CH₄ effluxes were not related to any of the measured environmental variables. 5. For all vegetation stands, the biomass above water level explained about 60% of the observed spatial variation in CH₄ emission, indicating the important role of plants as gas conduits and producers of substrates for methanogens in the anoxic sediment.     

Effects of an invasive crayfish on the littoral macroinvertebrates of large boreal lakes are habitat specific

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Structure and dynamics of zooplankton communities in the littoral zone of some North Carolina lakes

Species composition and seasonal, dynamics of zooplankton in the littoral zone were studied in three piedmont North Carolina lakes for one year. Nygaard's compound index indicated oligotrophic conditions in Belews Lake and eutrophic conditions in Salem Lake and Lake 150. A total of 32 species of zooplankton were found in the samples. Significantly fewer species of truely littoral zooplankton were present in the oligotrophic lake regardless of season (P < 0.05). Eutrophication appears to favor increased diversity in zooplankton of the littoral zone. Habitat heterogeneity, provided by a well developed littoral zone containing aquatic macrophytes, may be the major factor contributing to the maintenance of more taxa of zooplankton in eutrophic systems.Distinct taxa of Cladocera and Copepoda are present in the littoral zone, exhibiting seasonal dynamics that are largely different from the limnetic fauna. Such dynamics may result from predation by characteristically limnetic species of cyclopoid copepods which seek prey in the littoral zone. The numerical and seasonal abundance of predators and prey are inversely related and the true littoral taxa, represented in the present study by the genera Alona, Chydorus, Pleuroxus, Sida, Simocephalus, and Eucyclops, contained no predaceous species. The seasonal dynamics of these genera are very similar even in widely separated geographical regions, indicating that the factor(s) responsible operate independently of climatic and chemical variables. Intrazooplankton predation appears to be a more plausable explanation than segregation along depth which, although consistent with data from studies of limnetic waters, cannot be related to populations of zooplankton in shallow littoral areas.     

Distribution of macroinvertebrates in relation to physical and biological variables in the littoral zone of nine New Zealand lakes

Macroinvertebrates play a key role in the littoral zone of lakes. Macroinvertebrate community composition is closely linked to habitat conditions. To date, there have been few attempts to relate macroinvertebrates to habitat factors in lakes. In this study, nine mainly oligotrophic lakes from throughout New Zealand were surveyed for macroinvertebrates. The lakes were selected to represent a range of suspended sediment loading and lake level regimes. Within each lake, several sites were selected to provide a range of exposure to wave action. A multiple regression approach was taken to relate macroinvertebrate community composition and habitat characteristics. The results of the analysis suggest that the littoral zone of the lakes we studied could be divided into four general habitats. The first is the wave wash zone characterised by coarse substrates and macroinvertebrate taxa usually associated with lotic environments, such as Ephemeroptera and Plecoptera. The second habitat is associated with macrophytes and is limited at the top by wave action and at depth by light attenuation. In this zone, the snail Potamopyrgus antipodarum is dominant, along with Trichoptera and Odonata. At the base of the macrophytes is the detrital habitat characterised by fine, organic rich sediments and dominated by chironomids, oligochaetes and Trichoptera. At depths below the macrophyte zone, fine sediments are found, and bivalves such as the freshwater mussel Hyridella menziesi are common. While macroinvertebrate abundance can be highly variable, some general predictions of community structure can be made based on a few key environmental factors. Abundance of snails Odonata and Trichoptera was positively related to macrophyte biomass. Some macroinvertebrate groups such as oligochaetes, chironomids, snails and bivalves were more common in fine substrates, while Ephemeroptera were characteristic of coarse substrates. Detrital biomass was important for most of the macroinvertebrate groups studied showing a positive relationship for oligochaetes and Trichoptera and a negative relationship for Ephemeroptera and Plecoptera.     

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.     

Relationships between DOC concentration and epilithon stable isotopes in boreal lakes

1. Littoral biota in boreal lakes are known to assimilate epilithon. Being able to predict the stable isotopic composition of these alga will help to identify those systems in which δ¹³C and δ¹⁵N analysis can be used in foodweb investigations of allochthony and biomagnification.
2. In a survey of 15 boreal lakes, the concentration of dissolved organic carbon (DOC) explained 76% of the variation in epilithon δ¹³C, and 86% of the variation in epilithon δ¹⁵N.
3. Because both δ¹³C and δ¹⁵N values were depressed and similar to terrestrial values in humic (high DOC) lakes, it will be more difficult to successfully employ stable isotopic techniques for estimating allochthony in such systems. Lower δ¹⁵N values in humic lakes also indicate that trophic positions estimated by stable isotopes are not directly comparable to those of similar biota inhabiting clearwater lakes, unless autochthonous baseline corrections are made.     

Response of littoral vegetation on climate warming in the boreal zone; an experimental simulation

The impact of climate warming on the littoral zone of a boreal lake ecosystem was studied experimentally for three growing seasons in two artificial ponds (10×27 m) and in replicated chamber experiments. One pond was enclosed in a plastic greenhouse and another untreated pond served as a reference system. During the growing seasons temperature in the greenhouse was maintained at levels 2–3 °C higher than ambient with a computer-controlled ventilation system. One growing season prior to initiation of the experiment, a vegetated littoral zone with equal densities of water horsetail (Equisetum fluviatile) was established in both ponds. Although changes occurred in the species dominance (E. fluviatile - Alisma plantago-aquatica - Sparganium erectum spp. microcarpum - Elodea canadensis) within the three years of the study, the emergent macrophytes emerged earlier and grew better in the warmer conditions of the greenhouse pond compared with those in the reference pond. The difference in above-ground biomass throughout the growing seasons was >2 fold and after three experimental growing seasons the difference in below-ground biomass of macrophytes was 2.5-fold between the ponds. In replicated chamber experiments the biomass growth of E. fluviatile was also significantly higher in a 2–3 °C higher temperature than under ambient conditions. An ecosystem-scale induced change, characterized by a heavy growth of filamentous algae (mainly chlorophytes) was evident in the vegetated littoral zone of the greenhouse pond. A hypothesis that macrophyte rhizomes function as `phosphorus pumps' from the sediment and thus accelerate eutrophication in a warmer climate should be further studied.     

Detritus and nutrient dynamics in the shore zone of lakes: a review

The importance of detritus varies greatly among shore zones of lakes, but in a large majority of these regions detrital pathways prevail. Aside from a great spatial and seasonal variability, macrophytes and bottom sediments appear to be dominant stores of nutrients in these habitats. Macrophytes hold a central position in nutrient cycling in the shore-littoral lake zones. They are the main source of autochthonous detritus as they prevail in the total biomass of littoral organisms, and they are only rarely available as direct food of consumers. Various processes and interactions determine the role of macrophytes in nutrients dynamics. These are: the intensity of nutrient uptake and translocation, release of nutrients by healthy plants and from decomposing plants, exchange of elements between macrophytes and their periphyton, as well as interception of seston by macrophyte stands. Particular plant species differ in their time of dying and susceptibility to decomposition. The changes in decomposing material (size structure of particles and nutrient content) mean that detritus in various stages of decomposition differs in its role in trophic dynamics of shore-littoral lake zones. Several types of shore regions as regards detritus sources and retention level are discussed.     

Remote sensing evidence for the episodic transport of phosphorus from the littoral zone of a thermally stratified lake

• 1 Esthwaite Water in Cumbria is a small, thermally stratified lake fringed with beds of reed in the shallow littoral. In this study, we used a combination of in situ measurements and airborne remote sensing to investigate some of the physical processes influencing the transport of water and nutrients from the littoral zone.
• 2 The analysis of water samples collected from the reed beds, the littoral zone and the open water showed that significantly higher concentrations of dissolved reactive phosphorus (DRP) were frequently recorded in the reed beds in early summer.
• 3 Experiments with surface and near‐surface free‐running drogues demonstrated that the movement of water from the littoral zone was strongly influenced by the development of a secondary thermocline. When there was no secondary stratification, the surface currents generated by light winds seldom exceeded 2 cm s^‐1. When a secondary thermocline was present, surface current speeds of 5–10 cm s^‐1 were recorded even when the wind speed was less than 200 cm s^‐1.
• 4 A series of thermal surveys using a Daedalus Airborne Thematic Mapper (ATM) demonstrated that plumes of warm water frequently developed in the littoral zone when the weather was calm. Some of these plumes covered several hundred square metres and persisted for several hours. Others were only a few metres in width and acted as episodic ‘pumps’ that appeared and dispersed in less than an hour.

     

Oxidation of methane in boreal forest soils: a comparison of seven measures

Methane oxidation rates were measured in boreal forest soils using seven techniques that provide a range of information on soil CH₄ oxidation. These include: (a) short-term static chamber experiments with a free-air (1.7 ppm CH₄) headspace, (b) estimating CH₄ oxidation rates from soil CH₄ distributions and (c)²²²Rn-calibrated flux measurements, (d) day-long static chamber experiments with free-air and amended (+20 to 2000 PPM CH₄) headspaces, (e) jar experiments on soil core sections using free-air and (f) amended (+500 ppm CH₄) headspaces, and (g) jar experiments on core sections involving tracer additions of¹⁴CH₄. Short-term unamended chamber measurements,²²²Rn-calibrated flux measurements, and soil CH₄ distributions show independently that the soils are capable of oxidizing atmospheric CH₄ at rates ranging to < 2 mg m^–2 d^–1. Jar experiments with free-air headspaces and soil CH₄ profiles show that CH₄ oxidation occurs to a soil depth of 60 cm and is maximum in the 10 to 20 cm zone. Jar experiments and chamber measurements with free-air headspaces show that CH₄ oxidation occurs at low (< 0.9 ppm) thresholds. The¹⁴CH₄-amended jar experiments show the distribution of end products of CH₄ oxidation; 60% is transformed to CO₂ and the remainder is incorporated in biomass. Chamber and jar experiments under amended atmospheres show that these soils have a high capacity for CH₄ oxidation and indicate potential CH₄ oxidation rates as high as 867 mg m^–2 d^–1. Methane oxidation in moist soils modulates CH₄ emission and can serve as a negative feedback on atmospheric CH₄ increases.     

Beaver population fluctuations and tropospheric methane emissions in boreal wetlands

Measurements of net methane flux were made during the 1988 ice-free season (May–October) at a beaver-meadow complex in northern Minnesota, USA. The site included upland boreal forest, sedge meadow, submerged aquatic plants, and the open water of a beaver pond. Annual fluxes were 8–11 g C/m² in the permanently wetted zones and 0.2–0.4 g C/m² at the occasionally inundated meadow and forest sites. These data, when coupled with long-term (46 yr) data on beaver (Castor canadensis) population size and habitat alteration, suggest that about 1% of the recent rise in atmospheric methane may be attributable to pond creation by beaver in North America.     

Microbial decomposition of freshwater macrophytes in the littoral zone of lakes

The decomposition of several lake macrophytes was investigated under field conditions. Data on weight and phosphorus loss, numbers of microbial decomposers and their activity were obtained.Experiments were conducted in the littoral of two lakes with different levels of macrophyte development.Weight loss during 40–60 days of decomposition for fast-decomposing plants was 60–95% and after 365-day of incubation, Potamogeton perfoliatus L. lost nearly 100% of its initial weight. Slow-decomposing plants lost 20–50% of their initial weight after 40–60 days of incubation, and Phragmites australis (Cav.) Trin. ex Steud. lost 84% of its initial weight after 365 days.Total phosphorus content in plants did not decrease at the first stages of decomposition.The number of microbial decomposers utilizing both labile and resistant substrates increased 2–6 times during the first 5–25 days period. During this period the community was morphologically diverse and biochemically active (high level of microbial respiration). It coincided with the highest weight loss. After that period, the number of microorganisms utilizing labile substrates, as well as the rate of decomposition decreased.The part of macrophyte organic matter entering the biological cycle in two lakes made up 3.5% and 26% of phytoplankton primary production. Bacterial production on decomposing macrophytes was calculated at 4% and 51% of bacterioplankton production, respectively, in both lakes.     

The distribution of microcrustacea in the littoral zone of a freshwater lake

The distribution of microcrustacea in the water column, sediments and on different macrophyte species was examined in the littoral zone of Jack Lake, Nova Scotia, Canada. Large numbers of microcrustacea occurred in association with macrophytes, suggesting that this habitat should receive greater attention in future studies of microcrustacean numbers, biomass, and production. The relative abundance of different microcrustacea varied considerably among sediments, macrophytes and water column samples. Although microcrustacean species composition differed among macrophyte groups, consistent differences in absolute numbers per gram could not be demonstrated. Epiphytic microcrustacean community structure also varied among depth strata in Jack Lake. Few epiphytic and benthic microcrustacea migrated into the water column on a diurnal basis.     

Invertebrates of the littoral zone of Lake Glubokoe

This paper briefly reviews systematic, production and biocenotic studies on invertebrates of the littoral zone of Lake Glubokoe. It presents new data for a number of years on the spatial distribution and population dynamics of species (mainly cladocerans) constituting a zoocenosis confined to the lower surface of floating leaves of Nuphar luteum.     

Leaf litter colonization by invertebrates in the littoral zone of a small oligotrophic lake

The colonization of deciduous leaf litter by aquatic invertebrates was studied at Scott Lake in Algonquin Park, Ontario, Canada. Deciduous leaf packs were colonized after only 2 days submergence. The invertebrate community was dominated by chironomids (25–94% depending on sampling period), and to a lesser extent by oligochaetes, turbellarians, and mayflies. Collectors, such as the chironomids Dicrotendipes, Pseudochironomus, Paratanytarsus and Parakiefferiella were the dominant functional-feeding group suggesting that leaf litter is being used as habitat rather than a direct food source. Deciduous leaf litter lost a substantial amount of weight, due to leaching, after only 48 h submergence. Fall-shed beech (Fagus grandifolia) leaves decomposed more rapidly than fall-shed sugar maple (Acer saccharum) leaves with daily processing coefficients (k), determined using an exponential decay model, of 0.0058 and 0.0039, respectively. Conversely, conditioned maple leaves, defined as leaves remaining on the ground over winter, were processed faster than conditioned beech leaves, with coefficients of 0.0042 and 0.0014, respectively. It is speculated that inhibitory compounds have been leached from the maple leaves, allowing for faster leaf processing. This revised version was published online in July 2006 with corrections to the Cover Date.     

Role of lakes for organic carbon cycling in the boreal zone

We calculated the carbon loss (mineralization plus sedimentation) and net CO₂ escape to the atmosphere for 79 536 lakes and total running water in 21 major Scandinavian catchments (size range 437–48 263 km²). Between 30% and 80% of the total organic carbon that entered the freshwater ecosystems was lost in lakes. Mineralization in lakes and subsequent CO₂ emission to the atmosphere was by far the most important carbon loss process. The withdrawal capacity of lakes on the catchment scale was closely correlated to the mean residence time of surface water in the catchment, and to some extent to the annual mean temperature represented by latitude. This result implies that variation of the hydrology can be a more important determinant of CO₂ emission from lakes than temperature fluctuations. Mineralization of terrestrially derived organic carbon in lakes is an important regulator of organic carbon export to the sea and may affect the net exchange of CO₂ between the atmosphere and the boreal landscape.     

Methane (CH4) release from littoral wetlands of Boreal lakes during an extended flooding period

Lake littoral zones have a transitional nature and dynamic conditions, which are reflected in their CH₄ emissions. Thus, detailed studies are needed to assess the littoral CH₄ emissions in a regional scale. In this study, CH₄ fluxes were followed during the ice‐free seasons in 1998 and 1999 by using the static chamber method in the littoral zone of two lakes in Finland. An exceptionally high water level in 1998 caused an unusually long inundation in otherwise ephemerally flooded zone. The flooding was normal in year 1999. The factors controlling CH₄ emissions were examined and statistical response functions were constructed. Further, the effect of extended flooding on the littoral CH₄ budged was estimated. The methane flux was primarily regulated by the water level in grass and sedge dominated eulittoral zone, but not in infralittoral reed and water lily stands. Methane emissions in the sedge dominated zone decreased significantly, when the flood was high enough to submerge the venting structures of the plants. Besides water level, sediment temperature determined CH₄ emission. The cumulative CH₄ emissions from the whole littoral wetlands in wet year were 1.1 times (L. Kevätön), or 0.61 and 0.79 times (L. Mekrijärvi) those in dry year. The crucial factor was the discrepancy between the exceptional and the average water level. The extension of inundated area does not necessarily increase CH₄ emissions if the flood reaches infrequently inundated areas, which apparently have low CH₄ production potential. This is the case especially, if the emissions in lower zones simultaneously decrease due to high water level. Our study analyses these complex responses between CH₄ emissions and water level.     

太湖湖滨带生态系统健康评价

根据湖滨带生态系统的特点,运用综合健康指数法建立了湖滨带生态系统健康评价体系,由目标层、准则层、指标层构成,其中准则层由湖滨带水质状况、底泥状况、植被状况、其它生物状况(浮游动物、浮游植物、底栖动物)、岸带物理状况5项组成,指标层由总氮、总磷、溶解氧、挺水植物覆盖率等15项指标构成。采用专家打分法、熵值法分别确定了准则层、指标层的权重系数。对太湖湖滨带33个点位进行了采样分析,并进行无量纲化处理后应用到所建立的评价体系中。评价结果显示33个点位中为"很健康"、"健康"、"亚健康"、"疾病"、"严重疾病"的分别占0%、24.2%、21.2%、51.5%及3.0%,也即超过一半的点位处于"疾病"状态。只有东太湖刚刚超过"健康"分数的下限,东部沿岸、贡湖、南部沿岸均处于"亚健康"状态,而梅梁湾、竺山湾、西部沿岸属于"疾病"状态,且竺山湾的生态健康状态最差。该评价结果与太湖湖滨带各分区的实际调查情况相符合,评价方法可靠性、可行性较强,可为其它湖泊湖滨带的生态系统健康评价提供一定的参照。     

Biomass of planktonic crustaceans and the food of young cyprinids in the littoral zone of Lake Balaton

The littoral zone of Lake Balaton and its periphyton-zooplankton-fish communities have been investigated intensively in recent years. Total average number of crustacean plankton varied from 36 to 126 ind l^–1, their biomass from 0.49 to 1.86 mg ww l^–1 month^–1 at different areas of the littoral zone. In general, these values for the above parameters were higher in hypertrophic areas. 23 fish species occurred in the littoral zone with cyprinids dominating. The seasonal food spectra of Y-O-Y roach (Rutilus rutilus), white bream (Blicca bjoerkna) and bream (Abramis brema) were based mainly on planktonic crustaceans and benthic/periphytic invertebrates. According to the frequency of occurrence of crustaceans and other invertebrates, the food composition of young cyprinids differed significantly in the NE and SW-basins of the lake.     

Feeding strategy and growth of cyprinids in the littoral zone of Lake Balaton

The growth, diet and feeding strategy of five phytophil or phytolithophil species of Cyprinidae from the littoral habitats of Lake Balaton were investigated by examining their scales and foregut contents. The relationships between the total anterior radii of scales and the standard lengths were represented best by a power function for white bream Blicca bjoerkna , and linear functions for common bream Abramis brama , roach Rutilus rutilus and wild goldfish Carassius auratus gibelio , respectively. The backcalculated mean lengths for the first age groups of common bream, white bream and roach did not differ statistically from those obtained by direct observation on 0 group fish in late November 1995. Compared to other waters, common bream grows slowly, wild goldfish and roach rapidly, while the growth rate of white bream can be considered of medium speed in Lake Balaton. Common bream showed a generalized feeding pattern, consuming mainly chironomid larvae, detritus and Corophium curvispinun . Roach showed a clear shift between specialization for Dreissena and algae. Despite the dense population of D. polymorpha in the lake, the significance of the herbivorous adaptation of roach has not yet been made clear. Wild goldfish consumed mainly detritus but, in the open water region, it shifted to zooplankton. White bream preyed chiefly on D. polymorpha , but showed a mixed feeding pattern and utilized most of the available food resources. Carp had the most specialized feeding strategy and preyed mainly on D. polymorpha . According to the discriminant analysis, the five cyprinids exhibited significant food resource partitioning.