The benthic boundary layer approach and its application to Lake Päijänne,Finland

Resuspension of bottom sediments is the net result of a wide variety of different fluid mechanical processes with characteristic time and length scales that extend over six orders of magnitude. The sum of these effects is most heavily concentrated in a layer adjacent to the bottom called the benthic boundary layer (BBL). The physics of BBL must be understood before improved solutions to the resuspension problem are possible. Traditionally in lakes, sedimentation and resuspension have been modelled with the aid of equations which ignore the time and space variations of near bottom processes. This can lead to wrong estimates of material transport. With the exception of few recent studies, benthic boundary layer approach has been mainly applied to marine environments. The instrumentation has been a major problem for development of the theory and its applications, but during recent years some new instruments have been giving promising results. This paper discusses the applicability of the theory in Finnish lakes and presents results from Lake Päijänne.     

Salinity tolerance of three ostracode species (Crustacea:Ostracoda) of Iberian saline lakes

Laboratory experiments and field data were used to determine salinity tolerance limits of three ostracode species (Prionocypris aragonica, Eucypris mareotica and Heterocypris barbara) from Iberian saline lakes. Salinity tolerance appeared related to ionic composition and temperature. Implications for ostracode ecology and geographical distribution are evaluated.     

The neutralization of acidic coal mine lakes by additions of natural organic matter: a mesocosm test

Cylindrical polyethylene enclosures 3 m in length and 1 m in diameter reaching from the surface to the bottom were constructed in an acid (pH=3.1) lake on a coal surface mine in southern Illinois. Wheat straw was added to the enclosures to test the effects of dissimilatory sulfate reduction on water chemistry. Added straw increased sulfide concentrations, raised pH to 6.5, reduced O₂ and increased acid neutralizing capacity of the enclosed water columns when compared with a control enclosure and with the open lake. Generation of acid neutralizing capacity exceeded the standing stock of sulfide indicating that sulfide was removed either by precipitation of FeS or outgassing of H₂S. The pH and acid neutralizing capacity within the enclosures eventually returned to the level of the surrounding lake because of water exchange around the enclosure walls. Our results show that additions of organic matter to acid surface mine lakes result in the generation of acid neutralizing capacity.     

The eutrophication of shallow coastal lakes in Southwest England — understanding and recommendations for restoration,based on palaeolimnology,historical records,and the modelling of changing phosphorus loads

Palaeolimnological studies of sediments from Slapton Ley and Loe Pool, two coastal freshwater lakes in Southwest England, show that in the period since 1945, they have been eutrophicated by nutrient inputs from intensification of agriculture, but also from sewage effluent. Two simple models have been used to identify the main sources of catchment outputs, and in the case of Slapton Ley, to evaluate historical changes in land use, and their likely effect on lake trophic status.Restoration strategies may also be evaluated using the same models. They suggest that in order to reduce loads on either lake to within OECD permissible limits, not only will all sewage inputs need to be prevented, and non-phosphate detergents used, but also losses from agricultural land must be reduced. This could take the form of the keeping of fewer cattle (the main source of organic nitrogen and phosphorus in both catchments), or the zoning of the respective catchments so that steep slopes close to riparian zones are not used, as at present, for the grazing of livestock.A better option, however, would appear to be the establishment along most of the rivers draining into these lakes, of buffer strips of woodland at least 15 m wide. According to the models, this measure, along with treatment or diversion of sewage effluent, would reduce phosphorus loads upon the lakes to within acceptable limits.     

Organic matter mineralization in the pore water of a eutrophic lake (Aydat Lake,Puy de Dôme,France)

The chemical composition of the pore water from the sediment of a eutrophic lake is dominated by high concentrations of total dissolved CO₂ (up to 12 mM), reduced soluble iron (up to 2 mM) and dissolved silica (up to 1 mM). The pH lies within the range of 6.70 ± 0.02; this reflects that the pore water is efficiently buffered by the CO₂ acid/base system. This composition is directly related to the main diagenetic reactions which drive the organic matter mineralization i.e. methanogenesis and ferric oxides reduction. Other geochemical processes are of minor importance. A stoichiometric model based on these main reactions allow us: (i) to define a general formula for the organic matter which is close to Redfield's one for the C:N ratio, while the C:P ratio is much higher owing to a probable adsorption of phosphorus onto reactive surfaces of the solid and due to heterotrophic bacterial uptake; (ii) to calculate a global first order kinetic constant which drives the organo-polymers breakdown. Due to the strong influence on the trophic status of the lake caused by an excess of phosphate, special attention is devoted to this species. We show that the sediment-water interface is a source of dissolved phosphate when the hypolimnion is anoxic between May and November. This contribution represents about 17% of the river input and should be taken into account in any attempt toward lake restoration.     

Sedimentation in fluvial and lacustrine environments

Sedimentation in rivers is dominated by a complex set of physical processes, associated with the unidirectional flow of water. Variations in these processes give rise to different fluvial channel types, whose character can commonly be recognised in the ancient record. Chemical and biological processes are comparatively unimportant in fluvial sedimentation. In contrast, physical, chemical or biological processes can each dominate sedimentation in lakes. Physical (clastic) deposition dominates in high-latitude and mountain lakes (in which chemical and biological activity are low), and in lakes with high relief of the drainage basin and lake floor. Its variety reflects a range of processes influenced by river inflow, wave and current action, thermal and density effects. Economic benefits from the study of lake and river sedimentation include both resource and environmental aspects. An example is given of a mercury pollution study in a fluvial ecosystem. It shows that return to background levels can take place within a relatively short interval after cessation of pollutant input.     

Seasonality in Southern Hemisphere freshwater phytoplankton assemblages

Seasonal climatic cycles induce corresponding fluctuations in phytoplankton abundance and productivity at all latitudes, the magnitude of these fluctuations tending to increase with distance from the Equator. In equatorial regions seasonality is dependent on prevailing wind and rainfall patterns while annual temperature fluctuations exert increasing control over seasonal events at higher latitudes. The small annual temperature range of equatorial aquatic systems increases their sensitivity to localized climatic events which can bring about diel changes that exceed normal month-to-month variations. Long-term hydrological cycles with a periodicity greater than one year can also cause dramatic changes in equatorial and tropical aquatic systems leading to greater unpredictability.The factors regulating seasonal patterns of phytoplankton abundance and species composition in equatorial and low-latitude temperate regions of the Southern Hemisphere are examined and compared with similar features in the Northern Hemisphere. Despite the striking diversity of phytoplankton populations and the wide variety of habitats they occupy, seasonal succession follows a common sequence controlled, successively, by physical, chemical and biotic factors. This permits a high degree of predictability in the environmental conditions promoting growth of different taxa.Examination of Southern Hemisphere data indicates that, at class level, phytoplankton successional sequences in Southern Hemisphere aquatic systems are in agreement with the successional paradigm formulated for northern tropical and temperate latitudes. Diatoms characterize early successional episodes and these are followed by chlorophytes, and finally blue-green algae. Extreme habitat modification (e.g. hypertrophy, salinity) tends to lead to dominance of the habitat by a single taxon, often represented by a single species. Predictions of within-taxon species succession in phytoplankton assemblages are far less precise.     

The dynamics of freshwater phytoplankton stability in the Naroch Lakes (Belarus)

The phytoplankton community of the Belarus Lakes Naroch, Myastro and Batorino, which have a Trophic State Index of 42.3, 60.7 and 66.8, respectively, underwent drastic changes to their structure during the period between 1968 and 2012. Thanks to an extensive monitoring program, these changes were well-documented and were qualitatively interpreted as signs of the community destabilization. The main objective of this study was the quantification of the ecological stability of the phytoplankton community in the Naroch Lakes. The approach to the quantification of ecological stability was based on defining the stability index as an inverse of the Euclidean Distance between the current and the reference states of the algal community (EuD-approach). The stability of the phytoplankton community was characterized by two indices: a “combined” index (SI[Comb]), and a “total community” index (SI[TotB]). SI[Comb] was calculated based on the individual taxonomic group biomasses and thus characterizes the stability of a community structure. SI[TotB] was calculated based on the values of the total algal biomass. Analyses of the results of this study extended the plausibility of the EuD-approach for the quantification of lake phytoplankton stability and allowed us to identify the dynamics of the stability of the Naroch Lakes phytoplankton. For the Naroch Lakes, we observed relatively larger SI[TotB] values in comparison with the SI[Comb] values. The results enabled us to examine the relationship between the lake trophic status and the stability of the phytoplankton community.     

Widespread natural intraspecific variation in tissue stoichiometry of two freshwater molluscs: Effect of nutrient enrichment

A central premise of ecological stoichiometry is that consumers maintain relatively fixed elemental composition in their bodies, a process known as elemental homoeostasis. Although nutrient enrichment is a ubiquitous problem facing many freshwater lakes around the world, intraspecific variation in elemental composition of freshwater invertebrates and its relationship with nutrient loading have not been well addressed. Here, we examined carbon:nitrogen:phosphorus (C:N:P) stoichiometry of two widely distributed molluscs, Corbicula fluminea and Bellamya aeruginosa, from several subtropical shallow lakes across a nutrient gradient. Our results showed that these two species exhibited substantial natural intraspecific variation in tissue stoichiometry which can reach or even exceed the values among different freshwater taxa investigated before. Our results suggest that tissue P content presents the greatest variations, followed by N content, and lowest in C content. Tissue P content ranged about three-fold (0.56%–1.65%) and five-fold (0.41%–2.28%) for B. aeruginosa and C. fluminea, respectively. Correspondingly, N content ranged from 5.16% to 12.06% and from 6.47 to 11.36%, respectively. Tissue %P, C:P and N:P ratios were strongly correlated with PO₄³⁻-P and/or chlorophyll-a in the water column. Tissue N and P contents of B. aeruginosa and P content of C. fluminea increased with increasing lake trophic levels (mesotrophic to eutrophic to hypertrophic). These results suggest that the two molluscs can adjust their tissue stoichiometry in relation to nutrient enrichment. Relaxing the assumption of strict homeostasis may help them cope with potential stoichiometric constraints. The results provide additional clues to why these two species are successful invaders and widely distributed.