Alternative equilibria in shallow lakes

The turbidity of lakes is generally considered to be a smooth function of their nutrient status. However, recent results suggest that over a range of nutrient concentrations, shallow lakes can have two alternative equilibria: a clear state dominated by aquatic vegetation, and a turbid state characterized by high algal biomass. This bi-stability has important implications for the possibilities of restoring eutrophied shallow lakes. Nutrient reduction alone may have little impact on water clarity, but an ecosystem disturbance like foodweb manipulation can bring the lake back to a stable clear state. We discuss the reasons why alternative equilibria are theoretically expected in shallow lakes, review evidence from the field and evaluate recent applications of this insight in lake management.     

Alternative states in shallow lakes

Ecology of Shallow Lakes by Marten Scheffer Chapman and Hall (Population and Community Biology Series 22), 1998. ￡45.00/$79.65 hbk (xx+357 pages) ISBN 0 412 74920 3.     

Summer sedimentation in six shallow arctic lakes

The movement of sediment between the lake bottom and water column of shallow lakes can be sizeable due to the large potential for resuspension in these systems. Resuspended sediments have been shown to alter phytoplankton community composition and elevate water column production and nutrient concentrations. We measured the summer sedimentation rates of two lakes in 2003 and six lakes in 2004. All lakes were shallow and located in the Alaskan Arctic. In 2004, turbidity, light attenuation, total sediment:chlorophyll a mass in the sediment traps, and thermal stratification were also measured in each of the lakes. The sediment:chlorophyll a mass was much greater than if the sediment was derived from phytoplankton production in all of the lakes, indicating that the source of the sedimenting material was resuspension and allochthonous inputs. Consistent with these findings, the temporal variation in sedimentation rate was synchronous between most lakes, and sedimentation rate was positively related to wind speed and rainfall suggesting that sedimentation rate was strongly influenced by landscape-scale factors (e.g., wind and rain events). Two of the lakes are located on deposits of loess that accumulated during past glacial periods. These two lakes had sedimentation rates that were significantly greater and more variable than any of the other lakes in the study, as well as high turbidity and light attenuation. Our results indicate that sedimentation in these shallow arctic lakes is supported primarily by allochthonous inputs and resuspension and that landscape-scale factors (e.g., weather and geology) impact on the transport of materials between the lake bottom and water column. Handling editor: J. Saros     

Factors affecting light penetration in shallow lakes

Light conditions were studied in six lakes of the Danube Delta for a period of 2 years and were described as a function of 12 independent variables forming a data matrix with more than 1000 sample units. Light extinction was explained in percentage of 64% by phytoplankton, of 11% by detritus, of 7% by zooplankton, of 1.4% by dissolved organic matter and of 0.15 by bacterioplankton. The influence of mineral particles was insignificant. Equations are produced here for the relationship to water turbulence, wind intensity and lake depth. The threshold for full water turbulence was between 7 and 8% for a fluctuation domain of critical winds of 3.2–5.4 m s^-1 and for a depth domain of 1–3 m.     

Nitrate availability and hydrophyte species richness in shallow lakes

1. Submerged plant richness is a key element in determining the ecological quality of freshwater systems; it has often been reduced or completely lost. 2. The submerged and floating‐leaved macrophyte communities of 60 shallow lakes in Poland and the U.K. have been surveyed and species richness related to environmental factors by general linearised models. 3. Nitrogen, and more specifically winter nitrate, concentrations were most important in explaining species richness with which they were inversely correlated. Phosphorus was subsidiary. Such an inverse relationship is consistent with findings in terrestrial communities. Polish lakes, with less intensively farmed catchments, had greater richness than the U.K. lakes. 4. The richest U.K. communities were associated with winter nitrate‐N concentrations of up to about 1–2 mg L^?1 and may correspond with ‘good’ ecological quality under the terms of the European Water Framework Directive. Current concentrations in European lowlands are often much higher.     

Loading concentration models for phosphate in shallow lakes

A theoretical mathematical formula is presented, describing the phosphate concentration in water and sediments of shallow lakes as a function of loading. Experimentsin vitro are described and agree, in principle, with the expected shape of the relationship curve. The curve shows that the phosphate concentration rises slowly during the early stages of eutrophication and that, later, a much more rapid increase can be expected. The importance of the apatite system for eutrophic hard waters is demonstrated.     

Recent palaeolimnology of three shallow Danish lakes

Recent eutrophication histories of three shallow lakes (mean depths <3 m) were studied using palaeolimnological methods. Freeze-cores were dated using ²¹⁰Pb, ¹³⁷Cs, ¹³⁴Cs and ²⁴¹Am. Resultant chronologies were problematical at two sites (Væng Sø and Vesterborg Sø) due to sediment-water interface mixing, indicated by uniform ¹³⁷Cs profiles over the surface 20–30 cm. Sediments at Langesø and Vesterborg Sø have a high carbonate content, which together with the high mixing rate have resulted in diatom dissolution below 30 cm at Vesterborg Sø. Diatom stratigraphy indicates relatively small biological changes at both Væng Sø and Langesø both lakes have been eutrophic for at least the last 150–200 years. Væng Sø is dominated by planktonic diatoms together with high percentages of benthic Fragilaria spp., and Langesø by planktonic diatoms, especially Cyclostephanos dubius and Stephanodiscus parvus.Epilimnetic phosphorus concentrations were inferred using weighted averaging; at Væng Sø the diatom-inferred TP (DI-TP) concentrations were close to observed values in the early 1980s, but failed to record the post-biomanipulation decrease to 55 g l^-1 after 1988, presumably due to the smoothing of the sediment record by resuspension and mixing. At Langesø two increases in DI-TP suggest a two-phase enrichment of the lake, initial eutrophication 1880, and hypertrophy after 1950. The recent DI-TP values are lower than the annual observed values, and reasons for this are discussed. Despite problems associated with sediment mixing, the sediment records of these shallow systems can still be used for monitoring and for environmental reconstructions.     

An empirical model for sediment resuspension in shallow lakes

Suspended solids concentrations were measured at routine 2–3 week intervals and on additional windy days for at least one year in each of seven shallow (mean depth < 2 m) south Island, New Zealand lakes. Surface wave characteristics were estimated from water depths and local meteorological data using a shallow-water wave forecasting model for fetch-limited waves. Bottom shear stresses were computed from surface wave characteristics for the sampling stations and for a hypothetical lake-average station. The calculated shear stresses were, on average, much better predictors of suspended solids concentrations than alternative models using two different functions of wind speed, wave height²/depth or wavelength/depth. A combination of the sample station and lake average shear stresses provided slightly better predictions than the sample station values alone, suggesting that currents also contribute significantly to the concentration at a given point. Regressions of suspended solids on the combined functions had r ² values ranging from 0.74–0.73 in the seven lakes. The slopes of these regressions were negatively related to the settling velocity of the lowest quartile of the sediment, and to macrophyte biomass, in multiple regression (r ² = 0.94, p < 0.01).     

Spatial distribution and seasonal changes in methylmercury concentrations in shallow lakes

The environmental factors governing the distribution and behaviour of total and methylmercury within mercury contaminated shallow lakes, associated with the River Yare, Norfolk, UK, have been assessed in situ through the use of sediment cores. These were analysed for microbial and chemical changes on both a temporal and spatial scale. The distribution of total mercury proved to be site dependent and related to the hydrology, sediment transport dynamics, the degree of sediment disturbance and distance from the contamination source. The core profiles revealed a subsurface peak in total mercury with maximum concentrations residing at depths of 12 and 36 cm, depending upon location, with enrichment extending down to depths in excess of 88 cm. This vertical distribution was deemed to reflect historical emission in the late 1960s and early 1970s rather than post depositional migration effects. The distribution of methylmercury was distinct from that of its inorganic counterpart since it also displayed temporal variability with highest concentrations occurring in the spring and summer. Maximum concentrations also prevailed in the uppermost 12 cm of sediment, with peak loadings at or just below the sediment/water interface and detectable levels being restricted to the upper 36 cm. The temporal and spatial behaviour of methylmercury appeared to reflect variations in redox potential, the availability of the Hg²⁺ ion and the nature of microbial populations.     

Submerged vegetation development in two shallow, eutrophic lakes

Submerged macrophyte vegetation in two shallow lakes in the Netherlands, Lake Veluwemeer and Lake Wolderwijd, has been affected by eutrophication in the late 1960's and 1970's. Recent changes in the vegetation occurred in the period following lake restoration measures. Between 1987 and 1993, the dominance of Potamogeton pectinatus decreased, while Charophyte meadows expanded over the same time interval. The pattern of change of the dominant macrophyte species might result from changes in the underwater light climate. Seasonally persistent clear water patches associated with the Chara meadows have been observed in the last few years. The interaction between submerged macrophyte vegetation succession and water transparency in the lakes is discussed.     

Role of sediment and internal loading of phosphorus in shallow lakes

Current velocity is a pervasive feature of lotic systems, yet this defining environmental variable is rarely examined as a factor for regulating stream herbivory. To investigate how current modifies herbivory in the upper Colorado River, U.S.A., loops of electrified fencing wire were used to reduce in situ grazer densities on 30 × 30 cm tile substrates. After 45 d, electrified tiles had significantly fewer grazers (P = 0.03) and >2X more algal biomass than controls (P = 0.0002). Reduced grazing on electrified tiles yielded periphytic assemblages having more diatoms and chlorophytes, as well as greater algal species richness. Current velocity effects alone did not significantly regulate algal abundance; however, the interaction between current velocity and grazer exclusion resulted in more algae in slow vs. fast current (P = 0.02). Grazer abundances were similar between fast and slow current velocities, suggesting that grazers in the Colorado River differ in their ability to regulate algae across the current velocity gradient. Our results indicate that stream current-mediated herbivory in streams may be more important than is generally recognized.     

Bacterioplankton in the littoral and pelagic zones of subtropical shallow lakes

We measured bacterioplankton (phylotypes detected by fluorescent in situ hybridisation, morphometric forms, abundance and production) in samples collected in summer in the littoral and pelagic zones of 10 subtropical shallow lakes of contrasting area (from 13 to 80,800 ha). Compared to the pelagic zones, the littoral zones were overall characterised by higher macrophyte dominance and lower concentrations of total phosphorus and alkalinity and higher concentrations of dissolved organic carbon (DOC) and humic substances. Similarities of bacterial production and biomass turnover and density of active phylotypes and morphotype proportions were related to similarities in a set of environmental variables (including nutrients, humic substances content, predator density and phytoplankton biomass), and some additionally to lake area. Horizontal heterogeneity in bacterioplankton variables (littoral versus pelagic) increased with lake area. Bacterioplankton biomass and production tended to be lower in the littoral zone than in the pelagic zone despite higher concentrations of DOC and humic substances. A likely explanation is higher predation on bacterioplankton in the littoral zone, although allelophatic effects exerted by macrophytes cannot be excluded. Our results indicate that organic cycling via bacterioplankton may be less efficient in the littoral zone than in the pelagic zone of shallow lakes.     

Connectivity and cladoceran species richness in a metacommunity of shallow lakes

SUMMARY 1. The shallow ponds of the nature reserve 'De Maten' form a metacommunity, in which individual ponds are highly interconnected via a system of overflows and rivulets. This study reports on the relations between cladoceran species richness and (a) connectivity patterns and (b) local environmental variables.
2. No relation was found between local species richness and three connectivity variables or dispersal pathways.
3. Spatial configuration was related to richness, but was confounded by environmental variables for 2 of 3 years. In those 2 years, there was a significant linear relation between Secchi disc depth and species richness, suggesting an important impact of the clearwater/turbid state alternative equilibria in shallow lakes in determining cladoceran richness. Only in the year in which environmental variables were unimportant did connectivity between the ponds influence species richness.
4. These results suggest that local environmental variables related to the clearwater/turbid state alternative equilibria in shallow lakes are important in determining cladoceran species richness. Connectivity and dispersal of individuals between the different ponds only act secondarily by increasing the general species richness within a pond through dispersal from ponds with different environmental conditions.     

Nutrient balances and phytoplankton dynamics in two agriculturally loaded shallow lakes

The impact of agriculture was estimated on two shallow, eutrophic lakes, Lake Kotojärvi and Lake Villikkalanjärvi in southern Finland. The main emphasis was on phosphorus and nitrogen budgets and on the phytoplankton dynamics. Special attention was paid to internal P loading and blue-green algal blooms. The mean Tot-P load from agricultural land was 1.2 kg ha^-1 a^-1 in both basins and Tot-N loads were 19 kg ha^-1 a^-1 in L. Villikkalanjärvi and 12 kg ha^-1 a^-1 in L. Kotojärvi. The Tot-P input to L. Kotojärvi was on an average 0.62 g m^-2 a^-1 (per lake surface area), and the Tot-N input 9.1 g m^-2 a^-1. The corresponding inputs to L. Villikkalanjärvi were 3.1 and 57 g m^-2 a^-1, respectively. The annual variation followed the runoff volumes. About half of the Tot-P and one third of the Tot-N load was retained in L. Kotojärvi. In L. Villikkalanjärvi the retention was only 24% for Tot-P and 19% for Tot-N. The difference was very probably due to a longer theoretical retention time in L. Kotojärvi. In L. Villikkalanjärvi the mean concentration of Tot-P was 120 µg 1^-1 and that of Tot-N 1700 µg 1^-1 and the corresponding figures in L. Kotojärvi 67 and 990 µg 1^-1, respectively. The mean chlorophyll a concentration was, however, higher in L. Kotojärvi (26 µg 1^-1) than in L. Villikkalanjärvi (20 µg 1^-1). This was probably due to an internal P load in L. Kotojärvi: in 1988 the internal load of dissolved P was estimated to be as much as twofold the external load. In L. Villikkalanjärvi the internal dissolved P load was only up to 50% of the external input. In L. Kotojärvi the high internal P load coupled with a low DIN:DIP ratio resulted in a strong blue-green algal bloom in the summer of 1988. In L. Villikkalanjärvi blue-green algae were observed only in small amounts. Even in August 1990, when the DIN:DIP ratio was low enough to favor the occurrence of blue-green algae, they contributed only up to 10–15% of the total phytoplankton biomass.     

Balance between primary and bacterial production in North Patagonian shallow lakes

The shallow Andean North Patagonian lakes are suitable environments for the evaluation of autotrophic and heterotrophic production under a scenario of high radiation in high dissolved organic matter (DOM) systems. We aimed to study the balance between primary and bacterial production in three shallow Andean lakes, in a summer sampling (high irradiance condition). Our hypothesis is that two factors would interact: high light and high DOM, affecting bacteria and algae. We carried out experiments of bacterial production (BP) by measuring [¹⁴C]-l-leucine incorporation and PP by ¹⁴C uptake in two fractions: picophytoplankton and phytoplankton >2 μm. Cell abundance, chlorophyll a, nutrients, DOM, light, and temperature were also measured. The contribution of picophytoplankton to total primary production (PP) was, in general, very high exceeding 50%. Picophytoplankton was photosynthetically more efficient than the larger autotrophs in all lakes. We observed a decrease in PP at surface levels due the high solar irradiances, while BP was not affected. Changes in the PP:BP ratios were observed in relation to DOM content and light effect. Our data indicate that the amount of available DOM drives the balance between PP and BP. However, solar radiation should be included as an important factor since PP:BP ratio may decrease because of PP photoinhibition.     

Persistent internal phosphorus loading during summer in shallow eutrophic lakes

Nutrient availability, in particular of phosphorus (P), is a key factor for the structure and functioning of shallow lakes, and not least the sediment plays an important role by acting as both a nutrient source and sink. We used 21 years of monthly mass balance and lake water data from six shallow (mean depth = 1.2–2.7 m) and fast flushed (mean hydraulic retention time = 0.6–2.6 months) eutrophic Danish lakes (mean summer P concentrations ranging from 0.09 to 0.61 mg/l) to investigate long-term trends in yearly and seasonal patterns of P retention. To one of the lakes, the external P input was reduced by 70% in the early 1990s, whereas none of the other lakes have experienced major changes in external P loading for more than 20 years. All lakes showed a distinct seasonal pattern with high P concentrations and typically negative P retention during summer (up to ?300% of the external loading from May to August). During winter, P retention was overall positive (up to 50% of the external loading from December to April). Internal P loading from the sediment delayed lake recovery by approximately 10 years in the lake with the most recently reduced external loading, but in all the lakes net release of P from the sediment occurred during summer. P release in the six lakes has not abated during the past decade, indicating that the sediment of eutrophic and turbid shallow lakes remains a net source of P during summer. The seasonal variations in P retention became more pronounced with increasing P levels, and retention decreased with increasing temperature, but increased if clear water conditions were established.