Depth limits and minimum light requirements of freshwater macrophytes

1. Data for maximum colonization depth (Z_c) of five groups of submerged macrophytes and light attenuation were collected for forty-five Danish lakes and 108 non-Danish lakes. The macrophyte groups were bryophytes, charophytes, caulescent angiosperms, rosette-type angiosperms and Isoetes spp. 2. The data showed systematic differences among the groups in the relationship of Z_c to water transparency. In lakes with low transparency (Secchi disc transparency (Z_s) less than 7 m) caulescent angiosperms and charophytes penetrated deepest followed by bryophytes and Isoetes spp. In more transparent lakes bryophytes grew deepest, followed by charophytes, caulescent angiosperms and Isoetes spp. Rosette-type angiosperms had the lowest Z_c in all types of lakes. Charophytes and caulescent angiosperms had similar depth limits in lakes with Z_s < 4 m but charophytes grew deeper in more transparent lakes. The depth limits of both groups were independent of light penetration in lakes with very low transparency (Z_s < 1 m). The annual light exposure for the deepest growing macrophytes (bryophytes) was 20–95 mol photons m^–2. 3. The relationship between Z_c, macrophyte type and lake transparency could be explained by three distinct processes regulating Z_c. In lakes with low transparency (Z_s < 1 m), tall macrophytes (caulescent angiosperms and charophytes) compensate for light limitation by shoot growth towards the water surface and Z_c is therefore independent of transparency. In lakes with medium transparency (1 m < Z_s < 4 m) Z_c for angiosperms, charophytes and Isoetes spp. is constrained by light attenuation in the water column, corresponding to a linear relationship between Z_c and Z_s. This pattern also applies to bryophytes, despite lake transparency. In transparent lakes, the minimum light requirement at Z_c increased with increasing transparency for angiosperms, charophytes and Isoetes spp. 4. The minimum light requirements among submersed macrophytes (including marine macroalgae) depend on their plant-specific carbon value (plant biomass per unit of light-absorbing surface area) for the species/group, indicating that the light requirements of submersed plants are tightly coupled to the plants’ possibility to harvest light and hence to the growth form. 5. The light requirements increased on average 0.04% surface irradiance per degree increase in latitude corresponding to an average decrease in Z_c of 0.12 m per degree latitude.     

Shallow lakes theory revisited: various alternative regimes driven by climate,nutrients, depth and lake size

Shallow lakes have become the archetypical example of ecosystems with alternative stable states. However, since the early conception of that theory, the image of ecosystem stability has been elaborated for shallow lakes far beyond the simple original model. After discussing how spatial heterogeneity and fluctuation of environmental conditions may affect the stability of lakes, we review work demonstrating that the critical nutrient level for lakes to become turbid is higher for smaller lakes, and seems likely to be affected by climatic change too. We then show how the image of just two contrasting states has been elaborated. Different groups of primary producers may dominate shallow lakes, and such states dominated by a particular group may often represent alternative stable states. In tropical lakes, or small stagnant temperate waters, free-floating plants may represent an alternative stable state. Temperate shallow lakes may be dominated alternatively by charophytes, submerged angiosperms, green algae or cyanobacteria. The change of the lake communities along a gradient of eutrophication may therefore be seen as a continuum in which gradual species replacements are interrupted at critical points by more dramatic shifts to a contrasting alternative regime dominated by different species. The originally identified shift between a clear and a turbid state remains one of the more dramatic examples, but is surely not the only discontinuity that can be observed in the response of these ecosystems to environmental change.     

Long- and short-term dynamics of submerged macrophytes in two shallow eutrophk lakes

1. Periods with clear water and abundant submerged vegetation have alternated with periods of turbid water and sparse vegetation during recent decades in Lake Tåkern and Lake Krankesjön, two shallow, calcium-rich, moderately eutrophic lakes in southern Sweden, Between 1983 and 1991, submerged vegetation (predominant species: Chara tomentosa, Nitellopsis obtusa, Myriophyllum spicatum) covered about 50% of the open lake area in Lake Tåkern. In Lake Krankesjön, submerged vegetation was sparse during 1983–84, but increased continuously in the following years and covered about 50% of the open lake area by 1990 and 1991. Potamogeton pectinatus was the first species to expand in Lake Krankesjön, but was later replaced by C. tomentosa. 2. During 1983–84, turbidity was high in Lake Krankesjön, which indicated that submerged macrophytes were light-limited. During 1986–91, there was a negative correlation between the areal coverage of charophytes and angiosperms, indicating that competition for space had become an important limiting factor. The same negative correlation was found in Lake Tåkern for 1983–91. 3. Charophytes had much higher biomass per unit area than angiosperms in both lakes and reduced water movement considerably. This was probably one reason for the increase of water transparency in Lake Krankesjön during the spatial expansion of these plants. Charophytes also stored large amounts of phosphorus and nitrogen, Charophytes are probably superior competitors for both space and nutrients and thus have competitive advantage over angiosperms in this lake type. 4. In Lake Krankesjön, both P. pectinatus and C. tomentosa were negatively affected by high water level during the growing period. Total disappearance of submerged vegetation occurred in both lakes after catastrophic events (dry-out during summer or mechanical damage by ice) caused by extremely low water level. Changes in water level are thus one of the most important reasons for among-year fluctuations in areal coverage of submerged macrophytes in these lakes.     

Seasonal dynamics of macrophytes and phytoplankton in shallow lakes: a eutrophication‐driven pathway from plants to plankton?

1. Seasonal relationships between macrophyte and phytoplankton populations may alter considerably as lakes undergo eutrophication. Understanding of these changes may be key to the interpretation of ecological processes operating over longer (decadal‐centennial) timescales. 2. We explore the seasonal dynamics of macrophytes (measured twice in June and August) and phytoplankton (measured monthly May–September) populations in 39 shallow lakes (29 in the U.K. and 10 in Denmark) covering broad gradients for nutrients and plant abundance. 3. Three site groups were identified based on macrophyte seasonality; 16 lakes where macrophyte abundance was perennially low and the water generally turbid (‘turbid lakes’); 7 where macrophyte abundance was high in June but low in August (‘crashing’ lakes); and 12 where macrophyte abundance was high in both June and August (‘stable’ lakes). The seasonal behaviour of the crashing and turbid lakes was extremely similar with a consistent increase in nutrient concentrations and chlorophyll‐a over May–September. By contrast in the stable lakes, seasonal changes were dampened with chlorophyll‐a consistently low (<10–15 μg L^?1) over the entire summer. The crashing lakes were dominated by one or a combination of Potamogeton pusillus, Potamogeton pectinatus and Zannichellia palustris, whereas Ceratophyllum demersum and Chara spp. were more abundant in the stable lakes. 4. A long‐term loss of macrophyte species diversity has occurred in many shallow lakes affected by eutrophication. One common pathway is from a species‐rich plant community with charophytes to a species‐poor community dominated by P. pusillus, P. pectinatus and Z. palustris. Such compositional changes may often be accompanied by a substantial reduction in the seasonal duration of plant dominance and a greater tendency for incursions by phytoplankton. We hypothesise a slow‐enacting (10–100 s years) feedback loop in nutrient‐enriched shallow lakes whereby increases in algal abundance are associated with losses of macrophyte species and hence different plant seasonal strategies. In turn such changes may favour increased phytoplankton production thus placing further pressure on remaining macrophytes. This study blurs the distinction between so‐called turbid phytoplankton‐dominated and clear plant‐dominated shallow lakes and suggests that plant loss from them may be a gradual process.     

Total phosphorus thresholds for regime shifts are nearly equal in subtropical and temperate shallow lakes with moderate depths and areas

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Facilitation of clear-water conditions in shallow lakes by macrophytes: differences between charophyte and angiosperm dominance

A number of mechanisms result in a feedback between water clarity and macrophytes and, consequently, the occurrence of alternative stable states in shallow lakes. We hypothesize that bottom-up mechanisms and interactions within the benthic food web are more important in a charophyte-dominated clear-water state, while top-down mechanism and interactions in the planktonic food web prevail at angiosperm dominance. Charophytes, which dominate at lower nutrient concentrations and develop higher densities than most angiosperms, can have a higher influence on sedimentation, resuspension, and water column nutrients. During dominance of dense submerged vegetation like charophytes, zooplankton can be hampered by low food quality and quantity and by high predation pressure from juvenile fish, which in turn are favoured by the high refuge potential of this vegetation. Grazing pressure from zooplankton on phytoplankton can therefore be low in charophytes, but the main feedback in angiosperm-dominated ecosystems. Charophytes offer a higher surface than most angiosperms to periphyton, which favors benthic invertebrates. These support macrophytes by grazing periphyton and constitute a central link in a trophic cascade from fish to periphyton and macrophytes. To test these hypotheses, more experiments and field measurements comparing the effect of charophytes and angiosperms on water clarity are needed.     

Relationships between Fish Species Abundances and Water Transparency in Hypertrophic Turbid Waters of Temperate Shallow Lakes

We explored the relationships between Secchi disc depth and the abundance of fish species in very shallow, hypertrophic, turbid waters of Pampa Plain lakes, Argentine. We tested whether the abundance of any of the species present was associated with water transparency for lakes where water transparency, as measured by Secchi disc depth, ranged from 0.1 to 0.4 m. Overall, the abundance of five species (Cnesterodon decemmaculatus, Jenynsia multidentata, Corydoras paleatus, Pimelodella laticeps and Odontesthes bonariensis) seemed to be affected by this narrow gradient in water transparency. These findings represent an interesting result for turbid hypertrophic environments where narrow ranges in water transparency are traditionally neglected as important factors for fishes. We show, however, how water transparency patterns may be still important for some species in highly turbid waters with extremely narrow gradients in Secchi disc depth. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Consequences of Fish Kills for Long-Term Trophic Structure in Shallow Lakes: Implications for Theory and Restoration

Fish kills are a common occurrence in shallow, eutrophic lakes, but their ecological consequences, especially in the long term, are poorly understood. We studied the decadal-scale response of two UK shallow lakes to fish kills using a palaeolimnological approach. Eutrophic and turbid Barningham Lake experienced two fish kills in the early 1950s and late 1970s with fish recovering after both events, whereas less eutrophic, macrophyte-dominated Wolterton Lake experienced one kill event in the early 1970s from which fish failed to recover. Our palaeo-data show fish-driven trophic cascade effects across all trophic levels (covering benthic and pelagic species) in both lakes regardless of pre-kill macrophyte coverage and trophic status. In turbid Barningham Lake, similar to long-term studies of biomanipulations in other eutrophic lakes, effects at the macrophyte level are shown to be temporary after the first kill (c. 20 years) and non-existent after the second kill. In plant-dominated Wolterton Lake, permanent fish disappearance failed to halt a long-term pattern of macrophyte community change (for example, loss of charophytes and over-wintering macrophyte species) symptomatic of eutrophication. Important implications for theory and restoration ecology arise from our study. Firstly, our data support ideas of slow eutrophication-driven change in shallow lakes where perturbations are not necessary prerequisites for macrophyte loss. Secondly, the study emphasises a key need for lake managers to reduce external nutrient loading if sustainable and long-term lake restoration is to be achieved. Our research highlights the enormous potential of multi-indicator palaeolimnology and alludes to an important need to consider potential fish kill signatures when interpreting results.     

Macrophyte depth limits in North Island (New Zealand) lakes of differing clarity

The seasonal variation in water clarity, as indicated by the attenuation coefficient for photosynthetically active radiation, K _d (m^-1), was determined by monthly measurements for a year in 9 North Island, New Zealand lakes. K _d varied by a factor of 2 to 3 in 8 of the lakes, and a factor of 5 in one. Annual mean K _d (symbol% MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4saaaa!36BD!\[K\] _d) varied by a factor of approximately 15 between lakes. The maximum depth of water colonized by macrophytes (z _c)was also determined. Values of z _c were in the range 1.5–12.5 m. The relationship z _c =4.34/% MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4saaaa!36BD!\[K\]_d accounted for most (93 percent) of the variability in z _c , indicating that average annual clarity was probably a useful predictor of z _c in lakes in this region. The values of z _c in these North Island lakes were generally greater than values calculated using previously published empirical relationships derived for northern hemisphere groups of lakes. The extent to which these relationships underestimated z _c in the North Island lakes was broadly related to latitude. Estimated average irradiance at z _c in each lake was similar to compensation point irradiances reported previously for freshwater macrophytes.     

Ecological sensitivity of marl lakes to nutrient enrichment: evidence from Hawes Water,UK

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Maximum growing depth of macrophytes in Loch Leven,Scotland, United Kingdom,in relation to historical changes in estimated phosphorus loading

Eutrophication is common in shallow lakes in lowland areas. In their natural state, most shallow lakes would have clear water and a thriving aquatic plant community. However, eutrophication often causes turbid water, high algal productivity, and low species diversity and abundance of submerged macrophytes. A key indicator of the ecological state of lake ecosystems is the maximum growing depth (MGD) of aquatic plants. However, few studies have yet quantified the relationship between changes in external phosphorus (P) input to a lake and associated variation in MGD. This study examines the relationship between these variables in Loch Leven, a shallow, eutrophic loch in Scotland, UK. A baseline MGD value from 1905 and a series of more recent MGD values collected between 1972 and 2006 are compared with estimated P loads over a period of eutrophication and recovery. The results suggest a close relationship between changes in MGD of macrophytes and changes in the external P load to the loch. Variation in MGD reflected the ‘light history’ that submerged macrophytes had been exposed to over the 5-year period prior to sampling, rather than responding to short term, within year, variations in water clarity. This suggests that changes in macrophyte MGD may be a good indicator of overall, long term, changes in water quality that occur during the eutrophication and restoration of shallow lakes.     

Mesocosm experiments on nutrient and fish effects on shallow lake food webs in a Mediterranean climate

1. Nutrient and fish manipulations in mesocosms were carried out on food‐web interactions in a Mediterranean shallow lake in south‐east Spain. Nutrients controlled biomass of phytoplankton and periphyton, while zooplankton, regulated by planktivorous fish, influenced the relative percentages of the dominant phytoplankton species. 2. Phytoplankton species diversity decreased with increasing nutrient concentration and planktivorous fish density. Cyanobacteria grew well in both turbid and clear‐water states. 3. Planktivorous fish increased concentrations of soluble reactive phosphorus (SRP). Larger zooplankters (mostly Ceriodaphnia and copepods) were significantly reduced when fish were present, whereas rotifers increased, after fish removal of cyclopoid predators and other filter feeders (cladocerans, nauplii). The greatest biomass and diversity of zooplankton was found at intermediate nutrient levels, in mesocosms without fish and in the presence of macrophytes. 4. Water level decrease improved underwater light conditions and favoured macrophyte persistence. Submerged macrophytes (Chara spp.) outcompeted algae up to an experimental nutrient loading equivalent to added concentrations of 0.06 mg L^?1 PO₄‐P and 0.6 mg L^?1 NO₃‐N, above which an exponential increase in periphyton biomass and algal turbidity caused characean biomass to decline. 5. Declining water levels during summer favoured plant‐associated rotifer species and chroococcal cyanobacteria. High densities of chroococcal cyanobacteria were related to intermediate nutrient enrichment and the presence of small zooplankton taxa, while filamentous cyanobacteria were relatively more abundant in fishless mesocosms, in which Crustacea were more abundant, and favoured by dim underwater light. 6. Benthic macroinvertebrates increased significantly at intermediate nutrient levels but there was no relationship with planktivorous fish density. 7. The thresholds of nutrient loading and in‐lake P required to avoid a turbid state and maintain submerged macrophytes were lower than those reported from temperate shallow lakes. Mediterranean shallow lakes may remain turbid with little control of zooplankton on algal biomass, as observed in tropical and subtropical lakes. Nutrient loading control and macrophyte conservation appear to be especially important in these systems to maintain high water quality.     

Zooplankton Seasonal Abundance of South AmericanSaline Shallow Lakes

The central provinces of Argentina are characterized by the presence of a high number of shallow lakes, located in endorheic basins, many of which have elevated salinities as well as eutrophic or hypereutrophic condition. The zooplankton of four saline shallow lakes of the province of La Pampa was studied on a monthly basis during a 2‐year period to determine its temporal and spatial variation. The surface of these shallow lakes (<2.5 m depth) varied between 56.8 and 215.9 ha, and some have from 8.4 to 20.8 g · l^–1. The more saline lakes have “clear” water and the less saline lakes “turbid” water. Fishes, Jenynsia multidentata , were present in only two lakes during the last two months of the studied period. The zooplankton was composed of 17 taxa of Rotifera, 5 taxa of Cladocera and 4 taxa of Copepoda. The low diversity and the faunistic composition are characteristic of saline environments. Although the studied lakes share 38% of the species, the faunistic similarity was higher between the two least saline lakes. The lowest diversity was found in the two most saline lakes. All four shallow lakes were characterized by their very high zooplankton density, especially in the least saline lakes (<80000 ind · l^–1). The abundance is significantly correlated with the water transparency but not with salinity. The zooplankton temporal variation was characterized by the alternation of macro‐ and microzooplankton, probably regulated by competition and intrazooplanktonic predation. In each lake, the spatial abundance distribution of the macro‐ and microzooplankton was homogeneous. It was related to the shallow depht of the lakes and their polymictic condition. The Scheffer model on alternative states in shallow lakes acknowledges that it cannot be applied to saline lakes because Daphnia , the main responsible for the clear water state, is not tolerant to high salinity. Our study shows that the most saline lakes, where the halophylic Daphnia menucoensis is abundant, have also the most clear waters. Another difference that we found with regards to the mentioned model is that, in turbid lakes, it could not have had a top‐down control on macrozooplankton exerted by fishes because in these lakes fishes were practically absent. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effects of elevated turbidity on shallow lake fish communities

Synopsis We compared the fish communities of two shallow lakes in the lower Waikato River basin, North Island, New Zealand, to determine the effects of elevated suspended solids (SS) and collapse of submerged macrophytes. Lake Waahi was turbid (20–40 g m^-3 SS) and devoid of submerged macrophytes whereas Lake Whangape was clearer (5 g m^-3 SS) and dominated by submerged macrophytes. The lakes had similar fish species richness and had nine major species in common; representing eight families including Anguillidae, Retropinnidae, Galaxiidae, Eleotridae, Mugilidae, Ictaluridae, Poeciliidae, and Cyprinidae (two species). The only major fish that was absent from Lake Waahi was a lacustrine form of the common smelt, Retropinna retropinna, which disappeared after the lake became turbid in the late 1970s. CPUE, condition, and size of most species in Lake Waahi were similar to, or greater than, those in Lake Whangape. Lake Whangape clearly exceeded Lake Waahi only for CPUE of two species. Within Lake Whangape two species displayed significantly greater condition, and one species greater size, in a turbid arm of the lake than in the main basin. Apart from lacustrine Retropinna retropinna, the fish in these lakes appear well adapted to cope with, or to avoid, the direct toxic effects of suspended and settleable solids on sensitive early developmental stages. In Lake Waahi loss of cover and food provided by submerged macrophytes appears to have been compensated for by increased turbidity and an associated increase in the biomass of the mysid, Tenagomysis chiltoni (a major prey item).     

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.     

Reaction of large and shallow lakes Peipsi and Võrtsjärv to the changes of nutrient loading

More than 20-year monitoring of Estonian rivers reveals that the loading of nitrogen to large shallow lakes Peipsi (3,555 km², mean depth 7.1 m) and Võrtsjärv (270 km², mean depth 2.8 m) decreased substantially in the 1990s. Phosphorus loading decreased to a much smaller extent than nitrogen loading. In L. Võrtsjärv both N and P concentrations followed the decreasing trends of loading, which show the high sensitivity of large shallow lakes to catchment processes. Our study showed a positive relationship between P content in sediments and the relative depth of the lake. Assumingly the resilience of a lake in responding to the reduction of nutrient loading decreases together with the decrease of its relative depth. In L. Peipsi the concentration of P has not decreased since the 1990s. Our data show indirectly that P loading from Russia to L. Peipsi may have increased. The N/P ratio has decreased in both lakes. Cyanobacterial blooms have been common in both lakes already at the beginning of the 20th century. The blooms disappeared during heavy nitrogen loading in the 1980s but started again in L. Peipsi in recent years together with the drop of the N/P ratio. In L. Võrtsjärv the N/P ratio is higher and the ecosystem is more stable although the share of N₂-fixing cyanobacteria increased from the 1990s. Reappearing cyanobacterial blooms in L. Peipsi have caused fish-kills in recent years. In L. Peipsi summer/autumn fish-kills during water-blooms are a straightforward consequence of reduced nitrogen level at remaining high phosphorus level while in L. Võrtsjärv the climatic factors affecting water level are more critical––at low water level winter fish-kills may occur. In L. Võrtsjärv nutrient loading has decreased and water quality has improved, present ecological status seems to be mostly controlled by climatic factors through changes of water level. The most important measure to improve water quality in L. Peipsi would be the reduction of phosphorus loading from both Estonian and Russian subcatchments.     

Submerged macrophytes as indicators of the ecological quality of lakes

1. We analysed submerged macrophyte communities from 300 Danish lakes to determine the efficacy of different species, maximum colonisation depth (C_max) of plants as well as coverage and plant volume inhabited (PVI) as indicators of eutrophication. 2. Most species occurred at a wide range of phosphorus and chlorophyll a (Chla) concentrations, but some species of isoetids (Lobelia, Isoëtes) and Potamogeton (Potamogeton gramineus, Potamogeton alpinus and Potamogeton filiformis) were mainly found at low nutrient concentrations and hence may be considered as indicators of nutrient poor conditions. However, species typically found in nutrient‐rich conditions, such as Elodea canadensis and Potamogeton pectinatus, were also found at total phosphorus (TP) <0.02 mg P L^?1 and Chla <5 μg L^?1 and therefore cannot be considered as reliable indicators of eutrophic conditions. 3. Submerged macrophyte coverage, PVI and the C_max were negatively correlated with TP and Chla. However, variability among lakes was high and no clear thresholds were observed. At TP between 0.03 and 0.07 mg P L^?1 plant coverage in shallow lakes ranged from nearly 0 to 100%, whilst at concentrations between 0.10 and 0.20 mg P L^?1 only 29% of the lakes had coverage >10%. C_max was found to be a useful indicator only in deep lakes with unvegetated areas in the deeper part, whereas the use of coverage was restricted to shallow lakes or shallow areas of deep lakes. 4. Overall, submerged macrophytes responded clearly to eutrophication, but the metrics investigated here showed no well‐defined thresholds. We developed a simple index based on species richness, presence of indicator species, coverage and C_max, which might be used to track major changes in macrophyte communities and for lake classification.     

Clay-Turbid Interactions May Not Cascade—A Reminder for Lake Managers

Food web management is a frequently used lake restoration method, which aims to reduce phytoplankton biomass by strengthening herbivorous zooplankton through reduction of planktivorous fish. However, in clay‐turbid lakes several factors may reduce the effectivity of food web management. Increasing turbidity reduces the effectivity of fish predation and weakens the link between zooplankton and phytoplankton. Therefore, the effects of fish stock manipulations may not cascade to lower trophic levels as expected. Additionally, in clay‐turbid conditions invertebrate predators may coexist in high densities with planktivorous fish and negate the effects of fish reductions. For instance, in the stratifying regions of the clay‐turbid Lake Hiidenvesi, Chaoborus flavicans is the main regulator of cladocerans and occupies the water column throughout the day, although planktivorous Osmerus eperlanus is very abundant. The coexistence of chaoborids and fish is facilitated by a metalimnetic turbidity peak, which prevents efficient predation by fish. In the shallow parts of the lake, chaoborids are absent despite high water turbidity. We suggest that, generally, the importance of invertebrate predators in relation to vertebrate predators may change along turbidity and depth gradients. The importance of fish predation is highest in shallow waters with low turbidity. When water depth increases, the importance of fish in the top‐down regulation of zooplankton declines, whereas that of chaoborids increases, the change along the depth gradient being moderate in clear‐water lakes and steep in highly turbid lakes. Thus, especially deep clay‐turbid lakes may be problematic for implementing food web management as a restoration tool.     

A mosaic community of macrophytes for the ecological remediation of eutrophic shallow lakes

The restoration of macrophytes in eutrophic shallow lakes has been the focus of active research in the past decade. The approach of building up a mosaic community of macrophytes (MCMs) to improve water quality and remediate the ecosystems of lakes is proposed in this paper. Several species of floating, floating-leaved, and submerged macrophytes were introduced in experimental enclosures in eutrophic shallow lakes. These macrophytes were intercropped in small patches and formed mosaic communities of spatial and temporal combinations (spatial and seasonal mosaic patterns) in the lakes. Macrophytes can improve water transparency quickly and the MCM system can stabilize this clear water state over a long time in turbid eutrophic shallow lakes. The constructed MCM created heterogeneous habitats that are favourable for different macrophytes and for the growth and succession of other organisms, as well as for removing water pollutants. When the eutrophic water flowed through the MCM system at a retention time of 7 days, the removal efficiency rates of the MCM system for algae biomass, NH₄⁺–N, TN, TP and PO₄^3?–P were 58%, 66%, 60%, 72% and 80%, respectively.