Eutrophication of Lakes and Reservoirs

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V Istvanovics, Budapest University of Technology and Economics, Budapest, Hungary

© 2009 Elsevier Inc. All rights reserved.

Definition

Eutrophication is the process of enrichment of waters with excess plant nutrients, primarily phosphorus and nitrogen, which leads to enhanced growth of algae, periphyton, or macrophytes. Abundant plant growth produces an undesirable disturbance to the balance of organisms (structural and functional changes, decrease in biodiversity, higher chance for invasions, fish kills, etc.) and to the quality of water (cyanobacterial blooms, depletion of oxygen, liberation of corrosive gases, and toxins, etc.).

Introduction

Trophic status of a water body can roughly be assessed by using information about the concentration of the limiting nutrient (phosphorus), chlorophyll (an indicator of phytoplankton biomass), and transparency (dependent on both algal biomass and sediment resuspension, expressed as Secchi depth). The most widely accepted limits are those suggested by the Organization for Economic Cooperation and Development (OECD):

Trophic

Mean total,

Mean

Max.

Mean

category

Pfrgr1)

(mg chl-a l

-1) (mg chl-a l1)

Secchi

depth (m)

Oligotrophic

<10

<2.5

<8

>6

Mesotrophic

10-35

2.5-8

8-25

6-3

Eutrophic

>35

>8

>25

<3

During the last four decades, eutrophication has undoubtedly been the most challenging global threat to the quality of our freshwater resources. Survey of the International Lake Environmental Committee has indicated in the early 1990s that some 40-50% of lakes and reservoirs are eutrophicated. Many of these water bodies are extremely important for drinking water supply, recreation, fishery, and other economic purposes. Developing countries in hot and dry climate face particularly serious and rapidly increasing eutrophication-related ecological and economic challenges. Unlike this paper, eutrophication is not restricted to lakes and reservoirs. Large rivers, estuaries, coastal zones, and inland seas are also subject to this undesirable process.

Although nutrient cycling is far from being closed even in the most developed societies, eutrophication has been successfully reversed in several lakes by managing human nutrient emission (low-P detergents, P precipitation at sewage treatment plants, decreased fertilizer application, erosion control, etc.) and by cutting off nutrient loads to recipients (sewage diversion, buffer zones, etc.). Thus, eutrophication seems to be reversible. Nutrient management, however, does not result in an immediate oligotrophication (i.e., reversal of eutrophication) because of the resilience of the aquatic ecosystem. Various methods of in-lake physical, chemical, and biological interventions have been developed to facilitate the efficiency of load reduction, shorten the delay in recovery, and accelerate the rate of reversal. Early recognition of eutrophication, understanding the nutrient load-trophic response relationship in individual lakes, and planning the most suitable combination of management measures require an in-depth knowledge of functioning of aquatic ecosystems.

Basics of Eutrophication

Water bodies are open systems, in which autotrophic organisms, mainly algae and macrophytes, convert inorganic carbon into organic matter using the energy of solar radiation. In addition to C, other nutrients (oxygen, hydrogen, nitrogen, phosphorus, sulfur, silica, trace metals) are universally or taxon-specifically needed for primary production. The German chemist, von Liebig recognized in the nineteenth century that the yield of a plant is proportional to the amount of nutrient, which is available in the lowest supply relative to the plant's demand. This observation is known as the Law of the Minimum. Studying the elemental composition of phytoplankton in the English Channel, Redfield found that healthy algae contained C, N, and P atoms in an average ratio of 106:16:1 (40:7:1 by weight). This means that 1 g of P available in the water allows the algae to assimilate 40 g of C and thus increase fresh weight by about 400 g.

Algae, like any other plants, require nutrients in ratios, which are radically different from the elemental composition of their environment. In freshwater, the mid-summer average demand to supply ratio of P is up to 800 000; that of N is 300 000; that of C is 6000; and that of all other elements is below 1000. Consequently, P and N are the nutrients, which most often determine the carrying capacity of lakes and reservoirs. This has been verified experimentally during the 1970s by

Schindler and his team. In the Canadian Experimental Lake Area, untreated lakes were compared with nutrient enriched ones. Standing crop of algae increased with the supply of P. Nitrogen addition alone failed to similarly trigger phytoplankton growth.

Another large-scale study of the OECD (1982) also confirmed the biomass-limiting role of P by the means of statistical modeling. This study considered over 120, mostly temperate lakes and reservoirs. Mean in-lake concentration of total nutrients was calculated from the annual load corrected for flushing. Annual mean and maximum concentration of chlorophyll was regressed against the estimated nutrient concentration. The relations were highly significant for P. In most lakes, biomass was determined by the amount of P, while N was the controlling factor in only a few cases.

Chemical nature of major N and P ions explain why P determines the carrying capacity of lakes more often than N. Both nitrate and ammonium are much more mobile in soils and sediments than orthophosphate, since the latter is chemisorbed by clay minerals, iron(III) oxy-hydroxides, carbonates, etc. As a consequence, nitrogen compounds are preferentially washed out from the soils and phytoplankton in pristine temperate lakes tends to be P limited. For the same reason, terrestrial plants are most often N limited. In productive lowland wetlands dominated by dense stands of emergent plants that release O2 to the atmosphere, anaerobic conditions may develop in the water and denitrification speeds up. In such systems N deficiency prevails as frequently indicated by the presence of insectivorous plants. Carrying capacity of many subtropical and tropical lakes are also N determined because of the intense denitrification characteristic under warm climates.

However, widespread nutrient limitation is not a universal phenomenon in freshwaters. Besides nutrients, algal growth requires sufficient time and light. In several water bodies, biomass is determined by the availability of these latter factors. Such conditions prevail in running waters in which inorganic nutrients are usually in excess. To support abundant plankton, a river must either be sufficiently long or flow sufficiently slowly to permit seven to eight cell divisions required to raise an inoculum of several cells per milliliter to the order of a few thousand cells per milliliter. When the hydraulic residence time is increased by reservoir construction, the time available for 'undisturbed' algal growth and thus, the biomass may increase without any further nutrient enrichment. In addition to this, enhanced sedimentation within mainstream reservoirs improves light availability both in and downstream of the reservoir. Construction of a series of reservoirs on the upper Danube was recognized as a key factor in downstream eutrophica-tion of this large river.

Eutrophication is not merely an increase in the biomass of various organisms. Structural and functional changes accompany, and - as careful long term observations repeatedly testify - even precede quantitative changes at each trophic level.

Changes in Primary Producers

One of the most conspicuous and the best-known changes associated with eutrophication is the mass development of cyanobacteria, be it N2-fixing (Ana-baena, Aphanizomenon, Cylindrospermopsis, etc.) or nonfixing (Planktothrix, Microcystis, etc.). Many species may develop toxic strains (e.g., M. aeruginosa, Cylindrospermopsis raciborskii) and thus, large blooms may directly harm both other aquatic organisms and humans. An enormous scientific literature discusses the ecological traits leading to mass development of Cyanobacteria as well as their manifold influence on the functioning of the aquatic ecosystem. Without going into details, one can recognize three basic lines of adaptations in the background of cyano-bacterial success. Each bloom-forming species possesses a certain combination of the following traits:

1. Nutrients. Bloom-forming Cyanobacteria are capable of exploiting nutrient reserves that are unavailable, or not readily available, for most other algae. This is the reason why annual mean concentration of chlorophyll may show an abrupt rise upon the establishment of Cyanobacteria at high but 'steady' external nutrient load. Since the inorganic carbon concentrating mechanisms are highly efficient in the case of both CO2 and HCO3, Cyanobacteria continue to assimilate at high pH. Many species have high affinity for NH 4 uptake besides the ability to fix N2. Either fast maximum rate of P uptake, or exploitation of vertical nutrient gradients, facilitates P acquisition, thanks to buoyancy regulation. The relatively large size allows the storage of considerable amounts of excess C, N, and P beyond the actual needs of growth, thereby providing independence from the fluctuating supply.

2. Light. The lowest light saturation values of both photosynthesis and growth have been observed among bloom-forming cyanobacteria (e.g., Plankto-thrix rubescens, Cylindrospermopsis raciborskii). In general, their light requirement tends to be lower than that of the eukaryotic algae. Under calm conditions, buoyancy regulation allows optimal positioning of Cyanobacteria in the light gradient. Simple prokary-otic structure results in relatively low maintenance costs that both decreases the light demand and leaves more energy to acquire the limiting nutrient.

3. Low biomass loss. Buoyancy regulation prevents sinking loss of healthy Cyanobacteria even under calm conditions. Large size and morphology (large colonies, filaments) reduce zooplankton grazing to negligible levels. The decreased loss of Cyanobacteria is equivalent to the slowdown of nutrient regeneration and diminished internal supply of the limiting nutrient.

Summer blooms of Cyanobacteria cause a major shift in the seasonal pattern of phytoplankton biomass in temperate lakes. In oligo- and mesotrophic lakes, the biomass maximum occurs during the spring when temperature and light increase rapidly, and relatively large amounts of nutrients are delivered into the water by spring floods as well as the spring overturn in deep lakes. In comparison, the summer biomass of phytoplankton is lower as a result of the diminishing external and internal nutrient supply. In productive lakes, increased nutrient availability differentially enhances summer production and leads to a virtually monomodal temporal distribution of biomass with a summer maximum.

The most important functional changes associated with the dominance of bloom-forming Cyanobac-teria are (1) the involvement of formerly unavailable resources into aquatic production, (2) the decrease in the rate of turnover of nutrients, most importantly in that of the limiting ones, and (3) the decrease in the efficiency of energy transfer from primary producers to higher trophic levels. Because of these self-stabilizing functional changes, the shift from the noncyanobacteria dominated to Cyanobac-teria dominated late summer phytoplankton assemblages can be seen as alternative stable states of the aquatic ecosystem. An important manifestation of the alternative stable states is the hysteresis that occurs when the system is forced from the one state to the other.

Establishment of bloom-forming Cyanobacteria is the last step in the course of restructuring of phyto-plankton during eutrophication. Case Study 1 summarizes the history of compositional changes observed in a large, shallow temperate lake.

Submerged macrophytes may cover a significant portion of the area of shallow lakes that are protected from strong wave action or the small surface area of which restricts wind fetch. During eutrophication, such lakes may abruptly turn from a macrophyte-dominated clear water state into a phytoplankton-dominated turbid state. Similar to the case of Cyanobacteria, both states may prevail in a broad range of external nutrient loads since a number of feedback mechanisms stabilize the actual state. Thus, in the clear water state abundant macrophyte stands prevent sediment resuspension by dampening wind work and provide shelter for zooplankton. Visual predatory fish keep efficient control on planktivorous fish, thereby promoting the growth of zooplankton. In turn, grazing may significantly influence growth, biomass, and succession of phyto-plankton. In the turbid state, intense resuspension and shading by phytoplankton inhibits macrophyte growth and suppresses foraging of predatory fish. In the lack of sufficient refuge, zooplankton fall victim to plankti-vorous fish and phytoplankton are released from the top-down control.

Changes in Consumers

Increased primary production supports an elevated production and biomass of consumers. Structural changes in the phytoplankton, first of all those in the size distribution, exert a strong influence on pelagic herbivores by differentially favoring or suppressing one or the other group of zooplankton. Since detritus and associated bacteria make up the main food of most benthic invertebrates, the zoobenthos is less sensitive to eutrophication-related changes in algal composition. At the same time, presence of aquatic macrophytes is beneficial for both pelagic and benthic consumers because of either the mere maintenance of habitat patchiness or to more specific biotic interactions. The retreat of macrophytes during eutrophication may result in a drastic reduction in the species diversity of consumers.

The four main groups of freshwater zooplankton -protists, rotifers, cladocerans and copepods - partition food primarily on the basis of size. Most aquatic grazers consume any particles in the appropriate size range, be it algae, bacteria, another grazer, detritus, or inorganic particle. Size-selective predation by plankti-vorous fish inserts a top-down control on grazer populations, the larger zooplankton being more vulnerable to predation. In addition to the direct biological interactions, deterrent environmental effects of eutrophica-tion, including elevated turbidity, magnified daily and seasonal fluctuations in the oxygen concentration and pH may adversely affect the sensitive groups of zooplankton. Intense grazing alters species composition of phytoplankton by both selective removal of edible algae and nutrient regeneration. Because of this intricate net of interactions, eutrophication-related changes in the biomass, composition, size distribution, and seasonal pattern of various groups of zooplankton are highly lake-specific. The forthcoming discussion is restricted to a few general trends.

The most abundant freshwater zooplankters are crustaceans. Most cladocerans, such as Daphnia, are filter feeders whereas most cyclopoid and calanoid copepods are selective, raptorial grazers. In oligo-trophic lakes, the maximum community grazing rate is below 15% while in eutrophic lakes the entire volume of water can be filtered up to 4-5 times in a day (400-500%). Daphnia and other cladocerans usually account for up to 80% of the community grazing rate.

Filter feeders collect particulate matter from the water. Anatomy of feeding appendages prevents collecting particles smaller than 0.8 mm. The upper limit to ingestible particle size increase with the size of the animal up to about 45 mm in the case of spherical particles and larger in the case of nonspherical ones. Edible algae are small, naked green algae, nanoflagel-lates, cryptomonads, and certain diatoms. Algae that cannot be ingested are large, have biologically resistant cell walls and spines, or form colonies. Large, unicellular desmids and dinoflagellates, chain-forming diatoms, and colonial Cyanobacteria respresent this group. The copepods are inefficient at retaining small particles but can process much larger algae than cladocerans. Some copepods, for example, can break up chains of diatoms.

In oligo- and mesotrophic lakes, the spring bloom of phytoplankton is made up by small, fast growing, edible algae such as small diatoms, nanoflagellates, and small greens. This favors the growth of nonselec-tive cladocerans that - similar to their prey - are specialists at fast reproduction by parthenogenesis. Seasonal succession proceeds towards summer and autumn associations of large, slowly growing species such as dinoflagellates, gelatinous greens, colonial or filamentous Cyanobacteria. With the increasing abundance of large or noxious algae, slower growing but more selective copepods become more common. Collapse of algal blooms is associated with an increased availability of detritus and bacteria that constitute an appropriate food for cladocerans. Smaller blooms of diatoms and nanoplankton (<60 mm) during the autumn also promote the growth of filter feeders. As a general trend, the aquatic food chain gradually shifts during the succession from one based on living algae to one based on bacteria and detritus.

Compared with that in unfertile lakes, the shift from the dominance of nanoplankton to that of larger 'net' plankton takes place early in the season in eutrophic lakes. Moreover, filamentous Cyanobacteria that take over the dominance in the summer assemblages inhibit filtration of nonselective grazers by clogging up the feeding appendages. Thus, the share of grazing-resistant algae increases during eutrophication, accelerating the shift toward a bacteria- and detritus-based pelagic food chain that may result in an increasing dominance of cladocerans.

The tendency of an eutrophication-related increase in the cladoceran to copepod ratio is best seen in deep, stratified lakes. In naturally turbid reservoirs and shallow lakes zooplankton inherently rely to a greater extent on detritus and associated bacteria than in deep ones of the same trophic state. With increasing productivity, however, enhanced detritus availability disrupts the natural balance between pelagic and benthic food webs and the share of the latter increases in the total energy budget of both shallow and deep lakes.

The zoobenthos is an extremely diverse group comprising nearly all phyla from protists through large macroinvertebrates to vertebrates. Similar to the zooplankton, eutrophication-related changes in the composition, biomass, and seasonal dynamics of benthic assemblages are determined by food availability, predation, and indirect environmental effects, particularly oxygen concentration. Oxygen conditions in the hypo-limnion or in the uppermost sediment layer strongly depend on the downward flux of detritus, and the tolerance to low oxygen is extremely variable among benthic organisms. One of the pioneering discoveries of limnology was in the early twentieth century that the profundal benthic fauna is an excellent indicator of nutrient richness in deep lakes. Naumann postulated a direct relationship between phytoplankton and nutrient conditions in lakes and contrasted the extreme ends as 'eutrophic' (well-nourished) and 'oligotrophic' (poorly nourished). Thienemann recognized two lake types based on hypolimnetic oxygen concentrations and on correlated differences in the benthic chironomid fauna. The oligotrophic-eutrophic paradigm emerged originally from the crossing of these two lines of research. The oligotrophic water was deep with low nutrient supply, low algal production in the epilimnion, and low flux of detritus to the hypolimnion. Because of the small oxygen consumption, the hypolimnion had an orthograde oxygen profile and the corresponding stenoxybiont benthic fauna exploited mainly by white-fish (Coregonus). The eutrophic type was relatively shallow and nutrient-rich, water blooms appeared in the summer. The high flux of detritus to the shallow hypolimnion resulted in a fast depletion of oxygen. The oxygen profile was clinograde, and a euryoxybiont Chironomus fauna dominated the benthos. This paradigm had been rephrased during the 1950s and 1960s when eutrophication became a recognized environmental threat in the developed countries.

The two dominant groups of zoobenthos are the oligochaete worms and the dipteran chironomids. Although some oligochaetes are restricted to oligotro-phic waters, the tubificids can be extremely abundant in highly eutrophicated as well as in organically polluted lakes if some oxygen is available from time to time and toxic products of anaerobic metabolism do not accumulate in large quantities. In such systems, they benefit from both the rich nutrient supply and the lack of competition with other benthic animals that cannot tolerate the poor oxygen conditions. In contrast to the tubificids, relative abundance of the more oxygen demanding chironomid larvae decreases during eutrophication.

In deep, mesotrophic lakes the biomass of zoo-benthos exhibits two maxima: (1) a diverse fauna with high oxygen demand inhabits the littoral sediments and (2) less rich assemblages of species that tolerate low oxygen characterize the lower profundal zone. With increasing productivity, the littoral zone may loose its heterogeneity that results in a single pro-fundal biomass maximum of the zoobenthos. A further increase in fertility and the associated hypolimnetic oxygen deficit may then lead to the decline in the biomass of the benthic fauna in the profundal zone, too.

Restructuring of the zooplankton and zoobenthos during eutrophication should not be perceived as a chain of smooth transitions. On the contrary, abrupt compositional shifts may be associated with stepping over the threshold values of critical environmental variables, including habitat patchiness, food availability, or simple physical and chemical factors. Case Study 2 demonstrates the threshold effect of food availability in the compositional change of the chironomid fauna in a large, shallow lake.

The shift from the pelagic to the benthic food web implies basic alterations in the functioning of the freshwater ecosystem. On the one hand, the consumer control of phytoplankton diminishes for two reasons. First, the diet of zooplankton relies to a much greater extent on living algae than that of ben-thic animals. Second, a major change occurs in the seasonal pattern and magnitude of nutrient regeneration. The zooplankton regenerate nutrients directly into the trophogenic zone, even though the sedimentation of fecal pellets represents a net outward flux of nutrients from the epilimnion to the sediments. In contrast to this, benthic animals release nutrients to the sediments from where the flux to the water is primarily regulated by abiotic factors. When, however, large oligochaetes or chironomids are present in high densities, their burrowing activity considerably enhances the sediment-water exchange rates of various nutrients (O, N, P, etc.). In shallow lakes and in the littoral zone of deep ones, bioturbation may result in a significant increase in the internal load of nutrients. On the other hand, the annual production to biomass ratios are higher among planktivorous fishes than among bentivorous ones. Therefore, the overall efficiency of energy utilization decreases in the lake with the increasing proportion of the benthic food web.

Although the total biomass of fish tends to increase during eutrophication, the species composition shifts in undesirable directions. Most conspicuously, the relative abundance of visual predators drops drastically with increasing turbidity. This change was repeatedly shown to cascade down along the food chain to the phytoplankton. Unbalanced food availability may lead to enhanced mortality of the fry. Widely fluctuating oxygen conditions may result in mass killing of fish. Although intense fish production requires highly productive lakes, maintenance of a diverse native fish fauna conflicts with eutrophication.

Oligotrophication

Comprehensive studies of lake recovery from eutro-phication have shown that the trophic status of lakes is not a linear function of the external nutrient load. The same external load supports a higher biomass during oligotrophication than during eutrophication, that is a hysteresis can be observed (Figure 1). Sas and his team recognized four stages of recovery by examining 18 eutrophicated deep and shallow Western European lakes during the 1990s (Figure 1).

During Stage 1, the excess of available P is flushed out from the lake without any reduction in the standing crop of algae. This stage can be observed in lakes that received sufficiently high P loads for sufficiently long periods, and thus, phytoplankton biomass has no more been P-dependent. In such lakes, a considerable time may pass before P regains its biomass-limiting role. In deep lakes, the delay is essentially a function of the

Hysteresis (internal P load, biological resiliance)

Unused P capacity s a

io bi

Stage 1 No response

Hysteresis (internal P load, biological resiliance)

io bi

Stage 1 No response

Stage 2 Behavioral response

Stage 2 Behavioral response

Stage 3 Biomass Stage response Compositional response

External phosphorus load

Figure 1 Schematic representation of phytoplankton biomass as a function of the external nutrient load during eutrophication and oligotrophication with the four stages of lake recovery.

hydraulic residence time. In shallow lakes, where the excess of available P accumulates in the sediments, diagenetic processes and burial of P in deeper sediments are as, or even more important than flushing. A net annual internal P load is common among shallow lakes during the first years following the reduction of external load, whereas it is rare in the prerestoration period and does not occur in deep lakes.

During Stage 2, increasingly P limited algae disperse to greater depths in order to exploit vertical nutrient gradients characteristic of stratified lakes. As a result, transparency increases rapidly in spite of a negligible decrease in algal biomass. Shallow lakes do not provide such refuge options and therefore Stage 2 is lacking.

Stage 3 is the period of significant biomass decrease that occurs in proportion to load reduction. This phase is usually faster and more pronounced in stratified than in shallow lakes. The reason is the elevated internal load from the surface sediments of shallow lakes keep the 'memory' of past loading conditions.

In Stage 4, species composition of phytoplankton changes and a new steady state is established. Both absolute and relative abundance of Cyanobacteria decreases. Blooms disappear; species diversity and stability of the system increase. The delay in the retreat of Cyanobacteria may depend on the life history of species, gaining dominance during eutrophi-cation in various types of lakes.

The data of Sas (1989) allowed investigating the reaction of perennial Cyanobacteria, like Plank-tothrix. Suppression of periodically planktonic species may take a longer time than that of the perennial ones. Akinetes and colonies of many common bloom-forming genera (e.g., Aphanizomenon, Anabaena, Microcystis, Gloeotrichia) overwinter in the sediments. In most species, a small portion (up to 5-6%) of the benthic forms is required to initiate the growth of the planktonic population next year. Thus, the benthic reserves of Cyanobacteria that might increase orders of magnitude during eutrophication may inoculate the water for many years after substantial reduction in the external load. Case Study 1 also describes the behavior of periodically planktonic Cyanobacteria in a recovering shallow lake.

One can complete the earlier-mentioned list by adding Stage 5, which covers compositional changes at higher trophic levels. Similar to the restructuring of phytoplankton, delayed responses and hysteretic effects are characteristic of this process, too (cf. Case Study 2). For this reason, reduction of the external nutrient loads is only the first and most crucial step during eutrophication management in lakes, particularly in shallow ones. A series of measures have been developed that aim at speeding up recovery after load reduction by manipulating biotic interactions within the lake. If, however, load reduction is analogous to stopping a tooth, biomanipulation is certainly related to brain surgery.

Case Study 1

Three circumstances make large (596 km2), shallow (zmean = 3.1m) Lake Balaton an excellent case study when studying eutrophication-related changes in phytoplankton. First, floristic studies date back to the end of the nineteenth century, while Sebestyijn initiated quantitative phytoplankton research in the 1930s. Second, due to the specific morphological features (elongated shape, the main tributary enters at the southwestern end, the only outflow starts at the opposite end, relatively closed large-scale circulation patterns develop in the four basins under the influence of the dominant winds; Figure 2), the western areas become hypertrophic during the 1970s while the eastern ones remained mesotrophic. Third, after reducing the external P load by about 50% from 1.3 to 0.7 mg P m~2 day-1 during the late 1980s and early 1990s, the lake recovered surprisingly fast.

Although the development in the methods of phyto-plankton counting somewhat biases the comparability of long-term data, neither eutrophication nor oligo-trophication has substantially affected the eukaryotic algal flora. In the same time, appearance, disappearance, and reoccurrences of Cyanobacteria, especially Nostocales, have been detected during rapid changes in trophic conditions. Prior to eutrophication, three Aphanizomenon spp. (A. flos-aquae, A. klebahnii, A. gracile) and four or less Anabaena spp. represented the order in Lake Balaton. During the period of rapid eutrophication in the 1970s, many new species appeared abruptly in the flora, including A. aphanizo-menoides, A. issatschenkoi, Anabaenopsis elenkinii, Cylindrospermopsis raciborskii, and Rhaphidiopsis mediterranea. Most cyanobacteria present in the pre-eutrophication period maintained at least modest populations but A. gracile became virtually extinct. In the 1980s, when the ecosystem 'stabilized' at a high trophic level, the only newcomer was Anabaena contorta. Fast oligotrophication from the mid-1990s was coupled with frequent appearance of new species (Anabaena compacta, Anabena circinalis, Aphanizo-menon hungaricus, Anabaenopsis cunningtonii, Kom-vophoron constrictum), disappearance of some 'eutrophic' ones (Anabaenopsis elenkinii, Rhaphidiop-sis mediterranea), and reappearance of Aphanizome-non gracile.

A definite increase had already been observed in the biomass of the dominant summer alga, Ceratium i—Canal

WWTP with P precipitation ^^ Sewage diversion

Basin 1

2.3

3B

2750

Basin 2

2.9

144

1647

Basin 3

3.2

1B6

534

Basin 4

3.7

22B

249

Lake total:

596

51B0

WWTP with P precipitation ^^ Sewage diversion

Figure 2 Lake Balaton and its catchment. The most important management measures are also indicated. h, average water depth; a, surface area; aw, corresponding subwatershed area; WWTP, waste water treatment plant.

hirundinella between the 1930s and the 1950s. The annual mean biomass of phytoplankton increased from about 0.3 mg fresh weight per liter by a factor of 4. Up to the mid 1970, Cyclotella bodanica and C. ocellata dominated the spring plankton. Thereafter the abundance of pennate diatoms (Synedra acus and Nitzschia acicularis) increased conspicuously.

In the eastern areas of the lake, the biomass of summer phytoplankton showed a moderate further increase during the period of eutrophication compared with the 1950s, but annual maxima did not exceed 5mg l"1. In the same time, maxima reached 40-60 mg l"1 in the western areas during the early 1980. Interannual differences in biomass among successive years increased substantially in both areas. Prior to eutrophication, late summer assemblages were dominated by Ceratium hirundinella, Aphanizo-menon klebahnii, and Snowella lacustris. Depending on the wind regime, various meroplanctonic diatoms (Aulacoseira granulata, Cyclotella radiosa, C. ocel-lata, small Navicula and Nitzschia spp.) contributed significantly; in very windy years they could dominate. These species, however, have been gradually replaced by Cyanobacteria during eutrophication. The key cyanobacterium, Cylindrospermopsis raci-borskii is a subtropical species the akinetes of which require exceptionally high and narrow temperature range (22-24 °C) for germination when compared with other Cyanobacteria present in Lake Balaton. Warm conditions are incidental in this temperate lake, and this is certainly one of the main factors leading to the irregularity of C. raciborskii blooms.

C. raciborskii was first detected in Lake Balaton in 1978, and it produced its first large bloom in 1982.

This bloom was exceptional compared with cyano-bacterial blooms during the 1970s in two respects: (1) the maximum biomass exceeded those of the previous blooms by a factor of about 2, and (2) with a delay of about 3 weeks, the blooms spread to the mesotrophic areas. The external nutrient load was known in the westernmost basin from daily measurements since 1975. These data evidenced that C. raci-borskii achieved a much higher biomass than other N2-fixing Cyanobacteria under relatively constant external load conditions. This is clearly indicative of a superior exploitation of the available resources by C. raciborskii. Simple mass balance models indicated that in summers of C. raciborskii dominance, the internal P load was much higher than in other years. Indirect evidence suggests that it is not the enhanced internal P load that induces the blooms, but presence of C. raciborskii leads to an elevated internal load. Eastward extension of C. raciborskii blooms has repeatedly been observed in subsequent years including 1992 and 1994, when the external loads have already been reduced close to the present levels. It has been shown that dispersion along the longitudinal axis may result in an inoculation of the mesotrophic eastern areas from the eutrophic western ones.

Case Study 2

The water of shallow, wind-exposed and highly calcareous Lake Balaton is always turbid, the mean vertical light attenuation coefficient varies between 2 and 4 m"1. Gut content analysis of zooplankton revealed that grazers collect huge amounts of inorganic ballast and they starve even if the table is set in a near-optimal way. Because of this geomorphological drawback, community grazing rate of zooplankton is low (usually <5%), and phytoplankton production is channeled to fish primarily by zoobenthos. Investigation of the benthic macrofauna was rather sporadic in Lake Balaton before the mid-1990s. This sporadic information had to be supplemented with paleolim-nological data, as well as with comparative data obtained along the west-east trophic gradient in order to reconstruct composition and biomass of the benthic macrofauna during the preeutrophication period.

Gastropods (Potamopyrgus jenkinsi, Lithoglyphus naticoides) comprised a quantitatively important group of the macrobenthos until about the middle of the twentieth century. In the late 1990, however, no living gastropods could be collected during a detailed, long-term zoobenthos survey. Disappearance of gastropods started shortly after the invasion of Dreissena polymorpha in the early 1940s, most probably independent of eutrophication. At the present time, chir-onomids and oligochaetes are the dominant groups of the benthos. Tubificidae, however, make up less than 20% of the total benthic biomass.

Of the more than 50 chironomid taxa known from Lake Balaton, only seven can be found in the profun-dal sediments. Three of the latter species, Procladius cf. choreus, Tanypus punctipennis, and Chironomus balatonicus give 80-90% of the biomass and annual production. These three species responded specifically to eutrophication and oligotrophication, and the reaction could sufficiently be explained by differences in their feeding habits.

Procladius choreus is a predator. It is able to utilize a wide range of food, including macrobenthic animals, zooplankton, microphytobenthos, and detritus, while the most important food is meiozoobenthos. As a consequence, the biomass of P. choreus was more or less independent of the changes in trophic conditions both in time (eutrophication and oligotrophication) and in space (the west-east trophic gradient).

Tanypus punctipennis preys other chironomid larvae, but it may also ingests a great deal of plant material and detritus. Its biomass increases towards the mesotrophic areas of Lake Balaton. Prior to eutro-phication and in the mesotrophic eastern areas, a Procladius-Tanypus chironomid community was characteristic of Lake Balaton. The biomass of this community showed only a weak positive correlation with the biomass of phytoplankton.

Chironomus larvae are filter- and surface deposit feeders that take the advantage of the large amount of fresh detritus sedimenting after major algal blooms. In this way, their biomass and production strongly depends on the biomass of phytoplankton. In Lake

Balaton, there was a strong positive correlation between the spring biomass of Chironomus-dominated benthic community and the late summer phytoplank-ton biomass in the previous year. The relationship, however, was not linear. Chironomus larvae showed a presence-absence type response. When the late summer algal biomass exceeds 20-30 mg chl-a l-1, that is the annual primary production is higher than 220-250 g C m-2 year-1, Chironomus dominates the zoobenthos in the next year and the chironomid biomass attains high values (0.6-3.4gm-2). Below this level of primary production, Chironomus is absent, Tanypodinae dominate the zoobenthos, and the total benthic biomass varies from 0.3 to 0.5 gm-2.

One of the reasons why Chironomus depends so strongly on the last year's primary production in Lake Balaton is the low organic content (2-4%) of the sediments. This is due to the frequent resuspension that keeps the water and surface sediments permanently aerobic, thus enhancing bacterial decomposition. It was estimated that only 1.1-3.2% of the primary production was assimilated by benthic chir-onomids. Fast aerobic decomposition may also explain that chironomid densities are rather low in Lake Balaton when compared with other European lakes of similar trophic state.

The threshold-like shift in the dominance of Tany-pus to Chironomus with increasing trophy is accompanied by a 2.5-fold increase in energy transfer efficiency from the phytoplankton to chironomid fauna. The reason is that phytodetritiphagous Chiro-nomus directly harvests primary producers, while a trophic loop of one or two steps length connects predatory Tanypus to algae. Variability of the energy transfer efficiency is of vital importance for benthi-vorous fish. For example, stock density and growth rate of the dominant fish species, the common bream (Abramis brama) is basically influenced by chirono-mid production.

See also: Effects of Climate Change on Lakes; Lake Management, Criteria.

Further Reading

Istvanovics V and Somlyody L (2001) Factors influencing lake recovery from eutrophication—The case of basin 1 of Lake Balaton. Water Research 35: 729-735.

Istvanovics V, Clement A, Somlyody L, Specziar A, G-Toth L, and Padisak J (2007) Updating water quality targets for shallow Lake Balaton (Hungary), recovering from eutrophication. Hydrobio-logia 581: 305-318.

Padisak J and Reynolds CS (1998) Selection of phytoplankton associations in Lake Balaton, Hungary, in response to eutrophication and restoration measures, with special reference to the Cyano-prokaryotes. Hydrobiologia 384: 41-53.

Porter KG (1977) The plant-animal interface in freshwater ecosystems. American Scientist 65: 159-170.

Reynolds CS, Dokulil M, and Padisak J (eds.) (2000) The Trophic Spectrum Revisited. Springer.

Sas H (1989) Lake Restoration and Reduction of Nutrient Loading: Expectations, Experiences, Extrapolations. St Augustin: Academia Verlag Richarz.

Scheffer M, Hosper SH, Meijer M-L, Moss B, and Jeppesen E (1993) Alternative equilibria in shallow lakes. Trends in Ecology and Evolution 8: 275-279.

Schindler DW (1974) Eutrophication and recovery in experimental lakes: Implications for lake management. Science 184: 897-899.

Specziar A and Voros L (2001) Long-term dynamics of Lake Balaton's chironomid fauna and its dependence on the phytoplankton production. Archive für Hydrobiologie 152: 119-142.

Vollenweider RA and Kerekes JJ (1982) Background and summary results of the OECD cooperative programme on eutrophication. OECD report, Paris. Wetzel RG (2001) Limnology. Lake and River Ecosystems. San Diego: Academic Press.

Relevant Websites

http://www.ceep-phosphates.org (European Union Eutrophication Guidance Document information. (2006). SCOPE Newsletter No. 64). http://www.ilec.or.jp/eg/ http://www.umanitoba.ca/institutes/fisheries/

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  • anne
    How eutrophication affect light zonation in lake and reservoir?
    1 year ago

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