Lehmusluoto, P.
Expedition Indodanau, University of Helsinki, Finland
Mailing Address: Leppätie 4 A, FI-00780 Helsinki, FinlandTel: +358 (440) 484 066 E-mail: [email protected]
Background
Natural watershed depression lakes originate and were generally formed during the ice ages by tectonic or volcanic activities. In contrast, river-bottom lakes like floodplains, flooded forests and the man-induced development activities create the ever-increasing number of reservoirs and artificial water impoundments. The relation of lakes to the other terrestrial and aquatic water systems is illustrated in Figure 1.
Figure 1. Terrestrial and aquatic water systems (left) and two types of deep lakes, fully circulating and partially circulating internally loaded meromictic lakes (right) Forbes already in 1887 understood that “watershed” lakes are ancient isolated equilibriums and independent of the surrounding land and “river-bottom” lakes are appendages of rivers.
Some 100 years ago it was recognized that the temperature-dependent hydrodynamic and biogeochemical functions are driven by seasonal temperature changes and in the peculiarly behaving tropical lakes they are mostly irregularly wind-driven.
High temperature and the thermal barrier (metalimnion) between the upper water layer and deep water are important in structuring and distribution of substances and interrelationships and interaction of the ecosystem and bioactivity.
Watersheds of the river-dependent reservoirs are much larger in relation to the water surface area of the natural lakes. Because reservoirs are formed almost always in river valleys, the basins are usually narrow and elongated reservoir continuums from the riverine to transitional and to lake-like lacustrine conditions nearest the dam.
Whether naturally or artificially created, lakes share several common physical, chemical and biological characteristics, and the lake equilibriums alter, age and gradually dry out during the years. Lately land-use and agriculture related runoff, communal wastewaters and industrial effluents have taken a dominant polluter role with increasing consequences, such as siltation, eutrophication and bioaccumulation of harmful substances.
Watershed with water in a natural lake or made reservoir
+
Water as physical, chemical and biological intermediary
=
Watershed and natural lake/ made reservoir as an ecosystem
Figure 2. Building blocks, watershed and water (left) and the hydrodynamically and biogeochemically mediating water (center) and the entire holistic lake ecosystem (right)
For lakes nutrient-rich external loading from scattered and point sources is the main reason of eutrophication, causing algal blooms, reduced oxygen reserves and fish kills. Low oxygen levels or hypoxia affect fish production and oxygen controls release of phosphorus from the bottom sediment. In water, like in air, oxygen saturation must be at least 70-80% for the healthy higher life but continued anoxia in the near bottom water causes internal loading.
The rapidly increasing on-lake pressures of the cage/ pen fish farms directly pollute the lakes. In some areas airborne-precipitation and acid rain may also need attention.
Objectives
Limnology, “oceanography” of inland waters, the principal fresh and brackish water science is one of the three ecological divisions on earth. The management and protection and fisheries-oriented applied limnology are multidisciplinary, interdisciplinary and sometimes transdisciplinary ventures that meld the often descriptive building blocks of the natural sciences (see Figure 3 and 4) with the technical, economic and social issues (Figure 3).
Limnology is the causal-synthetic analytical science which builds on the holistic and integrated management of the water bodies; i.e. interrelationships with watershed, water and the living resources alike, in which air above, landscape around, impoundment of water and water as the mediation ground for biogeochemical building blocks of the ecosystem are concurrently considered as management objects (see Figure 2). The self-purification capacity as a collective expression must be abolished because in nature such a capacity rarely exists. The lake ecosystems are naturally developed “equilibriums which steadily self-adjust to accomplish the greatest good which the circumstances will permit for all parties”
(Forbes, 1887) and which must be properly conceptually understood (Figures 3, 4 and 6).
The best available technology mayfavor too efficient waste water treatment and the best environmental practices highlight the environmental issues but their goals may contradict when the controlling “excess” nitrogen load is removed. This may happen also in the
“heavily protected” lakes, where nitrogen pollution is needed to keep the N/ P-ratio high.
Although in the world there is a large number of guidelines and frameworks to manage the temperate lakes sustainably, Indonesia needs local climate and lake type and characteristic specific guidelines since they range from lowland tropical ones to high altitude alpine lakes.
Methodology
In the systemic lake management, first a water balance calculation is made and pressures from the air, watershed, on-lake and deep water and bottom sediment, the long-term log book of the lake, are assessed and quantified (Figure 3, left), after which the ecological situation is judged by comparing the physical, chemical and biological characteristics of the water body to the reference conditions (see Figure 4). Based on the assessments, and conceptual frameworks of the critical pathways is made and the areal loading is calculated.
Courtesy of NOOA, Freshwater Imperative, USA, 1995 Figure 3. Assessment of pressures from the watershed (left) and collection of information
from the various building blocks of the limnological assessment and preparing a management plan or, if necessary, restoration program for the lake ecosystem management (right)
If there are great changes in the characteristics of the ecosystem and the areal load exceeds the acceptable limits, a cause, consequence, correction, cost-benefit and concluding synthesis is made to improve or restore the ecosystem (see Figure 3, right).
Results
By studying the problem identification of the 15 various types of priority lakes selected by the Ministry of Environment for the national lake program to implement the Bali Declaration (2009), it became evident that problem identification needs improvements (Figure 1).
The Indonesian lakes hold some 500 km3 of water (world 175,000 km3) and reservoirs about 90 km3 (world 7,000 km3); in the rest of East/ Southeast Asia respectively 800 and 1,980 km3. They hold 72% of fresh water (Zainuddin Amilia, 2010) and may foil the water crisis (Rachmat Witoelar, 2009) and their environmental and socioeconomic value is high.
However, most of the required basic information is limited and fragmented into various institutions or such data and information are not at all available and/ or are longitudinally and depth wise seldom adequate, making assessments and management planning fraught with dangers. Despite the excellent start of the tropical limnology in Indonesia some 90 years ago by the Sunda Expedition and continued 20 years ago by the Expedition
Indodanau, the Indonesian national limnology is largely at the descriptive stages. To advance the evidence-based management of lakes, is necessary to expand the limnological focus also longitudinally and depth wise on physical, chemical and productivity/ bioactivity studies (Figure 4).
To speed-up switch from the descriptive to analytical, causal-synthetic studies, first it is necessary to understand the basic latitudinal differences between temperate and tropical lakes. As examples the relative metabolic rate, capacity of water to hold oxygen and the rate at which zero oxygen (anoxia) is reached are used (Figure 5).
These alone stress that water as media should be the first study object but most often, however, it is the last, because the diversity of aquatic life and forms of life and species attract more. Water has also at high temperatures the highest heat absorption capacity and lakes can also modify the local microclimate.
Figure 4. Lake, a water impoundment, and the net results of physical effects of light and wind (heat and thermal structure), biogeochemical metabolism (nutrients, oxygen, carbon dioxide, hydrogen sulfide, methane, etc.) and biological succession (algae, zooplankton, fish, etc.).
Figure 5. Latitudinal differences in metabolic rate, oxygen saturation (left) and anoxia development rate (right). In Indonesia, anoxia is not a direct indicator of eutrophication.
Biogeochemical functions and food webs
For basic biogeochemical functions, conceptual models have been developed to improve the understanding the key physical, chemical and biological processes occurring in lakes under non-stratified mixing conditions, where oxygen reserves are replenished (Figure 6, left) and in stratified conditions where oxygen reserves are consumed at various rates, (Figure 6, right). This happens in zero latitudes 4-10 times faster than in the temperate regions.
Figure 6. Light, temperature and wind form thermal stratification, drive turnover and mixing and biogeochemical metabolism govern oxygen conditions and the water ecosystem.
In temperate lakes mixing is cyclic while in tropical lakes turnovers/ mixing are irregular and often wind induced. Mixing of water cannot be overemphasized; it is vital to oxygen replenishment and for higher animal life like fish. Figure 7 (left) shows oxygen solubility and (right) how swift the changes in temperature and oxygen levels can be in 36 hours.
Courtesy of CWR-UWA, Australia, 2010 Figure 7. Oxygen solubility in various temperatures and altitudes (left) and the rapid
diurnal variation of temperature and oxygen concentrations in water (right, vertical lines show midnight).
Internal waves
An internal wave (seiche) or sloping of the thermal barrier (thermocline), first recorded in 1519 but widely unknown in Indonesia, is generated by a heavy wind/ rainfall on one end of a lake/ reservoir and oscillations are damped by frictional resistance of water (Figure 8).
Courtesy of EU WFD, 2001
Suspended inorganic solids heterotrophs
aquatic carnivores
autotrophs herbivores/
bacteriovores organic
inorganic
terrestrial carnivores external inputs
NO3-, NH4+, R-NH2+, H2PO4-, R-OPO3H
mineralisation NH4+
H2PO4
-NO3 -Inorganic
solids
denitrification N2 internal inputs N-fixation
N2
temperature
Dissolved oxygen
Key excretion consumption sedimentation dissolved Particulate reaction
nitrification
terrestrial carnivores external inputs
NO3-, NH4+, R-NH2+, H2PO4-, R-OPO3H
Hypolimnion Epilimnion (mixed layer)
Metalimnion (thermocline) Suspended
inorganic solids
aquatic carnivores autotrophs herbivores/
bacteriovores organic
inorganic
N2
denitrification
internal inputs NO 3-nitrification heterotrophs N2
N-fixation
mineralisation H2PO4
NH4+ - dissolution
Key excretion consumption sedimentation dissolved Particulate reaction
Dissolved oxygen temperature
Figure 8. The schematic internal wave (left) and recorded wave in Lake Earn, Scotland (right). The black layer in the lake is the oscillating thermocline, the green dot the node of the wave and the red dot shows the shallow areas where fish farms are in greatest danger
In the uninodal seiche, the thermocline slope and vertical water displacement is greatest at windward and leeward ends, least at the node. These waves may also create turbulence in the near bottom water and enhance internal loading.The internal waves may also be a cause of the fish kills in Saguling, Cirata and Jatiluhur reservoirs, where the fish farms are located in the shallow bays and the anoxic water starts at the depth of 2-3 meters. In Figure 9, the excess number of farm units shown in three shallow areas may block oxygen replenishment routes; a part of the fish kills. Locating the units in the mainstream and covering only 1-5% of the surface area allows better oxygen exchange and is less risky for fish farmers (see Figure 8).
Laguna de Bay, The Philippines Cirata Jatiluhur Courtesy of Bulletin Today, May 2, 1982 Courtesy of Google Earth, 2011
Figure 9. Excess fish farms in Laguna de Bay in 1982; Cirata and Jatiluhur in 2011 In fish farming, the relationship between the areal loading and phosphorus treatment rate shows highest retention in lakes which receive least pollution per lake surface area. The capacity sharply declines when the loading increases and exceeds the treatment capacity and the lake develops eutrophic. This is already happening in some Indonesian lakes.
Discussion and Conclusions
Lakes equilibriums have a limited tolerance/ capacity (erroneously called self-purification, resilience or carrying capacity) to adjust to and treat external, on-lake and in-lake influences to change. After a certain pollution stress, this threshold is exceeded and the internal loading becomes a significant factor to control. Consequently, the slowly progressing eutrophication may suddenly be a visible and sensed reality (algal blooms, fish kills, smell,
etc.) causing loss of e.g. environmental, economic, recreational, tourist and water and land value. When the point pollution is brought under control, on-lake and internal loads will become the main sources of nutrients and the significance of the agricultural run-off increases.
Primary productivity, fisheries and sustainability of fish resources
Eutrophication has both positive and negative consequences in lakes. Can artificial eutrophication be controlled in lakes, most probably not? The ranges of primary productivity of tropical and temperate freshwater bodies in Figure 10 are based on the data from Hill and Rai (1982) and Tundisi (1983).
In the graph (left), the range for tropical lakes is not necessarily correct for Indonesian large, deep lakes because in such lakes the range of species number is 5-40 and in temperate lakes 18-75 and primary productivity/ phytoplankton biomass is minimal; Batur 2.4 mg l-1, Toba 0.2-0.4, Maninjau 0.2, Diatas 0.05, and Matano 0.002. If fish farms are introduced on such lakes, they do not have enough capacity to treat the nutrient load and dilution is insignificant due to the long water renewal time. In natural lakes, the first fish farms cause local, in the long-term lake-wide eutrophication, which may be difficult to restore.
In Figure 10 (right) primary productivity and fish yields (the dotted and dashed lines) represent the theoretically calculated yields, “potensi”, the lowest curve shows the typical annual fish yields from tropical fresh waters and the dots/ curve represent tilapia yields of the fertilized ponds (Almazan & Boyd, 1978). This “potensi” and actual yields are quite apart from each other: Attempts to gain higher yields than the natural productivity are environmentally and economically harmful. The “potensi ikan KJA” for large lakes is 150-200 kg m-2 a-1 (DKP, 2005) goes beyond the vertical scale and would need uniformly fertilized lakes (Figure 11, left), which is not feasible for lake sustainability.
Courtesy of Almazan and Boyd, 1978
Figure 10. Overestimation of the biological productivity of tropical lakes (left) and fisheries yields (right) may create situations, where fishing efforts are not adjusted to the carrying capacity of the fish stocks. There are also wide gaps in the theoretical and actual fish yields (red arrow) and the actual and artificially nourished fish yields (green arrow). Blue color below the horizontal axis indicates oligotrophic and green/ bluegreen eutrophic water.
Figure 11 (left) suggests acceptable (dotted line) and ideal (solid line) total phosphorus concentrations for different use purposes of water. However, the ecosystem capacity to treat nutrients is low and meso- and eutrophic lakes (left, green vertical line) and lakes with total phosphorus concentrations above 50-100 µg l-1 (left, red vertical line) are beyond most restoration methods, requiring the best phosphorus load reduction.
The plans of DKP to improve fish yields, livelihoods and export revenue is “suicidal” in lakes like Maninjau, where the oxygen rich and immediately oxygen demanding negative oxygen layers meet at ~50 m depth. The direct organic oxygen consuming load from surface enhances “pathological” hypoxia, anoxia and fish kills, harming the multi-uses of lakes.
This means that the competent authorities should, before any fish farming units are located on any lake in Indonesia, evaluatethe combined areal load of total phosphorus and total nitrogen (g m-2 a-1) from i) external diffuse runoff and point sources, ii) on-lake fish farms and iii) internally from the deep water and/ or bottom sediment.
Like Maninjau, many other deep lakes are similar "sleeping bears" with great amounts of dormant gases, nutrients and other substances blocked into the stagnant deep water below the oxygen rich surface water. These internally loaded lakes should not be provoked by fish farms, oxygenation or mixing the layers; instead the deep water could be siphoned out.
Figure 11. Fertility as total phosphorus concentration vs. fisheries expectations in various climates (left). In Indonesian lakes phosphorus levels are generally low, thus the line for fisheries in warm climates is misleading (green line). Characteristics of Lake Maninjau before fish farming (middle, down and right) & increase of on-lake fish farms (middle, up).
Also water level lowering by increased water withdrawal may reduce hydrostatic pressure and induce diffusive nutrient flux, i.e. “limnic eruption”, from the deep water into the middle/ upper water and earthquakes may aggregate gas bubbling and upward lift of nutrients.
Oxygen and fish
Basically, in lakes only capture fisheries should be supported, especially in lakes which are permanently stratified and have a naturally developed anoxic hypolimnion, which volume is larger than that of epilimnion. There is circumstantial evidence that since 1929 and 1993, the present oxygen conditions in Lake Maninjau are alarming, since the minimum oxygen
requirements for fish is in that temperature at night 7.0-7.5 mg l-1, ~90% saturation, and 5-6 mg l-l to survive. Specific attention should be given to lakes in small islands.
For optimal fish yields, diurnal oxygen saturation range is 80%-110% because at night metabolism and larger fish populations use-up the oxygen reserves. In unfavorable conditions pathological hypoxia, anoxia and fish kills cause great losses in farming (see Figures 5, 9 and 11, right). If the incremental areal nutrient load and biological oxygen demand exceed the treatment capacity of a lake ecosystem and Zoxic/ Zanoxic volume ratios are 0-40%/ 100-60%, permits should be considered and granted permits reassessed.
Conclusions
Freshwater is a finite resource and, in fact, both water demand and pollution are increasing.
Thus much less freshwater is available than the potential shows (Wetzel, 2001). It makes it a great challenge to implement the objectives of the Bali Declaration. Lake managers andthe competent authorities need a substantial knowledge-base on lakes, which requires research, studies and continued monitoring.Only if problems are correctly identified and quantified, the consequences and losses can be eased by the evidence-based management/
restoration programs. This includes hydrological conditions (water security), keeping-up ecosystem equilibrium and fisheries capacity (sustainability) and especially the multipurpose use values of lakes and of the surrounding land. Lakes and reservoirs are basically for capture and leisure fisheries, which have to be based on the natural productivity. For sustainability of their values, it is better to locate fish farms in river systems or on land.
Regarding assessment of Lake Toba problems: i) the excellent study of the catchment ecology should have been connected to the lake and quantify the annual inflow, silt transport and nutrient loads from rivers (Zulkifli Nasution, 2006) and ii) the House probe team defined declining water level and sedimentation as damages (The Jakarta Post, 2010).
However, they are inadequate evidences; the arbitrators should have also focused on the water ecosystem.
To improve evaluation of the ecological quality and changes in lakes, basic research and long-term monitoring are needed. In this respect, three educational programs are essential:
i) basic limnology for fine-scale structures, functions and net results of the water ecosystem, ii) management-oriented limnology for understanding the causal-synthetic mechanisms of ecosystems and finding solutions to alleviate problems and iii) fisheries-oriented limnology for understanding trophic-dynamics, fish production and sustainability of fish populations.
References
Amilia, Z. 2010. Lake Toba subject of House probe. The Jakarta Post, 17 May 2010.
Almazan, G. and C. E. Boyd. 1978. Plankton production and tilapia yields in ponds. Elsevier.
Bali Declaration. 2009. The Ministry of Environment, Jakarta.
Bulletin Today. May 2, 1982.
Center for Water Resources (CWR). 2011.
DKP. 2005. The view of Indonesian lakes. The opportunity of aquaculture development.
European Commission. Water Framework Directive. 2001. EC, Brussels.
Forbes, Stephen A. 1887. The lake as a microcosm. Bull. Sci. Assoc., (Preoria, IL).Google Earth. 2011.
Hill, G. and H. Rai. 1982. Establishing the pattern of heterotrophic bacterial activity in three Central Amazonian lakes, Hydrobiologia 86: 1-2.
ILEC. 1999. Reservoir water quality management. Guidelines in lake management, Vol. 9.
Lehmusluoto, P. et. al. 1999. Limnology in Indonesia. From the legacy of the past to the prospects for the future. Limnology in Developing Countries, 2.
Lehmusluoto, P. 2000. Lake Toba, the first sound science initiative to abate change in the lake environment. Research and Monitoring for Basin Management Decisions.
Nasution, Z. 2006. Ecology of Lake Toba catchment area for sustainable agriculture.
NOOA. 1995. The freshwater imperative, USA.
The Jakarta Post. 17 May 2010.
Tundisi, J. 1983. A review of basic ecological processes interacting with production and standing-stock of phytoplankton in lakes and reservoirs in Brazil. Hydrobiologia 100, 1.
Wetzel, R. G. 2001. Limnology, lake and river Ecosystems. 3rd Edition. Academic Press.
Witoelar, R. 2009. Lakes could foil RI water crisis. The Jakarta Post, 13 August 2009.