Eutrophication of aquatic ecosystems: causes and consequences

Transcription

1 Eutrophication of aquatic ecosystems: causes and consequences Eutrophication is one of the most widely spread environmental problems in aquatic ecosystems 1

2 Eutrophication = increased production due to external nutrient loading oint loading Atmospheric deposition Diffuse loading from the catchment area Groundwater discharge Accumulation of nutrients in aquatic systems is a natural phenomenon (natural eutrophication) Natural background loading - external loading from the catchment area without human impact Human impact has greatly accelerated the accumulation of nutrients to aquatic ecosystems (anthropogenic eutrophication, cultural eutrophication) Modification of catchment areas for agricultural purposes Steep increase in external loading in the 1950 s rapid industralization after the second world war 2

3 For instance Lake Jyväsjärvi alomäki ym. (2004) Lake Tuusulanjärvi Vesijärvi (Lahti) Lake Hiidenvesi 3

4 The Baltic Sea Lake Taihu, China Lake Erie, USA B_4fTAhVB8ywKHdYFBOwQ_AUIBigB&biw=1536&bih=832#imgrc=4Q16ppowSuFKeM:&spf= YfTAhWF_iwKHbnKA6gQ_AUIBigB&biw=1536&bih=832#spf=1 4

5 oint loading has substantially decreased since the 1980 s development of industrial processes treatment of waste waters legislation No considerable improvement in the overall state of waters since the 1990 s 5

6 Diffuse loading has not shown a considerable decrease äästölähteet istemäinen kuormitus Fosfori t/a Typpi t/a Fosfori % Typpi % Massa- ja paperiteollisuus ,3 3,3 Muu teollisuus ,5 1,6 Yhdyskunnat ,7 16,6 Kalankasvatus ,0 0,9 Turkistarhaus ,5 0,7 Turvetuotanto ,6 0,9 istemäinen kuormitus yhteensä ,5 24,0 Hajakuormitus Maatalous ,1 47,6 Haja-asutus ,1 4,3 Metsätalous ,6 5,1 Hajakuormitus yhteensä ,8 57,1 Laskeuma ,7 18,9 Kuormitus yhteensä ,0 100,0 Luonnon huuhtouma Teollisuus, kalankasvatus ja yhdyskunnat v Tiedot perustuvat VAHTI-tietojärjestelmän tietoihin Muut päästölähteet ja luonnon huuhtouma SYKEn laskema arvio. Diffuse loading makes up c. 80% of all the loading to our waters At the moment, agriculture may produce 70% of the humaninduced - loading 6

7 The impacts of eutrophication? Why increases in productivity and nutrient concentrations are considered harmful? Why don t we want to fertilize lakes to increase food production? Eutrophication causes changes in all components of the food web When concentration increases In phytoplankton The percentage of cyanobacteria usually increases Cyanobacteria: need a high concentration of nutrients before they reach the maximal growth rate low mortality (low sedimentation rate, not favourable food for zooplankton) well adapted to low light intensitites (increased water turbidity in eutrophic waters) nitrogen fixation effective use of bicarbonate (elevated ph values in eutrophic conditions) 7

8 In zooplankton The percentage of large cladocerans and calanoid copepods decreases The percentage of cyclopoid copepods and rotifers increases selective predation by the dense fish stocks lack of favourable food for cladocerans Large cladocerans are the most effective consumers of phytoplankton: when they decrease, phytoplankton biomass with a given concentration usually increases In fish populations The percentage of predatory fish decreases decrease in water transparency effectivity of visual predation oxygen depletion selective fishing The percentage of planktivores and benthivores increases Along with eutrophication, a larger percentage of fish turn to zooplankton oxygen depletion decreased availability of benthic food Decrease in the growth rate of fish Increase of small-sized cyprinids generalistic feeding habits (use of plant material) ability to tolerate low oxygen concentrations 8

9 In the littoral plant communities Submerged species become less abundant decrease in light intensity increase of epiphytes increase in sediment resuspension, decrease of refuges for zooplankton Floating-leaved and free-floating species increase 9

10 10

11 The changes caused by eutrophication depend on the individual characteristics of the ecosystem For instance water depth Deep, stratifying Shallow, non-stratifying When water depth is high enough, the water column stratifies according to water temperature (density of the water) epilimnion metalimnion, thermocline hypolimnion There is usually no supply of oxygen for hypolimnion during stratification Wetzel (1983) 11

12 Oxygen deficits Stratification during summer oxygen deficit in the hypolimnion No stratification no oxygen deficit Littoral vegetation No considerable change in the area colonized by macrophyetes when water transparency changes Large change in the abundance of macrophytes 12

13 The state of aquatic ecosystems in the future? recovery from the effects of external nutrient loading possibilities to restore the ecosystems development of diffuse loading climate change (changes in run-off, changes in ice-cover and stratification) Recovery after the reduction of external loading?? 13

14 Although the external loading is considerably reduced, the lake does not often recover Enonselkä basin, Lake Vesijärvi In1976, external loading was reduced from 1.5 g m -2 a -1 to 0.15 g m -2 a -1 (critical loading 0.3 g m -2 a -1 ) Total concentration of the water decreased from 150 µg l -1 to 50 µg l

15 Internal loading The nutrients accumulated in the lake recycle within the system causing intensive production Internal loading is caused by oxygen depletion and consequent release of Fe-bound gas formation and bubbling in the sediment (e.g. methane) elevated water ph and consequent liberation of from the sediment particles bioturbation caused by biota nutrient excretion by biota foraging on the sediment sediment resuspension caused by waves and water currents the reduction of macrophytes 15

16 The largest storage of nutrient is in the bottom sediment It is usually impossible to remove the sediment nutrients Kymijärvi in in the the water column (µg/l) Tot. Dissolved Tot. Dissolved Dissolved in the sediment (µg/l) depth (m) depth (m) 5 seidment depth (cm)

17 Most restoration methods aim to prevent the contact of sediment with the illuminated and warm productive layer Aeration/oxygenation Chemical inactivation of Reduction of fish-induced internal loading by effective fishing It is however very difficult to isolate the sediment nutrients, because numerous mechanisms accelerate the release of from the sediment If oxygen concentration above the bottom is increased through aeration, sediment resuspension caused by wind, water currents and biota maintain internal loading Although external loading is reduced, internal loading caused by oxygen depletion accelerates recycling Intensive photosynthesis elevates water ph, which accelerates release from sediment particles independently of oxygen concentration Cyanobacteria are problematic food items for zooplankton, and phytoplankton biomass does not always decrease although zooplankton biomass increases due to fish biomass reduction 17

18 Eutrophication = increase in nutrient concentration = changes in physical-chemical conditions Changes in biological interactions Do the interactions between aquatic organisms return to the pre-disturbation state if the physical chemical conditions are restored (e.g. reduction of nutrient concentration)? Usually no, or the effect is temporary The ecosystem is resistant to changes A lake can receive strong external loading before it is severely eutrophicated When the system turns to a eutrophic state, it resists changes to a less eutrophic state 18

19 The long term effect of restoration initiatives can only be described only for a few lakes Considering the sustainability of effects over 8-10 years, the cases of success are very limited Effects of climate change? Scheffer et al. (2001): along the climate change shallow lakes will become more clear (growth of macrophytes, increase in the grazing rate of zooplankton) Jeppesen et al. (2003): along the climate change lakes will become more turbid (changes in water level, increased internal loading, changes in fish assemblages) 19

20 Shorter ice-cover periods effects on nutrient cycling and thermal properties of the water column Invasion of new species Changes in the light environment (predator-prey interactions) It has been predicted, that in the northern countries changes the annual external loading are not very large But the timing of nutrient supply to lakes will change considerably as loading will be more concentrated to the winter months Water framework directive by EU All waters should be at least in a good ecological state by

21 w=1920&bih=1040#imgrc=qyefyetbobfeom:&spf= ? 2027? Evaluation of eutrophication risk Concept of critical loading (Vollenweider 1976) Frequently used in lakes External -loading is related to the water retention time of the lake. ermeable loading and dangerous loading (leading to rapid eutrophication) are determined (for instance 1 g m -2 a -1 ) If water retention time is short (water exchange is rapid), the lake can tolerate more nutrient loading compared with a situation when retention time is long Based on the assumption that in a eutrophic lake - concentration exceeds 20 µg/l 21

22 Using phytoplankton biomass as the symptom of eutrophication makes models much more complicated Arhonditsis et al. (2007) Circumstances in the catchment area have to be taken into account Land use (field percentage, forest cover etc.) Amount of livestock in the catchment area Density of the human population 22