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Soft-water macrophytes and ecosystems: why are they so vulnerable to environmental changes?
Aquatic Botany 73 (2002) 285–286
Editorial
Soft-water macrophytes and ecosystems: why are they so vulnerable to environmental changes? Introduction
Soft-water lakes are common in the boreal and temperate zones of the northern hemisphere, and also on high elevations in the (sub)tropic regions. They are poor in (calcium) bicarbonate and generally also poor in nutrients. In large parts of northern America and Europe, soft-water ecosystems have developed on granite bedrock with a low weathering rate, whilst in the Atlantic and sub-Atlantic regions of western Europe shallow soft-water lakes can be found on non-calcareous sandy deposits or in bogs. Most soft-water bodies are mainly fed by rainwater, which partly explains their oligotrophic status. Already for more than a century, soft-water lakes received the attention of many botanists because of their diverse vegetation. It is not very easy for plants to survive in oligotrophic soft-water lakes, because of the low availability of carbon dioxide (CO2 ), nitrogen and phosphorus in the water layer. When isoetid species are exposed to lake water containing ambient CO2 levels, they show a negative net photosynthesis (Madsen et al., this issue). Characteristic soft-water plants like isoetids have developed morphological and biochemical adaptations to enable successful growth in carbon-limited oligotrophic lakes. Species such as Lobelia dortmanna, Littorella uniflora and Isoetes spp. for instance, are able to absorb free CO2 via their the roots (in the rhizospere CO2 levels are generally 10–100 times higher as compared to the water layer). In addition, these plants recapture part of the photo-respired CO2 in their extensive lacunal system and the rest is released by the roots (Wium-Andersen and Andersen, 1972; Søndergaard and Sand-Jensen, 1979; Roelofs et al., 1984; Pedersen and Sand-Jensen, 1995). Photosynthetically produced oxygen is released into the sediment through the roots, in this way stimulating mineralisation and nitrification rates (Risgaard-Petersen and Jensen, 1997). Isoetid species only develop well if nitrate is the main nitrogen source (Schuurkes et al., 1986). As a result of the high nitrogen losses, caused by strong denitrification in isoetid communities, the system can remain stable and oligotrophic for many centuries. During the last century, however, there has been a dramatic decline in soft-water lakes dominated by characteristic soft-water macrophytes, and at present many species are endangered. The question arises why these ecosystems are more vulnerable to environmental changes than other aquatic ecosystems. I think that the answer lies in the specific hydrology, hydrochemistry and biogeochemistry of these lakes and their catchments. The poorly buffered status of 0304-3770/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 7 7 0 ( 0 2 ) 0 0 0 3 9 - 6
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Editorial / Aquatic Botany 73 (2002) 285–286
both lake and catchment causes sensitivity to acid deposition. As a result, many lakes have become acidified in the second part of the last century. However, also the opposite process, alkalinisation, occurs in many lakes for instance due to bicarbonate leaching from limed agricultural land in the surroundings, or because of liming of lakes. Another threat is the high atmospheric nitrogen load originating from intense stockbreeding and traffic in densely populated regions. Finally, the expected doubling of the atmospheric CO2 levels in the next century, resulting in global climatic changes, will provide another threat to carbon-limited soft-water lakes in the near future. In recent decades, many studies have been carried out on macrophyte dominated soft-water lakes, resulting in numerous scientific papers. The editors of Aquatic Botany decided to bring this knowledge together in one special issue. The various topics in soft-water macrophyte research are presented in six comprehensive review papers. Kevin Murphy reviews the plant communities and plant diversity in soft-water lakes of northern Europe. Next, Alfons Smolders and co-authors focus on the biogeochemistry of the isoetid environment. Tom Madsen and his colleagues discuss carbon acquisition and carbon dynamics for aquatic isoetids. In the paper following, Gertie Arts gives an overview of the deterioration of Atlantic soft-water macrophyte communities, as caused by acidification, eutrophication and alkalinisation. After this, Tor Erik Brandrud discusses the effects of liming on soft-water macrophytes. Finally, Emiel Brouwer and his co-authors review the restoration of aquatic macrophyte communities in acidified and eutrophied soft-water lakes. I am convinced that these papers provide a clear answer to the question why soft-water ecosystems are so vulnerable to environmental changes, and put prevailing methods for conservation and recovery into a rational perspective. References Madsen, T.V., Olesen, B., Bagger, J. (this issue). Carbon acquisition and carbon dynamics by aquatic isoetids. Aquat. Bot. Pedersen, O., Sand-Jensen, K., 1995. Diel pulses of O2 and CO2 in sandy lake sediments inhabited by Lobelia dortmanna. Ecology 76, 1533–1545. Risgaard-Petersen, N., Jensen, K., 1997. Nitrification and denitrification in the rhizosphere of the aquatic macrophyte Lobelia dortmanna L. Limnol. Oceanogr. 42, 529–537. Roelofs, J.G.M., Schuurkes, J.A.A.R., Smits, A.J.M., 1984. Impact of acidification and eutrophication on macrophyte communities in soft waters. Part II. Experimental studies. Aquat. Bot. 18, 389–411. Schuurkes, J.A.A.R., Kok, C.J., Den Hartog, C., 1986. Ammonium and nitrate uptake by aquatic plants from poorly buffered environments. Aquat. Bot. 24, 131–146. Søndergaard, M., Sand-Jensen, K., 1979. Carbon uptake by leaves and roots of Littorella uniflora (L.) Aschers. Aquat. Bot. 6, 1–12. Wium-Andersen, S., Andersen, J.M., 1972. The influence of vegetation on the redox profile of the sediment of Grane Langsø, a Danish Lobelia lake. Limnol. Oceanogr. 17, 948–952.
J.G.M. Roelofs Department of Aquatic Ecology and Environmental Biology University of Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The Netherlands E-mail address: [email protected] (J.G.M. Roelofs)
Editorial
Soft-water macrophytes and ecosystems: why are they so vulnerable to environmental changes? Introduction
Soft-water lakes are common in the boreal and temperate zones of the northern hemisphere, and also on high elevations in the (sub)tropic regions. They are poor in (calcium) bicarbonate and generally also poor in nutrients. In large parts of northern America and Europe, soft-water ecosystems have developed on granite bedrock with a low weathering rate, whilst in the Atlantic and sub-Atlantic regions of western Europe shallow soft-water lakes can be found on non-calcareous sandy deposits or in bogs. Most soft-water bodies are mainly fed by rainwater, which partly explains their oligotrophic status. Already for more than a century, soft-water lakes received the attention of many botanists because of their diverse vegetation. It is not very easy for plants to survive in oligotrophic soft-water lakes, because of the low availability of carbon dioxide (CO2 ), nitrogen and phosphorus in the water layer. When isoetid species are exposed to lake water containing ambient CO2 levels, they show a negative net photosynthesis (Madsen et al., this issue). Characteristic soft-water plants like isoetids have developed morphological and biochemical adaptations to enable successful growth in carbon-limited oligotrophic lakes. Species such as Lobelia dortmanna, Littorella uniflora and Isoetes spp. for instance, are able to absorb free CO2 via their the roots (in the rhizospere CO2 levels are generally 10–100 times higher as compared to the water layer). In addition, these plants recapture part of the photo-respired CO2 in their extensive lacunal system and the rest is released by the roots (Wium-Andersen and Andersen, 1972; Søndergaard and Sand-Jensen, 1979; Roelofs et al., 1984; Pedersen and Sand-Jensen, 1995). Photosynthetically produced oxygen is released into the sediment through the roots, in this way stimulating mineralisation and nitrification rates (Risgaard-Petersen and Jensen, 1997). Isoetid species only develop well if nitrate is the main nitrogen source (Schuurkes et al., 1986). As a result of the high nitrogen losses, caused by strong denitrification in isoetid communities, the system can remain stable and oligotrophic for many centuries. During the last century, however, there has been a dramatic decline in soft-water lakes dominated by characteristic soft-water macrophytes, and at present many species are endangered. The question arises why these ecosystems are more vulnerable to environmental changes than other aquatic ecosystems. I think that the answer lies in the specific hydrology, hydrochemistry and biogeochemistry of these lakes and their catchments. The poorly buffered status of 0304-3770/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 7 7 0 ( 0 2 ) 0 0 0 3 9 - 6
286
Editorial / Aquatic Botany 73 (2002) 285–286
both lake and catchment causes sensitivity to acid deposition. As a result, many lakes have become acidified in the second part of the last century. However, also the opposite process, alkalinisation, occurs in many lakes for instance due to bicarbonate leaching from limed agricultural land in the surroundings, or because of liming of lakes. Another threat is the high atmospheric nitrogen load originating from intense stockbreeding and traffic in densely populated regions. Finally, the expected doubling of the atmospheric CO2 levels in the next century, resulting in global climatic changes, will provide another threat to carbon-limited soft-water lakes in the near future. In recent decades, many studies have been carried out on macrophyte dominated soft-water lakes, resulting in numerous scientific papers. The editors of Aquatic Botany decided to bring this knowledge together in one special issue. The various topics in soft-water macrophyte research are presented in six comprehensive review papers. Kevin Murphy reviews the plant communities and plant diversity in soft-water lakes of northern Europe. Next, Alfons Smolders and co-authors focus on the biogeochemistry of the isoetid environment. Tom Madsen and his colleagues discuss carbon acquisition and carbon dynamics for aquatic isoetids. In the paper following, Gertie Arts gives an overview of the deterioration of Atlantic soft-water macrophyte communities, as caused by acidification, eutrophication and alkalinisation. After this, Tor Erik Brandrud discusses the effects of liming on soft-water macrophytes. Finally, Emiel Brouwer and his co-authors review the restoration of aquatic macrophyte communities in acidified and eutrophied soft-water lakes. I am convinced that these papers provide a clear answer to the question why soft-water ecosystems are so vulnerable to environmental changes, and put prevailing methods for conservation and recovery into a rational perspective. References Madsen, T.V., Olesen, B., Bagger, J. (this issue). Carbon acquisition and carbon dynamics by aquatic isoetids. Aquat. Bot. Pedersen, O., Sand-Jensen, K., 1995. Diel pulses of O2 and CO2 in sandy lake sediments inhabited by Lobelia dortmanna. Ecology 76, 1533–1545. Risgaard-Petersen, N., Jensen, K., 1997. Nitrification and denitrification in the rhizosphere of the aquatic macrophyte Lobelia dortmanna L. Limnol. Oceanogr. 42, 529–537. Roelofs, J.G.M., Schuurkes, J.A.A.R., Smits, A.J.M., 1984. Impact of acidification and eutrophication on macrophyte communities in soft waters. Part II. Experimental studies. Aquat. Bot. 18, 389–411. Schuurkes, J.A.A.R., Kok, C.J., Den Hartog, C., 1986. Ammonium and nitrate uptake by aquatic plants from poorly buffered environments. Aquat. Bot. 24, 131–146. Søndergaard, M., Sand-Jensen, K., 1979. Carbon uptake by leaves and roots of Littorella uniflora (L.) Aschers. Aquat. Bot. 6, 1–12. Wium-Andersen, S., Andersen, J.M., 1972. The influence of vegetation on the redox profile of the sediment of Grane Langsø, a Danish Lobelia lake. Limnol. Oceanogr. 17, 948–952.
J.G.M. Roelofs Department of Aquatic Ecology and Environmental Biology University of Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The Netherlands E-mail address: [email protected] (J.G.M. Roelofs)