TY - JOUR

T1 - Influence of sediment organic enrichment and water alkalinity on growth of aquatic isoetid and elodeid plants

AU - Raun, Ane-Marie Løvendahl

AU - Borum, Jens

AU - Jensen, Kaj Sand

N1 - Keywords:freshwater macrophytes;organic enrichment;oxygen stress;recovery;root anchorage

PY - 2010

Y1 - 2010

N2 - 1. Lake eutrophication has increased phytoplankton blooms and sediment organic matter. Among higher plants, small, oligotrophic rosette species (isoetids) have disappeared, while a few tall, eutrophic species (elodeids) may have persisted. Despite recent reduction of nutrient loading in restored lakes, the vegetation has rarely regained its former composition and coverage. Patterns of recovery may depend on local alkalinity because HCO3- stimulates photosynthesis of elodeids and not of isoetids. In laboratory growth experiments with two isoetids (Lobelia dortmanna and Littorella uniflora) and two elodeids (Potamogeton crispus and P. perfoliatus), we test whether organic enrichment of lake sediments has a long-lasting influence by: (i) reducing plant growth because of oxygen stress on plant roots and (ii) inhibiting growth more for isoetids than elodeids. We also test whether (iii) increasing alkalinity (from 0.17 to 3.20 meq. L-1) enhances growth and reduces inhibition of organic sediment enrichment for elodeids but not for isoetids.2. In low organic sediments, higher oxygen release from roots of isoetids than elodeids generated oxic conditions to greater sediment depth for Lobelia (4.3 cm) and Littorella (3.0 cm) than for Potamogeton species (1.6-2.2 cm). Sediment oxygen penetration depth fell rapidly to 0.4-1.0 cm for all four species at even modest organic enrichment and oxygen consumption in the sediments. Roots became shorter and isoetid roots became thicker to better supply oxygen to apical meristems.3. Growth of elodeids was strongly inhibited across all levels of organic enrichment of sediments being eight-fold lower at the highest enrichment compared to the unenriched control. Leaf biomass of isoetids increased three-fold by moderate organic enrichment presumably because of greater CO2 supply from sediments being their main CO2 source. At higher organic enrichment, isoetid biomass was reduced, leaf chlorophyll declined up to 10-fold, root length declined from 7 to <2 cm and mortality rose (up to 50%) signalling high plant stress.4. Lobelia was not affected by HCO3- addition in accordance with its use of sediment CO2. Biomass of elodeids increased severalfold by rising alkalinity from 0.17 to 3.20 meq. L-1 in accordance with their use of HCO3- for photosynthesis, while the negative impact of organically enriched sediments remained.5. Overall, root development of all four species was so strongly restricted in sediments enriched with labile organic matter that plants if growing in situ may lose root anchorage. Other experiments demonstrate that this risk is enhanced by greater water content and reduced consolidation in organically rich sediments. Therefore, formation of more muddy and oxygen-demanding sediments during eutrophication will impede plant recovery in restored lakes while high local alkalinity will help elodeid recovery.

AB - 1. Lake eutrophication has increased phytoplankton blooms and sediment organic matter. Among higher plants, small, oligotrophic rosette species (isoetids) have disappeared, while a few tall, eutrophic species (elodeids) may have persisted. Despite recent reduction of nutrient loading in restored lakes, the vegetation has rarely regained its former composition and coverage. Patterns of recovery may depend on local alkalinity because HCO3- stimulates photosynthesis of elodeids and not of isoetids. In laboratory growth experiments with two isoetids (Lobelia dortmanna and Littorella uniflora) and two elodeids (Potamogeton crispus and P. perfoliatus), we test whether organic enrichment of lake sediments has a long-lasting influence by: (i) reducing plant growth because of oxygen stress on plant roots and (ii) inhibiting growth more for isoetids than elodeids. We also test whether (iii) increasing alkalinity (from 0.17 to 3.20 meq. L-1) enhances growth and reduces inhibition of organic sediment enrichment for elodeids but not for isoetids.2. In low organic sediments, higher oxygen release from roots of isoetids than elodeids generated oxic conditions to greater sediment depth for Lobelia (4.3 cm) and Littorella (3.0 cm) than for Potamogeton species (1.6-2.2 cm). Sediment oxygen penetration depth fell rapidly to 0.4-1.0 cm for all four species at even modest organic enrichment and oxygen consumption in the sediments. Roots became shorter and isoetid roots became thicker to better supply oxygen to apical meristems.3. Growth of elodeids was strongly inhibited across all levels of organic enrichment of sediments being eight-fold lower at the highest enrichment compared to the unenriched control. Leaf biomass of isoetids increased three-fold by moderate organic enrichment presumably because of greater CO2 supply from sediments being their main CO2 source. At higher organic enrichment, isoetid biomass was reduced, leaf chlorophyll declined up to 10-fold, root length declined from 7 to <2 cm and mortality rose (up to 50%) signalling high plant stress.4. Lobelia was not affected by HCO3- addition in accordance with its use of sediment CO2. Biomass of elodeids increased severalfold by rising alkalinity from 0.17 to 3.20 meq. L-1 in accordance with their use of HCO3- for photosynthesis, while the negative impact of organically enriched sediments remained.5. Overall, root development of all four species was so strongly restricted in sediments enriched with labile organic matter that plants if growing in situ may lose root anchorage. Other experiments demonstrate that this risk is enhanced by greater water content and reduced consolidation in organically rich sediments. Therefore, formation of more muddy and oxygen-demanding sediments during eutrophication will impede plant recovery in restored lakes while high local alkalinity will help elodeid recovery.

U2 - 10.1111/j.1365-2427.2010.02423.x

DO - 10.1111/j.1365-2427.2010.02423.x

M3 - Journal article

VL - 55

SP - 1891

EP - 1904

JO - Freshwater Biology

JF - Freshwater Biology

SN - 0046-5070

IS - 9

ER -