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 HCO 3 ) 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 CO 2 supply from sediments being their main CO 2 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 HCO 3 ) addition in accordance with its use of sediment CO 2 .Biomass of elodeids increased severalfold by rising alkalinity from 0.17 to 3.20 meq. L )1 in accordance with their use of HCO 3 ) 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 Correspondence: Ane Løvendahl Raun, Freshwater 1891 and oxygen-demanding sediments during eutrophication will impede plant recovery in restored lakes while high local alkalinity will help elodeid recovery.
We investigated the influence of light, nutrients, and organic matter on gross primary production (GPP), ecosystem respiration (R), and net ecosystem production (NEP = GPP ‐ R) in a dystrophic forest lake and an open eutrophic lake. Forest vegetation reduced incoming irradiance (20%) and wind speed (34%) in dystrophic Gribsø, having thermal stratification 1 month longer than in eutrophic Slotssø. While Gribsø had nutrient‐limited phytoplankton during most of the year, Slotssø only experienced nutrient depletion during algal blooms. Colored dissolved organic matter (CDOM) absorbed most light (average 82%) in dystrophic Gribsø, while phytoplankton and other particles absorbed most light (45%) in eutrophic Slotssø. GPP and NEP were positively related to irradiance in both lakes. However, because of higher CDOM absorbance, three times more light was needed to attain autotrophy in Gribsø, being net heterotrophic (NEP < 0) for 79% of all days, compared to 59% in Slotssø. This difference vanished when NEP was scaled to light absorption by pigments, although the eutrophic lake maintained a higher photon yield. Metabolic rates varied much more in Slotssø, where higher light and nutrient availability facilitated occasional phytoplankton blooms, while low light and nutrient availability in Gribsø dampened temporal variability. Both lakes were annually net heterotrophic with similar annual areal rates (NEP, ‐14 mol C m−2). Net heterotrophy in dystrophic Gribsø derives from high import of organic carbon‐rich water, while heterotrophy in eutrophic Slotssø is fueled by degradation of sediment pools of organic matter accumulated under previous hypereutrophic conditions, emphasizing the importance of lake history on the contemporary metabolic state.
Constraints imposed by the spherical form and gelatinous matrix of centimeter-thick colonies of the cyanobacterium Nostoc zetterstedtii on its functional properties were tested by examining the scaling of its composition, light absorption, photosynthesis, and respiration to individual size. In three summer experiments with colonies collected from the bottom of oligotrophic lakes of low inorganic carbon concentrations (dissolved inorganic C, DIC), metabolism and pigment density of colonies were scaled to their surface area as most algal filaments were confined to a 2-mm-thick outer shell. Nostoc absorbed 96% of incident light from the surface to the center because of high areal pigment density, but absorbed photons were used with low quantum efficiency (11-38 mmol O 2 mol 21 photon) and photosynthesis was low relative to dark respiration (2.0-5.4). Therefore, N. zetterstedtii is threatened by reduced light availability and only extended to lake depths receiving about 12% of surface irradiance, whereas mosses, characeans, and angiosperms with thin photosynthetic tissues grew deeper (3.1-7.5% of surface irradiance). Nostoc ameliorated the restrictions of low lake DIC and long diffusion paths by active transport that could extract most external DIC, accumulate DIC in the colony 150-fold above external concentrations, and retain respiratory CO 2 . The energy cost of solute transport and gel formation in Nostoc colonies and extensive self shading restrict their potential growth, whereas colony formation should prevent grazing and increase longevity and nutrient recirculation. Nostoc zetterstedtii has become one of rarest freshwater macroalgae because of widespread lake eutrophication reducing water transparency and increasing competition from taller and faster-growing stands of filamentous algae and higher plants.
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