1. Data for maximum colonization depth (Zc) of five groups of submerged macrophytes and light attenuation were collected for forty‐five Danish lakes and 108 non‐Danish lakes. The macrophyte groups were bryophytes, charophytes, caulescent angiosperms, rosette‐type angiosperms and Isoetes spp. 2. The data showed systematic differences among the groups in the relationship of Zc to water transparency. In lakes with low transparency (Secchi disc transparency (Zs) less than 7 m) caulescent angiosperms and charophytes penetrated deepest followed by bryophytes and Isoetes spp. In more transparent lakes bryophytes grew deepest, followed by charophytes, caulescent angiosperms and Isoetes spp. Rosette‐type angiosperms had the lowest Zc in all types of lakes. Charophytes and caulescent angiosperms had similar depth limits in lakes with Zs < 4 m but charophytes grew deeper in more transparent lakes. The depth limits of both groups were independent of light penetration in lakes with very low transparency (Zs < 1 m). The annual light exposure for the deepest growing macrophytes (bryophytes) was 20–95 mol photons m–2. 3. The relationship between Zc, macrophyte type and lake transparency could be explained by three distinct processes regulating Zc. In lakes with low transparency (Zs < 1 m), tall macrophytes (caulescent angiosperms and charophytes) compensate for light limitation by shoot growth towards the water surface and Zc is therefore independent of transparency. In lakes with medium transparency (1 m < Zs < 4 m) Zc for angiosperms, charophytes and Isoetes spp. is constrained by light attenuation in the water column, corresponding to a linear relationship between Zc and Zs. This pattern also applies to bryophytes, despite lake transparency. In transparent lakes, the minimum light requirement at Zc increased with increasing transparency for angiosperms, charophytes and Isoetes spp. 4. The minimum light requirements among submersed macrophytes (including marine macroalgae) depend on their plant‐specific carbon value (plant biomass per unit of light‐absorbing surface area) for the species/group, indicating that the light requirements of submersed plants are tightly coupled to the plants’ possibility to harvest light and hence to the growth form. 5. The light requirements increased on average 0.04% surface irradiance per degree increase in latitude corresponding to an average decrease in Zc of 0.12 m per degree latitude.
The aim of this study was to evaluate seasonal and diurnal variability in pH and inorganic carbon in shallow-water macroalgal habitats and to evaluate the importance of high pH for macroalgal photosynthesis. Seasonal variations in pH, oxygen saturation and inorganic carbon concentration were measured at an exposed and a sheltered shallow-water (0 to 1 m) macroalgal habitat. Daytime pH was significantly higher in spring, summer and autumn than in winter at both study sites. Diurnal measurements at the most exposed site showed significantly higher pH during the day than during the night. The diurnal variations were largest in shallow water and decreased with increasing water depth. High pH resulted in periodically low concentrations of available inorganic carbon in summer (as low as 1.3 mmol [CO 2 + HCO 3 -] l -1). Photosynthesis as a function of inorganic carbon concentrations was measured at pH 8 and 9.3 for 4 common macroalgal species (Fucus vesiculosus, F. serratus, Ceramium rubrum, Ulva sp.). Photosynthesis in these species was not limited by natural concentrations of inorganic carbon, but maximum photosynthesis and inorganic carbon concentrations at saturation were lower when measured at pH 9.3 than at pH 8. Our results suggest that pH is higher in natural shallow-water habitats than previously thought, and that high pH has a direct effect on photosynthesis that cannot be accounted for by low availability of inorganic carbon. KEY WORDS: pH · Seasonal variations · Diurnal variations · Macroalgae · PhotosynthesisResale or republication not permitted without written consent of the publisher
ABSTRACT. The patch dynamics (recruitment, growth and mortality) of seagrass Cymodocea nodosa were examined at the landscape scale (Le, the scale at which disturbances occur) by means of 3-dimensional (X. Y dimensions and depth) mapping of shoot internal density and age structure within the basic units (patches) forming the landscape. Highly skewed patch size and age distributions ~n d icated a high exponential patch mortality rate ( m = 0.82 * 0 07 yr-l) and a slightly higher exponential patch recruitment rate (1.04 yr-l) that ensures the maintenance and fast turnover of this population of patches Patch growth (i.e. increase in number of shoots per patch) proceeded at an exponential growth rate of 2 28 t 0 14 yr.', indicating a doubling time of 11 1 d for the shoot population within each patch Moreover, average patch growth rate accelerates with increasing patch size and age. This selfacceleration of patch growth means that this seagrass species has a remarkable potential for rapid space occupation due to its intense clonal growth. The results obtained emphasize the power of a landscape approach for describing the intense dynamics of colonising seagrass populations.
Photosynthesis-irradiance relationships of macroalgal communities and thalli of dominant species in shallow coastal Danish waters were measured over a full year to test how well community production can be predicted from environmental (incident irradiance and temperature) and community variables (canopy absorptance, species number and thallus metabolism). Detached thalli of dominant species performed optimally at different times of the year, but showed no general seasonal changes in photosynthetic features. Production capacity of communities at high light varied only 1.8-fold over the year and was unrelated to incident irradiance, temperature and mean thallus photosynthesis, while community absorptance was a highly significant predictor. Actual rates of community photosynthesis were closely related to incident and absorbed irradiance alone. Community absorptance in turn was correlated to canopy height and species richness. The close relationship of community photosynthesis to irradiance is due to the fact that (1) large differences in thallus photosynthesis of individual species are averaged out in communities composed of several species, (2) seasonal replacement of species keeps communities metabolically active, and (3) maximum possible absorptance at 100% constrains the total photosynthesis of all species. Our results imply that the photosynthetic production of macroalgal communities is more predictable than their complex and dynamic nature suggest and that predictions are possible over wide spatial scales in coastal waters by measurements of vegetation cover, incoming irradiance and canopy absorptance.
T. 2004. Importance of canopy structure on photosynthesis in single-and multi-species assemblages of marine macroalgae. Á/ Oikos 107: 422 Á/432. Plant communities utilize available irradiance with different efficiency depending not only on their photosynthetic characteristics but also on the canopy structure and density. The importance of canopy structure are well studied in terrestrial plant communities but poorly studied in aquatic plant communities. The objective of this study was to evaluate macroalgal community photosynthesis in artificial constructed communities of one to four species with different morphologies along a range of leaf (i.e.0/thallus) area densities. In a laboratory set-up we measured net photosynthesis and dark respiration in constructed assemblages of macroalgae, excluding effects other than photosynthesis of individual tissue and distribution of photons in the canopy from influencing metabolism. We hypothezised that 1) canopy structure determines the actual rates of photosynthesis relative to the optimal rates and 2) multi-species communities attain higher maximum photosynthetic rates than single species communities. We found that differences in canopy structure outweighed large differences in tissue photosynthesis resulting in relatively similar maximum community photosynthetic rates among the different single and multi-species assemblages (20.1 Á/40.5 mmol O 2 m (2 s (1 ). Canopy structure influenced community photosynthesis both at low and high leaf area densities because it determines the ability of macroalgae to use the photosynthetic potential of their individual tissues. Due to an averaging effect the photosynthetic rate at high leaf area density was more similar among multi-species community than among singlespecies communities. Multi-species communities had, on average, a slightly higher photosynthetic production than expected from photosynthesis of single species communities. Moreover multi-species communities were capable of exposing new tissue to irradiance up to high densities thereby avoiding a decrease in net photosynthesis. This finding suggests that multi-species communities may be able to maintain higher biomass per unit ground area than single-species communities.
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