Birgit Olesen

Bioenergy potential of Ulva lactuca: Biomass yield, methane production and combustion

The biomass production potential at temperate latitudes (56°N), and the quality of the biomass for energy production (anaerobic digestion to methane and direct combustion) were investigated for the green macroalgae, Ulva lactuca. The algae were cultivated in a land based facility demonstrating a production potential of 45T (TS) ha(-1) y(-1). Biogas production from fresh and macerated U. lactuca yielded up to 271 ml CH(4) g(-1) VS, which is in the range of the methane production from cattle manure and land based energy crops, such as grass-clover. Drying of the biomass resulted in a 5-9-fold increase in weight specific methane production compared to wet biomass. Ash and alkali contents are the main challenges in the use of U. lactuca for direct combustion. Application of a bio-refinery concept could increase the economical value of the U. lactuca biomass as well as improve its suitability for production of bioenergy.

Demography of shallow eelgrass (Zostera marina) populations: shoot dynamics and biomass development

Although Zostera marina (eelgrass) is a widespread and well examined seagrass we provide here a first study of shoot demography and growth, conducted by following cohorts of eelgrass shoots during an annual cycle in six perennial populations in Limfjorden, Denmark. The formation of leaf shoots on side-branches occurred throughout the year but peaked in May-June and declined at reduced light availability. Maximum mortality of over-wintering leaf shoots coincided with the flowering season, whereas new leaf shoots had the highest mortality in late summer and most were lost at an early age (mean half-life of 50 days) presumably due to shading within the dense stands (...)

Dynamics of seagrass stability and change

To the casual observer, seagrass meadows often appear to be uniform landscapes with limited structure. Belying this appearance, seagrass meadows contain considerable structure and dynamics (cf. den Hartog, 1971). Seagrass meadows, at any one time, consist of a nested structure of clones, possibly fragmented into different ramets, each supporting a variable number of shoots. Thus, although apparently rather static, seagrass meadows are highly dynamic landscapes maintained through the continuous recruitment of new clones to the meadow, and the growth and the turnover of the shoots they contain. Therefore, the intense dynamics of seagrass ecosystems results from the combination of processes operating at various scales, which—if balanced—maintain a rather stable ecosystem. Often, however, the various processes responsible for meadow dynamics are either unbal-

Biogeochemical conditions in sediments enriched by organic matter from net-pen fish farms in the Bolinao area, Philippines

Sedimentation and sediment metabolism was measured at eight active milkfish fish pens and at one abandoned site in the Bolinao area, Philippines in order to examine the interactions between sediment and water in this shallow coastal zone. The rates of sedimentation were high in the area due to siltation, but the activities in the fish pens also contributed to enhanced sedimentation as indicated by the difference between the abandoned and active sites. The sediment metabolism appeared to decrease with increasing rates of sedimentation indicating that the microbial activity reached a saturation level in the fish pen sediments. Anaerobic processes dominated the organic matter decomposition, and sulfate reduction rates are among the highest measured in fish farm sediments. The rates decreased with increasing organic loading despite high concentrations of sulfate (>10 mM) at all sites. Presence of methane bubbles in the sediments suggests that sulfate reduction and methanogenesis were coexisting. The sediment metabolism was significantly reduced at the abandoned site indicating that the stimulation of microbial activities is due to active fish production. The anaerobic activity remained high at the abandoned site indicating that the sediment biogeochemical conditions remain affected long time after fish production has ceased.

Recolonization dynamics in a mixed seagrass meadow: The role of clonal versus sexual processes

Recolonization dynamics from disturbance on a Philippine mixed seagrass meadow, containing species spanning more than 10-fold in rhizome elongation rates and reproductive effort, was examined by following the recovery of a 1,200 m2 gap over 2.5 yr. The objective was to assess the contribution of contrasting species to the recovery process and to evaluate the importance of sexual versus vegetative colonization. Large, slow-growing species,Thalassia hemprichii andEnhalus acoroides, that produce large, broadly-dispersed seeds dominated sexual colonization with a total of 2,643 and 210 seedlings, respectively, recruiting to the area. Despite very rapid turnover of sexual recruits, the high frequency of seedling establishment ensured successful development of new patches in areas devoid of vegetation, leading to a scattered and evenly distributed presence of vegetation inside the gap. The small seagrass speciesCymodocea rotundata andHalodule uninervis, characterized by fast rhizome elongation rates but low reproductive output and limited seed dispersal, were the major contributors to the overall 450 m2 increase in vegetation cover through fast lateral extension (144±6 cm yr−1) from meadow edge and surviving patches, forming a compact vegetation cover in one edge of the denuded area. We conclude that contrasting recruitment strategies in the mixed-species seagrass community examined have implications for colonization potential at different spatial scales. Fast clonal growth is only an efficient mechanism for colonization of disturbances within established meadows (small gaps), whereas the large species, which combined high reproductive output with high seed dispersal capacity, may act to accelerate the colonization process in large gaps or distant from established meadows.

Distribution, structure and function of Nordic eelgrass (Zostera marina) ecosystems: implications for coastal management and conservation

This paper focuses on the marine foundation eelgrass species, Zostera marina, along a gradient from the northern Baltic Sea to the north-east Atlantic. This vast region supports a minimum of 1480 km2 eelgrass (maximum >2100 km2), which corresponds to more than four times the previously quantified area of eelgrass in Western Europe. Eelgrass meadows in the low salinity Baltic Sea support the highest diversity (4–6 spp.) of angiosperms overall, but eelgrass productivity is low (<2 g dw m-2 d-1) and meadows are isolated and genetically impoverished. Higher salinity areas support monospecific meadows, with higher productivity (3–10 g dw m-2 d-1) and greater genetic connectivity. The salinity gradient further imposes functional differences in biodiversity and food webs, in particular a decline in number, but increase in biomass of mesograzers in the Baltic. Significant declines in eelgrass depth limits and areal cover are documented, particularly in regions experiencing high human pressure. The failure of eelgrass to re-establish itself in affected areas, despite nutrient reductions and improved water quality, signals complex recovery trajectories and calls for much greater conservation effort to protect existing meadows. The knowledge base for Nordic eelgrass meadows is broad and sufficient to establish monitoring objectives across nine national borders. Nevertheless, ensuring awareness of their vulnerability remains challenging. Given the areal extent of Nordic eelgrass systems and the ecosystem services they provide, it is crucial to further develop incentives for protecting them.

Carbon acquisition and carbon dynamics by aquatic isoetids

Abstract Isoetids are small, submerged vascular plants that often dominate the vegetation of soft-water lakes. The isoetids have short stiff leaves and large roots that might make up more than 50% of plant biomass. Leaves and roots have a system of internal gas spaces that allow efficient intra-plant gas transport and are the morphological basis for the remarkable use of sediment CO 2 in photosynthesis by isoetids. This ability allows the plants to exploit the rich CO 2 pool in the sediment of soft-water lakes and thereby partly ameliorate the low CO 2 availability in the bulk water. The transport of CO 2 from roots to leaves is by diffusion in the lacunae. Diffusion is relatively slow over longer distances and appears to set an upper limit to the leaf-length of isoetids. Quantitatively, the use of sediment CO 2 accounts for most of the inorganic carbon assimilated by isoetids and only at very high CO 2 concentrations in the bulk water significant amounts are taken up through the leaf surface. Physiologically some of the isoetids are unique among submerged macrophytes by featuring the crassulacean acid metabolism (CAM). CAM in isoetids is considered a carbon conserving mechanism and is an ecological advantage in typical isoetid habitats. CAM allows the plants to take up CO 2 from the water at night when competition from other plants is eliminated and, since daytime CO 2 uptake is unaffected by CAM, the time-slot during which CAM-isoetids can assimilate inorganic carbon is expanded compared to non-CAM species. The unique morphological and physiological traits and adaptations of isoetids allow them to prosper under the conditions prevailing in soft-water lakes. Still, growth is restrained by low resource availability and by an inherent low capacity for growth. Isoetids grow throughout the year with higher rates in summer and lower in winter. Annual leaf turnover rates are low and correspond to a leaf lifetime as long as 700 days in some species. A close relationship between production and mortality of leaves results in a relatively constant biomass over the year with only slightly higher values in summer than winter. However, the small stature and low weight of isoetid plants only allow moderate standing biomass per ground area despite high plant densities and accordingly the low leaf turnover rates result in low annual primary production in isoetid populations.

Distribution, structure and function of Nordic eelgrass (Zostera marina) ecosystems

This paper focuses on the marine foundation eelgrass species, Zostera marina, along a gradient from the northern Baltic Sea to the north-east Atlantic. This vast region supports a minimum of 1480 km2 eelgrass (maximum >2100 km2), which corresponds to more than four times the previously quantified area of eelgrass in Western Europe. Eelgrass meadows in the low salinity Baltic Sea support the highest diversity (4–6 spp.) of angiosperms overall, but eelgrass productivity is low (<2 g dw m-2 d-1) and meadows are isolated and genetically impoverished. Higher salinity areas support monospecific meadows, with higher productivity (3–10 g dw m-2 d-1) and greater genetic connectivity. The salinity gradient further imposes functional differences in biodiversity and food webs, in particular a decline in number, but increase in biomass of mesograzers in the Baltic. Significant declines in eelgrass depth limits and areal cover are documented, particularly in regions experiencing high human pressure. The failure of eelgrass to re-establish itself in affected areas, despite nutrient reductions and improved water quality, signals complex recovery trajectories and calls for much greater conservation effort to protect existing meadows. The knowledge base for Nordic eelgrass meadows is broad and sufficient to establish monitoring objectives across nine national borders. Nevertheless, ensuring awareness of their vulnerability remains challenging. Given the areal extent of Nordic eelgrass systems and the ecosystem services they provide, it is crucial to further develop incentives for protecting them.

Seasonal sea ice cover as principal driver of spatial and temporal variation in depth extension and annual production of kelp in Greenland

We studied the depth distribution and production of kelp along the Greenland coast spanning Arctic to sub-Arctic conditions from 78 °N to 64 °N. This covers a wide range of sea ice conditions and water temperatures, with those presently realized in the south likely to move northwards in a warmer future. Kelp forests occurred along the entire latitudinal range, and their depth extension and production increased southwards presumably in response to longer annual ice-free periods and higher water temperature. The depth limit of 10% kelp cover was 9–14 m at the northernmost sites (77–78 °N) with only 94–133 ice-free days per year, but extended to depths of 21–33 m further south (73 °N–64 °N) where >160 days per year were ice-free, and annual production of Saccharina longicruris and S. latissima, measured as the size of the annual blade, ranged up to sevenfold among sites. The duration of the open-water period, which integrates light and temperature conditions on an annual basis, was the best predictor (relative to summer water temperature) of kelp production along the latitude gradient, explaining up to 92% of the variation in depth extension and 80% of the variation in kelp production. In a decadal time series from a high Arctic site (74 °N), inter-annual variation in sea ice cover also explained a major part (up to 47%) of the variation in kelp production. Both spatial and temporal data sets thereby support the prediction that northern kelps will play a larger role in the coastal marine ecosystem in a warmer future as the length of the open-water period increases. As kelps increase carbon-flow and habitat diversity, an expansion of kelp forests may exert cascading effects on the coastal Arctic ecosystem.

Invasion strategies in clonal aquatic plants: are phenotypic differences caused by phenotypic plasticity or local adaptation?

BACKGROUND AND AIMS The successful spread of invasive plants in new environments is often linked to multiple introductions and a diverse gene pool that facilitates local adaptation to variable environmental conditions. For clonal plants, however, phenotypic plasticity may be equally important. Here the primary adaptive strategy in three non-native, clonally reproducing macrophytes (Egeria densa, Elodea canadensis and Lagarosiphon major) in New Zealand freshwaters were examined and an attempt was made to link observed differences in plant morphology to local variation in habitat conditions. METHODS Field populations with a large phenotypic variety were sampled in a range of lakes and streams with different chemical and physical properties. The phenotypic plasticity of the species before and after cultivation was studied in a common garden growth experiment, and the genetic diversity of these same populations was also quantified. KEY RESULTS For all three species, greater variation in plant characteristics was found before they were grown in standardized conditions. Moreover, field populations displayed remarkably little genetic variation and there was little interaction between habitat conditions and plant morphological characteristics. CONCLUSIONS The results indicate that at the current stage of spread into New Zealand, the primary adaptive strategy of these three invasive macrophytes is phenotypic plasticity. However, while limited, the possibility that genetic diversity between populations may facilitate ecotypic differentiation in the future cannot be excluded. These results thus indicate that invasive clonal aquatic plants adapt to new introduced areas by phenotypic plasticity. Inorganic carbon, nitrogen and phosphorous were important in controlling plant size of E. canadensis and L. major, but no other relationships between plant characteristics and habitat conditions were apparent. This implies that within-species differences in plant size can be explained by local nutrient conditions. All together this strongly suggests that invasive clonal aquatic plants adapt to a wide range of habitats in introduced areas by phenotypic plasticity rather than local adaptation.