Vår forskning omfattar en rad ämnen, från marin landskapsekologi till marin växtfysiologi. Mina personliga forskningsintressen är främst marin fotosyntes och primärproduktion. Vi arbetar främst i östra Afrika, men även i tempererade miljöer, och studerar där hur de marina växtekosystemen påverkas av miljöförändringar, såsom föroreningar och havsförsurning.

I urval från Stockholms universitets publikationsdatabas

• Methane emissions from macrophyte beach wrack on Baltic seashores

  2023. Mats Björk (et al.). Ambio 52 (1), 171-181

  Artikel

  Beach wrack of marine macrophytes is a natural component of many beaches. To test if such wrack emits the potent greenhouse gas methane, field measurements were made at different seasons on beach wrack depositions of different ages, exposure, and distance from the water. Methane emissions varied greatly, from 0 to 176 mg CH₄-C m⁻² day⁻¹, with a clear positive correlation between emission and temperature. Dry wrack had lower emissions than wet. Using temperature data from 2016 to 2020, seasonal changes in fluxes were calculated for a natural wrack accumulation area. Such calculated average emissions were close to zero during winter, but peaked in summer, with very high emissions when daily temperatures exceeded 20 °C. We conclude that waterlogged beach wrack significantly contributes to greenhouse gas emissions and that emissions might drastically increase with increasing global temperatures. When beach wrack is collected into heaps away from the water, the emissions are however close to zero.

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• Nordic Blue Carbon Ecosystems: Status and Outlook

  2022. Dorte Krause-Jensen (et al.). Frontiers in Marine Science 9

  Artikel

  Vegetated coastal and marine habitats in the Nordic region include salt marshes, eelgrass meadows and, in particular, brown macroalgae (kelp forests and rockweed beds). Such habitats contribute to storage of organic carbon (Blue Carbon – BC) and support coastal protection, biodiversity and water quality. Protection and restoration of these habitats therefore have the potential to deliver climate change mitigation and co-benefits. Here we present the existing knowledge on Nordic BC habitats in terms of habitat area, C-stocks and sequestration rates, co-benefits, policies and management status to inspire a coherent Nordic BC roadmap. The area extent of BC habitats in the region is incompletely assessed, but available information sums up to 1,440 km² salt marshes, 1,861 (potentially 2,735) km² seagrass meadows, and 16,532 km² (potentially 130,735 km², including coarse Greenland estimates) brown macroalgae, yielding a total of 19,833 (potentially 134,910) km². Saltmarshes and seagrass meadows have experienced major declines over the past century, while macroalgal trends are more diverse. Based on limited salt marsh data, sediment C-stocks average 3,311 g C_org m^-2 (top 40-100 cm) and sequestration rates average 142 g C_org m^-2 yr^-1. Eelgrass C-stocks average 2,414 g C_org m^-2 (top 25 cm) and initial data for sequestration rates range 5-33 g C_org m^-2, quantified for one Greenland site and one short term restoration. For Nordic brown macroalgae, peer-reviewed estimates of sediment C-stock and sequestration are lacking. Overall, the review reveals substantial Nordic BC-stocks, but highlights that evidence is still insufficient to provide a robust estimate of all Nordic BC-stocks and sequestration rates. Needed are better quantification of habitat area, C-stocks and fluxes, particularly for macroalgae, as well as identification of target areas for BC management. The review also points to directives and regulations protecting Nordic marine vegetation, and local restoration initiatives with potential to increase C-sequestration but underlines that increased coordination at national and Nordic scales and across sectors is needed. We propose a Nordic BC roadmap for science and management to maximize the potential of BC habitats to mitigate climate change and support coastal protection, biodiversity and additional ecosystem functions.

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• Methane Emissions From Nordic Seagrass Meadow Sediments

  2022. Maria E. Asplund (et al.). Frontiers in Marine Science 8

  Artikel

  Shallow coastal soft bottoms are important carbon sinks. Submerged vegetation has been shown to sequester carbon, increase sedimentary organic carbon (C_org) and thus suppress greenhouse gas (GHG) emissions. The ongoing regression of seagrass cover in many areas of the world can therefore lead to accelerated emission of GHGs. In Nordic waters, seagrass meadows have a high capacity for carbon storage, with some areas being recognized as blue carbon hotspots. To what extent these carbon stocks lead to emission of methane (CH₄) is not yet known. We investigated benthic CH₄ emission (i.e., net release from the sediment) in relation to seagrass (i.e. Zostera marina) cover and sedimentary C_org content (%) during the warm summer period (when emissions are likely to be highest). Methane exchange was measured in situ with benthic chambers at nine sites distributed in three regions along a salinity gradient from ∼6 in the Baltic Sea (Finland) to ∼20 in Kattegat (Denmark) and ∼26 in Skagerrak (Sweden). The net release of CH₄ from seagrass sediments and adjacent unvegetated areas was generally low compared to other coastal habitats in the region (such as mussel banks and wetlands) and to other seagrass areas worldwide. The lowest net release was found in Finland. We found a positive relationship between CH₄ net release and sedimentary C_org content in both seagrass meadows and unvegetated areas, whereas no clear relationship between seagrass cover and CH₄ net release was observed. Overall, the data suggest that Nordic Zostera marina meadows release average levels of CH₄ ranging from 0.3 to 3.0 μg CH₄ m^–2 h^–1, which is at least 12–78 times lower (CO₂ equivalents) than their carbon accumulation rates previously estimated from seagrass meadows in the region, thereby not hampering their role as carbon sinks. Thus, the relatively weak CH₄ emissions from Nordic Z. marina meadows will not outweigh their importance as carbon sinks under present environmental conditions.

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• Impacts of land-use change and urban development on carbon sequestration in tropical seagrass meadow sediments

  2022. Martin Dahl (et al.). Marine Environmental Research 176

  Artikel

  Seagrass meadows store significant carbon stocks at a global scale, but land-use change and other anthropogenic activities can alter the natural process of organic carbon (C_org) accumulation. Here, we assessed the carbon accumulation history of two seagrass meadows in Zanzibar (Tanzania) that have experienced different degrees of disturbance. The meadow at Stone Town has been highly exposed to urban development during the 20th century, while the Mbweni meadow is located in an area with relatively low impacts but historical clearing of adjacent mangroves. The results showed that the two sites had similar sedimentary C_org accumulation rates (22–25 g m⁻² yr⁻¹) since the 1940s, while during the last two decades (∼1998 until 2018) they exhibited 24–30% higher accumulation of C_org, which was linked to shifts in C_org sources. The increase in the δ¹³C isotopic signature of sedimentary C_org (towards a higher seagrass contribution) at the Stone Town site since 1998 points to improved seagrass meadow conditions and C_org accumulation capacity of the meadow after the relocation of a major sewage outlet in the mid–1990s. In contrast, the decrease in the δ¹³C signatures of sedimentary C_org in the Mbweni meadow since the early 2010s was likely linked to increased C_org run-off of mangrove/terrestrial material following mangrove deforestation. This study exemplifies two different pathways by which land-based human activities can alter the carbon storage capacity of seagrass meadows (i.e. sewage waste management and mangrove deforestation) and showcases opportunities for management of vegetated coastal C_org sinks.

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• Seagrass meadows mixed with calcareous algae have higher plant productivity and sedimentary blue carbon storage

  2022. Olivia J. Kalokora (et al.). Ecology and Evolution 12 (2)

  Artikel

  Seagrass meadows capture and store large amounts of carbon in the sediment beneath, thereby serving as efficient sinks of atmospheric CO₂. Carbon sequestration levels may however differ greatly among meadows depending on, among other factors, the plant community composition. Tropical seagrass meadows are often intermixed with macroalgae, many of which are calcareous, which may compete with seagrass for nutrients, light, and space. While the photosynthetic CO₂ uptake by both seagrasses and calcareous algae may increase the overall calcification in the system (by increasing the calcium carbonate saturation state, Ω), the calcification process of calcareous algae may lead to a release of CO₂, thereby affecting both productivity and calcification, and eventually also the meadows’ carbon storage. This study estimated how plant productivity, CaCO₃ production, and sediment carbon levels were affected by plant community composition (seagrass and calcareous algae) in a tropical seagrass-dominated embayment (Zanzibar, Tanzania). Overall, the patterns of variability in productivity differed between the plant types, with net areal biomass productivity being highest in meadows containing both seagrass and calcareous algae. Low and moderate densities of calcareous algae enhanced seagrass biomass growth, while the presence of seagrass reduced the productivity of calcareous algae but increased their CaCO₃ content. Sedimentary carbon levels were highest when seagrasses were mixed with low or moderate cover of calcareous algae. The findings show that plant community composition can be an important driver for ecosystem productivity and blue carbon sequestration. 

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• A temporal record of microplastic pollution in Mediterranean seagrass soils

  2021. Martin Dahl (et al.). Environmental Pollution 273

  Artikel

  Plastic pollution is emerging as a potential threat to the marine environment. In the current study, we selected seagrass meadows, known to efficiently trap organic and inorganic particles, to investigate the concentrations and dynamics of microplastics in their soil. We assessed microplastic contamination and accumulation in Pb-210 dated soil cores collected in Posidonia oceanica meadows at three locations along the Spanish Mediterranean coast, with two sites located in the Almeria region (Agua Amarga and Roquetas) and one at Cabrera Island (Santa Maria). Almeria is known for its intense agricultural industry with 30 000 ha of plastic-covered greenhouses, while the Cabrera Island is situated far from urban areas. Microplastics were extracted using enzymatic digestion and density separation. The particles were characterized by visual identification and with Fourier-transformed infrared (FTIR) spectroscopy, and related to soil age-depth chronologies. Our findings showed that the microplastic contamination and accumulation was negligible until the mid-1970s, after which plastic particles increased dramatically, with the highest concentrations of microplastic particles (MPP) found in the recent (since 2012) surface soil of Agua Amarga (3819 MPP kg(-1)), followed by the top-most layers of the soil of the meadows in Roquetas (2173 kg(-1)) and Santa Maria (68-362 kg(-1)). The highest accumulation rate was seen in the Roquetas site (8832 MPP m(-2) yr(-1)). The increase in microplastics in the seagrass soil was associated to land-use change following the intensification of the agricultural industry in the area, with a clear relationship between the development of the greenhouse industry in Almeria and the concentration of microplastics in the historical soil record. This study shows a direct linkage between intense anthropogenic activity, an extensive use of plastics and high plastic contamination in coastal marine ecosystems such as seagrass meadows. We highlight the need of proper waste management to protect the coastal environment from continuous pollution.

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• Dynamics and fate of blue carbon in a mangrove-seagrass seascape

  2021. Maria E. Asplund (et al.). Landscape Ecology 36, 1489-1509

  Artikel

  Context Seagrass meadows act as efficient natural carbon sinks by sequestering atmospheric CO2 and through trapping of allochthonous organic material, thereby preserving organic carbon (C-org) in their sediments. Less understood is the influence of landscape configuration and transformation (land-use change) on carbon sequestration dynamics in coastal seascapes across the land-sea interface. Objectives We explored the influence of landscape configuration and degradation of adjacent mangroves on the dynamics and fate of C-org in seagrass habitats. Methods Through predictive modelling, we assessed sedimentary C-org content, stocks and source composition in multiple seascapes (km-wide buffer zones) dominated by different seagrass communities in northwest Madagascar. The study area encompassed seagrass meadows adjacent to intact and deforested mangroves. Results The sedimentary C-org content was influenced by a combination of landscape metrics and inherent habitat plant- and sediment-properties. We found a strong land-to-sea gradient, likely driven by hydrodynamic forces, generating distinct patterns in sedimentary C-org levels in seagrass seascapes. There was higher C-org content and a mangrove signal in seagrass surface sediments closer to the deforested mangrove area, possibly due to an escalated export of C-org from deforested mangrove soils. Seascapes comprising large continuous seagrass meadows had higher sedimentary C-org levels in comparison to more diverse and patchy seascapes. Conclusion Our results emphasize the benefit to consider the influence of seascape configuration and connectivity to accurately assess C-org content in coastal habitats. Understanding spatial patterns of variability and what is driving the observed patterns is useful for identifying carbon sink hotspots and develop management prioritizations.

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• Managing Seagrasses for Resilience to Climate Change

  2008. Mats Björk (et al.).

  Bok

  There is growing evidence that seagrasses are experiencing

  declines globally due to anthropogenic

  threats (Short and Wyllie-Echeverria 1996, Duarte

  2002, Orth et al. 2006). Runoff of nutrients and

  sediments that affect water quality is the greatest

  anthropogenic threat to seagrass meadows,

  although other stressors include aquaculture, pollution,

  boating, construction, dredging and landfill

  activities, and destructive fishing practices. Natural

  disturbances such as storms and floods can

  also cause adverse effects. Potential threats from

  climate change include rising sea levels, changing

  tidal regimes, UV radiation damage, sediment

  hypoxia and anoxia, increases in sea temperatures

  and increased storm and flooding events.

  Thus, seagrass meadows, the ecosystems that

  they support and the ecosystem services that they

  provide are threatened by a multitude of environmental

  factors that are currently changing or will

  change in the future.

  Seagrasses are flowering plants that thrive in shallow

  oceanic and estuarine waters around the world.

  Descendants of terrestrial plants that re-entered

  the ocean between 100 and 65 million years ago,

  seagrasses have leaves, stems, rhizomes (horizontal

  underground runners) and roots. Although

  there are only about 60 species of seagrassesworldwide, these plants play an important role in

  many shallow, near-shore, marine ecosystems.

  Seagrass meadows provide ecosystem services

  that rank among the highest of all ecosystems on

  earth. The direct monetary outputs are substantial

  since highly valued commercial catches such

  as prawns and fish are dependent on these systems.

  Seagrasses provide protective shelter for

  many animals, including fish, and can also be a

  direct food source for manatees and dugongs,

  turtles, water fowl, some herbivorous fish and sea

  urchins. The roots and rhizomes of seagrasses

  also stabilise sediments and prevent erosion while

  the leaves filter suspended sediments and nutrients

  from the water column. Seagrass meadows

  are thus linked to other important marine habitats

  such as coral reefs, mangroves, salt marshes and

  oyster reefs.

  This paper presents an overview of seagrasses,

  the impacts of climate change and other threats to

  seagrass habitats, as well as tools and strategies

  for managers to help support seagrass resilience.

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• Sensitivity of Photosynthesis to Warming in Two Similar Species of the Aquatic Angiosperm Ruppia from Tropical and Temperate Habitats

  2021. Lina M. Rasmusson (et al.). Sustainability 13 (16)

  Artikel

  Climate change-related events, such as marine heatwaves, are increasing seawater temperatures, thereby putting pressure on marine biota. The cosmopolitan distribution and significant contribution to marine primary production by the genus Ruppia makes them interesting organisms to study thermal tolerance and local adaptation. In this study, we investigated the photosynthetic responses in Ruppia to the predicted future warming in two contrasting bioregions, temperate Sweden and tropical Thailand. Through DNA barcoding, specimens were determined to Ruppia cirrhosa for Sweden and Ruppia maritima for Thailand. Photosynthetic responses were assessed using pulse amplitude-modulated fluorometry, firstly in short time incubations at 18, 23, 28, and 33 degrees C in the Swedish set-up and 28, 33, 38, and 43 degrees C in the Thai set-up. Subsequent experiments were conducted to compare the short time effects to longer, five-day incubations in 28 degrees C for Swedish plants and 40 degrees C for Thai plants. Swedish R. cirrhosa displayed minor response, while Thai R. maritima was more sensitive to both direct and prolonged temperature stress with a drastic decrease in the photosynthetic parameters leading to mortality. The results indicate that in predicted warming scenarios, Swedish R. cirrhosa may sustain an efficient photosynthesis and potentially outcompete more heat-sensitive species. However, populations of the similar R. maritima in tropical environments may suffer a decline as their productivity will be highly reduced.

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• An experimental assessment of algal calcification as a potential source of atmospheric CO2

  2020. Olivia J. Kalokora (et al.). PLoS ONE 15 (4)

  Artikel

  Marine vegetated ecosystems such as seagrass meadows are increasingly acknowledged as important carbon sinks based on their ability to capture and store atmospheric carbon dioxide, thereby contributing to climate change mitigation. Most studies on carbon storage in marine ecosystems have focused on organic carbon, leaving inorganic carbon processes such as calcification unaccounted for, despite of their critical role in the global carbon budget. This is probably because of uncertainties regarding the role of calcification in marine carbon budgets as either atmospheric CO2 source or sink. Here, we conducted a laboratory experiment to investigate the influence of a calcifying alga (Corallina officinalis L.) on seawater carbon content, using a non-calcifying alga (Ulva lactuca L.) as a control. In a first part, algae were incubated separately while measuring changes in seawater pH, total alkalinity (TA) and total dissolved inorganic carbon (DIC). The amount of carbon used in photosynthetic uptake and production of CaCO3 was then calculated. In a second, directly following, part the algae were removed and DIC levels were allowed to equilibrate with air until the pH stabilized and the loss of CO2 to air was calculated as the difference in total DIC from the start of part one, to the end of the second part. The results showed that C. officinalis caused a significant and persistent reduction in total dissolved inorganic carbon (DIC), TA and seawater pH, while no such permanent changes were caused by U. lactuca. These findings indicate that calcification can release a significant amount of CO2 to the atmosphere and thereby possibly counteract the carbon sequestration in marine vegetated ecosystems if this CO2 is not re-fixed in the system. Our research emphasises the importance of considering algal calcification in future assessments on carbon storage in coastal areas.

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• Blue Carbon Storage in Tropical Seagrass Meadows Relates to Carbonate Stock Dynamics, Plant–Sediment Processes, and Landscape Context

  2017. Martin Gullström (et al.). Ecosystems (New York. Print)

  Artikel

  Globally, seagrass ecosystems are considered major blue carbon sinks and thus indirect contributors to climate change mitigation. Quantitative estimates and multi-scale appraisals of sources that underlie long-term storage of sedimentary carbon are vital for understanding coastal carbon dynamics. Across a tropical–subtropical coastal continuum in the Western Indian Ocean, we estimated organic (C_org) and inorganic (C_carb) carbon stocks in seagrass sediment. Quantified levels and variability of the two carbon stocks were evaluated with regard to the relative importance of environmental attributes in terms of plant–sediment properties and landscape configuration. The explored seagrass habitats encompassed low to moderate levels of sedimentary C_org (ranging from 0.20 to 1.44% on average depending on species- and site-specific variability) but higher than unvegetated areas (ranging from 0.09 to 0.33% depending on site-specific variability), suggesting that some of the seagrass areas (at tropical Zanzibar in particular) are potentially important as carbon sinks. The amount of sedimentary inorganic carbon as carbonate (C_carb) clearly corresponded to C_org levels, and as carbonates may represent a carbon source, this could diminish the strength of seagrass sediments as carbon sinks in the region. Partial least squares modelling indicated that variations in sedimentary C_org and C_carb stocks in seagrass habitats were primarily predicted by sediment density (indicating a negative relationship with the content of carbon stocks) and landscape configuration (indicating a positive effect of seagrass meadow area, relative to the area of other major coastal habitats, on carbon stocks), while seagrass structural complexity also contributed, though to a lesser extent, to model performance. The findings suggest that accurate carbon sink assessments require an understanding of plant–sediment processes as well as better knowledge of how sedimentary carbon dynamics are driven by cross-habitat links and sink–source relationships in a scale-dependent landscape context, which should be a priority for carbon sink conservation.

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• Depth-specific fluctuations of gene expression and protein abundance modulate the photophysiology in the seagrass Posidonia oceanica

  2017. Gabriele Procaccini (et al.). Scientific Reports 7

  Artikel

  Here we present the results of a multiple organizational level analysis conceived to identify acclimative/adaptive strategies exhibited by the seagrass Posidonia oceanica to the daily fluctuations in the light environment, at contrasting depths. We assessed changes in photophysiological parameters, leaf respiration, pigments, and protein and mRNA expression levels. The results show that the diel oscillations of P. oceanica photophysiological and respiratory responses were related to transcripts and proteins expression of the genes involved in those processes and that there was a response asynchrony between shallow and deep plants probably caused by the strong differences in the light environment. The photochemical pathway of energy use was more effective in shallow plants due to higher light availability, but these plants needed more investment in photoprotection and photorepair, requiring higher translation and protein synthesis than deep plants. The genetic differentiation between deep and shallow stands suggests the existence of locally adapted genotypes to contrasting light environments. The depth-specific diel rhythms of photosynthetic and respiratory processes, from molecular to physiological levels, must be considered in the management and conservation of these key coastal ecosystems.

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• Contribution of seagrass plants to CO2 capture in a tropical seagrass meadow under experimental disturbance

  2017. Diana Deyanova (et al.). PLoS ONE 12 (7)

  Artikel

  Coastal vegetative habitats are known to be highly productive environments with a high ability to capture and store carbon. During disturbance this important function could be compromised as plant photosynthetic capacity, biomass, and/or growth are reduced. To evaluate effects of disturbance on CO2 capture in plants we performed a five-month manipulative experiment in a tropical seagrass (Thalassia hemprichii) meadow exposed to two intensity levels of shading and simulated grazing. We assessed CO2 capture potential (as net CO2 fixation) using areal productivity calculated from continuous measurements of diel photosynthetic rates, and estimates of plant morphology, biomass and productivity/respiration (P/R) ratios (from the literature). To better understand the plant capacity to coping with level of disturbance we also measured plant growth and resource allocation. We observed substantial reductions in seagrass areal productivity, biomass, and leaf area that together resulted in a negative daily carbon balance in the two shading treatments as well as in the high-intensity simulated grazing treatment. Additionally, based on the concentrations of soluble carbohydrates and starch in the rhizomes, we found that the main reserve sources for plant growth were reduced in all treatments except for the low-intensity simulated grazing treatment. If permanent, these combined adverse effects will reduce the plants' resilience and capacity to recover after disturbance. This might in turn have long-lasting and devastating effects on important ecosystem functions, including the carbon sequestration capacity of the seagrass system.

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• Effects of waterlogging, salinity and light on the productivity of Bruguiera gymnorrhiza and Heritiera littoralis seedlings

  2017. M. M. Mangora, M. S. P. Mtolera, Mats Björk. African Journal of Marine Science 39 (2), 167-174

  Artikel

  This study aimed to establish the effects of waterlogging, salinity and light on the early development of mangroves. Seedlings of Bruguiera gymnorrhiza (L.) Lamk. and Heritiera littoralis Dryand were exposed to 12 weeks of waterlogging, during which time growth and photosynthesis were measured every two weeks. The salinity of the water inundation ranged from fresh water to full-strength sea water (salinity 35). Seedlings were exposed to either full sunlight of 1 500 mu mol photon m(-2) s(-1) (SD 397) at midday or shade conditions of 325 mu mol photon m(-2) s(-1) (SD 40) of light at midday, to explore whether the plants would be differently affected by prolonged waterlogging in increased salinities and under different light conditions. Heritiera littoralis was more sensitive to waterlogging, salinity and light, displaying a least relative growth rate of 0.127 g g(-1) week(-1) (SE 0.032) under shade, and 0.025 g g(-1) week(-1) (SE 0.021) in full light; while under shade, photosynthesis continued only in fresh water, but photosynthetic yield decreased from 0.7 to 0.4 with increasing duration of waterlogging. By 12 weeks, all H. littoralis seedlings treated with any saltwater mixture had died. Bruguiera gymnorrhiza seedlings maintained a moderate rate of photosynthesis throughout inundation in both shade and full light, with yields of 0.7 and 0.3, respectively. Furthermore, B. gymnorrhiza survived waterlogging in up to 66% seawater, and maintained comparable relative growth rates of 0.164 g g(-1) week(-1) (SE 0.066) with 0.083 g g(-1) week(-1) (SE 0.065) and 0.074 g g(-1) week(-1) (SE 0.036) with 0.052 g g(-1) week(-1) (SE 0.037) under shade and in full light between fresh water and the highest salinity conditions, respectively. These results suggest that B. gymnorrhiza is broadly tolerant, making it a potential candidate species for restoring vulnerable mangrove forests.

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• Effects of shading and simulated grazing on carbon sequestration in a tropical seagrass meadow

  2016. Martin Dahl (et al.). Journal of Ecology 104 (3), 654-664

  Artikel

  1. There is an ongoing world-wide decline of seagrass ecosystems, one of the world's most efficient carbon sink habitats. In spite of this, there is a clear lack of studies experimentally testing the effects of anthropogenic disturbances on carbon sequestration of seagrass systems. 2. We assessed the effects of two disturbances of global concern on the carbon sink function in a five-month in situ experiment within a tropical seagrass (Thalassia hemprichii) meadow by testing the impacts of shading and simulated grazing at two levels of intensity using shading cloths and clipping of shoot tissue. We measured the effects of these disturbances on the carbon sequestration process by assessing the net community production (NCP), carbon and nitrogen content in tissue biomass, and organic matter and THAA (total hydrolysable amino acids) in the sediment down to 40 cm depth. 3. Treatments of high-intensity shading and high-intensity clipping were similarly impacted and showed a significantly lower NCP and carbon content in the below-ground biomass compared to the seagrass control. No significant effects were seen in organic carbon, total nitrogen, C:N ratio and THAA in the sediment for the seagrass treatments. However, both clipping treatments showed different depth profiles of carbon and THAA compared to the seagrass control, with lower organic carbon and THAA content in the surface sediment. This can be explained by the clipping of shoot tissue causing a less efficient trapping of allochthonous carbon and reduced input of shredded seagrass leaves to the detritus sediment layer. In the clipping plots, erosion of the surface sediment occurred, which was also most likely caused by the removal of above-ground plant biomass. 4. Synthesis. Our findings show that during the course of this experiment, there were no impacts on the sedimentary carbon while the high-intensity disturbances caused a clear depletion of carbon biomass and reduced the seagrass meadow's capacity to sequester carbon. From a long-term perspective, the observed effect on the carbon biomass pool in the high-intensity treatments and the sediment erosion in the clipping plots may lead to loss in sedimentary carbon.

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• The role of O-2 as an electron acceptor alternative to CO2 in photosynthesis of the common marine angiosperm Zostera marina L.

  2016. Pimchanok Buapet, Mats Björk. Photosynthesis Research 129 (1), 59-69

  Artikel

  This study investigates the role of O-2 as an electron acceptor alternative to CO2 in photosynthesis of the common marine angiosperm Zostera marina L. Electron transport rates (ETRs) and non-photochemical quenching (NPQ) of Z. marina were measured under saturating irradiance in synthetic seawater containing 2.2 mM DIC and no DIC with different O-2 levels (air-equilibrated levels, 3 % of air equilibrium and restored air-equilibrated levels). Lowering O-2 did not affect ETR when DIC was provided, while it caused a decrease in ETR and an increase in NPQ in DIC-free media, indicating that O-2 acted as an alternative electron acceptor under low DIC. The ETR and NPQ as a function of irradiance were subsequently assessed in synthetic seawater containing (1) 2.2 mM DIC, air-equilibrated O-2; (2) saturating CO2, no O-2; and (3) no DIC, air-equilibrated O-2. These treatments were combined with glycolaldehyde pre-incubation. Glycolaldehyde caused a marked decrease in ETR in DIC-free medium, indicating significant electron flow supported by photorespiration. Combining glycolaldehyde with O-2 depletion completely suppressed ETR suggesting the operation of the Mehler reaction, a possibility supported by the photosynthesis-dependent superoxide production. However, no notable effect of suppressing the Mehler reaction on NPQ was observed. It is concluded that during DIC-limiting conditions, such as those frequently occurring in the habitats of Z. marina, captured light energy exceeds what is utilised for the assimilation of available carbon, and photorespiration is a major alternative electron acceptor, while the contribution of the Mehler reaction is minor.

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• Sediment Properties as Important Predictors of Carbon Storage in Zostera marina Meadows

  2016. Martin Dahl (et al.). PLoS ONE 11 (12)

  Artikel

  Seagrass ecosystems are important natural carbon sinks but their efficiency varies greatly depending on species composition and environmental conditions. What causes this variation is not fully known and could have important implications for management and protection of the seagrass habitat to continue to act as a natural carbon sink. Here, we assessed sedimentary organic carbon in Zostera marina meadows (and adjacent unvegetated sediment) in four distinct areas of Europe (Gullmar Fjord on the Swedish Skagerrak coast, Asko in the Baltic Sea, Sozopol in the Black Sea and Ria Formosa in southern Portugal) down to similar to 35 cm depth. We also tested how sedimentary organic carbon in Z. marina meadows relates to different sediment characteristics, a range of seagrass-associated variables and water depth. The seagrass carbon storage varied greatly among areas, with an average organic carbon content ranging from 2.79 +/- 0.50% in the Gullmar Fjord to 0.17 +/- 0.02% in the area of Sozopol. We found that a high proportion of fine grain size, high porosity and low density of the sediment is strongly related to high carbon content in Z. marina sediment. We suggest that sediment properties should be included as an important factor when evaluating high priority areas in management of Z. marina generated carbon sinks.

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• Photorespiration and carbon limitation determine productivity in temperate seagrasses

  2013. Pimchanok Buapet (et al.). PLoS ONE 8 (12)

  Artikel

  The gross primary productivity of two seagrasses, Zostera marina and Ruppia maritima, and one green macroalga, Ulva intestinalis, was assessed in laboratory and field experiments to determine whether the photorespiratory pathway operates at a substantial level in these macrophytes and to what extent it is enhanced by naturally occurring shifts in dissolved inorganic carbon (DIC) and O2 in dense vegetation. To achieve these conditions in laboratory experiments, seawater was incubated with U. intestinalis in light to obtain a range of higher pH and O2 levels and lower DIC levels. Gross photosynthetic O2 evolution was then measured in this pretreated seawater (pH, 7.8–9.8; high to low DIC:O2 ratio) at both natural and low O2concentrations (adjusted by N2 bubbling). The presence of photorespiration was indicated by a lower gross O2 evolution rate under natural O2 conditions than when O2 was reduced. In all three macrophytes, gross photosynthetic rates were negatively affected by higher pH and lower DIC. However, while both seagrasses exhibited significant photorespiratory activity at increasing pH values, the macroalga U. intestinalis exhibited no such activity. Rates of seagrass photosynthesis were then assessed in seawater collected from the natural habitats (i.e., shallow bays characterized by high macrophyte cover and by low DIC and high pH during daytime) and compared with open baymouth water conditions (where seawater DIC is in equilibrium with air, normal DIC, and pH). The gross photosynthetic rates of both seagrasses were significantly higher when incubated in the baymouth water, indicating that these grasses can be significantly carbon limited in shallow bays. Photorespiration was also detected in both seagrasses under shallow bay water conditions. Our findings indicate that natural carbon limitations caused by high community photosynthesis can enhance photorespiration and cause a significant decline in seagrass primary production in shallow waters.

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• A blueprint for blue carbon

  2011. Elizabeth Mcleod (et al.). Frontiers in Ecology and the Environment 9 (10), 552-560

  Artikel

  Recent research has highlighted the valuable role that coastal and marine ecosystems play in sequestering carbon dioxide (CO(2)). The carbon (C) sequestered in vegetated coastal ecosystems, specifically mangrove forests, seagrass beds, and salt marshes, has been termed blue carbon. Although their global area is one to two orders of magnitude smaller than that of terrestrial forests, the contribution of vegetated coastal habitats per unit area to long-term C sequestration is much greater, in part because of their efficiency in trapping suspended matter and associated organic C during tidal inundation. Despite the value of mangrove forests, seagrass beds, and salt marshes in sequestering C, and the other goods and services they provide, these systems are being lost at critical rates and action is urgently needed to prevent further degradation and loss. Recognition of the C sequestration value of vegetated coastal ecosystems provides a strong argument for their protection and restoration; however, it is necessary to improve scientific understanding of the underlying mechanisms that control C sequestration in these ecosystems. Here, we identify key areas of uncertainty and specific actions needed to address them.

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• Seagrass photosynthesis controls rates of calcification and photosynthesis of calcareous macroalgae in a tropical seagrass meadow

  2009. Immaculate Sware Semesi, Sven Beer, Mats Björk. Marine Ecology Progress Series 382, 41-47

  Artikel

  Diel fluctuations in seawater pH can be >1 pH unit (7.9 to >8.9) in the seagrass meadows of Chwaka Bay (Zanzibar, Tanzania). The high daily pH values are generated by the photosynthetic activity of the bay’s submerged seagrasses and macroalgae, and maintained by the relatively low, tide-dominated, water exchange rate. Since pH in principle can affect rates of both calcification and photosynthesis, we investigated whether diel variations in pH caused by photosynthesis could affect rates of calcification and photosynthesis of the calcareous red (Hydrolithon sp. and Mesophyllum sp.) and green (Halimeda renschii) algae growing within these meadows. This was done by measuring rates of calcification and relative photosynthetic electron transport (rETR) of the algae in situ in open-bottom incubation cylinders either in the natural presence of the rooted seagrasses or after the leaves had been removed. The results showed that seagrass photosynthesis increased the seawater pH within the cylinders from 8.3–8.4 to 8.6–8.9 after 2.5 h (largely in conformity with that of the surrounding seawater), which, in turn, enhanced the rates of calcification 5.8-fold for Hydrolithon sp. and 1.6-fold for the other 2 species. The rETRs of all algae largely followed the irradiance throughout the day and were (in Mesophyllum sp.) significantly higher in the presence of seagrasses despite the higher pH values generated by the latter. We conclude that algal calcification within seagrass meadows such as those of Chwaka Bay is considerably enhanced by the photosynthetic activity of the seagrasses, which in turn increases the seawater pH.

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