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Coastal marshes are key habitats contributing to organic carbon (OC) storage but remain understudied in Nordic regions regarding Blue Carbon processes. This study quantified OC stocks in above- and below-ground plant biomass and in the top 50 cm-soil across 12 grazed and ungrazed marshes, spanning a major environmental gradient, and assessed how biotic (plant communities, livestock grazing) and abiotic (soil properties, environmental conditions) drivers shape OC storage. Soil OC stocks accounted for ∼ 73 % of total OC in grazed sites and ∼ 63 % in ungrazed ones and was higher in grazed sites (99.7 ± 57.9 Mg ha−1) than in ungrazed sites (78.2 ± 44.2 Mg ha−1). Grazing and the large-scale environmental gradient strongly structured plant communities, partly by regulating reed (Phragmites australis), prevalent in ungrazed sites. Abiotic soil properties were major large-scale drivers of soil OC storage, while grazing affected soil OC storage indirectly through plant composition. Soil OC increased with finer textures, whereas vegetation and grazing effects were variable and locally expressed. Aboveground OC stocks were reduced by grazing, both directly through biomass removal and indirectly by reducing reed dominance. Belowground OC stocks were driven by plant community composition and indirectly by grazing effects on vegetation. Root biomass was concentrated in the top 15 cm in grazed sites and deeper (15–50 cm) in ungrazed sites, reflecting contrasting plant strategies. Overall, soil OC stocks in Nordic coastal marshes fall within the lower range of global estimates. These findings highlight the need to consider soil processes, grazing and environmental gradients in the sustainable management of Nordic coastal marshes and their carbon storage potential.

Poecilogonous species show variation in developmental mode, with larvae that differ both morphologically and ecologically. The spionid polychaete Pygospio elegans shows variation in developmental mode not only between populations, but also seasonally within populations. We investigated the consequences of this developmental polymorphism on the spatial and seasonal genetic structure of P. elegans at four sites in the Danish Isefjord‐Roskilde‐Fjord estuary at six time points, from March 2014 until February 2015. We found genetic differentiation between our sampling sites as well as seasonal differentiation at two of the sites. The seasonal genetic shift correlated with the appearance of new size cohorts in the populations. Additionally, we found that the genetic composition of reproductive individuals did not always reflect the genetic composition of the entire sample, indicating that variance in reproductive success among individuals is a likely explanation for the patterns of chaotic genetic patchiness observed during this and previous studies. The heterogeneous, unpredictable character of the estuary might maintain poecilogony in P. elegans as a bet‐hedging strategy in the Isefjord‐Roskilde‐Fjord complex in comparison with other sites where P. elegans are expected to be fixed to a certain mode of development. We investigated the population genetic structure of the poecilogonous polychaete Pygospio elegans in the Danish Isefjord‐Roskilde‐Fjord estuary complex. Pygospio elegans showed seasonal and spatial population genetic structure, but seasonal structure varied among the sampling sites. The seasonal genetic switch correlated with the arrival of new size cohorts.

Pygospio elegans is an opportunistic, wide-spread spionid polychaete that reproduces asexually via fragmentation and can produce benthic and pelagic larvae, hence combining different developmental modes in one species. We documented the density, size distribution, and reproductive activity of P. elegans at four sites in the Danish Isefjord-Roskilde Fjord estuary complex, where all modes of reproduction were reported. We compared population dynamics of this species to environmental parameters such as salinity, temperature, and sediment characteristics (grain size, sorting, porosity, water content, organic content, C/N). We observed that new cohorts—resulting either from sexual or asexual reproduction—appeared in spring and fall, and old ones disappeared in late summer and winter. Sexual reproduction occurred from September until May, and although their timing was variable, there were two reproductive peaks at three sites. At those sites, we also observed a switch in larval developmental mode. Asexual reproduction peaked in April. While the seasonal dynamics can be related to temperature to a large extent, the differences in population dynamics among sites also correlated with sediment structure and salinity. Populations from sites with coarse and heterogeneous sediment had high levels of sexual reproduction. At the site with lower salinity, intermediate and benthic larvae were present during winter in contrast to pelagic larvae found at the other sites. However, we could not identify one clear environmental factor determining the mode of development. At present, it remains unclear to what degree genetic background contributes to mode of development. Hence, whether the differences in developmental mode are the result of genetically different cohorts will be further investigated.

The effect of bioturbation by the lugworm Arenicola marina on uptake and distribution of cadmium in sediment was assessed using laboratory sediment cores. Carrier-free 109Cd was added to the water phase each day. Bioturbation (irrigation) was measured using bromide (Br–) as a tracer for water movement. In cores without lugworms all Cd was found in the surface sediment where it continued to build up over 16 d of exposure. In cores containing lugworms a distinct peak of Cd was found both at the sediment surface and, after a few days, at the feeding pocket of the worm (10 to 15 cm depth). During the 16 d of exposure this subsurface peak broadened and eventually Cd was found in all depths from top to feeding depth of the individual worm. Compared to sediment cores without worms, the presence of lugworms more than doubled the rate of removal of Cd from water to sediment. This was attributed to an increased turnover of sediment (due to feeding activity), an increased sediment surface area (due to fecal casts, head shaft, tube and irrigation of the whole burrow) and an increased contact of Cd-labelled water with potential binding sites in the sediment due to irrigation. Exposure to 1 ppm Cd reduced the fractional rate at which lugworms removed Cd from the water (as % of Cd in the water). The total Cd flux to the sediment, however, was much greater due to the higher Cd concentrations in the water. Water fluxes estimated using Br– as a solute tracer revealed a 10- to 20- fold increase in water exchange of the sediments when lugworms were present. This enhanced water flux was not affected by exposure of lugworms to 1 ppm Cd. The results indicate that the presence of bioturbating infauna influences both the uptake rates of trace metals in near-shore sediments and the distribution of those metals.

Organic matter decomposition and nitrogen cycling in coastal marine sediments, processes important for controlling productivity in coastal waters, were shown to be controlled by seasonal cycles of temperature, inputs of organic matter to the sediment, and activities of macrobenthic animals. Rates of benthic respiration (O $\\sb2$consumption) and dissolved inorganic nitrogen (DIN) release for silt-clay sediments in Buzzards Bay, Massachusetts, were measured in laboratory incubations of natural sediment cores. Benthic flux rates generally followed the annual temperature cycle and ranged from 6 to 24 mmol O $\\sb2$m $\\sp{-2}$d $\\sp{-1}$and 0 to 3.2 mmol N m $\\sp{-2}$d $\\sp{-1}$ . After the deposition of phytoplankton detritus to the sediment in spring, however, benthic respiration was high despite low water temperature. Low DIN release rates at the same time were due to high rates of denitrification or N uptake by benthic organisms. Results of macrofaunal manipulation experiments in large sediment cores demonstrated seasonal differences in the effect of benthic macrofauna on benthic processes. In a summer experiment, the presence of benthic animals increased organic matter mineralization and nutrient cycling rates 2-3 fold (from 11-14 to 22-25 mmol O $\\sb2$m $\\sp{-2}$d $\\sp{-1}$for benthic respiration and from 0.8-0.9 to 2.1-2.9 mmol N m $\\sp{-2}$d $\\sp{-1}$for DIN release), while in a fall experiment, benthic animals had no effect on these rates. In both experiments, benthic animals increased porewater irrigation rates but had no effect on SO $\\sb4\\sp{-2}$reduction rates. When both macrofauna and organic matter inputs were manipulated, the amount and decomposability of organic matter within sediments affected the ability of macrofauna to stimulate organic matter decomposition and DIN release rates. When organic matter quality and quantity were either high or low in sediments, macrofaunal stimulation of benthic flux rates was negligible. Benthic macrofauna stimulated microbial processes only when sediment organic matter was of intermediate quality and quantity. Although benthic animals stimulated rates of organic matter decomposition and DIN release only at certain times and under certain conditions, animals were always important for redistributing dissolved and particulate materials within sediments. ftn*All degree requirements completed in 1991, but degree will be granted in 1992.

The term ‘bioturbation’ is frequently used to describe how living organisms affect the substratum in which they live. A closer look at the aquatic science literature reveals, however, an inconsistent usage of the term with increasing perplexity in recent years. Faunal disturbance has often been referred to as particle reworking, while water movement (if considered) is referred to as bioirrigation in many cases. For consistency, we therefore propose that, for contemporary aquatic scientific disciplines, faunal bioturbation in aquatic environments includesall transport processes carried out by animals that directly or indirectly affect sediment matrices. These processes include both particle reworking and burrow ventilation.With this definition, bioturbation acts as an ‘umbrella’ term that covers all transport processes and their physical effects on the substratum. Particle reworking occurs through burrow construction and maintenance, as well as ingestion and defecation, and causes biomixing of the substratum. Organic matter and microorganisms are thus displaced vertically and laterally within the sediment matrix. Particle reworking animals can be categorized as biodiffusors, upward conveyors, downward conveyors and regenerators depending on their behaviour, life style and feeding type. Burrow ventilation occurs when animals flush their open- or blind-ended burrows with overlying water for respiratory and feeding purposes, and it causes advective or diffusive bioirrigation ex change of solutes between the sediment pore water and the overlying water body. Many bioturbating species perform reworking and ventilation simultaneously. We also propose that the effects of bioturbation on other organisms and associated processes (e.g. microbial driven biogeochemical transformations) are considered within the conceptual framework of ecosystem engineering.

Draining wetland landscapes accelerates climate change, and multilateral support is therefore needed to speed up the transition to new land uses. This paper examines perceptions of ecosystem services (ES) in wetland areas in scientific and civic assessments. The case study area is Denmark’s largest drained wetland system, which is notable for its carbon sequestration potential. The area’s transformation efforts involving public participation offer a unique chance to examine differences between scientific and civic perceptions of ES. This exceptional case is ideal for revealing contextual differences, trade-offs, and controversies between scientific and civic perceptions of ES. Millennium ES Assessment and CICES are used as a conceptual framework for understanding and mapping human–nature interactions in a nature park. However, these systems are, in practice, not sufficiently developed to identify how citizens understand and value ES in real life. Therefore, we analyse perceptions using interviews, collaborative mapping, and media analysis. We compare these to scientific ES mappings based on local data, literature reviews, and fieldwork. The paper concludes that (1) scientific ES asymmetries are important; (2) environmental blind spots in scientific ES are due to its approach to knowledge collection; (3) citizens’ blind spots are due to their everyday life focus and tabooing the issue of local climate mitigation; and (4) science-based ES assessments and accounts are disconnected from local ES controversies. We argue that identifying ES controversies through various scientific methods may improve climate mitigation and restoration efforts if community planning becomes involved.

We investigated the invasion of the non-native polychaete Marenzelleria viridis in a shallow Danish estuary, Odense Fjord. Three datasets with different spatial and temporal resolution were examined to describe the invasion of M. viridis and to investigate its effect on the native benthic community with focus on the 2 common polychaetes, Nereis (Hediste) diversicolor and Arenicola marina. Marenzelleria viridis colonized Odense Fjord rapidly, and within 3 yr it had spread to similar to 50% of the estuary. The population development of M. viridis in Odense Fjord followed the 'boom-bust' pattern that is typical for many invaders. M. viridis is now firmly established and has reached an overall abundance of 100 to 200 individuals (ind.) m super(-2) with local maxima of up to 1200 ind. m super(-2). Its distribution is apparently regulated by abiotic parameters that prevent its establishment in the oligohaline and more silty parts of Odense Fjord. There was a positive interaction between M. viridis and the native A. marina. Otherwise the introduction of M. viridis was synchronous with a decrease of several macroinvertebrates species, especially N. diversicolor. The latter is still the dominant species in Odense Fjord, but its density has decreased by >60% in areas colonised by M. viridis. We do not expect that N. diversicolor disappears completely in this estuary, but it will probably be displaced to refuge areas where M. viridis cannot survive. Decrease in the population size of a key native species such as N. diversicolor might have significant ecological implications at the ecosystem level with respect to biodiversity and nutrient cycling.

We investigated the invasion of the non-native polychaeteMarenzelleria viridisin a shallow Danish estuary, Odense Fjord. Three datasets with different spatial and temporal resolution were examined to describe the invasion ofM. viridisand to investigate its effect on the native benthic community with focus on the 2 common polychaetes,Nereis (Hediste) diversicolorandArenicola marina.Marenzelleria viridiscolonized Odense Fjord rapidly, and within 3 yr it had spread to ~50% of the estuary. The population development ofM. viridisin Odense Fjord followed the ‘boom-bust’ pattern that is typical for many invaders.M. viridisis now firmly established and has reached an overall abundance of 100 to 200 individuals (ind.) m−2with local maxima of up to 1200 ind. m−2. Its distribution is apparently regulated by abiotic parameters that prevent its establishment in the oligohaline and more silty parts of Odense Fjord. There was a positive interaction betweenM. viridisand the nativeA. marina. Otherwise the introduction ofM. viridiswas synchronous with a decrease of several macroinvertebrates species, especiallyN. diversicolor. The latter is still the dominant species in Odense Fjord, but its density has decreased by >60% in areas colonised byM. viridis. We do not expect thatN. diversicolordisappears completely in this estuary, but it will probably be displaced to refuge areas whereM. viridiscannot survive. Decrease in the population size of a key native species such asN. diversicolormight have significant ecological implications at the eco system level with respect to biodiversity and nutrient cycling.