Thomas Bruun Valdemarsen

Influence of benthic macrofauna community shifts on ecosystem functioning in shallow estuaries

We identify how ecosystem functioning in shallow estuaries is affected by shifts in benthic fauna communities. We use the shallow estuary, Odense Fjord, Denmark, as a case study to test our hypotheses that (1) shifts in benthic fauna composition and species functional traits affect biogeochemical cycling with cascading effects on ecological functioning, which may (2) modulate pelagic primary productivity with feedbacks to the benthic system. Odense Fjord is suitable because it experienced dramatic shifts in benthic fauna community structure from 1998 to 2008. We focused on infaunal species with emphasis on three dominating burrow-dwelling polychaetes: the native Nereis (Hediste) diversicolor and Arenicola marina, and the invasive Marenzelleria viridis. The impact of functional traits in the form of particle reworking and ventilation on biogeochemical cycles, i.e. sediment metabolism and nutrient dynamics, was determined from literature data. Historical records of summer nutrient levels in the water column of the inner Odense Fjord show elevated concentrations of NH4+ and NO3- (DIN) during the years 2004-2006, exactly when the N. diversicolor population declined and A. marina and M. viridis populations expanded dramatically. In support of our first hypothesis, we show that excess NH4+ delivery from the benthic system during the A. marina and M. viridis expansion period enriched the overlying water in DIN and stimulated phytoplankton concentration. The altered benthic-pelagic coupling and stimulated pelagic production may, in support of our second hypothesis, have feedback to the benthic system by changing the deposition of organic material. We therefore advice to identify the exact functional traits of the species involved in a community shift before studying its impact on ecosystem functioning. We also suggest studying benthic community shifts in shallow environments to obtain knowledge about the drivers and controls before exploring deep-water environments.

Effect of temperature on biogeochemistry of marine organic‐enriched systems: implications in a global warming scenario

Coastal biogeochemical cycles are expected to be affected by global warming. By means of a mesocosm experiment, the effect of increased water temperature on the biogeochemical cycles of coastal sediments affected by organic-matter enrichment was tested, focusing on the carbon, sulfur, and iron cycles. Nereis diversicolor was used as a model species to simulate macrofaunal bioirrigation activity in natural sediments. Although bioirrigation rates of N. diversicolor were not temperature dependent, temperature did have a major effect on the sediment metabolism. Under organic-enrichment conditions, the increase in sediment metabolism was greater than expected and occurred through the enhancement of anaerobic metabolic pathway rates, mainly sulfate reduction. There was a twofold increase in sediment metabolism and the accumulation of reduced sulfur. The increase in the benthic metabolism was maintained by the supply of electron acceptors through bioirrigation and as a result of the availability of iron in the sediment. As long as the sediment buffering capacity toward sulfides is not surpassed, an increase in temperature might promote the recovery of organic-enriched sediments by decreasing the time for mineralization of excess organic matter.

Biogeochemical malfunctioning in sediments beneath a deep-water fish farm

We investigated the environmental impact of a deep water fish farm (190 m). Despite deep water and low water currents, sediments underneath the farm were heavily enriched with organic matter, resulting in stimulated biogeochemical cycling. During the first 7 months of the production cycle benthic fluxes were stimulated >29 times for CO(2) and O(2) and >2000 times for NH(4)(+), when compared to the reference site. During the final 11 months, however, benthic fluxes decreased despite increasing sedimentation. Investigations of microbial mineralization revealed that the sediment metabolic capacity was exceeded, which resulted in inhibited microbial mineralization due to negative feed-backs from accumulation of various solutes in pore water. Conclusions are that (1) deep water sediments at 8 °C can metabolize fish farm waste corresponding to 407 and 29 mmol m(-2) d(-1) POC and TN, respectively, and (2) siting fish farms at deep water sites is not a universal solution for reducing benthic impacts.

Impact of deep-water fish farms on benthic macrofauna communities under different hydrodynamic conditions.

In this study the environmental impacts of two fish farms located over deep water (180-190 m) were compared. MC-Farm was located at a site with slightly higher water currents (mean current speed 3-5 cms(-1)) than LC-farm (<2 cms(-1)). Macrofauna composition, bioirrigation and benthic fluxes (CO2 and NH4(+)) were quantified at different stages of the production cycle, revealing very different impact of the two farms. Macrofauna abundance and bioirrigation were stimulated compared to a non-impacted reference site at MC-farm, while macrofauna diversity was only moderately reduced. In contrast, macrofauna communities and related parameters were severely impoverished at LC-Farm. This study suggests that deep-water fish farms should not be sited in low current areas (<2 cms(-1)), since this will hamper waste dispersal and aggravate environmental impacts. On the other hand, fish farming at slightly more dynamic sites can lead to stimulated benthic macrofauna communities and only moderate environmental impacts.

Carbon mineralization pathways and bioturbation in coastal Brazilian sediments.

Carbon mineralization processes and their dependence on environmental conditions (e.g. through macrobenthic bioturbation) have been widely studied in temperate coastal sediments, but almost nothing is known about these processes in subtropical coastal sediments. This study investigated pathways of organic carbon mineralization and associated effects of macrobenthic bioturbation in winter and summer (September 2012 and February 2014) at the SE Brazilian coast. Iron reduction (FeR) was responsible for 73–81% of total microbial carbon mineralization in September 2012 and 32–61% in February 2014. Similar high rates of FeR have only been documented a few times in coastal sediments and can be sustained by the presence of large bioturbators. Denitrification accounted for 5–27% of total microbial carbon mineralization while no SO42− reduction was detected in any season. Redox profiles suggested that conditions were less reduced in February 2014 than in September 2012, probably associated with low reactivity of the organic matter, higher rates of aerobic respiration and bioirrigation by the higher density of small-macrofauna. Bioturbation by small macrofauna may maintain the sediment oxidized in summer, while large-sized species stimulate the reoxidation of reduced compounds throughout the year. Therefore, bioturbation seems to have an important role modulating the pathways of carbon mineralization in the area.

Carbon degradation in agricultural soils flooded with seawater after managed coastal realignment

Should be rewritten in a new version of the manuscript Author response: The abstract of the revised manuscript will be thoroughly revised.

Responses of an Agricultural Soil Microbiome to Flooding with Seawater after Managed Coastal Realignment

Coastal areas have become more prone to flooding with seawater due to climate-change-induced sea-level rise and intensified storm surges. One way to cope with this issue is by “managed coastal realignment”, where low-lying coastal areas are no longer protected and instead flooded with seawater. How flooding with seawater impacts soil microbiomes and the biogeochemical cycling of elements is poorly understood. To address this, we conducted a microcosm experiment using soil cores collected at the nature restoration project site Gyldensteen Strand (Denmark), which were flooded with seawater and monitored over six months. Throughout the experiment, biogeochemical analyses, microbial community fingerprinting and the quantification of marker genes documented clear shifts in microbiome composition and activity. The flooding with seawater initially resulted in accelerated heterotrophic activity that entailed high ammonium production and net removal of nitrogen from the system, also demonstrated by a concurrent increase in the abundances of marker genes for ammonium oxidation and denitrification. Due to the depletion of labile soil organic matter, microbial activity decreased after approximately four months. The event of flooding caused the largest shifts in microbiome composition with the availability of labile organic matter subsequently being the most important driver for the succession in microbiome composition in soils flooded with seawater.

Benthic macrofauna bioturbation and early colonization in newly flooded coastal habitats

How will coastal soils in areas newly flooded with seawater function as habitat for benthic marine organisms? This research question is highly relevant as global sea level rise and coastal realignment will cause flooding of soils and form new marine habitats. In this study, we tested experimentally the capacity of common marine polychaetes, Marenzelleria viridis, Nereis (Hediste) diversicolor and Scoloplos armiger to colonize and modify the biogeochemistry of the newly established Gyldensteen Coastal Lagoon, Denmark. All tested polychaetes survived relatively well (28–89%) and stimulated carbon dioxide release (TCO2) by 97–105% when transferred to newly flooded soils, suggesting that soil characteristics are modified rapidly by colonizing fauna. A field survey showed that the pioneering benthic community inside the lagoon was structurally different from the marine area outside the lagoon, and M. viridis and S. armiger were not among the early colonizers. These were instead N. diversicolor and Polydora cornuta with an abundance of 1603 and 540 ind m-2, respectively. Considering the species-specific effects of N. diversicolor on TCO2 release and its average abundance in the lagoon, we estimate that organic carbon degradation was increased by 219% in the first year of flooding. We therefore conclude that early colonizing polychaetes modify the soils and may play an important role in the ecological and successional developments, e.g. C cycling and biodiversity, in newly flooded coastal ecosystems. Newly flooded soils have thus a strong potential to develop into well-functioning marine ecosystems.

Carbon oxidation and bioirrigation in sediments along a Skagerrak-Kattegat-Belt Sea depth transect

Partitioning of electron acceptors and macrofaunal bioirrigation were assessed in sediments from 4 stations along a Skagerrak−Kattegat−Belt Sea depth transect. Sediment was examined for benthic fauna composition and abundance, sediment−water fluxes (O2, dissolved inorganic carbon [DIC], NH4, and NO3), anaerobic reactions (carbon oxidation [Cox], sulfate reduction [SR], manganese reduction [MnR], iron reduction [FeR], and ammonification [Nmin]), porewater profiles (O2, DIC, SO4 and NH4), and solid phase profiles (organic content, Fe and Mn). Deep stations had less than half the abundance of benthic fauna than shallow stations, while the Belt Sea station was azoic due to bottom-water hypoxia. Solute fluxes and anaerobic reactions showed order-of-magnitude lower rates in sediment from deep than from shallow water. In general, anaerobic Cox in sediments along the Skagerrak−Kattegat−Belt Sea transect is dominated by SR (>50%) in shallow water and decreases gradually when moving into deeper water and reaches 0 at ~700 m depth. FeR increases from 0 in shallow water to ~50% at ~600 m, but rapidly reaches 0 again at 700 m. MnR is close to 0 down to at least 500 m and increases to complete dominance at 700 m. The contribution of denitrification is generally below 10% at all depths. Bioirrigation — quantified as non-local exchange through diagenetic modelling—is proportional to fauna biomass and functional traits. The Br− tracer approach to determine bioirrigation on newly extracted sediment onboard a ship is not recommended. It is concluded that enhanced downward trans location of O2 into anoxic sediment through bioirrigation is the major mechanism reoxidizing subsurface metals in the deep Skagerrak and Kattegat.

Stable C and N Isotope Composition of Primary Producers and Consumers Along an Estuarine Salinity Gradient: Tracing Mixing Patterns and Trophic Discrimination

The mixing pattern along a summer salinity gradient in the estuary Odense Fjord was evaluated using nutrient concentrations as well as 13C and 15N isotope signatures of suspended and sediment organic matter, immobile macrophytes (Fucus vesiculosus and Ruppia maritima), and benthic fauna (Mya arenaria, Hediste (Nereis) diversicolor, and Arenicola marina). Trophic discrimination (Δ13C and Δ15N) of the infaunal consumers (suspension feeders and detritivores) was assessed from the obtained mixing patterns along the estuarine gradient. Correspondence between salinity, DIC, and DIN in Odense Fjord implies conservative mixing as also evident from linear relationships between salinity and δ13C and δ15N signatures of most living organic pools. Isotope signatures of suspended organic matter (i.e., diatoms) indicate that the river to marine DIC and DIN end-members have daily/weekly δ13C and δ15N averages during summer from − 10 to 0‰ and 10–12 to 0–5‰, respectively. Stable isotope signatures of long-lived macrophytes stationary at specific locations in Odense Fjord showed δ13C levels that were about 7‰ higher than for suspended particles and 3–4‰ higher than for sediment organic matter, while no such difference was evident for δ15N. The food of invertebrate consumers (M. arenaria, H. diversicolor, and A. marina) determined from the estuarine δ13C and δ15N patterns provided the first ever reported trophic discrimination of these animals. Thus, Δ13C was 1.9, 1.6, and 1.3‰ and Δ15N was 4.4, 5.0, and 3.5‰ for the three species, respectively. Accordingly, benthic suspension and deposit feeders in Odense Fjord are largely supported by a diet consisting of benthic and pelagic microalgae, however, with a possible slight shift in diet proportions or to other food sources in the lower reaches of the estuarine gradient.