Diffusion scale dependent change in anaerobic carbon and nitrogen mineralization: True effector experimental artifact?

Burrowing macrofauna is known to influence the diffusion properties and solute transport within sediments as well as promoting aerobic microbial processes. It has recently been argued that changes in diffusion scales also affect anaerobic microbial processes in sediments. We tested this contention on coastal sandy sediment using a widely employed sediment plug flux-incubation technique. Our results confirm that volume specific microbial carbon and nitrogen mineralization apparently are enhanced (6–15 and 3–9 times, respectively) in thin (1 mm) compared with thick (10 cm) sediment plugs incubated in anoxic seawater. However, differential accumulation of inhibiting substances among plug sizes cannot explain this phenomenon, as the same pattern was observed for plugs incubated in anoxic porewater. Instead, we found that microbial activity in the overlying water and biofilms along the walls of incubation chambers affects the results significantly. A low, but constant production of 3–7 (TCO2) and 0.5 (NH4+) nmol cm−3 d−1 in the water phase and 51–140 (TCO2) and 4–16 (NH4+) nmol cm−2 d−1in biofilms irrespective of plug thickness can explain a large part of the apparent diffusion scale dependent change in volume specific solute production in the sediment. Model considerations show that sediment independent water phase and biofilm reactions occurring in the incubation chambers result in highly overestimated volume specific rates for thin plugs. The overestimate is negligible for plugs thicker than 1 cm because the total reactions occurring within these larger sediment plugs exceed those in the water and biofilms considerably. We therefore conclude that most of the apparent inverse relationship between diffusion scales and anaerobic microbial processes in sandy sediment found by the use of flux-incubation chambers is an experimental artifact rather than a true diffusion scale dependent effect

A productive start for new centre

I 2011 kunne fiskere og forskere stå på broen på fiskebåten og følge med på direkte videooverføring av fisk som gikk inn i trålen. Tråldører med justerbare luker og et system som automatisk identifiserer og sorterer fisk er andre nyvinninger som har vært med på å gi CRISP-senteret en god start. :: In 2011, fishers and researchers could stand on the bridge of a fishing vessel and watch live video of the fish entering the trawl net. Trawl doors with adjustable hatches and a system that automatically identifies and sorts fish are two of the other innovations that have helped to give the CRISP centre a good start

Effects of temperature and organic pollution on nutrient cycling in marine sediments

Increasing ocean temperature due to climate change is an important anthropogenic driver of ecological change in coastal systems. In these systems sediments play a major role in nutrient cycling. Our ability to predict ecological consequences of climate change is enhanced by simulating real scenarios. Based on predicted climate change scenarios, we tested the effect of temperature and organic pollution on nutrient release from coastal sediments to the water column in a mesocosm experiment. PO43− release rates from sediments followed the same trends as organic matter mineralization rates, increased linearly with temperature and were significantly higher under organic pollution than under nonpolluted conditions. NH4+ release only increased significantly when the temperature rise was above 6 °C, and it was significantly higher in organic polluted compared to nonpolluted sediments. Nutrient release to the water column was only a fraction from the mineralized organic matter, suggesting PO43− retention and NH4+ oxidation in the sediment. Bioturbation and bioirrigation appeared to be key processes responsible for this behavior. Considering that the primary production of most marine basins is N-limited, the excess release of NH4+ at a temperature rise > 6 °C could enhance water column primary productivity, which may lead to the deterioration of the environmental quality. Climate change effects are expected to be accelerated in areas affected by organic pollution.CS was supported by the Ministerio de Educación y Ciencia of Spain

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.M. Holmer was supported by FNU 09-071369, C. Sanz-Lázaro was supported by a predoctoral grant from the Ministerio de Educación y Ciencia of Spain, and T. Valdemarsen was supported by EU-project Thresholds (Contract No. 003933)