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Using the non-invasive aquatic eddy covariance technique, we provide the first oxygen (O2) uptake rates from within coral gardens at the Condor seamount (Azores). To explore some of the key drivers of the benthic O2 demand, we obtained benthic images, quantified local hydrodynamics, and estimated phototrophic biomass and deposition dynamics with a long-term moored sediment trap. The coral gardens were dominated by the octocorals Viminella flagellum and Dentomuricea aff. meteor. Daily rates of O2 uptake within 3 targeted coral garden sites (203 to 206 m depth) ranged from 10.0 ± 0.88 to 18.8 ± 2.0 mmol m-2 d-1 (mean ± SE) and were up to 10 times higher than 2 local sandy reference sites within the seamount summit area. The overall mean O2 uptake rate for the garden (13.4 mmol m-2 d-1) was twice the global mean for sedimentary habitats at comparable depths. Combined with parallel ex situ incubations, the results suggest that the octocorals might contribute just ~5% of the observed O2 uptake rates. Deposition of particulate organic matter (POM) assessed by the sediment trap accounted for less than 10% of the O2 demand of the coral garden, implying a substantial POM supply circumventing the deployed traps. Our results expand the database for carbon turnover rates in cold-water coral habitats by including the first estimates from these largely understudied coral gardens.

Sediment resuspension is a common process in dynamic coastal settings, but its implications for remineralization and carbon turnover in seagrass meadows are poorly understood. Here, we assessed eelgrass Zostera marina metabolism in the Baltic Sea (SW Finland) using benthic flume-chambers and aquatic eddy covariance to critically evaluate the drivers of benthic O-2 exchange during dynamic flow conditions. During quiescent weather conditions, the 2 methods resolved similar metabolic rates and net ecosystem autotrophy (+/- 11. However, elevated flow speeds and sediment resuspension halfway through the study induced a 5-fold increase in the O-2 uptake rates measured by eddy covariance, whereas chamber fluxes remained relatively unchanged. Following particle resettlement, instruments were redeployed and the benthic O-2 uptake resolved by both techniques was just similar to 302 uptake up to 6-fold, mainly due to the reoxidation of reduced compounds (e.g. FeSx). This process was fully captured by the eddy O-2 fluxes, but not by the chamber incubation. Consequently, the chamber and eddy net ecosystem metabolism amounted to - 17 and -824 mmol C m(-2), respectively, throughout the study period. The rapid reoxidation and long-term effects of resuspension on benthic O-2 dynamics highlight the importance of fully capturing dynamic conditions when assessing the overall carbon turnover in coastal habitats. Future studies on the biogeochemical functioning of coastal environments should aim to capture the natural frequency and duration of resuspension events. Sediment resuspension is a common process in dynamic coastal settings, but its implications for remineralization and carbon turnover in seagrass meadows are poorly understood. Here, we assessed eelgrass Zostera marina metabolism in the Baltic Sea (SW Finland) using benthic flume-chambers and aquatic eddy covariance to critically evaluate the drivers of benthic O2 exchange during dynamic flow conditions. During quiescent weather conditions, the 2 methods resolved similar metabolic rates and net ecosystem autotrophy (±11% of each other). However, elevated flow speeds and sediment resuspension halfway through the study induced a 5-fold increase in the O2 uptake rates measured by eddy covariance, whereas chamber fluxes remained relatively unchanged. Following particle resettlement, instruments were redeployed and the benthic O2 uptake resolved by both techniques was just ~30% of the values measured before resuspension. Laboratory investigations revealed sediment resuspension could potentially increase benthic O2 uptake up to 6fold, mainly due to the reoxidation of reduced compounds (e.g. FeSx). This process was fully captured by the eddy O2 fluxes, but not by the chamber incubation. Consequently, the chamber and eddy net ecosystem metabolism amounted to -17 and -824 mmol C m-2, respectively, throughout the study period. The rapid reoxidation and long-term effects of resuspension on benthic O2 dynamics highlight the importance of fully capturing dynamic conditions when assessing the overall carbon turnover in coastal habitats. Future studies on the biogeochemical functioning of coastal environments should aim to capture the natural frequency and duration of resuspension events.

It is well established that benthic infauna alter sediment biogeochemistry, but the importance of their behavioural patterns in biogeochemical processes has only recently been fully appreciated. Using arrays of O2 microoptodes, siphon imaging, accelerometer loggers and different incubation approaches, we investigated the importance of behavioural patterns of the soft-shell clam Mya arenaria for benthic O2 and nitrogen dynamics. The investigations resolved a new behavioural component of buried M. arenaria: pedal water ejection (PWE). During PWE, the clams contracted their valves while briefly closing the siphon to expel oxic water previously accumulated within the mantle cavity through the pedal gape. Consequently, an upward-moving oxic plume embedded most of the shell, and oxia at the shell−sediment interface was observed for 15% of the total time of investigation. The buried clams displayed 2 additional behavioural stages: resting (R) and ventilation (V) that occurred for 57 and 28% of the time, respectively. During the V and PWE stages, the siphon of the clam was elongated above the sediment surface and surrounded by a dynamic oxic halo; both behavioural stages thus markedly increase the sediment oxygenation. The resolved irrigation patterns enhanced the benthic exchange of O2 and NH4 + as well as the benthic denitrification rate, presumably via increased benthic NO3 − supply and production. Particularly, the extensive sediment irrigation induced by the PWE stage has important implications for benthic nutrient cycling as well as for sediment oxidation and biogeochemical function of coastal sediments.

The copepod Calanus hyperboreus, a key species of Arctic marine ecosystems, has a partially anoxic gut that is suspected to host anaerobic microbial activities. So far, however, only dead specimens have been studied in which gut processes are quickly abolished by microbial carcass degradation. Here, live specimens were exposed to different feeding regimes and ambient oxygen levels to study the controls on copepod-associated microbial nitrogen cycling in 15N-enrichment experiments. Algae-fed copepods exhibited consistently high rates of denitrification and low or variable rates of dissimilatory nitrate reduction to ammonium (or nitrite), nitrification, and nitrous oxide release. Thus, live C. hyperboreus contribute to pelagic fixed-nitrogen loss through denitrificationthat is otherwise absent from oxygenated Arctic surface waters. The feeding-related controls on copepod-associated anaerobic nitrogen cycling aresupported by significantly reduced rates in starved copepods. In addition, freshly released fecal pellets showed similar patterns of nitrogen cycling as fedcopepods. Unlike in previous studies on copepod carcasses, low ambient oxygen levels did not stimulate anaerobic nitrogen cycling in live specimens, supportingthat the C. hyperboreus gut is anoxic, irrespective of ambient oxygen levels. Fecal pellets and carcasses retained denitrification activity for 4 d of simulated sinking. We estimate that during the time of the Arctic spring bloom, pelagic denitrification associated with live specimens, fecal pellets, and carcassesof C. hyperboreus is equivalent to ~12% of the benthic fixed-nitrogen loss. This estimate increases to ~28%, provided that the abundant sibling species C.glacialis and C. finmarchicus exhibit denitrification activities in proportion to their smaller body size

Sinking phycodetrital aggregates can contribute to anaerobic nitrogen turnover as they may represent oxygen-depleted microbial hot spots in otherwise oxygenated waters. However, the dynamics of anaerobic nitrogen cycling during the long descent of aggregates through oxic or hypoxic waters are unknown. Thus, model aggregates prepared from the diatom Skeletonema marinoi were allowed to age for 4 d at high and low ambient O2 levels (70 and 15% air saturation, respectively), and changes in nitrogen transformations and microbial community structure were followed. At both O2 levels, denitrification and dissimilatory NO3– reduction to NO2– (DNRN) were the most important processes of aggregate-associated anaerobic nitrogen cycling. However, at 70% air saturation, rates of anaerobic N cycling were lower and decayed towards 0 after an early rise, whereas at 15% air saturation, they remained constantly high at average production rates of 0.66 nmol N2-N aggregate (aggr.)–1 h–1 and 0.26 nmol NO2– aggr.–1 h–1. At both O2 levels, but more pronouncedly at 70% air saturation, the microbial community underwent succession as expressed by an increase in (1) relative abundance of specific bacterial taxonomic units; (2) bacterial diversity; and (3) prokaryotic abundance. Probably, a higher carbon oxidation rate at high ambient O2 level progressively selected for microbes capable of using complex carbon polymers. Taken together, the occurrence of anoxic aggregate centers may be ephemeral at high ambient O2 levels, but persistent at low ambient O2 levels, indicating that sinking phycodetrital aggregates can remain sinks for bioavailable N in the oceans for several days, especially in O2‑depleted settings

Flow of organic carbon (OC) and nitrogen through a sea cage trout farm was calculated on the basis of detailed studies of the farming operation, water circulation, OC and nutrient transport and recycling processes in sediment. A third of the OC and nitrogen provided by fish food was incorporated into fish biomass, which is more than has been found in previous studies. Most OC input was respired by the fish (52 to 70%), and ~63% of the associated nitrogen was lost as dissolved inorganic nitrogen (DIN), potentially stimulating pelagic primary production. Approx. 6% of carbon and 5% of nitrogen derived from fish food settled on the seabed, where it was either mineralized or accumulated in the sediment. Based on transect measurements of diagenetic activity, the farm footprint was found to cover an area ~10 times the farm area. OC mineralization in the sediment increased linearly with increasing food input; the divergence between carbon efflux and oxygen uptake in sediment likewise increased with increasing food input, reflecting an increasing level of sediment reduction. Directly below the farm, the dissolved organic carbon (DOC) efflux was high (on average 53% of dissolved inorganic carbon efflux), indicating that DOC efflux is an important pathway for benthic carbon release below aquaculture farms. Overall, microbial processes removed 56 and 38% of OC and nitrogen, respectively, that settled to the seabed. During a 39 d break in farming activity, due to the combined effect of mineralization and resuspension of surface sediment, sediment conditions improved considerably.

We investigated the microalgal community in sea ice and in the water column of Kobbe- fjord, West Greenland, through an entire sea ice season. Temporal variation in physical (photosynthet- ically active radiation (PAR), temperature, brine volume) and chemical (salinity, nutrient concentra- tion) properties confirmed that sea ice is a very dynamic habitat. Nevertheless, a viable sea ice algal community was present throughout the year, with a species succession from flagellate dominance (di- noflagellates and cryptophytes) in December to February, followed by Chaetoceros simplex (a centric diatom) in March and pennate diatoms in May. Sea ice algal composition, biomass and productivity were influenced by PAR and brine volume, with colonization (enhanced by large brine volume) appar- ent during sea ice melt. Sea ice chlorophyll levels were generally low (

Bacterial and virus-like particle (VLP) abun- dances and physical and chemical parameters were measured in reef water and sediments over a 10 d period, coinciding with mass coral spawning at Heron Island, Great Barrier Reef. Bacterial abundances in reef water increased 2-fold after spawning and remained elevated for 3 d, before declining to below pre-spawning values. Reef water VLP abundances were also elevated 2 d after spawning; however, VLP abundances exhibited a general decline over the study. Dissolved oxygen (DO) and total nitrogen (TN) concentrations appeared to be dominant factors driving reef water bacterial and VLP dynamics. Sediment bacterial and VLP abundances exceeded those in the water column by up to 3 orders of magnitude and exhibited strong positive correlations for all investigated sediment depths. While short-lived peaks in bacterial and VLP abundances within sediments lagged behind water column trends by 2 d, reef water to- tal phosphorus (TP) concentrations were strongly corre- lated with sediment bacterial and VLP abundances. Shifts in bacterial and VLP abundances in reef water and sediments during the study corresponded with 2 distinct periods: one prior to, and one after the first night of in- tense spawning. Scavenging by sedimenting coral spawn material is proposed as a direct mechanism contributing to these shifts, by removing bacteria and VLPs from the water column. The input of organic matter and associated nutrients from mass coral spawning, and the immediate and strongly correlated responses of bacteria and VLPs, indicate that viruses are important players in nutrient cy- cling processes in coral reefs.

The peer-review system is overloaded. This causes problems for reviewers and editors. The focus of this Theme Section (TS) is misuse of the peer-review system by repeated resubmission of unchanged manuscripts (mss). A number of editors and experienced reviewers were invited for comments. Most contributors have seen examples of authors resubmitting mss to new journals after rejection without considering the criticisms of former reviewers. No contributor objects to resubmission of mss to other journals, but all object to authors resubmitting a rejected ms practically unchanged to another journal. For some journals this is not a serious problem, but for others this practice is common and it needs to be stopped. It is fair to give authors a chance for a second opinion on their mss. Most authors take reviewer's reports into consideration before sending the ms to another journal, but many never inform the new journal that the ms had previously been rejected.

A study of the variation in sea ice algal distribution and production was performed dur- ing June and early July 1999 in Young Sound, northeastern Greenland. Measurements were per- formed in situ by divers using an underwater pulse-amplitude-modulated (PAM) fluorometer, oxygen microelectrodes and 14 C incubations. In early June, Young Sound was covered by ~1.6 m of sea ice and ~30 cm of dry snow, which reflected ~75% of the incident light. These conditions resulted in low irradiance below the sea ice (

Photosynthetic performance and cellular chlorophyll a (chl a) content were studied in the mixotrophic dinoflagellate Fragilidium subglobosum during transition from phototrophic to mixotrophic growth and back again. Experiments were conducted at an irradiance of 50 μE m -2 s -1 and, for mixotrophic growth, in the presence of Ceratium tripos or C. lineatum. After 4 to 8 d in mixotrophic culture, cells of F. subglobosum had reduced both their cellular chl a content to 47 % and the photosynthetic performance to ∼42% of that found in phototrophic cultures. Net production of chl a in phototrophic and mixotrophic F. subglobosum cells was the same, indicating that feeding in F. subglobosum does not repress chl a production in mixotrophic cells. Thus, the reduction in cellular chl a content observed in mixotrophic F. subglobosum cells was caused by an increase in growth rate due to feeding. In cultures close to steady state, the photosynthetic performance at low prey concentrations was similar to values obtained by monocultures of F. subglobosum. However, above a certain prey concentration, photosynthetic performance decreased exponentially with an increase in prey concentration. The application of the traditional 14 C labeled HCO 3 - technique to measure photosynthesis in mixotrophically grown cells underestimated rates by 5 to 12%, probably due to fixation of CO 2 produced from respiration of ingested carbon. In phototrophic and mixotrophic F. subglobosum cells 46 and 28% of the assimilated carbon is used for respiration, respectively. This indicates that F. subglobosum needs more energy to synthesize, maintain and run the photosynthetic apparatus than the heterotrophic apparatus involved in prey capture, digestion and assimilation.

By 3 different approaches we demonstrate the ability of filter feeding ciliates (Euplotes spp. and Uronema marinum) to enhance interstitial solute transports. By a simple diffusion analogy we quantify the effect as a function of cell densities. Ciliate densities often encountered in biofilms, microbial mats or estuarine sediments may enhance the transport coefficients of biogeochemical important solutes like, for instance, O 2 by a factor of 1.1 to 10 above the diffusive coefficient. In less productive environments coastal or shelf sedimentsl ciliate densities are usually too low to cause any significant effect on the diffusive mediated solute transport, at least in the case of small fast-diffusing molecules such as O 2 .

The 0, dynamics at the base of biofilms was studied using planar optodes Biofllms were grown directly on the optodes and the 2-dimensional distnbution of O2 at the base of biofilms was resolved at a spatial resolution of 30 X 30 pm, using a CCD camera The average 0, saturahon at the base decreased and the heterogeneity increased as blofllms developed In mature blofilms heterogeneous 0, distributions were caused by clusters of high biomass which had low O2 saturations surrounded by 0,-nch voids and channels The O2 distribution at the base of biofilms was highly dependent on the free flow veloclty above the biofilm e g m a 400 pm thick biofllm the average 0, saturation increased from 0 to 23 1 % air saturation as the free flow velocity increased from 6 2 to 35 1 cm S ' Addition of glucose to a concentrat~on of 2 n1M in the water phase at maximum flow velocity caused the 0, consumption rate to increase and the base of the blofilm to go anoxlc The insertion of an O2 microelectrode into a biofdm caused the 0, saturation at the base of the biofiln~ to increase by approximately 25 pM This effect, presumably caused by hydrodynamic disturbances, typically extended several mm away from the position of the mcrosensor tip The presented data show for the first tune the true distnbution of O2 at the basis of heterogeneous biofilms and demonstrate the great potential of planar optodes for the study of solute dynamcs within biofllms at a very high spabal and temporal resolution

A new tool ('planar optrodes') for measuring fine scale 2-dimensional O-2 distributions in benthic communities is presented and discussed. The sensor consists of an O-2-quenchable fluorophore cast onto a 25 x 55 mm, 175 mu m thick transparent PVC sheet, which can be introduced into sediments and microbial mats. The O-2 sensitive fluorescence emitted by the planar optrode was measured with a high resolution CCD camera. A 3-point calibration was performed for each pixel in the acquired images to calculate the 2-dimensional oxygen distribution with high accuracy. Calibrated images covered an area of 13 x 17 mm and the set-up allowed the 2-dimensional distribution of O-2 to be resolved with a spatial resolution of 26 mu m. The sensitivity of the sensor had excellent long-term stability. The planar optrode was used to record the oxygen equilibration between small gas bubbles and surrounding water with a temporal resolution of 20 s. The sensor was also used to measure the vertical and horizontal O-2 distribution in an intertidal sediment. The obtained distribution correlated well with O-2 concentration microprofiles measured with an O-2 microelectrode. The data showed pronounced differences in the O-2 penetration depths due to patchy distribution of highly labile organic carbon and showed differences in thickness of the diffusive boundary layer due to small topographic structures on the sediment surface. Potential applications of the planar optrode are presented and discussed.

Although O2 microsensors used for study of benthic photosynthesis have tip diameters of less than 10 pm and stem diameters of about 150 pm at a distance of 0.5 cm from the tip, they do change the local flow pattern in the overlying water and thereby reduce the thickness of the diffusive boundary layer (DBL). The reduced DBL resulted in a lower diffusional resistance towards export of 0, from communities with a net production of Oa, and insertion of a microsensor in such a community therefore resulted in lowered O2 concentrations. The total diffusion flux out of the photosynthetic layer was not significantly dffected by the presence of the microsensor. The higher upward flux into the water phase was counterbalanced by a reduced downward flux into the sediment. The O2 profiles and diffusion fluxes were only affected when the surface was smooth, while the effect of microsensor insertion was undetectable when the surface was made irregular by small tufts, bubbles, etc. In the investigated cyanobacteria-dominated mats, the gross photosynthetic rates as measured by the light/dark shift technique were unaffected by the presence of a microsensor.