Ursula Witte

Benthic biogeochemistry: state of the art technologies and guidelines for the future of in situ survey

Sediment and water can potentially be altered, chemically, physically and biologically as they are sampled at the seafloor, brought to the surface, processed and analysed. As a result, in situ observations of relatively undisturbed systems have become the goal of a growing body of scientists. Our understanding of sediment biogeochemistry and exchange fluxes was revolutionized by the introduction of benthic chambers and in situ micro-electrode profilers that allow for the direct measurement of chemical fluxes between sediment and water at the sea floor and for porewater composition. Since then, rapid progress in the technology of in situ sensors and benthic chambers (such as the introduction of gel probes, voltammetric electrodes or one- and two-dimensional optodes) have yielded major breakthroughs in the scientific understanding of benthic biogeochemistry. This paper is a synthesis of discussions held during the workshop on sediment biogeochemistry at the “Benthic Dynamics: in situ surveillance of the sediment–water interface” international conference (Aberdeen, UK—March 25–29, 2002). We present a review of existing in situ technologies for the study of benthic biogeochemistry dynamics and related scientific applications. Limitations and possible improvement (e.g., technology coupling) of these technologies and future development of new sensors are discussed. There are countless important scientific and technical issues that lend themselves to investigation using in situ benthic biogeochemical assessment. While the increasing availability of these tools will lead research in yet unanticipated directions, a few emerging issues include greater insight into the controls on organic matter (OM) mineralization, better models for the understanding of benthic fluxes to reconcile microelectrode and larger-scale chamber measurements, insight into the impacts of redox changes on trace metal behavior, new insights into geochemical reaction pathways in surface sediments, and a better understanding of contaminant fate in nearshore sediments.

Europe’s Grand Canyon: Nazaré submarine canyon

The Nazare submarine canyon extends similar to 210 km westward from the coast of Portugal, down to a water depth of > 4300 m. The considerable habitat heterogeneity found throughout the canyon is affected by strong currents and high turbidity, especially in the upper parts of the canyon. The canyon morphology comprises steep slopes, scarps, terraces, and overhangs, and a deeply incised thalweg is found in the lower part of the canyon. The seabed within the canyon is composed of varying proportions of rock and sediments that range from sand to fine mud. This great variation in physical environment is reflected by the varied fauna inhabiting the canyon. Diversity tends to decrease with depth, but there is also continual replacement of species with increasing water depth. Certain groups, such as the gorgonians and sea lilies, tend to be found on rocky surfaces, while large protozoans dominate the sediments at 3400-m depth. In addition to describing the fauna of Nazare Canyon, we discuss experiments undertaken as part of the HERMES project to elucidate the ecosystem function processes operating in the deeper parts of the canyon.

Benthic-pelagic coupling in the Greenland-Norwegian Sea and its effect on the geological record

The sedimentation pattern of organic material in the Greenland-Norwegian Sea is reflected in the surface sediments, although less than 0.5% of the organic matter is buried in the sediment. Maximum fluxes and benthic responses are observed during June and/or August/September, following the pattern of export production in the pelagial zone. The annual remineralization rate on the Vøring Plateau is 3.0 g C m−2 a −1 Freshly settled phytodetritus, as detected by chlorophyll measurements, is rapidly mixed into the sediment and decomposed. It stimulates the activity of benthic organisms, especially foraminifera. The mixing coefficient for this material is Db=0.2 cm2 d−1, which is two to three orders of magnitude higher than that estimated from radiotracer methods. The effect on the geological record, however, is likely to be small. Chlorophyll-containing particles are at first very evenly distributed on the seafloor. After partial decomposition and resuspension, a secondary redistribution of particles occurs which can result in the formation of a high accumulation area, with an up to 80-fold increase in the sedimentation rate by lateral advection. This is mainly due to physical processes, because biodeposition mediated by benthic animals increases sedimentation by only a factor of two or three.

Scavenger assemblages under differing trophic conditions : a case study in the deep Arabian Sea

Baited cameras and traps were deployed at four stations in the deep Arabian Sea to investigate the composition of the necrophagous fauna and to evaluate whether regional differences in trophic conditions are reflected by differing scavenger assemblages. The ophidiid fish Barathrites iris, the large lysianassoid amphipod Eurythenes gryllus, the aristeid prawn Plesiopenaeus armatus, and zoarcid fishes of the genus Pachycara were abundant at the bait at all stations. The ophidiid Holcomycteronus aequatorius, the liparid fish Paraliparis sp., and galatheid crabs of the genus Munidopsis occurred in considerable numbers at single sites. Trap catches further contained lysianassoid amphipods of the genera Paralicella, Abyssorchomene and Paracallisoma. In contrast to scavenger assemblages of the Atlantic and Pacific Ocean, macrourid fishes were virtually absent at the bait. E. gryllus and B. iris consumed the main proportion of the bait, while consumption was at most moderate in all other taxa. Feeding strategies of the respective taxa are inferred from their behavior at the bait and discussed with regard to the profit that can be drawn from food falls. Differences between stations were pronounced with respect to species dominating bait consumption. E. gryllus appeared in highest numbers at the bait in the productive northern and central Arabian Sea where a relatively high availability of food items is expected to sustain high population densities. High numbers of B. iris in the least productive southern part indicate their ability to persist under food-poor conditions and may correspond to a high dependency on food falls. E. gryllus and B. iris both occurred in smaller numbers in the particularly productive western Arabian Sea. This may reflect a reduced dependency on food falls, due to an access to alternative food sources, rather than small population densities. Smaller numbers of E. gryllus and B. iris resulted in slower bait consumption and gave Pachycara spp. the opportunity to contribute considerably to bait consumption. The relation between scavenger assemblages and trophic conditions is discussed with respect to results obtained under differing trophic regimes in the Atlantic and Pacific Ocean.

Resource quality affects carbon cycling in deep-sea sediments

Deep-sea sediments cover ∼70% of Earths surface and represent the largest interface between the biological and geological cycles of carbon. Diatoms and zooplankton faecal pellets naturally transport organic material from the upper ocean down to the deep seabed, but how these qualitatively different substrates affect the fate of carbon in this permanently cold environment remains unknown. We added equal quantities of 13C-labelled diatoms and faecal pellets to a cold water (−0.7 °C) sediment community retrieved from 1080 m in the Faroe-Shetland Channel, Northeast Atlantic, and quantified carbon mineralization and uptake by the resident bacteria and macrofauna over a 6-day period. High-quality, diatom-derived carbon was mineralized >300% faster than that from low-quality faecal pellets, demonstrating that qualitative differences in organic matter drive major changes in the residence time of carbon at the deep seabed. Benthic bacteria dominated biological carbon processing in our experiments, yet showed no evidence of resource quality-limited growth; they displayed lower growth efficiencies when respiring diatoms. These effects were consistent in contrasting months. We contend that respiration and growth in the resident sediment microbial communities were substrate and temperature limited, respectively. Our study has important implications for how future changes in the biochemical makeup of exported organic matter will affect the balance between mineralization and sequestration of organic carbon in the largest ecosystem on Earth.

Enhanced benthic activity in sandy sublittoral sediments: Evidence from 13C tracer experiments

Abstract In situ and on-board pulse-chase experiments were carried out on a sublittoral fine sand in the German Bight (southern North Sea) to investigate the hypothesis that sandy sediments are highly active and have fast turnover rates. To test this hypothesis, we conducted a series of experiments where we investigated the pathway of settling particulate organic carbon through the benthic food web. The diatom Ditylum brightwellii was labelled with the stable carbon isotope 13C and injected into incubation chambers. On-board incubations lasted 12, 30 and 132 h, while the in situ experiment was incubated for 32 h. The study revealed a stepwise short-term processing of a phytoplankton bloom settling on a sandy sediment. After the 12 h incubation, the largest fraction of recovered carbon was in the bacteria (62%), but after longer incubation times (30 and 32 h in situ) the macrofauna gained more importance (15 and 48%, respectively), until after 132 h the greatest fraction was mineralized to CO2 (44%). Our findings show the rapid impact of the benthic sand community on a settling phytoplankton bloom and the great importance of bacteria in the first steps of algal carbon processing.

Patterns and Determinants of the Distribution and Structure of Benthic Faunal Assemblages in the Northern North Atlantic

The distribution and structure of zoobenthic communities have been investigated in the northern North Atlantic. The principal goal of these studies is to assess the degree to which benthic community patterns depend on and/ or mediate carbon flux between the pelagic and benthic realms, as well as between seabed, sediment-water interface and benthic boundary layer. A common rationale is that these patterns integrate the impact of environmental factors over longer periods of time and reflect relatively long-lasting or predictably recurrent environmental states, thus providing clues to the relative significance of potential community determinants on a time scale of months to years. Since 1992, several meso-scale field studies have been carried out in three regions at the East Greenland continental margin between 68° N and 81° N at water depths ranging from 40 to 3,700 m. A suite of sampling methods was employed (corers, trawls, seabed imaging) to adequately probe various benthic community fractions, such as foraminifers, poriferans, macrobenthic endofauna, peracarid crustaceans and megabenthic epifauna.A depth zonation in the faunal composition, accompanied by a shift in the predominance of different feeding types and a significant decline in biomass and abundance by as much as two and three orders of magnitude was the most conspicuous general pattern detected. However, in terms of species richness, no common trend for water depth or latitude was perceivable. The general depth zonation of the macrobenthos as well as the spatial concordance of high macrobenthic abundance and biomass with relatively productive hydrographic zones, such as marginal ice zones, polynyas and anti-cyclonic gyres, provide evidence for the importance of water column processes and, hence, for subsequent food availability as major determinants for benthic assemblages and the significance of pelago -benthic coupling in the study area in general. However, for megafaunal species such as echinoderms, community patterns on a 10-1an scale and the dispersion of organisms on a 100-m scale, are best explained by seafloor properties. There is no evidence of direct pelago-benthic coupling, irrespective of water depth. These contrasting findings emphasize that the relative importance of potential community determinants can change with both spatial scale and life traits, e.g. body size, mobility and feeding ecology, of the organisms considered.

Negative Priming Effect on Organic Matter Mineralisation in NE Atlantic Slope Sediments

The priming effect (PE) is a complex phenomenon which describes a modification (acceleration or retardation) in the mineralisation rate of refractory organic matter (OM) following inputs of labile material. PEs are well-studied in terrestrial ecosystems owing to their potential importance in the evolution of soil carbon stocks but have been largely ignored in aquatic systems despite the fact that the prerequisite for their occurrence, i.e. the co-existence of labile and refractory OM, is also true for sediments. We conducted stable isotope tracer experiments in continental margin sediments from the NE Atlantic (550–950 m) to study PE occurrence and intensity in relation to labile OM input. Sediment slurries were treated with increasing quantities of the 13C-labelled diatom Thalassiosira rotula and PE was quantified after 7, 14 and 21 days. There was a stepwise effect of diatom quantity on its mineralisation although mineralisation efficiency dropped with increasing substrate amounts. The addition of diatomaceous OM yielded a negative PE (i.e. retardation of existing sediment OM mineralisation) at the end of the experiment regardless of diatom quantity. Negative PE is often the result of preferential utilisation of the newly deposited labile material by the microbial community (“preferential substrate utilization”, PSU) which is usually observed at excessive substrate additions. The fact that PSU and the associated negative PE occurred even at low substrate levels in this study could be attributed to limited amounts of OM subject to priming in our study area (∼0.2% organic carbon [OC]) which seems to be an exception among continental slopes (typically >0.5%OC). We postulate that PEs will normally be positive in continental slope sediments and that their intensity will be a direct function of sediment OC content. More experiments with varying supply of substrate targeting C-poor vs. C-rich sediments are needed to confirm these hypotheses.

The reproductive cycle of the sponge Halichondria panicea Pallas (1766) and its relationship to temperature and salinity

Abstract The reproductive cycle of the sponge Halichondria panicea was investigated at Boknis Eck in the Western Kiel Bight over 2 yr, and over 1 yr on the island Helgoland in the North Sea and was compared with material collected earlier at Tjarno at the Swedish west coast. Temperature and salinity were monitored at the different stations in order to determine whether different hydrographic conditions could be linked to differences in seasonal patterns of the reproductive cycle. The results showed H. panicea to be gonochoristic at all stations. The ratio of males to females varied between populations and years, but females predominated consistently. Comparison of the reproductive cycle in the years 1987 and 1989 at the brackish-water station Boknis Eck shows that high ambient winter and spring temperatures in 1989 resulted in a shorter period of oogenesis and earlier larval release, as compared to the year 1987. The reproductive cycle of the sponges at the fully marine station at Helgoland in 1989 and the brackish-water station at Tjarno in 1978 under temperature and salinity conditions similar to those at Boknis Eck in 1989 showed the same seasonal pattern. We conclude that the lower salinities found at both Boknis Eck and Tjarno do not cause a delay or slow-down of reproduction at these stations as compared to Helgoland. Under favourable temperature conditions, oogenesis was highly synchronous within the different populations, while it was more spread over time under the low ambient water temperatures at Boknis Eck in 1987.

Instantaneous benthic response to different organic matter quality: In situ experiments in the Benguela Upwelling System

Abstract The benthic community in continental slope and deep-sea sediments of the Benguela Upwelling System was supplied with 13C-labelled organic matter (OM) of two different qualities using a benthic chamber lander. Freeze-dried cultures of Skeletonema costatum served as ‘fresh’ OM. ‘Altered’ OM of the same material had been additionally dialysed to remove low-molecular weight compounds. In order to investigate the benthic response pattern, mineralization of labelled OM, uptake by macrofauna and incorporation into bacteria were followed over 18–36 h. Total oxygen uptake was not affected beyond natural variation by the OM addition. Mineralization dominated the 13C-labelled phytodetritus processing, constituting 71–95% of the total processed OM. Bacterial incorporation of phytodetrital carbon exceeded macrofaunal uptake at all stations. Stations situated in a major centre of OM deposition showed phytodetritus processing rates on average twice as high as outside the depocentre. Phytodetritus processing was 1.5, 2.5 and 4.3 times higher for fresh than for altered OM at 605, 1019 and 1335 m water depth, respectively. Our observations clearly indicate the importance of OM quality on mineralization rates.