Variable Importance of Macrofaunal Functional Biodiversity for Biogeochemical Cycling in Temperate Coastal Sediments

Coastal marine systems are currently subject to a variety of anthropogenic and climate-change-induced pressures. An important challenge is to predict how marine sediment communities and benthic biogeochemical cycling will be affected by these ongoing changes. To this end, it is of paramount importance to first better understand the natural variability in coastal benthic biogeochemical cycling and how this is influenced by local environmental conditions and faunal biodiversity. Here, we studied sedimentary biogeochemical cycling at ten coastal stations in the Southern North Sea on a monthly basis from February to October 2011. We explored the spatio-temporal variability in oxygen consumption, dissolved inorganic nitrogen and alkalinity fluxes, and estimated rates of nitrification and denitrification from a mass budget. In a next step, we statistically modeled their relation with environmental variables and structural and functional macrobenthic community characteristics. Our results show that the cohesive, muddy sediments were poor in functional macrobenthic diversity and displayed intermediate oxygen consumption rates, but the highest ammonium effluxes. These muddy sites also showed an elevated alkalinity release from the sediment, which can be explained by the elevated rate of anaerobic processes taking place. Fine sandy sediments were rich in functional macrobenthic diversity and had the maximum oxygen consumption and estimated denitrification rates. Permeable sediments were also poor in macrobenthic functional diversity and showed the lowest oxygen consumption rates and only small fluxes of ammonium and alkalinity. Macrobenthic functional biodiversity as estimated from bioturbation potential appeared a better variable than macrobenthic density in explaining oxygen consumption, ammonium and alkalinity fluxes, and estimated denitrification. However, this importance of functional biodiversity was manifested particularly in fine sandy sediments, to a lesser account in permeable sediments, but not in muddy sediments. The strong relationship between macrobenthic functional biodiversity and biogeochemical cycling in fine sandy sediments implies that a future loss of macrobenthic functional diversity will have important repercussions for benthic ecosystem functioning.     

Response of Estuarine Biofilm Microbial Community Development to Changes in Dissolved Oxygen and Nutrient Concentrations

The information content and responsiveness of microbial biofilm community structure, as an integrative indicator of water quality, was assessed against short-term changes in oxygen and nutrient loading in an open-water estuarine setting. Biofilms were grown for 7-day periods on artificial substrates in the Pensacola Bay estuary, Florida, in the vicinity of a wastewater treatment plant (WWTP) outfall and a nearby reference site. Substrates were deployed floating at the surface and near the benthos in 5.4 m of water. Three sampling events covered a 1-month period coincident with declining seasonal WWTP flow and increasing dissolved oxygen (DO) levels in the bottom waters. Biomass accumulation in benthic biofilms appeared to be controlled by oxygen rather than nutrients. The overriding effect of DO was also seen in DNA fingerprints of community structure by terminal restriction fragment length polymorphism (T-RFLP) of amplified 16S rRNA genes. Ribotype diversity in benthic biofilms at both sites dramatically increased during the transition from hypoxic to normoxic. Terminal restriction fragment length polymorphism patterns showed pronounced differences between benthic and surface biofilm communities from the same site in terms of signal type, strength, and diversity, but minor differences between sites. Sequencing of 16S rRNA gene clone libraries from benthic biofilms at the WWTP site suggested that low DO levels favored sulfate-reducing prokaryotes (SRP), which decreased with rising oxygen levels and increasing overall diversity. A 91-bp ribotype in the CfoI-restricted 16S rRNA gene T-RFLP profiles, indicative of SRP, tracked the decrease in relative SRP abundance over time.     

Consequences of Increasing Hypoxic Disturbance on Benthic Communities and Ecosystem Functioning

Disturbance-mediated species loss has prompted research considering how ecosystem functions are changed when biota is impaired. However, there is still limited empirical evidence from natural environments evaluating the direct and indirect (i.e. via biota) effects of disturbance on ecosystem functioning. Oxygen deficiency is a widespread threat to coastal and estuarine communities. While the negative impacts of hypoxia on benthic communities are well known, few studies have assessed in situ how benthic communities subjected to different degrees of hypoxic stress alter their contribution to ecosystem functioning. We studied changes in sediment ecosystem function (i.e. oxygen and nutrient fluxes across the sediment water-interface) by artificially inducing hypoxia of different durations (0, 3, 7 and 48 days) in a subtidal sandy habitat. Benthic chamber incubations were used for measuring responses in sediment oxygen and nutrient fluxes. Changes in benthic species richness, structure and traits were quantified, while stress-induced behavioral changes were documented by observing bivalve reburial rates. The initial change in faunal behavior was followed by non-linear degradation in benthic parameters (abundance, biomass, bioturbation potential), gradually impairing the structural and functional composition of the benthic community. In terms of ecosystem function, the increasing duration of hypoxia altered sediment oxygen consumption and enhanced sediment effluxes of NH₄ ⁺ and dissolved Si. Although effluxes of PO₄ ³⁻ were not altered significantly, changes were observed in sediment PO₄ ³⁻ sorption capability. The duration of hypoxia (i.e. number of days of stress) explained a minor part of the changes in ecosystem function. Instead, the benthic community and disturbance-driven changes within the benthos explained a larger proportion of the variability in sediment oxygen- and nutrient fluxes. Our results emphasize that the level of stress to the benthic habitat matters, and that the link between biodiversity and ecosystem function is likely to be affected by a range of factors in complex, natural environments.     

Environmental Drivers of Benthic Flux Variation and Ecosystem Functioning in Salish Sea and Northeast Pacific Sediments

The upwelling of deep waters from the oxygen minimum zone in the Northeast Pacific from the continental slope to the shelf and into the Salish Sea during spring and summer offers a unique opportunity to study ecosystem functioning in the form of benthic fluxes along natural gradients. Using the ROV ROPOS we collected sediment cores from 10 sites in May and July 2011, and September 2013 to perform shipboard incubations and flux measurements. Specifically, we measured benthic fluxes of oxygen and nutrients to evaluate potential environmental drivers of benthic flux variation and ecosystem functioning along natural gradients of temperature and bottom water dissolved oxygen concentrations. The range of temperature and dissolved oxygen encountered across our study sites allowed us to apply a suite of multivariate analyses rarely used in flux studies to identify bottom water temperature as the primary environmental driver of benthic flux variation and organic matter remineralization. Redundancy analysis revealed that bottom water characteristics (temperature and dissolved oxygen), quality of organic matter (chl a:phaeo and C:N ratios) and sediment characteristics (mean grain size and porosity) explained 51.5% of benthic flux variation. Multivariate analyses identified significant spatial and temporal variation in benthic fluxes, demonstrating key differences between the Northeast Pacific and Salish Sea. Moreover, Northeast Pacific slope fluxes were generally lower than shelf fluxes. Spatial and temporal variation in benthic fluxes in the Salish Sea were driven primarily by differences in temperature and quality of organic matter on the seafloor following phytoplankton blooms. These results demonstrate the utility of multivariate approaches in differentiating among potential drivers of seafloor ecosystem functioning, and indicate that current and future predictive models of organic matter remineralization and ecosystem functioning of soft-muddy shelf and slope seafloor habitats should consider bottom water temperature variation. Bottom temperature has important implications for estimates of seasonal and spatial benthic flux variation, benthic–pelagic coupling, and impacts of predicted ocean warming at high latitudes.     

Effects of sedimentation from water-based drill cuttings and natural sediment on benthic macrofaunal community structure and ecosystem processes

In this study, we aim at investigating the role of physical disturbance in effects of water-based drill cuttings on benthic ecosystems. Today, most of the cuttings discharged from oil and gas installations contain water-based drilling muds, rather than oil-based or synthetic muds. Drill cuttings with water-based muds are assumed to cause only marginal effects on the benthos, mainly resulting from sedimentation. However, this statement has not been experimentally tested, which is the purpose of the present work. Natural sediment particles and water-based drill cuttings were added to benthic communities in layer thicknesses of 3-24 mm in a mesocosm set-up. During the following 6 months, changes in benthic community structure and fluxes of oxygen and nutrients across the sediment water interface were studied. There was a significant reduction in number of taxa, abundance, biomass and diversity of macrofauna with increasing thickness of drill cuttings, which was not observed for the natural sediment particles. The drill cuttings also influenced oxygen consumption and oxygen penetration depth in the sediment, and it was concluded that an organic compound in the drill cuttings initiated a typical eutrophication response. Fluxes of phosphate and silicate were, however, similarly affected by the two types of particles, and maximum fluxes occurred in sediments treated with thin layers (3-6 mm) of particles. As the response of water-based drill cuttings in the present study was a result of factors other than physical disturbance, we recommend a reconsideration of the assumption that water-based drill cuttings only cause sedimentation (burial) effects.     

Functioning of a Shallow-Water Sediment System during Experimental Warming and Nutrient Enrichment

Effects of warming and nutrient enrichment on intact unvegetated shallow-water sediment were investigated for 5 weeks in the autumn under simulated natural field conditions, with a main focus on trophic state and benthic nitrogen cycling. In a flow-through system, sediment was exposed to either seawater at ambient temperature or seawater heated 4°C above ambient, with either natural or nutrient enriched water. Sediment–water fluxes of oxygen and inorganic nutrients, nitrogen mineralization, and denitrification were measured. Warming resulted in an earlier shift to net heterotrophy due to increased community respiration; primary production was not affected by temperature but (slightly) by nutrient enrichment. The heterotrophic state was, however, not further strengthened by warming, but was rather weakened, probably because increased mineralization induced a shortage of labile organic matter. Climate-related warming of seawater during autumn could therefore, in contrast to previous predictions, induce shorter but more intensive heterotrophic periods in shallow-water sediments, followed by longer autotrophic periods. Increased nitrogen mineralization and subsequent effluxes of ammonium during warming suggested a preferential response of organisms driving nitrogen mineralization when compared to sinks of ammonium such as nitrification and algal assimilation. Warming and nutrient enrichment resulted in non-additive effects on nitrogen mineralization and denitrification (synergism), as well as on benthic fluxes of phosphate (antagonism). The mode of interaction appears to be related to the trophic level of the organisms that are the main drivers of the affected processes. Despite the weak response of benthic microalgae to both warming and nutrient enrichment, the assimilation of nitrogen by microalgae was similar in magnitude to rates of nitrogen mineralization. This implies a sustained filter function and retention capacity of nutrients by the sediment.     

Relative influences of submersed macrophytes and bioturbating fauna on biogeochemical processes and microbial activities in freshwater sediments

1. Invertebrates and aquatic plants often play a key role in biogeochemical processes occurring at the water–sediment interface of aquatic ecosystems. However, few studies have investigated the respective influences of plants and bioturbating animals on ecological processes (nutrient fluxes, benthic oxygen uptake, microbial activities) occurring in freshwater sediments. 2. We developed a laboratory experiment in aquaria to quantify the effects of (i) one invertebrate acting as a bioturbator (Tubifex tubifex); (ii) one submersed plant with a high sediment‐oxygenating potential (Myriophyllum spicatum) and (iii) one submersed plant with a low sediment‐oxygenating potential (Elodea canadensis). 3. The tubificid worms significantly increased the fluxes of nitrogen at the water–sediment interface (influx of nitrate, efflux of ammonium), whereas the two plant species did not have significant influences on these nitrogen fluxes. The differences in nitrogen fluxes between tubificid worms and plants were probably due to the bioirrigation process caused by T. tubifex, which increased water exchanges at the water–sediment interface. Tubifex tubifex and M. spicatum produced comparable reductions of nutrient concentrations in pore water and comparable stimulations of benthic oxygen uptake and microbial communities (percentages of active eubacteria and hydrolytic activity) whereas E. canadensis had a very weak influence on these variables. These differences between the two plants were due to their contrasting abilities to increase oxygen in sediments by radial oxygen losses (release of oxygen from roots). 4. Our study suggests that the bioirrigation process and radial oxygen loss are major functional traits affecting biogeochemical functioning at the water–sediment interface of wetlands.     

Macrophyte communities and their impact on benthic fluxes of oxygen, sulphide and nutrients in shallow eutrophic environments

The impact of macrophyte communities on benthic fluxes has been analyzed in three shallow coastal environments: Etang du Prévost (Mediterranean coast of France), characterized by the large floating macro-alga Ulva rigida; Certes fishponds (Bassin d'Arcachon), covered by Ruppia cirrhosa; and the inner intertidal mud-flat in the Arcachon Bay (French Atlantic coast), which has extensive Zostera noltii meadows. In these bodies of water, primary production is dependent primarily on the dominant seagrasses and macroalgae that are also responsible for the large quantity of organic matter deposited on the sediment surface. In 1993 and 1994, fluxes of oxygen, sulphide and nutrients were measured in early and late summer, which were selected in order to represent the production and decomposition phases of the dominant macrophytes. Experimental work was undertaken to measure: (1) standing crop of dominant macroalgae and rooted phanerogams and the elemental and macromolecular composition of plant biomass; (2) benthic fluxes of oxygen, sulphide, nitrogen and phosphorus using incubation of multiple dark and light benthic chambers; (3) water-sediment profiles of free-sulphide in sediment cores with rooted phanerogams (Ruppia) as well as with floating Seaweeds (Ulva).At the selected sampling sites, in addition to external (tides) and/or internal (sediment reactivity) factors, we observed differences in benthic fluxes which were clearly related to growth patterns and structure of the macrophyte communities. The Z. noltii meadows were stable and characterized by slow growth and almost constant biomass. In the more sheltered sampling station in the Certes fishponds, R. cirrhosa beds showed a summer decrease due to extensive epiphyte growth. During the decomposition phase, significant fluxes of free-sulphide occurred inside the dark benthic chambers, probably due to the metabolism of the epiphytic layer. In the Etang du Prévost, U. rigida achieved high biomass levels, even though the macroalgal beds exhibited a patchy distribution due to wind action and the hydrodynamics of the lagoon. In the decomposition phase, which was coincident with the annual dystrophic crisis the rapid decomposition of Ulva led to high fluxes of free sulphide.The shift from the production to decomposition phase resulted in substantial changes in nutrient recycling only in the macro-algal-dominated system. During the growth period dissolved inorganic nitrogen and phosphorus were kept at low levels due to macrophyte uptake. In contrast during the decomposition phase when the macroalgal biomass was mineralised, nitrogen and phosphorus were rapidly recycled. The same processes did not occur in the Certes fishponds probably because of the greater internal buffering capacity linked either to plant morphology/physiology or to the properties of the sediment.     

Temporal variations in microbenthic metabolism and inorganic nitrogen fluxes in sandy and muddy sediments of a tidally dominated bay in the northern Wadden Sea

Factors controlling seasonal variations in benthic metabolism (O₂ flux) and dissolved inorganic nitrogen (DIN) fluxes were examined during a 12–14 month period at three intertidal Wadden Sea stations. Since the flux measurements were made as small-scale laboratory core incubations, the results are primarily related to the microbenthic community (microalgae, bacteria, micro-, meio- and small macrofauna) and cannot be considered representative of the total benthic community in the Wadden Sea. Furthermore, it has to be emphasized that light intensity during day-time simulations were constant and saturating at all times. Benthic primary production and oxygen uptake appeared to be temperature dependent with a ‘seasonal Q₁₀’ of 1.7–1.8 and 2.7–4.3, respectively. Inundation had no effect on oxygen fluxes as evidenced by similar sediment respiration with and without water cover. A stronger temperature dependence of primary production in muddy than in sandy sediment indicated that the overall control in the latter may be complex due to factors like macrofaunal grazing and nutrient availability. Benthic respiration may not be controlled by temperature alone, as sedimentary organic matter content correlated significantly with both temperature and benthic respiration. Annual gross primary production in high intertidal sandy sediment was 10 and 50% higher than in low intertidal sandy and muddy sediments, respectively. Since annual benthic community respiration was 2 times higher in muddy than sandy sediments, the annual net primary production was about 0 in the former and 17–19 mol C m^?2 yr^?1 in the latter. However, heterotrophic contribution by larger faunal components as well as removal of organic carbon by waves and tidal currents, which are not included here, may balance the budget at the sandy stations. There was no or only weak relationships between (light and dark) DIN exchange and factors like temperature, sedimentary organic content, and oxygen fluxes. Factors related to nutrient fluxes, such as denitrification and nutrient concentration in the overlying water, may have hampered any such relationships. In fact, DIN fluxes at all three stations appeared to be strongly controlled by DIN concentrations in the overlying water. On an annual basis, the sediment appeared to be a net sink for DIN.     

The functional group approach to bioturbation: The effects of biodiffusers and gallery-diffusers of the Macoma balthica community on sediment oxygen uptake

The Macoma balthica community, which is widely distributed in intertidal soft sediments bordering the north Atlantic, is dominated by two functional groups with different sediment mixing modes: the biodiffusers M. balthica and Mya arenaria and the gallery-diffuser Nereis virens. To compare the effects of these two groups on sediment oxygen uptake rates, we used experimental microcosms with identical biovolumes to measure the influence of each species on oxygen uptake. The two biodiffusers had similar effects on oxygen uptake in spite of different space occupation and different feeding, ventilation and burrowing modes. Biodiffusers and gallery-diffusers had different effects on oxygen uptake. Periodic ventilation by the gallery-diffusers stimulated the oxygen uptake by the sediment more than the steady activities of the biodiffusers. Temporal variation in oxygen fluxes in bioturbated microcosms was linked to construction and maintenance of biogenic structures. The results confirm that the functional group approach to bioturbation is a useful tool for quantifying the effects of intertidal benthic communities on benthic fluxes.     

Assessing scales of variability in benthic diatom community structure

Sources of variability such as sampling method, sample preparation, and sample analysis (taxonomy) might affect our ability to measure differences in community structure between sites in environmental effects studies. It is therefore important to ensure that changes in community structure related to the physical or chemical differences between sites are not hidden by other sources of variability within a site. The goal of this study was to quantify the amount of variability in benthic diatom community structure related to sampling and laboratory procedures. Our results showed that variability in community structure was minimal among replicate microscope slides (< 1%) and among samples collected within a site (1.8%). Variability in community structure was substantially higher between sites located in one stream (16.6%), and highest across streams (59.6%). This suggests that field sampling and laboratory methods do not contribute a large amount of variation into our analyses of benthic diatom community structure across sites.     

Assessment of Littoral Benthic Invertebrate Communities at the Land–Water Interface in Lakes Recovering from Severe Acid- and Metal-Damage

Benthic invertebrate communities within confluence sites, or areas of sediment deposition, are shaped by the input of catchment products including coarse woody debris, organic and inorganic particulates, and contaminants, but these sites also appear to be potential “hotspots” where recolonization of severely damaged ecosystems begins. Two species of leaf packs and a sweep netting technique were used to assess benthic invertebrate communities across a gradient of 14 confluence sites in 3 recovering lakes near the copper and nickel smelters in Sudbury, Canada. Environmental variables including delta habitat composition, delta area and length, and composition of deposited materials were used to detect spatial patterns in littoral benthic invertebrate communities. Benthic invertebrate community relationships with water chemistry were also assessed. Partial redundancy analysis (pRDA) showed that all sampling methods detected similar gradients of increasing invertebrate community richness and diversity as area and length of the sediment delta and the surface organic matter abundance increased. Two-way nested ANOVAs showed significant differences (p < .05) in taxa richness and diversity metrics among sites. Of the three methods, the benthic invertebrate community measurements from the birch leaf packs provided the strongest correlations with measures of organic matter inputs or habitat characteristics of the confluence zones. These correlations suggest that tree planting in riparian areas, or organic matter or macrophyte additions to littoral zones, may enhance littoral benthic invertebrate richness and diversity in acid and metal damaged lakes.     

Analysis of macrobenthic communities in Flanders,Belgium, using a stepwise input variable selection procedure with artificial neural networks

The effect of environmental conditions on river macrobenthic communities was studied using a dataset consisting of 343 sediment samples from unnavigable watercourses in Flanders, Belgium. Artificial neural network models were used to analyse the relation among river characteristics and macrobenthic communities. The dataset included presence or absence of macroinvertebrate taxa and 12 physicochemical and hydromorphological variables for each sampling site. The abiotic variables served as input for the artificial neural networks to predict the macrobenthic community. The effects of the input variables on model performance were assessed in order to identify the most diagnostic river characteristics for macrobenthic community composition. This was done by consecutively eliminating the least important variables and, when beneficial for model performance, adding previously removed ones again. This stepwise input variable selection procedure was tested not only on a model predicting the entire macrobenthic community, but also on three models, each predicting an individual taxon. Additionally, during each step of the stepwise leave-one-out procedure, a sensitivity analysis was performed to determine the response of the predicted macroinvertebrate taxa to the input variables applied. This research illustrated that a combination of input variable selection with sensitivity analyses can contribute to the development of reliable and ecologically relevant ANN models. The river characteristics predicting presence or absence of the benthic macroinvertebrates best were the Julian day, conductivity, and dissolved oxygen content. These conditions reflect the importance of discharges of untreated wastewater that occurred during the period of investigation in nearly all Flemish rivers.     

Sediment quality of North Carolina estuaries: an integrative assessment of sediment contamination, toxicity, and condition of benthic fauna

Sediment quality of North Carolina estuaries was evaluated using synoptic data on sediment chemistry, toxicity, and macroinfaunal community structure from 175 subtidal stations sampled during the summers of 1994–1997. The study area included Currituck, Albemarle, and Pamlico Sounds; estuarine portions of major rivers (e.g., Chowan, Roanoke, Tar-Pamlico, Neuse, New, Cape Fear); and numerous smaller tributaries and coastal embayments between the Virginia and South Carolina borders. A probabilistic sampling design permitted statistical estimation of the spatial extent of degraded versus non-degraded condition across these estuaries. Over half (54 ± 7%) of the surveyed area had high sediment quality characterized by healthy benthic assemblages and low levels of sediment contamination and toxicity. The remaining 46% showed evidence of significant stress in one or more of the above sediment-quality-triad components. While this is a sizable area, portions of it (27 ± 6%) were represented by sites with no connection between presence of stressors and adverse biological responses. Only 19% of the total area showed evidence of an impaired benthos coupled to significant pollution exposure (high sediment contamination, toxicity, or both). Impaired benthic condition was more closely linked to sediment contamination than to low dissolved oxygen (based on instantaneous oxygen measurements). The most pervasive contaminants were the metals arsenic, mercury, chromium, and nickel; the pesticides lindane, dieldrin, DDT, and DDT derivatives; and total PCBs. Degraded condition in all three components of the sediment quality triad co-occurred in <10% of the study area, suggesting that strong contaminant-induced effects on the benthos are limited to a small (yet ecologically significant) percentage of total estuarine area. The spatial extent of sediment contamination and toxicity was much less in these estuaries in comparison to other U.S. coastal regions where similar studies have been performed.     

Benthic fauna bio-irrigation effects on nutrient regeneration in fish farm sediments

Impacts of organic enrichment and a modified benthic fauna community (caused by fish farming) on benthic mineralization rates and nutrient cycling were studied in sediments at one Danish and one Cypriote fish farm. Sediment organic matter concentration and macrofauna community composition were manipulated in microcosms and changes in total benthic metabolism (oxygen consumption, TCO₂ production), anaerobic metabolism (sulfate reduction rates), nutrient fluxes and sediment parameters were followed for a period of 3 weeks. Mineralization rates were found to be highly correlated with irrigation velocities and largest fauna effects were found in the Danish sediments with the large and active irrigating climax species (Nereis diversicolor and Macoma balthica). Eastern Mediterranean climax species (Glycera rouxii and Naineris laevigata) also stimulated mineralization rates but to a smaller extent due to lower irrigation, whereas the opportunistic species (Capitella in Danish sediment and Hermodice carunculata in Cypriote sediment) showed less effect on mineralization. Ammonium and phosphate release increased with increasing irrigation velocities, but much less in Cyprus indicating higher nutrient retention at the ultra-oligotrophic location compared to eutrophic Danish site. Irrigation velocities, and thus mineralization rates, increased by organic matter loading, indicating larger fauna-induced oxidation in enriched environments. The result implies that a change in fauna structure in fish farm sediment towards smaller opportunistic polychaete species with lower irrigation will result in slower mineralization rates and potentially increase accumulation of organic waste products.     

Comparing benthic biogeochemistry at a sandy and a muddy site in the Celtic Sea using a model and observations

Results from a 1D setup of the European Regional Seas Ecosystem Model (ERSEM) biogeochemical model were compared with new observations collected under the UK Shelf Seas Biogeochemistry (SSB) programme to assess model performance and clarify elements of shelf-sea benthic biogeochemistry and carbon cycling. Observations from two contrasting sites (muddy and sandy) in the Celtic Sea in otherwise comparable hydrographic conditions were considered, with the focus on the benthic system. A standard model parameterisation with site-specific light and nutrient adjustments was used, along with modifications to the within-seabed diffusivity to accommodate the modelling of permeable (sandy) sediments. Differences between modelled and observed quantities of organic carbon in the bed were interpreted to suggest that a large part (>90%) of the observed benthic organic carbon is biologically relatively inactive. Evidence on the rate at which this inactive fraction is produced will constitute important information to quantify offshore carbon sequestration. Total oxygen uptake and oxic layer depths were within the range of the measured values. Modelled depth average pore water concentrations of ammonium, phosphate and silicate were typically 5–20% of observed values at the muddy site due to an underestimate of concentrations associated with the deeper sediment layers. Model agreement for these nutrients was better at the sandy site, which had lower pore water concentrations, especially deeper in the sediment. Comparison of pore water nitrate with observations had added uncertainty, as the results from process studies at the sites indicated the dominance of the anammox pathway for nitrogen removal; a pathway that is not included in the model. Macrofaunal biomasses were overestimated, although a model run with increased macrofaunal background mortality rates decreased macrofaunal biomass and improved agreement with observations. The decrease in macrofaunal biomass was compensated by an increase in meiofaunal biomass such that total oxygen demand remained within the observed range. The permeable sediment modification reproduced some of the observed behaviour of oxygen penetration depth at the sandy site. It is suggested that future development in ERSEM benthic modelling should focus on: (1) mixing and degradation rates of benthic organic matter, (2) validation of benthic faunal biomass against large scale spatial datasets, (3) incorporation of anammox in the benthic nitrogen cycle, and (4) further developments to represent permeable sediment processes.     

Cost/benefit and the effect of sample preservation procedures on quantitative patterns in benthic ecology

Some benthic assemblages studies have tested the effects of different preservation procedures on biomass, but their influence on quantitative patterns (number of species and abundance) is still unclear. We evaluated the influence of two sample preservation procedures on quantitative patterns in benthic ecology. Ten sampling points were systematically interspersed on two types of sediment (sandy and muddy). At each sediment type, samples from five sampling points were fixed in 10% formalin, and the other five points were preserved in 70% ethanol (without previous fixation). Three replicates were collected at each sampling point, and samples were washed in 0.5 mesh size and sorted in laboratory. A cost/benefit analysis was performed considering the washing time in laboratory and the costs of substances. A total of 1970 individuals were collected (muddy sediment: 132; sandy sediment: 1838), belonging to 121 taxa (muddy: 49; sandy: 83). Assemblages preserved in ethanol were composed of 795 individuals and 80 taxa, while those fixed with formalin had 1173 individuals and 94 taxa. Polychaeta predominated as the most abundant group for both preservation procedures. For the whole benthic community, significant differences occurred only between sediment types. Significant differences in the number of individuals of polychaetes were observed for the different preservation procedures in sandy sediment. Ethanol has the best cost/benefit ratio in both sediment types due to additional costs to attend safety requirements for formalin-fixed samples. Further studies should evaluate how quantitative patterns are affected by exposure time of preservation, anesthesia interaction, and morphological deformations (e.g. impossibility of identification).     

A comparison of two nitrification inhibitors used to measure nitrification rates in estuarine sediments

Abstract: Nitrification rates were measured using intact sediment cores from South San Francisco Bay and two different nitrification inhibitors: acetylene and methyl fluoride. Sediment oxygen consumption and ammonium and nitrate fluxes were also measured in these cores. Four experiments were conducted in the spring, and one in the fall of 1993. There was no significant difference in nitrification rates measured using the two inhibitors, which suggests that methyl fluoride can be used as an effective inhibitor of nitrification. Nitrification was positively correlated with sediment oxygen consumption and numbers of macrofauna. This suggests that bioturbation by macrofauna is an important control of nitrification rates. Irrigation by the tube-dwelling polychaete, Asychis elongata , which dominates the benthic biomass at this location, appears particularly important. Ammonium fluxes out of the sediment were greatest about one week after the spring bloom, while nitrification peaked about one month later.     

Bacterial diversity in organically-enriched fish farm sediments

The bacterial diversity and community structure within both organically enriched and adjacent, unimpacted, near-shore marine sediments at two fish farms in southern Tasmania, Australia, was examined using 16S rRNA gene clone library construction and analysis. Sediments at both caged and reference sites at both farms showed a very high level of microbial diversity. Over 900 clones were analysed and grouped into 631 unique phylotypes. Reference sites were dominated by Delta- and Gammaproteobacteria and the Cytophaga-Flavobacteria-Bacteroides group. Cage site sediments were also dominated by these phylotypes, as well as members of the Alpha- and Epsilonproteobacteria. Diversity and coverage indices indicated that the actual diversity of the sediments was much greater than that detected, despite a large sampling effort. All libraries were shown to be statistically different from one another (P < 0.05). Many phylotypes did not group with cultured bacteria, but grouped with other environmental clones from a wide array of marine benthic environments. Diversity and evenness indices suggested that although both parameters changed after farming, diverse communities were present in all sediments. The response of the microbial community to organic load suggested that random, rather than predictable, succession events determine community composition and diversity, and that sediment type may influence bacterial community and sediment response to organic perturbation.     

Succession and collapse of macrozoobenthos in a subtropical hypertrophic lake under restoration (Lake Rodó, Uruguay)

We studied the succession patterns of the benthic community following a whole-lake restoration experiment in a subtropical hypertrophic lake (Lake Rodó, 34°55′ S 56°10′ W, Montevideo, Uruguay). The restoration measures involved diversion of the main inlet and removal of upper 1-m sediment and biomanipulation of the fish community. Between January 1997 and November 1999, we sampled sediments seasonally to analyse changes in benthos in relation to other abiotic and biotic characteristics of the system. The benthic community of the lake was composed of three families and nine genera. The maximum density (646 ind m⁻²), as well as the maximum taxonomic richness (six), were observed 1 month after the lake was refilled. Since 1998, the benthic abundance decreased considerably and continuously and a total absence of benthic organisms was registered by the end of the year. The low abundance of macroinvertebrates during 1997 could be explained by the food preferences of the dominant fish species, and the high fish biomass at the beginning of the biomanipulation process. However, the most relevant physico-chemical temporal patterns were the increase of organic matter and nutrients in the sediment and the fluctuations of oxygen and nitrate in the deepest layer of the water column. The disappearance of benthos was related to these temporal changes. These results stress the importance of the increase of organic matter for the changes in the physico-chemical environment, and its importance in the benthic succession and possible collapse. We suggest that in hypertrophic lakes, the effects of organic matter enrichment in the sediment can be even more relevant than fish predation in shaping the zoobenthos.