The effects of chemical pollution on the bioturbation potential of estuarine intertidal mudflats

Bioturbation by benthic infauna has important implications for the fate of contaminants as well as for changes to the sediment structure, chemistry and transport characteristics. There is an extensive literature dealing with the influence of sedimentary variables on the structure and function of infaunal marine and estuarine organisms but less is known of the converse, the influence of biota on sedimentary structure. Although some work has been carried out regarding spatial and temporal patterns of bioturbation, little attention has been given to the effects of pollution. The paper gives a framework of animal sediment relationships in an intertidal environment and discusses the general role of macrofauna in structuring and modifying sedimentary features. A brief outline of the various techniques used for quantifying the degree of bioturbation is given and some of these techniques have then been used to demonstrate the effect of a petrochemical discharge on the bioturbation potential of intertidal communities in the Humber estuary, eastern England. These studies indicate an increase in bioturbation with increasing distance from the source of pollution, not only because of differences in abundance, animal size and depth of activity but also because of the difference in species composition between the communities. As a means of interpreting the responses, the species present have been broadly classified in terms of their feeding strategy and sediment modification potential. The paper concludes by discussing the potential impact, in terms of effect on sediment transport, of selectively removing the different guilds (by pollution). Received: 8 February 1999 / Received in revised form: 10 May 1999 / Accepted: 14 May 1999     

Density of Monoporeia affinis and biogeochemistry in Baltic Sea sediments

This study focused on effects from Monoporeia affinis reworking and ventilation activities on benthic fluxes and mineralization processes during a simulated bloom event. The importance of M. affinis density for benthic solute (O₂, ΣNO₂⁻ + NO₃⁻, NH₄⁺ and HPO₄²⁻) fluxes and sediment reactivity (mobilization of NH₄⁺ and HPO₄²⁻) following additions of organic material to the sediment surface was experimentally investigated using sediment-water and closed sediment (jar) incubations. Three different densities of M. affinis were used to resemble a low, medium and high density situation (1300, 2500 and 6400 ind. m^− 2, respectively) of a natural amphipod community. The degradation of phytodetritus (Tetraselmis sp., 5 g C m^− 2) added to the sediment surface was followed over a period of 20 days. Benthic solute fluxes of O₂, ΣNO₂⁻ + NO₃⁻ and NH₄⁺ were generally progressively stimulated with increasing number of M. affinis, while no such correlation was found for HPO₄²⁻. Solute fluxes were initially enhanced 1 to 2 days after the addition of phytodetritius, caused by mineralization of the most labile organic material and a food-stimulated irrigation by the amphipods. There was no effect from the activity of M. affinis on total denitrification (Dtot = Dn + Dw) or denitrification utilizing nitrate from coupled nitrification/denitrification (Dn) for any of the densities examined. Denitrification utilizing overlying water nitrate (Dw) was only about 10% of Dtot. Dw was significantly enhanced for the highest M. affinis density investigated. The reactivity of the sediment decreased progressively with increasing density of M. affinis and with time of the experiment. However, enhanced ammonium production at least 6 days after the organic addition indicated excretion of N-containing organic compounds by M. affinis. In conclusion, large spatial and temporal variations in density of M. affinis may be of significant importance for benthic solute fluxes and overall mineralization of organic material in Baltic Sea sediments.     

The gut microenvironment of sediment-dwelling Chironomus plumosus larvae as characterised with O2, pH, and redox microsensors

We devised a set-up in which microsensors can be used for characterising the gut microenvironment of aquatic macrofauna. In a small flow cell, we measured microscale gradients through dissected guts (O₂, pH, redox potential [E _h ]), in the haemolymph (O₂), and towards the body surface (O₂) of Chironomus plumosus larvae. The gut microenvironment was compared with the chemical conditions in the lake sediment in which the animals reside and feed. When the dissected guts were incubated at the same nominal O₂ concentration as in haemolymph, the gut content was completely anoxic and had pH and E _h values slightly lower than in the ambient sediment. When the dissected guts were artificially oxygenated, the volumetric O₂-consumption rates of the gut content were at least 10× higher than in the sediment. Using these potential O₂-consumption rates in a cylindrical diffusion–reaction model, it was predicted that diffusion of O₂ from the haemolymph to the gut could not oxygenate the gut content under in vivo conditions. Additionally, the potential O₂-consumption rates were so high that the intake of dissolved O₂ along with feeding could be ruled out to oxygenate the gut content. We conclude that microorganisms present in the gut of C. plumosus cannot exhibit an aerobic metabolism. The presented microsensor technique and the data analysis are applicable to guts of other macrofauna species with cutaneous respiration.     

Effects of drill cuttings on biogeochemical fluxes and macrobenthos of marine sediments

Experimental work was performed on drill cuttings sampled from off-shore drilling operations. The cuttings contained remnants of two different types of drilling muds: a water-ilmenite based mud and an olefin-barite based mud. In a pilot experiment, a gradient of up to 65 mm thick layers of water-ilmenite based cuttings were added to 78 cm² core samples. In another set-up, 3 mm layers of water-ilmenite and olefin-barite based cuttings were added to replicate 1000 cm² box-core samples with natural benthic communities transferred from the Oslofjord, S.E. Norway. Boxes with no addition and addition of 3 mm “clean” sediment were used for control. Increased consumption of oxygen and nitrate indicated the presence of degradable organic phases in both mud systems. The release of silicate showed a general decrease with increasing thickness of the cuttings layer, but maximum rates in the 3.1 mm treatment indicated increased bioturbation at low and moderate doses. The initial (field) composition of the macrobenthic communities was maintained throughout the 3 months experimental period, and at community level, no significant difference was observed between the four treatments at the end of the exposure period. However, after grouping the treatments into “clean” (untreated control and 3 mm clean sediment) and “cuttings” (3 mm water-ilmenite and 3 mm olefin-barite based cuttings), multivariate statistical analyses revealed a significant difference in community composition between clean and cuttings treatments, and three taxa showed significantly reduced abundances in the cuttings treatments. Chemical toxicity of mud components is assumed to be small, and the observed effect was more likely a result of physical properties such as the size or shape of cuttings particles.     

Estimating lagoonal biodiversity in Greece: comparison of rapid assessment techniques

An attempt is made to compare the results of different rapid biodiversity assessment techniques at the pan-Mediterranean, sectorial and local levels. A uniform multivariate pattern exists at the pan-Mediterranean and national (sectorial) levels: lagoons can be different when they host only a few species, but as species numbers increase, lagoons become homogenous in composition. Multivariate techniques cannot distinguish anthropogenically-impacted lagoons from those, which are naturally disturbed. In the pan-Mediterranean context it is the higher taxonomic levels, but in the national and local context it is the most abundant macrobenthic groups (polychaetes, molluscs and crustaceans) and meiobenthos which provide patterns closest to that derived from the species level. Taxonomic distinctness indices applied to polychaete and mollusc inventories provide meaningful results at most levels and scales of observation. These indices seem to be robust enough to discriminate anthropogenically impacted from naturally disturbed lagoons.     

Response of deposit-feeders to exclusion of epibenthic predators in a Mediterranean intertidal flat

Deposit-feeders are common components of macrofaunal assemblages in intertidal soft sediments. Predation has been considered to have a central role in affecting their distribution and population dynamics. This study investigates the effect of epibenthic predators on deposit-feeders, inhabiting the shallow layers of the sediment (surface and subsurface) and also the deepest layers (e.g., burrowing shrimp Upogebia pusilla; Petagna). The experiment was conducted in summer 2000 (August-September) at three different sites on an intertidal flat in Mediterranean Sea. In the field, predators were excluded using cages, placed on the surface of the sediment. It was predicted that under the cages, (i) abundances of animals would increase and (ii) species composition of assemblages would change as an effect of the exclusion of predators. Potential artefacts caused by the cages were controlled using partial cages (PC). Composition of organic matter and porosity were also analysed under PC and in natural controls to test whether the presence of cages would change sediment characteristics on the surface. At the end of the experiment, there were no significant differences in the composition of organic matter between PC and the undisturbed sediment (UC). After 8 weeks from the beginning of the experiment, exclusion of predators enhanced the abundance of U. pusilla. There were, however, no clear-cut changes in the species composition of macrofaunal assemblages and densities of animals did not increase under the cages. Indeed, some animals (Oligochaeta, Brania oculata, and Tanais dulongii) were less abundant under the cages (EC) than outside (PC and UC). We propose that predation might play a role in regulating interspecific relationships among some surface deposit-feeders and the burrowing shrimp U. pusilla.     

Benthic macrofauna in Tasmanian estuaries: scales of distribution and relationships with environmental variables

Information on the distribution of species richness, faunal density, biomass and estimated productivity of benthic invertebrates in Tasmanian estuaries was quantified at a variety of spatial and temporal scales to assess general hypothesis relating community metrics to such environmental variables as salinity, seagrass biomass and sediment particle size. An associated aim was to assess appropriate scales of investigation for soft-sediment biota distributed in estuaries, including whether patterns identified at individual sites, estuaries, tidal levels or times are likely to have more general relevance. Faunal biomass and productivity varied principally at between-estuary (10 to 1000 km) and replicate-sample (1 m) scales, indicating that these two community metrics were largely responding to estuary-wide effects, such as nutrient loading, and to microhabitat features, rather than to locality characteristics at intermediate scales such as salinity, anoxia or sediment particle size. By contrast, faunal density showed greater response to tidal height (1 to 100 m) and to factors distributed at the locality scale within estuary (10 km) than to factors between estuary. Both faunal density and species richness in estuaries declined over three- and fivefold ranges down the shore from high water mark to the shallow sublittoral, while estimated productivity and biomass showed highest overall levels at low water mark. The greatest component of variance in species richness was associated with tidal height, with variance then distributed approximately evenly between other spatial scales examined. At the low-tide and shallow subtidal levels, species richness, faunal biomass and estimated productivity were all highly correlated with salinity and biomass of macrophytes, whereas faunal density was highly correlated with biomass of macrophytes only. Relationships between environmental and biological variables examined were poorly defined at high tidal levels. Seasonal plus interannual variance was much lower than spatial variance—a clear indication that sampling effort in studies would generally be better directed across a range of localities than for a single locality to be repeatedly investigated over time.     

Benthic fauna in tropical tidal flats of Hinchinbrook Channel, NE Australia: diversity, abundance and their spatial and temporal variation

A survey of the benthic infauna (macro- and mesofauna) in tidalflats of Hinchinbrook Channel, north-east Australia, was carried out toassess the species diversity and individual abundances as well as theirtemporalvariations. Two sites were surveyed at five occassions from November 1988 toOctober 1991. In addition, spatial distributions over the intertidal gradientwere investigated once in May 1990. During the entire survey, over 200 specieswere recorded, with 53 species found on average at each sampling occassion.Species densities ranged from 4.0 to 8.2 species 177cm^–2 for macrofauna and from 4.1 to 11.8 species10 cm^–2 for mesofauna, but varied little betweensites. The average infaunal diversity (H') was 2.53. Individual abundancesreached median values of 14.0 individuals 177cm^–2for macrofauna and 14.5 individuals 10 cm^–2 formesofauna. Changes in total abundances were not related to season andinconsistent at the two study sites, with significantly higher abundancesoccurring either at site A or B on single sampling dates. Transects sampled inMay 1990 showed varying species compositions and abundances in the upper, midand lower intertidal, but no defined benthic communities were revealed bymultivariate analyses. Temporal variations in the benthic assemblages of thetwotidal flats in Hinchinbrook Channel were higher than variations between sites,with a separation of assemblages before and after 1989. Problems to assess andinterpret temporal changes in species rich tropical benthic communities arediscussed.     

Chironomus plumosus larvae increase fluxes of denitrification products and diversity of nitrate-reducing bacteria in freshwater sediment

Benthic invertebrates affect microbial processes and communities in freshwater sediment by enhancing sediment-water solute fluxes and by grazing on bacteria. Using microcosms, the effects of larvae of the widespread midge Chironomus plumosus on the efflux of denitrification products (N₂O and N₂ + N₂O) and the diversity and abundance of nitrate- and nitrous-oxide-reducing bacteria were investigated. Additionally, the diversity of actively nitrate- and nitrous-oxide-reducing bacteria was analyzed in the larval gut. The presence of larvae increased the total effluxes of N₂O and N₂ + N₂O up to 8.6- and 4.2-fold, respectively, which was mostly due to stimulation of sedimentary denitrification; incomplete denitrification in the guts accounted for up to 20% of the N₂O efflux. Phylotype richness of the nitrate reductase gene narG was significantly higher in sediment with than without larvae. In the gut, 47 narG phylotypes were found expressed, which may contribute to higher phylotype richness in colonized sediment. In contrast, phylotype richness of the nitrous oxide reductase gene nosZ was unaffected by the presence of larvae and very few nosZ phylotypes were expressed in the gut. Gene abundance of neither narG, nor nosZ was different in sediments with and without larvae. Hence, C. plumosus increases activity and diversity, but not overall abundance of nitrate-reducing bacteria, probably by providing additional ecological niches in its burrow and gut.     

Structuring factors in a marine soft bottom community during eutrophication—an experiment with radio-labelled phytodetritus

The quantitative development and uptake of radio-labelled phytodetritus in benthic macro- and meiofauna was studied in a 5-month experiment in two mesocosms, one of which received a single large nutrient (N and P) addition, while the other served as control. In reponse to the 12-fold increase in phytoplankton biomass noted after 2 weeks and the resulting enhanced accumulation of fresh phytodetritus, the abundance and biomass of the polychaetes Mediomastus ambiseta and Polydora ligni and the mud anemone Cerianthiopsis americanus increased significantly in the enriched tank. The abundances of P. ligni and M. ambiseta increased 37-fold and 12-fold, respectively, within the first two months of the experiment. No other macrofaunal or meiofaunal taxa showed any consistent positive quantitative response to the increased input of phytodetritus. In the control tank no considerable change in the benthic community structure was noted. The measurements of radio-label uptake within the benthic fauna showed that the quantitatively most successful species utilized fresh phytodetritus highly. However, a high degree of utilization of fresh detritus was also shown by taxa that did not respond quantitatively within the 5 month of the experiment, and almost all taxa showed a preference for fresh detritus over older organic material. Within the benthic meiofauna, kinorhynchs and especially foraminiferans showed a remarkably low preference for fresh detritus. A budget calculation comparing the total amounts of labelled organic carbon bound in animal tissue and in the sediment indicated that at any time at least 75% of this carbon was available for assimilation by deposit feeders. These results suggest that factors other than the availability of food, such as competition for space by a few opportunistic macrofauna species, limited the response of other species within this benthic community to the increased input of phytodetritus.