Benthic Mesocosm Research Articles

Responses at various levels of ecological hierarchy indicate acclimation to sequential sublethal heatwaves in a temperate benthic ecosystem.

Marine heatwaves have caused massive mortality in coastal benthic ecosystems, altering community composition. Here, we aim to understand the effects of single and sequential sublethal heatwaves in a temperate benthic ecosystem, investigating their disturbance on various levels of ecological hierarchy, i.e. individual physiology, trophic groups' biomass and ecosystem carbon fluxes. To do so, we performed a near-natural experiment using outdoor benthic mesocosms along spring/summer, where communities were exposed to different thermal regimes: without heatwaves (0HW), with one heatwave (1HW) and with three heatwaves (3HWs). Gastropods were negatively impacted by one single heatwave treatment, but the exposure to three sequential heatwaves caused no response, indicating ecological stress memory. The magnitude of ecosystem carbon fluxes mostly decreased after 1HW, with a marked negative impact on mesograzers' feeding, while the overall intensity of carbon fluxes increased after 3HWs. Consumers' acclimation after the exposure to sequential heatwaves increased grazing activity, representing a threat for the macroalgae biomass. The evaluation of physiological responses and ecological interactions is crucial to interpret variations in community composition and to detect early signs of stress. Our results reveal the spread of heatwave effects along the ecological hierarchical levels, helping to predict the trajectories of ecosystem development.This article is part of the theme issue 'Connected interactions: enriching food web research by spatial and social interactions'.

Benthic–Pelagic Coupling in the Oligotrophic Eastern Mediterranean: A Synthesis of the HYPOXIA Project Results

Benthic–pelagic coupling studies have shown that the response of the benthic system to eutrophication is subject to complex nonlinear dynamics with specific thresholds beyond which abrupt changes in the response of the ecosystem occur and time lags between inputs and responses. The “HYPOXIA: Benthic–pelagic coupling and regime shifts” project aimed to investigate how nutrient input in the water column results in ecological processes of eutrophication, which may lead to significant, irreversible changes in the eastern Mediterranean marine ecosystems within a short period of time. The project included analysis of historical water and benthic data, field sampling, and mesocosm experiments. From the project results, it can be concluded that nutrient inputs are quickly capitalized by small phytoplankton species in the water column resulting in the bloom of specific species with high nutrient uptake capabilities. When Eutrophic Index values (calculated using nutrient and chlorophyll-a concentrations) cross the moderate-to-poor threshold, the precipitating organic matter can cause observable effects on the benthic system. Depending on eutrophication intensity and persistence, the effects can start from microbenthos, meiofauna, and macrofauna increase in abundance and biomass to significant changes in the community structure. The latter includes the proliferation of macrofaunal opportunistic species, an increase in deposit feeders, and the high risk of ecosystem quality degradation. However, contrary to other regions of the world, no water hypoxia or benthic dead zones were observed as chlorophyll-a and O2 concentrations showed a positive correlation. This is caused by the high photosynthetic activity of the phytoplankton and microphytobenthos, the increased bioturbation of macrofauna, and the increased abundance of sediment deposit-feeding species, which quickly consume the excess organic matter. Eastern Mediterranean coastal ecosystems show high resilience to the adverse effects of eutrophication, preventing hypoxia and azoic conditions when eutrophication is the only source of environmental disturbance.

Antioxidative Properties of Baltic Sea Keystone Macroalgae (Fucus vesiculosus, Phaeophyceae) under Ocean Warming and Acidification in a Seasonally Varying Environment.

Simple SummaryThe brown seaweed Fucus vesiculosus is the dominant and the most ecologically crucial primary producer and habitat founder in the Baltic Sea. In the shallow coastal zone, F. vesiculosus is particularly exposed to strongly and rapidly changing environmental conditions due to global change. This study examines how single and joint effects of elevated seawater temperature and pCO2 levels influence F. vesiculosus in all four seasons, using benthic mesocosms. The antioxidative properties and the sensitivity of F. vesiculosus photosynthetic performance to oxidative stress under different global change scenarios were assessed. F. vesiculosus tolerated strong hydrogen peroxide stress in all seasons, as reflected in high antioxidative enzyme activities and a low degree of membrane lipid peroxidation. Forecasted warming affected the antioxidative properties of F. vesiculosus stronger than acidification, causing significantly increased lipid peroxidation under elevated temperatures in all seasons. However, pCO2 levels modulated the oxidative stress of F. vesiculosus under warming. Overall, summer heatwaves reaching lethal temperatures in shallow waters will most likely determine the persistence of Baltic F. vesiculosus. The keystone macroalga Fucus vesiculosus (Phaeophyceae), dominating shallow hard bottom zones, encounters a strongly and rapidly changing environment due to anthropogenic change over the last decades in the Baltic Sea. Thus, in four successive benthic mesocosm experiments, the single and joint effects of increased temperature (Δ + 5 °C) and pCO2 (1100 ppm) under ambient irradiances were experimentally tested on the antioxidative properties of western Baltic F. vesiculosus in all seasons. The antioxidative properties (superoxide dismutase activity and lipid peroxidation) as well as the sensitivity of F. vesiculosus photosynthetic performance (i.e., effective quantum yield) to oxidative stress under these global change scenarios were seasonally examined. F. vesiculosus exhibited high and relatively constant photosynthetic performance under artificial hydrogen peroxide (H2O2) stress in all seasons. High activities of superoxide dismutase and a relatively low degree of the biomarker for lipid peroxidation (malondialdehyde concentration) were found in F. vesiculosus. Thus, Baltic F. vesiculosus is equipped with a high antioxidative potential to tolerate strong oxidative stress for at least short periods. Antioxidative properties of F. vesiculosus were more strongly affected by warming than by acidification, resulting in significantly increased malondialdehyde concentrations under elevated temperature levels in all seasons. Oxidative stress was enhanced in F. vesiculosus under warming but seem to be modulated by seasonally varying environmental conditions (e.g., high and low irradiances) and pCO2 levels. However, more frequent summer heatwaves reaching and surpassing lethal temperatures in shallow coastal waters may determine the F. vesiculosus population’s overall persistence in the Baltic Sea.

Effects of DOC addition from different sources on phytoplankton community in a temperate eutrophic lake: An experimental study exploring lake compartments

A mesocosm experiment was conducted in a temperate eutrophic lake with the hypotheses: 1) the addition of a labile form of DOC would trigger a more pronounced response in phytoplankton biomass and composition compared with a non-labile form; 2) DOC addition would increase phytoplankton biomass by co-inserting organic nutrients for phytoplankton growth; 3) DOC addition would change phytoplankton composition, in particular towards mixotrophic taxa due to higher DOC availability; and that 4) there would be differences in phytoplankton responses to DOC addition, depending on whether sediment was included or not. We used two types of mesocosms: pelagic mesocosms with closed bottom, and benthic mesocosms open to the sediment. The experiment ran for 29 days in total. The DOC addition occurred once, at Day 1. Besides the control, there were two treatments: HuminFeed® (non-labile DOC) at a concentration of 2 mg L−1, and a combination of 2 mg L−1 HuminFeed® and 2 mg L−1 DOC from alder leaf leachate (labile). Responses were detected only in the treatment with alder leaf extract. Ecosystem processes responded immediately to DOC addition, with the fall in dissolved oxygen and pH indicating an increase in respiration, relative to primary production (Day 2). In contrast, there was a delay of a few days in structural responses in the phytoplankton community (Day 6). Phytoplankton biomass increased after DOC addition, probably boosted by the phosphorus released from alder leaf extract. Changes in phytoplankton composition towards mixotrophic taxa were not as strong as changes in biomass, and happened only in the pelagic mesocosms. With the DOC addition, diatoms prevailed in benthic mesocosms, while the contribution of colonial buoyant cyanobacteria increased in the pelagic ones. This study points towards the necessity to look in greater detail at specific responses of phytoplankton to DOC concentration increases considering lake-habitat and sediment influence.

Seasonal Photophysiological Performance of Adult Western Baltic Fucus vesiculosus (Phaeophyceae) Under Ocean Warming and Acidification

Shallow coastal marine ecosystems are exposed to intensive warming events in the last decade, threatening keystone macroalgal species such as the bladder wrack (Fucus vesiculosus, Phaeophyceae) in the Baltic Sea. Herein, we experimentally tested in four consecutive benthic mesocosm experiments, if the single and combined impact of elevated seawater temperature (Δ + 5°C) and pCO2 (1100 ppm) under natural irradiance conditions seasonally affected the photophysiological performance (i.e., oxygen production, in vivo chlorophyll a fluorescence, energy dissipation pathways and chlorophyll concentration) of Baltic Sea Fucus. Photosynthesis was highest in spring/early summer when water temperature and solar irradiance increases naturally, and was lowest in winter (December to January/February). Temperature had a stronger effect than pCO2 on photosynthetic performance of Fucus in all seasons. In contrast to the expectation that warmer winter conditions might be beneficial, elevated temperature conditions and sub-optimal low winter light conditions decreased photophysiological performance of Fucus. In summer, western Baltic Sea Fucus already lives close to its upper thermal tolerance limit and future warming of the Baltic Sea during summer may probably become deleterious for this species. However, our results indicate that over most of the year a combination of future ocean warming and increased pCO2 will have slightly positive effects for Fucus photophysiological performance.

A review of mesocosm experiments on heavy metals in marine environment and related issues of emerging concerns.

Mesocosms are real-world environmental science tools for bridging the gap between laboratory-scale experiments and actual habitat studies on ecosystem complexities. These experiments are increasingly being applied in understanding the complex impacts of heavy metals, ocean acidification, global warming, and oil spills. The insights of the present review indicate how metals and metal-bound activities impact on various aspects of ecological complexities like prey predator cues, growth, embryonic development, and reproduction. Plankton and benthos are used more often over fish and microbes owing to their smaller size, faster reproduction, amenability, and repeatability during mesocosm experiments. The results of ocean acidification reveal calcification of plankton, corals, alteration of pelagic structures, and plankton blooms. The subtle effect of oil spills is amplified on sediment microorganisms, primary producers, and crustaceans. An overview of the mesocosm designs over the years indicates that gradual changes have evolved in the type, size, design, composition, parameters, methodology employed, and the outputs obtained. Most of the pelagic and benthic mesocosm designs involve consideration of interactions within the water columns, between water and sediments, trophic levels, and nutrient rivalry. Mesocosm structures are built considering physical processes (tidal currents, turbulence, inner cycling of nutrients, thermal stratification, and mixing), biological complexities (population, community, and ecosystem) using appropriate filling containers, and sampling facilities that employ inert materials. The principle of design is easy transportation, mooring, deployment, and free floating structures besides addressing the unique ecosystem-based science problems. The evolution of the mesocosm tools helps in understanding further advancement of techniques and their applications in marine ecosystems.

Future warming and acidification effects on anti-fouling and anti-herbivory traits of the brown alga Fucus vesiculosus (Phaeophyceae).

Human-induced ocean warming and acidification have received increasing attention over the past decade and are considered to have substantial consequences for a broad range of marine species and their interactions. Understanding how these interactions shift in response to climate change is particularly important with regard to foundation species, such as the brown alga Fucus vesiculosus. This macroalga represents the dominant habitat former on coastal rocky substrata of the Baltic Sea, fulfilling functions essential for the entire benthic community. Its ability to withstand extensive fouling and herbivory regulates the associated community and ecosystem dynamics. This study tested the interactive effects of future warming, acidification, and seasonality on the interactions of a marine macroalga with potential foulers and consumers. F.vesiculosus rockweeds were exposed to different combinations of conditions predicted regionally for the year 2100 (+∆5°C, +∆700μatm CO2 ) using multifactorial long-term experiments in novel outdoor benthic mesocosms ("Benthocosms") over 9-12-week periods in four seasons. Possible shifts in the macroalgal susceptibility to fouling and consumption were tested using consecutive bioassays. Algal susceptibility to fouling and grazing varied substantially among seasons and between treatments. In all seasons, warming predominantly affected anti-fouling and anti-herbivory interactions while acidification had a subtle nonsignificant influence. Interestingly, anti-microfouling activity was highest during winter under warming, while anti-macrofouling and anti-herbivory activities were highest in the summer under warming. These contrasting findings indicate that seasonal changes in anti-fouling and anti-herbivory traits may interact with ocean warming in altering F.vesiculosus community composition in the future.

Even moderate nutrient enrichment negatively adds up to global climate change effects on a habitat-forming seaweed system

Coastal marine ecosystems have been under high anthropogenic pressure and it can be assumed that prevalent local perturbation interacts with rising global stressors under proceeding climate change. Understanding their effective pathways and cumulative effects is of high relevance not only with regard to future risk assessment, but also for current ecosystem management. In benthic mesocosms, we factorially tested the effects of one global (combined elevated seawater temperature and CO2 concentration) and one local (nutrient enrichment) stressor on a common coastal Baltic seaweed system (Fucus vesiculosus). Both treatments in combination had additive negative impacts on the seaweed—epiphyte—mesograzer system by altering its regulatory mechanisms. That is, warming decreased the biomass of two mesograzer species (weakened top-down control), whereas moderate nutrient enrichment increased epiphyte biomass (intensified bottom-up control), which ultimately resulted in a significant biomass reduction of the foundation seaweed. Our results suggest that climate change impacts might be underestimated if local pressures are disregarded. Furthermore, they give implication for local ecological management as the mitigation of local perturbation may limit climate change impacts on marine ecosystems.

Tidal benthic mesocosms simulating future climate change scenarios in the field of marine ecology

AbstractIncreasing human activities cause local to global changes in sea surface temperatures, ocean acidity, eutrophication, and rising sea levels. Many laboratory experiments investigate the effects of these regime shifts on single species and single stressors, showing variable responses within and among species, while different combinations of stressors can have synergistic, additive or antagonistic effects. Large‐scale multi‐species and multi‐stressor experiments can more reliably predict future ecosystem changes. A unique mesocosm facility was developed and set up at the AWI Wadden Sea Station – Sylt, Northern Germany to investigate the particular effects of future climate changes on predominant marine intertidal communities. Each of 12 benthic mesocosms serves as an independent experimental unit with novel techniques of tide and current simulations as well as multi parameter measurement systems to simulate multi‐factorial climate change scenarios including the combination of warming, acidification, nutrient enrichment, and sea level rise. Temperature, pH, oxygen, and salinity can be continuously monitored and logged, while discretely collected samples of total alkalinity, light availability, chlorophyll a (Chl a), nutrients and seston supplement these online datasets. Herein we demonstrate the functionality of the new benthic mesocosm system including first experimental results on the responses of Fucus vesiculosus forma mytili, and its associated community to the combination of warming, ocean acidification, and increased nutrient enrichment.

Temperature effects on seaweed-sustaining top-down control vary with season.

Rising seawater temperature and CO2 concentrations (ocean acidification) represent two of the most influential factors impacting marine ecosystems in the face of global climate change. In ecological climate change research, full-factorial experiments performed across seasons in multispecies, cross-trophic-level settings are essential as they permit a more realistic estimation of direct and indirect effects as well as the relative importance of the effects of both major environmental stressors on ecosystems. In benthic mesocosm experiments, we tested the responses of coastal Baltic Sea Fucus vesiculosus communities to elevated seawater temperature and CO2 concentrations across four seasons of one year. While increasing [CO2] levels had only minor effects, warming had strong and persistent effects on grazers, and the resulting effects on the Fucus community were found to be season dependent. In late summer, a temperature-driven collapse of grazers caused a cascading effect from the consumers to the foundation species, resulting in overgrowth of Fucus thalli by epiphytes. In fall/winter (outside the growing season of epiphytes), intensified grazing under warming resulted in a significant reduction in Fucus biomass. Thus, we were able to confirm the prediction that future increases in water temperatures will influence marine food-web processes by altering top-down control, but we were also able to show that specific consequences for food-web structure depend on the season. Since F. vesiculosus is the dominant habitat-forming brown algal system in the Baltic Sea, its potential decline under global warming implies a loss of key functions and services such as provision of nutrient storage, substrate, food, shelter, and nursery grounds for a diverse community of marine invertebrates and fish in Baltic Sea coastal waters.

Dose-dependent response of a benthic system to biodeposition from suspended blue mussel (Mytilus edulis) culture

This study reports the results of a field experiment using benthic mesocosms that examined dose-dependent effects of mussel biodeposition on the benthic environment. Mesocosms were placed in the natural sea bottom and subjected to one of eight levels of biodeposition (from 0 to 1400musselsm−2). Most analyses indicated non-linear (i.e., threshold) effects. Sediment characteristics changed significantly between 200 and 400musselsm−2 as did multivariate community structure. Community structure effects were characterised by changes in abundances of species that are very sensitive or tolerant to organic loading. The multivariate AZTI Marine Biotic Index (M-AMBI) indicated that the benthic status changed from High to Good in all mesocosms receiving biodeposits. Sediments acted as a sink for oxygen (O2), but results suggest O2 sediment demand was not sensitive enough to evaluate organic loading impacts. Results from this and improved experiments can be used to determine the environmental carrying capacity of sites for bivalve culture.

The impact of two species of bioturbating shrimp (Callianassa subterranea and Upogebia deltaura) on sediment denitrification

In a benthic mesocosm experiment, the effects of two species of burrowing Thalassinidean shrimps (Callianassa subterranea and Upogebia deltaura) on rates of sediment denitrification were determined using the isotope pairing technique. Denitrification rate (Dtot) and coupled nitrification–denitrification (Dn) were shown to be significantly enhanced by the presence of U. deltaura by 2·9 and 3·3 times respectively, relative to control measurements. For U. deltaura the stimulation of the denitrification rate was found to be significantly related to the size of the animal (F=5·81, P=0·042). No deviation from the rates determined in control cores for either Dtot or Dn was observed for those cores inhabited by C. subterranea. The increase in Dtot with U. deltaura was considered to be the result of a combination of different factors, including; the direct extension of the sediment–water interface and an increase in oxygenation of the sediments and solute transport, as a result of the ventilating activities of the animal itself.

Importance of bioturbators for biodiversity maintenance: indirect effects of fishing disturbance

The relative impact on macrofaunal diversity of 7 naturally co-occurring bioturbators has been examined in a benthic mesocosm experiment. The bioturbators chosen were selected because of their potential vulnerability to the disturbances associated with demersal trawling. The experiment was conducted at the Solbergstrand mesocosm (Norwegian Institute for Water Research) using subtidal sediment collected from Bjornehodebukta, a small sheltered bay in the Oslofjord. For 3 of the bioturbating species (Brissopsis lyrifera, Aphrodita aculeata and Amphiura chiajei) a positive, linear relationship was demonstrated between bioturbator abundance and measures of species rich- ness (number of species and Margalef species richness). It was suggested that the presence of these bioturbating species increased oxygen penetration into the sediment, leading to an enhancement in the ability of benthic systems to process organic material. This had a beneficial effect on the associ- ated fauna in terms of maintaining levels of diversity. For Nephtys caeca, highest species diversity was seen in the low abundance treatments. Predation by N. caeca on small polychaetes is proposed as a possible explanation of the lower diversity in high-density treatments. It is concluded that B. lyrifera, A. aculeata, A. chiajei and N. caeca are functionally similar with respect to setting the diversity of the associated macrobenthic community and may therefore fulfill the same role within the benthic ecosystem. The presence of Calocaris macandreae, Nuculana minuta and Astarte sulcata had no detectable effect on diversity. Trawling has been shown to reduce the abundance of bioturbating species, and the current study demonstrates important implications of this loss on the maintenance of diversity. In addition, the interactions between bioturbation, diversity and organic enrichment high- light the need for management of anthropogenic impacts within the coastal environment in a holistic way rather than by isolation.

Design and evaluation of benthic mesocosms for coastal lagoons

We present the design of mesocosms built to run experiments in a coastal lagoon of the southern Atlantic Ocean, in Uruguay. Cylindrical mesocosms (1.7 m high, 2.26 m diameter) were built using supporting structures of iron grating (15 x 15 cm) to avoid damage from strong winds. The iron structures were still in good condition for over four years. Within the iron grating, four types of materials were tested for the walls. A single galvanized iron sheet, painted, and covered with thermic polyethylene on both sides proved to be the most suitable design under conditions of low water levels. This design remained in good conditions for more than 6 months. In the short and medium term, i.e. weeks to months, light, Secchi disk transparency, temperature and dissolved oxygen were similar inside and outside the mesocosms, indicating that the experimental set-up did not cause any artifacts. However, very short-term experiments may be affected by the water heating due to its confinement, while in the long-term, i.e. more than one year, the enclosures’ walls suffer major deterioration. A wind-driven agitator was tested to simulate the water mixing within the enclosures. However, light penetration was highly reduced with the mixing system. Temperature profiles inside and outside the polyethylene enclosures showed that they were vertically mixed. Differences in dissolved oxygen concentration between surface and bottom within the enclosures as well as between enclosures and the lagoon were small.

The interaction between physical disturbance and organic enrichment: An important element in structuring benthic communities

The interaction between physical disturbance and organic enrichment, with respect to its effect on the diversity and community structure of a macroinfaunal assemblage, has been examined in a benthic mesocosm experiment. The experiment was conducted at the Solbergstrand mesocosm (Norwegian Institute for Water Research) using subtidal sediment collected from Bjørnhordenbukta, a small sheltered bay in Oslofjørd. Ninety‐eight areas of homogenized sediment were subjected to one of seven levels of organic enrichment, combined with one of seven different frequencies of physical disturbance, each replicated once. This structured matrix of physical disturbance and organic enrichment treatments demonstrated the combined effects of these factors to be nonadditive. Diversity was lower than expected when low frequencies of physical disturbance acted in conjunction with high levels of organic enrichment or when high frequencies of physical disturbance were combined with low levels of organic enrichment. Diversity was higher than expected when both disturbance and enrichment were either high or low. The implications of this interaction between physical disturbance and organic enrichment for the application of the dynamic equilibrium model (Huston 1979) to sediment communities are discussed. Multivariate analysis also showed community structure to be significantly affected by physical disturbance, organic enrichment, and interactions between the two. It is concluded that strong interactions between physical disturbance and organic enrichment, coupled with both small‐ and large‐scale variability in these factors, could promote heterogeneity and diversity in benthic infaunal assemblages. However, this remains to be tested in field conditions. Additionally, interactions between physical disturbance and organic enrichment may have important implications for matters of coastal zone management.

The use of benthic mesocosms for the assessment of sediment contamination

The diagnostic capability of changes in the benthic macroinvertebrate community composition to determine the cause in contaminated sediments were addressed using box core mesocosms subjected to cadmium, atrazine and nutrient enrichment. Intact cores of lake sediment were collected from Lake Erie at four occasions during 1996 and were returned to laboratory where they were maintained and manipulated. The benthic community composition of the artificially polluted mesocosms was compared with field data, collected at the same time as the intact cores, and to untreated control boxes. Community composition was related to the measured environmental variables using multi-dimensional scaling (MDS) and principle axis correlation (PCC). Community responses to different treatments were both different and consistent, suggesting that the community structure of these mesocosms could be used to identify the primary source of impact.

Bioirrigation modeling in experimental benthic mesocosms

Burrow irrigation by benthic infauna affects chemical mass transfer regimes in marine and estuarine sediments. The bioirrigation facilitates rapid exchange of solutes between oxygenated overlying water and anoxic pore water, and thus promotes biogeochemical reactions that include degradation of sedimentary organic matter and reoxidation of reduced species. A comprehensive understanding of chemical mass transfer processes in aquatic sediments thus requires a proper treatment of bioirrigation. We investigated bioirrigation processes during early diagenesis using laboratory benthic mesocosms. Bioirrigation was carried out in the mesocosms by Schizocardium sp., a funnel-feeding enteropneust hemichordate that builds and ventilates a U-shaped burrow. Interpretation of the laboratory results was aided by a two-dimensional multicomponent model for transport and reactions that explicitly accounts for the depth-dependent distribution of burrows as well as the chemical mass transfers in the immediate vicinity of burrow walls. Our study shows that bioirrigation significantly affects the spatial distributions of pore water solutes. Moreover, bioirrigation promotes burrow walls to be the site of steep geochemical gradients and rapid chemical mass transfer. Our results also indicate that the exchange function, a, widely used in one-dimensional bioirrigation modeling, can accurately describe the bioirrigation regimes if its depth attenuation is coupled to the depth-dependent distribution of burrows. In addition, this study shows that the multicomponent 2D reaction-transport model is a useful research tool that can be used to critically evaluate common biogeochemical assumptions such as the prescribed depth dependencies of organic matter degradation rate and C/N ratio, as well as the lack of macrofaunal contribution of metabolites to the pore water.

Experimental evidence for the role of Brissopsis lyrifera (Forbes, 1841) as a critical species in the maintenance of benthic diversity and the modification of sediment chemistry

The effects on infaunal diversity and sediment chemistry of bioturbation/feeding activity by different densities of the heart urchin Brissopsis lyrifera are quantified in an experiment conducted in the benthic mesocosm facility of the Norwegian Institute for Water Research at Solbergstrand, Norway. Using sediment from Bjørnhordenbukta, a small, sheltered bay in Oslofjørd, areas were subjected to 20 weeks of continuous disturbance from urchins at densities equivalent to 28 and 71 individuals m −2, whilst other areas remained undisturbed. Low density treatments, reflecting the natural field densities observed during collection of the sediment, produced higher infaunal β diversity than the heavily disturbed or control treatments and this could be attributed to a decrease in competitive exclusion. This is consistent with the predictions of the intermediate disturbance hypothesis ( Connell, 1978). Bioturbation also caused a significant change in the chemistry of the surface sediment increasing oxygenation, decreasing the rates of denitrification and increasing the precipitation of phosphate. It is concluded that the disturbance activity of Brissopsis lyrifera may play a vital role in the maintenance of regional diversity and in the mediation of geochemical processes.

Benthic microalgae and nutrient dynamics in wave-disturbed environments in Marmion Lagoon, Western Australia, compared with less disturbed mesocosms

The relationships between the standing stock of marine benthic microalgae and physical variables and nutrients in the water column and porewaters were explored by comparing a wave-exposed field location at Marmion Lagoon, Western Australia, to less disturbed benthic mesocosms. For field stations in Marmion Lagoon, multiple regressions of chlorophyll a concentrations against the PCA scores (PCA1, PCA2, PCA3) indicated chlorophyll a was only significantly negatively related to PCA1 scores, which itself was dominated by temperature, salinity, DO, NO x , SiO 4 and SRP from the water column. Concentrations of chlorophyll a in surface sediments at field stations were significantly positively correlated to decreased temperature and salinity and negatively correlated to increased water column nutrients over the duration of the experiment. For mesocosms, chlorophyll a concentrations were not significantly related to any of the PC axes, indicating that shifts in temperature, salinity and nutrients in water column and porewaters were not responsible for the observed distribution of chlorophyll a in surface sediments. Benthic microalgae standing stock did not respond in similar ways in mesocosms and field stations to changes in nutrient status in the water column and surface sediments. There was a visible difference in the level of disturbance of surface sediments. In field stations, sand was resuspended and ripples developed on surface sediments in response to winter storm swells. In mesocosms, where disturbance of sediment was reduced, a surface layer of detritus and living benthic microalgae developed on the sediment surface and appeared to reduce the rate of nutrient efflux out of the sediment.

Experimental evidence of effects of the heart urchin Brissopsis lyrifera on associated subtidal meiobenthic nematode communities

An enclosure experiment was carried out in the benthic mesocosm facility of the Norwegian Institute for Water Research at Solbergstrand, Norway, to determine the effects of the predatory/disturbance activities of the heart urchin Brissopsis lyrifera on natural meiobenthic nematode communities. Four 1 m 2 boxes, filled to a depth of 22 cm with fresh sediment containing meiobenthic communities which had been collected from a 30 m deep site in the Oslofjord, were placed in the mesocosm. Large areas of the boxes were partitioned off with coarse mesh, 30 cm diameter cages which allowed meiofauna to move freely around within the box but retained the Brissopsis within the caged areas. Brissopsis were added to the cages at two densities (equivalent to 28 and 71 individuals m −2, respectively) with two replicates of each treatment per box (i.e. four cages per box). Meiofauna was sampled inside and outside the cages of each box after 20 weeks. Multivariate nematode community structure varied between the uncaged sediment and the Brissopsis cages and between the cages with different Brissopsis densities. Univariate measures of nematode community structure such as abundance were unaffected by the Brissopsis treatments. The more abundant species generally decreased in the presence of Brissopsis with the exception of Odontophora sp. whose abundance increased with increasing Brissopsis density. The nematode abundance in different feeding groups varied between treatments suggesting that Brissopsis altered the functional composition of the nematode community. High densities of Brissopsis are quite patchy in the Oslofjord and Brissopsis may act only locally as a keystone species. However the presence of these localised high density patches of Brissopsis in the Oslofjord may increase the regional heterogeneity of nematode communities.