Christian Nozais

Predation and sediment disturbance effects of the intertidal polychaete Nereis virens (Sars) on associated meiofaunal assemblages

A microcosm experiment was carried out to determine the effects of the activity of the burrowing polychaete Nereis virens (Sars) on the associated meiofauna. The sediment basin (76×41 cm) was filled with 10 cm of sandy sediment previously sieved with a 1-mm mesh to remove any undesired macrofauna and macrodetritus. Fifteen 13-cm long polyvinyl-chloride (PVC) tubes (I.D.=10 cm) were pushed into the sediment to partition treatments. Nereis were added to the tubes at two densities, low (N=1) and high (N=3). Five tubes were used as controls (no Nereis), while two sets of five tubes were used for the low (L) and high (H) density treatments, respectively. After 14 days, meiofauna was sampled by coring. Cores were cut into three slices: surface (0–1 cm), subsurface (1–5 cm), and deep (5–10). High densities of Nereis (H) significantly affected nematodes, harpacticoid copepods, and nauplii abundance. However, lower abundances were found only in the top cm of the sediment. Moreover, a significant number of dead nematodes found in this sediment layer of treatment H allowed a distinction between sediment disturbance effects and predation effects. Sediment disturbance caused by Nereis may be related to an intensive “ploughing” of surface sediment during food-searching activity. Diversity indices were affected only in the top cm of the sediment with generally lower values in treatment H. Differences in the relative survival of the different feeding groups were found in treatment H, where microvores and deposit feeders respectively showed greater and lower survival. Multivariate analysis (multidimensional scaling) revealed significant differences in nematode species composition among treatments in all sediment layers. It is concluded that N. virens significantly affects meiofauna mostly by disturbance of the top cm of the sediment where its predation represents an influent force as well. The structure of nematode assemblages in subsurface and deeper sediment layers is also affected, most likely by changes in redox conditions caused by the bioirrigating effects of Nereis burrows.

PARTICULATE DIMETHYLSULFOXIDE IN ARCTIC SEA-ICE ALGAL COMMUNITIES: THE CRYOPROTECTANT HYPOTHESIS REVISITED

The particulate‐phase concentrations of dimethyl sulfoxide (DMSOp) and dimethylsulfoniopropionate (DMSPp) in sea‐ice algal communities from the North Water, northern Baffin Bay, were examined from April to June 1998. The concentrations of these compounds were measured in the bottom 2 cm of the ice at 36 locations throughout this region and are compared with results from water‐column samples collected for a complementary study. In general, levels of DMSPp (8.66–987 nmol·L−1, average 126 nmol·L−1) in sea‐ice algal communities were slightly less than those found in bottom sea‐ice algal communities from other polar locations but greater than those found in phytoplankton in other polar environments or at more temperate latitudes. Furthermore, DMSPp  :chl a ratios (0.02–14.8 nmol·μg−1, average 1.91 nmol·μg−1) in the sea‐ice algal community were slightly less than those found in other polar environments. DMSOp was measured for the first time in sea‐ice algal communities. DMSOp concentrations varied from 1.35 to 102 nmol·L−1 (average 13.7 nmol·L−1). DMSOp:chl a ratios varied from 0.01 to 4.5 nmol·μg−1 (average 0.22 nmol·μg−1) and were significantly lower than the DMSPp:chl a ratios observed in this study. It has been hypothesized that DMSO can act as a cryoprotector in phytoplankton cells. However, the low concentrations of DMSO observed in the ice algae during this study indicate that intracellular concentrations of DMSO are unlikely to have a significant influence on the freezing point depression of intracellular fluids.

UV Effects on Marine Planktonic Food Webs: A Synthesis of Results from Mesocosm Studies

Abstract UV irradiance has a broad range of effects on marine planktonic organisms. Direct and indirect effects on individual organisms have complex impacts on food-web structure and dynamics, with implications for carbon and nutrient cycling. Mesocosm experiments are well suited for the study of such complex interrelationships. Mesocosms offer the possibility to conduct well-controlled experiments with intact planktonic communities in physical, chemical and light conditions mimicking those of the natural environment. In allowing the manipulation of UV intensities and light spectral composition, the experimental mesocosm approach has proven to be especially useful in assessing the impacts at the community level. This review of mesocosm studies shows that, although a UV increase even well above natural intensities often has subtle effects on bulk biomass (carbon and chlorophyll), it can significantly impact the food-web structure because of different sensitivity to UV among planktonic organisms. Given the complexity of UV impacts, as evidenced by results of mesocosm studies, interactions between UV and changing environmental conditions (e.g. eutrophication and climate change) are likely to have significant effects on the function of marine ecosystems.

Mucus composition and bacterial communities associated with the tissue and skeleton of three scleractinian corals maintained under culture conditions

Corals live in close association with bacterial communities, but the nature of the relationship is still poorly understood. In this study, three scleractinian coral species, Galaxea fascicularis, Pavona cactus and Turbinaria reniformis were incubated under different laboratory conditions, and the composition of the bacterial community associated with their tissue or skeleton was compared between species or between species and seawater using denaturing gradient gel electrophoresis (DGGE). The amount of dissolved organic carbon (DOC) excreted and the mucus glycoconjugate composition were also determined for each species. The aim of the study was to assess if the bacterial community composition was species-specific or linked either to the seawater composition, or to the quality and quantity of carbon released by each coral. Results obtained showed that DOC release was significantly different ( P G. fascicularis . Also, the mucus of G. fascicularis and P. cactus mainly contained galactose and glucose whereas the mucus of T. reniformis contained more glucose and xylose. Cluster analyses of microbial community composition showed that the bacterial community was species-specific in the coral tissue but not in the skeleton, in all conditions. It remained specific when corals were incubated in the same or in different aquaria, and under different seawater renewal rates. Since DOC release rates and bacterial composition were both different according to the coral species considered, a link might be suggested between the two parameters. Sequencing of DGGE bands indicated that some bacterial phylotypes were consistently retrieved in all samples of a given species.

Simulation of Ozone Depletion Using Ambient Irradiance Supplemented with UV Lamps

Abstract In studies of the biological effects of UV radiation, ozone depletion can be mimicked by performing the study under ambient conditions and adding radiation with UV-B lamps. We evaluated this methodology at three different locations along a latitudinal gradient: Rimouski (Canada), Ubatuba (Brazil) and Ushuaia (Argentina). Experiments of the effect of potential ozone depletion on marine ecosystems were carried out in large outdoor enclosures (mesocosms). In all locations we simulated irradiances corresponding to 60% ozone depletion, which may produce a 130–1900% increase in 305 nm irradiance at noon, depending on site and season. Supplementation with a fixed percentage of ambient irradiance provides a better simulation of irradiance increase due to ozone depletion than supplementation with a fixed irradiance value, particularly near sunrise and sunset or under cloudy skies. Calculations performed for Ushuaia showed that, on very cloudy days, supplementation by the square-wave method may produce unrealistic irradiances. Differences between the spectra of the calculated supplementing irradiance and the lamp for a given site and date will be a function of the time of day and may become more or less pronounced according to the biological weighting function of the effect under study.

Body size as a predictor of species loss effect on ecosystem functioning

There is an urgent need to develop predictive indicators of the effect of species loss on ecosystem functioning. Body size is often considered as a good indicator because of its relationship to extinction risk and several functional traits. Here, we examined the predictive capacity of species body size in marine and freshwater multitrophic systems. We found a significant, but weak, effect of body size on functioning. The effect was much stronger when considering the effect of body size within trophic position levels. Compared to extinctions ordered by body size, random extinction sequences had lower multiple species loss effects on functioning. Our study is the first to show experimentally, in multitrophic systems, a more negative impact of ordered extinction sequences on ecosystem functioning than random losses. Our results suggest apparent ease in predicting species loss effect on functioning based on easily measured ecological traits that are body size and trophic position.

Response of microphytobenthos to flow and trophic variation in two South African temporarily open/closed estuaries

Abstract Variations in microphytobenthic biomass were investigated in relation to major environmental factors in the Mdloti and Mhlanga temporarily open/closed estuaries (TOCEs), South Africa, from March 2002 to March 2003. Both estuaries receive different volumes of treated sewage waters. This has caused eutrophication and an increase in the frequency of mouth opening, particularly at the Mhlanga. No significant differences were found in microphytobenthic chl a concentrations between the two estuaries. At the Mdloti, microphytobenthic biomass varied considerably, with values ranging from 1.33 to 131 mg chl a m-2 during the open phase, and from 18 to 391 mg chl a m-2 during the closed phase. At the Mhlanga, microphytobenthic biomass ranged from 7.0 to 313 mg chl a m-2 during the open phase, and from 1.7 to 267 mg chl a m-2 during the closed phase. Unlike in the Mdloti, the higher microphytobenthic biomass values at the Mhlanga were not always associated with the closed mouth state. For the duration of the study, the Mdloti functioned as a typical temporarily open/closed system, with prolonged open and closed phases, while the Mhlanga behaved more like a permanently open estuary than a TOCE (it lacked a period of prolonged mouth closure).

Identity effects dominate the impacts of multiple species extinctions on the functioning of complex food webs

Understanding the impacts of species extinctions on the functioning of food webs is a challenging task because of the complexity of ecological interactions. We report the impacts of experimental species extinctions on the functioning of two food webs of freshwater and marine systems. We used a linear model to partition the variance among the multiple components of the diversity effect (linear group richness, nonlinear group richness, and identity). The identity of each functional group was the best explaining variable of ecosystem functioning for both systems. We assessed the contribution of each functional group in multifunctional space and found that, although the effect of functional group varied across ecosystem functions, some functional groups shared common effects on functions. This study is the first experimental demonstration that functional identity dominates the effects of extinctions on ecosystem functioning, suggesting that generalizations are possible despite the inherent complexity of interactions.

Macroinvertebrates on coarse woody debris in the littoral zone of a boreal lake

Logging activity was a regular practice in the boreal forest of Quebec during the 19th century and may have had an impact on the temporal dynamics of aquatic coarse woody debris (CWD) and associated organisms. The dynam- ics of white cedar (Thuja occidentalis) CWD inputs from the riparian environment in a boreal lake in Eastern Quebec, Canada, over the past 350 years were reconstructed and differences in the macroinvertebate communities according to CWD age, season of sampling (spring, summer and autumn), depth and site were investigated. It was hypothesised that CWD macroinvertebrate community structure would change with CWD age, season and depth, but not among sites. No significant correlation was found between CWD age and macroinvertebrate densities and taxa number. The macroinverte- brate community was highly variable in space and time. Season was the main factor influencing taxa composition and the relative densities of individuals. The mean density was more than twofold greater in autumn than in spring and summer (1046, 1049 and 2335 individuals m −2 in spring, summer and autumn respectively). Density and taxa number decreased with depth, but site did not appear to influence the community. As CWD inputs increased during the log-driving period, impacts on macroinvertebrate communities were likely to be important and should be documented across the boreal zone.

DIEL VARIATIONS IN OPTICAL PROPERTIES OF IMANTONIA ROTUNDA (HAPTOPHYCEAE) AND THALASSIOSIRA PSEUDONANA (BACILLARIOPHYCEAE) EXPOSED TO DIFFERENT IRRADIANCE LEVELS(1).

Diel variations of cellular optical properties were examined for cultures of the haptophyte Imantonia rotunda N. Reynolds and the diatom Thalassiosira pseudonana (Hust.) Hasle et Heimdal grown under a 14:10 light:dark (L:D) cycle and transferred from 100 μmol photons · m−2 · s−1 to higher irradiances of 250 and 500 μmol photons · m−2 · s−1. Cell volume and abundance, phytoplankton absorption coefficients, flow‐cytometric light scattering and chl fluorescence, and pigment composition were measured every 2 h over a 24 h period. Results showed that cell division was more synchronous for I. rotunda than for T. pseudonana. Several variables exhibited diel variability with an amplitude >100%, notably mean cell volume for the haptophyte and photoprotective carotenoids for both species, while optical properties such as flow‐cytometric scattering and chl a–specific phytoplankton absorption generally showed <50% diel variability. Increased irradiance induced changes in pigments (both species) and mean cell volume (for the diatom) and amplified diel variability for most variables. This increase in amplitude is larger for pigments (factor of 2 or more, notably for cellular photoprotective carotenoid content in I. rotunda and for photosynthetic pigments in T. pseudonana) than for optical properties (a factor of 1.5 for chl a–specific absorption, at 440 nm, in I. rotunda and a factor of 2 for the absorption cross‐section and the chl a–specific scattering in T. pseudonana). Consequently, diel changes in optical properties and pigmentation associated with the L:D cycle and amplified by concurrent changes in irradiance likely contribute significantly to the variability in optical properties observed in biooptical field studies.