Guglielmo Tita

Differential dispersal rates in an intertidal meiofauna assemblage

Meiofaunal nematodes and copepods disperse passively with sediment bedload, and copepods also display active emergence and reentry behavior. Epigrowth-feeders may be the nematode feeding group most susceptible to passive transport because they live closest to the sediment surface. We used bottom traps at a nematode-dominated intertidal mudflat in Maine, USA, to test the hypotheses that (1) meiofauna taxa disperse in relative proportions different from those of the ambient community; (2) copepods have the highest relative dispersal rate (number of individuals trap−1 day−1 ambient individual−1) and are not as tightly linked as other taxa to sediment flux; and (3) epigrowth-feeders have the highest nematode relative dispersal rate. Results supported all three hypotheses. Nematodes accounted for 95.8% of the individuals in cores, but only 38.9% of the individuals in traps. Copepods accounted for 1.5% of the individuals in cores, but 56.7% of the individuals in traps. Less common taxa also had different relative proportions in cores and traps, as did nematode feeding groups and individual species. The relative dispersal rate was far higher for copepods than for any other taxonomic group, and the absolute (number of individuals trap−1 day−1) and bulk (number of individuals g sediment−1 trap−1 day−1) dispersal rates for copepods were equal to those of the 65-fold more abundant nematodes and higher than those for all other taxa. The non-selective deposit-feeders were the most abundant nematode feeding group in the ambient community, but the epigrowth-feeders as a group and as individual species had the highest absolute, relative, and bulk dispersal rates. Non-metric multidimensional scaling (MDS) using analysis of similarity (ANOSIM) and species similarity percentages (SIMPER) reflected these differences between ambient and dispersing nematode assemblages. Significant positive regression relationships between sediment weight and the number of individuals captured in traps for nematodes and some other taxa indicated that they moved passively in the bedload. Lack of a significant regression relationship for copepods suggested an active behavioral component to dispersal. Meiofauna populations in this soft-bottom community were highly dynamic, demonstrating that the role of dispersal must be included in any consideration of the ecology of soft-bottom systems at local and regional spatial scales.

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.

Size spectra, body width and morphotypes of intertidal nematodes: an ecological interpretation

Nematode species from three intertidal assemblages (St Lawrence Estuary, Quebec, Canada) were studied in order to form an ecological interpretation of three allometric attributes: body width, size spectra, and morphotypes. The three assemblages were characterized by a very similar sediment grain median (Md) but different silt–clay proportions: A1 (upper-tidal level; Md=122 μm; silt=34.8%), A3 (mid-tidal level; Md=182 μm; silt=12.8%), and A5 (lower-tidal level; Md=122 μm; silt=6.8%). Silt–clay proportions were an influential factor in determining the mean nematode body width, used as a morphological discriminant between burrowing and interstitial organisms. A plot of the number of species vs the body width-classes showed two peaks: between 19.3 and 22.6 μm (interstitial), and between 32.0 and 45.5 μm (burrowers). As for the size spectra, in sandy sediments the mean nematode individual biomass was smaller than in muddy sediments. As a consequence, the estimated mean individual respiration rate was greater in muddy (A1=2.26 nl O 2  h −1 ) than sandy sediments (A3=1.25 nl O 2  h −1 ; A5=1.12 nl O 2  h −1 ). In contrast, estimated metabolic ratios were lower in A1 (2.78 nl O 2  h −1  μg −1 dry weight, DW) than in A3 (2.95 nl O 2  h −1  μg −1 DW) and A5 (3.01 nl O 2  h −1  μg −1  DW) suggesting different productivity and/or physiological adaptations to different lifestyles (burrowing vs interstitial) between species inhabiting muddy or sandy sediments. Morphotypes (body width/body length ratio=w/l ratio) were found to be associated with feeding groups. Small w/l ratios were typical of microvores, while greater ratios were typical of epigrowth feeders and predators. Ciliate-feeders, deposit-feeders and facultative predators had intermediate ratios. A morphotype food-related hypothesis is proposed: the species morphotype reflects the quality of exploited food; a small w/l ratio (i.e. long gut) would favour digestive efficiency and would be an adaptation to low quality food (microvores); inversely, a greater w/l ratio (i.e. short gut) would be an adaptation to high quality food (epigrowth-feeders and predators).

Intertidal meiofauna of the St Lawrence estuary (Quebec, Canada): diversity, biomass and feeding structure of nematode assemblages

The meiofauna of the St Lawrence estuary was investigated in the intertidal zone of the Parc du Bic (Quebec, Canada). Five nematode assemblages were distinguished by a cluster analysis: A1 and A2 (uppertide level); A3 (mid-tide level); A4 and A5 (low-tide level). Discriminant function analysis showed that exposure time during low tide was the most important environmental factor in determining diierences between assemblages. Chlorophyll-a, phaeopigments, sediment water content, and per cent of silt followed in the same order. Nematode densities (400^1500 ind 10 cm 72 ) were found to be lower than those generally reported for other estuarine intertidal zones of the eastern Atlantic coast. Mean nematode biomass in the ¢ve assemblages ranged between 96 � 14 and 248 � 86m gC org 10 cm 72 . Deposit feeders were generally the dominant nematode feeding group in terms of abundance and biomass. Correlation of epigrowth-feeders with chlorophyll-a and phaeopigments, respectively, suggested that in the upper-tide level, old or partially degraded phytodetritus contribute more to the diet of this nematode feeding group; and in the low-tide level epigrowth-feeders may rely more on ‘fresher’ phytodetritus.

EFFECTS OF DIESEL AND INTERACTIONS WITH COPPER AND OTHER METALS IN AN ESTUARINE SEDIMENT MICROBIAL COMMUNITY

Estuarine sediment microcosms were treated with combinations of diesel, copper (at two levels), and a mixture of heavy metals (mercury, cadmium, lead, and chromium; at two levels) mimicking the contaminant loadings found in harbor sediments. The effects on the microbial community were monitored by polar lipid fatty acid analysis. Diesel addition increased microbial biomass, caused shifts in some fatty acid structural groups, and decreased starvation biomarkers. Incorporation of diesel hydrocarbons into lipids was expressed as an increase in the proportion of odd-carbon-number fatty acids. No treatment with the metals mixture (mercury, cadmium, lead, and chromium) alone significantly changed any parameter derived from the polar lipid fatty acids, but the increase in microbial biomass from diesel addition was higher with the metals mixture, possibly because of indirect effects caused by reductions in grazing resulting from metal-induced toxicity to bacteriovorous nematodes. Copper also modified the effects of diesel addition, preventing biomass increase but not diesel degradation, suggesting that some of the energy gained from diesel oxidation was expended combating copper toxicity. In the present study, observations indicate that metals in general, and copper in particular, can modify the response of sedimentary microorganisms to petroleum-hydrocarbon contaminants.