Abstract. The biomass distributions of marine benthic metazoans (meio-to macro-fauna, 1 µg-32 mg wet weight) across three contrasting sites were investigated to test the hypothesis that allometry can consistently explain observed trends in biomass spectra. Biomass (and abundance) size spectra were determined from observations made at the Faroe-Shetland Channel (FSC) in the Northeast Atlantic (water depth 1600 m), the Fladen Ground (FG) in the North Sea (150 m), and the hypoxic Oman Margin (OM) in the Arabian Sea (500 m). Observed biomass increased with body size as a power law at FG (scaling exponent, b = 0.16) and FSC (b = 0.32), but less convincingly at OM (b = 0.12 but not significantly different from 0). A simple model was constructed to represent the same 16 metazoan size classes used for the observed spectra, all reliant on a common detrital food pool, and allowing the three key processes of ingestion, respiration and mortality to scale with body size. A micro-genetic algorithm was used to fit the model to observations at the sites. The model accurately reproduces the observed scaling without needing to include the effects of local influences such as hypoxia. Our results suggest that the size-scaling of mortality and ingestion are dominant factors determining the distribution of biomass across the meio-to macrofaunal size range in contrasting marine sediment communities. Both the observations and the model results are broadly in agreement with the "metabolic theory of ecology" in predicting a quarter power scaling of biomass across geometric body size classes.

The benthic body size miniaturization hypothesis states that deep-sea communities are dominated by organisms of smaller body size, although some ¢eld studies have produced contradictory results. Using appropriate sample sets, this study tests this hypothesis by contrasting the benthic communities of the Fladen Ground (North Sea, 150 m) and the Faroe^Shetland Channel (1600 m). Samples were collected for large (500 mm) and small macrofauna (250^355 mm), meiofauna (45 mm) as well as an intermediate sized 'mesofauna' (180 mm) to ensure comprehensive coverage of the full meio-and macro-faunal body size-range. The body size structure of the benthos was compared using two methods. The more widely used average individual biomass method involves dividing the total sample biomass by sample abundance. Additionally, body size accumulation curves were constructed by assigning all specimens into a logarithmic size-class and then plotting the cumulative percentage of individuals present in each size-class. The results seem to support the hypothesis that the deep-sea environment is a small organism habitat. Although these ¢ndings only represent two locations, the overall body size accumulation curves clearly display a statistically signi¢cant shift towards smaller body sizes at the deeper site. The magnitude of the e¡ect is appreciable with median metazoan body size reducing from 14.3 mg wet weight in the Fladen Ground to 3.8 mg wet weight in the Faroe^Shetland Channel. The average individual biomass measurements are shown to be of limited value and can lead to potentially misleading conclusions if the underlying sizestructure is not analysed in detail.

The biomass distributions of marine benthic metazoans (meio-to macro-fauna, 1 µg-32 mg wet weight) across three contrasting sites were investigated to test the hypothesis that allometry can consistently explain observed trends in biomass spectra. Biomass (and abundance) size spectra were determined from observations made at the Faroe-Shetland Channel (FSC) in the Northeast Atlantic (water depth 1600 m), the Fladen Ground (FG) in the North Sea (150 m), and the hypoxic Oman Margin (OM) in the Arabian Sea (500 m). Observed biomass increased with body size as a power law at FG (scaling exponent, b = 0.16) and FSC (b = 0.32), but less convincingly at OM (b = 0.12 but not significantly different from 0). A simple model was constructed to represent the same 16 metazoan size classes used for the observed spectra, all reliant on a common detrital food pool, and allowing the three key processes of ingestion, respiration and mortality to scale with body size. A micro-genetic algorithm was used to fit the model to observations at the sites. The model accurately reproduces the observed scaling without needing to include the effects of local influences such as hypoxia. Our results suggest that the size-scaling of mortality and ingestion are dominant factors determining the distribution of biomass across the meio-to macrofaunal size range in contrasting marine sediment communities. Both the observations and the model results are broadly in agreement with the "metabolic theory of ecology" in predicting a quarter power scaling of biomass across geometric body size classes.

The response of a deep-water megafaunal assemblage to sedimentation disturbance from hydrocarbon drilling was investigated using remotely operated vehicle video off the Atlantic coast of Venezuela. This was the first assessment of megafauna in bathyal waters in this region. A two-way analysis of variance (ANOVA) design was used to assess patterns in density and assemblage structure both temporally, before and after the drilling event, and spatially, at different distances from the disturbance. High levels of sedimentation occurred within a radius of 20 to 50 m from the drilling site. Megafaunal densities were reduced with high levels of disturbance (from 0.60 m−2 to 0.17 m−2 <20 m from the drilling site). The responses of motile and sessile fauna were different. Sessile fauna were most common (77% total) and reflected trends for total density. Motile megafaunal density was generally higher after drilling (up to double the pre-drill density). Species richness was reduced by disturbance and proximity to the disturbance. Multivariate ANOVA revealed significant differences in assemblage composition with distance and before and after drilling but no interaction. This was most likely a result of variable species-specific responses to disturbance. Megafaunal densities were generally much higher than reported densities from comparable depths in the Gulf of Mexico or from deeper locations in the Caribbean Sea. The responses to sedimentation disturbance were generally less obvious than observed elsewhere and may result from the fauna being adapted to the naturally high levels of sedimentation deriving from the Orinoco River.

<p><strong>Abstract.</strong> Factors controlling biomass distributions in marine benthic organisms (meio- to macro-fauna, 1 μg–32 mg wet weight) were investigated through observations and allometric modelling. Biomass (and abundance) size spectra were measured at three locations: the Faroe-Shetland Channel in the north-east Atlantic (FSC, water depth 1600 m, September 2000); the Fladen Ground in the North Sea (FG, 150 m, September 2000); and the hypoxic Oman Margin (OM, 500 m, September 2002) in the Arabian Sea. Biomass increased with body size through a power law at FG (allometric exponent, <i>b</i> = 0.16) and at FSC (<i>b</i> = 0.32), but less convincingly at OM (<i>b</i> was not significantly different from −1/4 or 0). Our results question the assumption that metazoan biomass spectra are bimodal in marine sediments. <br><br> The model incorporated 16 metazoan size classes, as derived from the observed spectra, all reliant on a common detrital food pool. All physiological (ingestion, mortality, assimilation and respiration) parameters scaled to body size following optimisation to the data at each site, the resulting values being consistent within expectations from the literature. For all sites, body size related changes in mortality played the greatest role in determining the trend of the biomass size spectra. The body size trend in the respiration rate was most sensitive to allometry in both mortality and ingestion, and the trend in body size spectra of the production: biomass ratio was explained by the allometry in ingestion. <br><br> Our results suggest that size-scaling mortality and ingestion are important factors determining the distribution of biomass across the meiofauna to macrofauna size range in marine sedimentary communities, in agreement with the general observation that biomass tends to accumulates in larger rather than smaller size classes in these environments.</p>