Jonathan Grant

Particle clearance and absorption of phytoplankton and detritus by the sea scallop Placopecten magellanicus (Gmelin)

Abstract Diets of cultured phytoplankton (Tahitian Isochrysis aff. galbana and Chaetoceros gracilis Schutt), fresh and “aged” kelp powder ( Laminaria longicruris de la Pyl.) and resuspended sediment were fed to sea scallops ( Placopecten magellanicus Gmelin) from Georges Bank. The majority of particles from each diet were within the size range effectively retained by sea scallops (>5/ gmm ). Based solely on net and gross absorption efficiency ( AE ) data as an index of nutritional quality, the following ranking was observed: “aged” kelp debris > C . gracilis = I . galbana > fresh kelp > resuspended sediment. While only a small fraction of the total sediment organic matter was utilized (mean net AE = 9 %), an average of 50 % of available nitrogen was absorbed. Resuspended paniculate matter may play an important role in the energy gain and nitrogen demands of this species. Despite the ability of P. magellanicus to efficiently digest “aged” kelp debris, it was consumed at a much lower rate than phytoplankton and did not contribute significantly to the dietary requirements of the sea scallop. These results demonstrate the importance of phytoplankton as a diet for the sea scallop, but indicate that detrital particles can contribute to energy gain during periods when phytoplankton are less available to meet energy demands.

Transport of recently settled soft-shell clams (Mya arenaria L.) in laboratory flume flow

Abstract A laboratory flume was used to examine the retention of juvenile soft-shell clams (2 wk postsettlement) in sediment when exposed to flow. Clams resisted erosion until the initiation of sediment transport, after which they were rapidly advected from sections of test substrate. Comparisons between living and killed clams indicated that burrowing behavior was instrumental for maintaining position in flow ≤ the critical erosion velocity for sediment movement. The ability of low density, shallow-burrowing juvenile bivalves to avoid transport as bedload or resuspended particles is probably minimal during erosional periods, despite that fact that the position of larger, deeper living individuals may be unaffected. In energetic regimes, infaunal recruitment patterns can be dominated by hydrodynamic forces affecting juveniles.

Carbon and nitrogen scope for growth as a function of diet in the sea scallop Placopecten magellanicus

Laboratory feeding experiments with the sea scallop Placopecten magellanicus were carried out to compare scope for growth (SFG) to measured growth and determine the effect of diet on carbon and nitrogen SFG. Diets consisting of cultured phytoplankton, kelp detritus, and resuspended sediment were provided daily for 52 days (October-December). Measurements of clearance rate, absorption efficiency, respiration, O/N ratio, and carbon and nitrogen content of diets and scallop tissue were used to construct carbon and nitrogenbudgets for each diet. Growth coefficients were calculated from change in tissue weight during the study period.

An in situ estimation of the effect of wind-driven resuspension on the growth of the mussel Mytilus edulis L.

Previous studies have suggested that the effect of near-bottom seston depletion on individual growth in dense beds of suspension-feeding mussels Mytilus edulis L. may be alleviated by particle enhancement caused by resuspension. As a test of the relative importance of depletion and resuspension, mussels were grown at two intertidal sites, 0 and 1 m off bottom (St Lawrence Estuary, Quebec), with the same immersion time, but with differing phytoplankton concentrations and resuspended loads. Growth at both sites was measured according to two independent methods in order to distinguish between effects of resuspended particles and vertical phytoplankton gradients. The first method involved the estimation of gain in tissue weight and was resuspension-dependent since mussel growth integrated effects of wind events as well as those of calm weather. In the second method, growth was estimated from scope for growth (SFG) measurements carried out only in calm weather, to yield resuspension-free results. Growth at both sites was compared by computing the ratio of growth (0 m : 1 m) for each method, an approach not dependent on agreement of absolute growth rates between the two methods. Results indicated that the net effect of wind events and resuspension did not have a significant effect on growth during May–July and May–November study periods, and that 0 m : 1 m growth and SFG ratios were usually <1. This suggests that decreased mussel growth at the bottom of the water column was primarily caused by phytoplankton depletion near the mussels, a process which could not be ameliorated by resuspended food supplies.

Benthic Metabolism and the Quality of Sediment Organic Carbon

AbstractWe have used changes in oxygen uptake of sediment cores incubated in the laboratory and receiving no particulate input to examine the utilization of labile organic carbon by the benthos. A model is described that predicts a decline in sediment oxygen consumption after primary labile organic carbon (G01) is depleted. Oxygen consumption by cores taken from an intertidal sandflat in Nova Scotia and incubated in darkness (11–13°C) declined after about two months. G01 organic carbon was estimated as ∼8% of total sediment carbon (GT) in the upper 1 cm of sediment. Anaerobic metabolism (based on CO2 production), a significant portion of community metabolism, increased rather than declined during the incubations. Similar calculations were performed using published data on cores from Narragansett Bay. At this location, organic matter in the G01 fraction was 5% of GT. A less labile organic carbon fraction (G02) was estimated to be 44% of GT based on declining oxygen consumption after 27 days. Refractory car...

Sediment resuspension rates, organic matter quality and food utilization by sea scallops (Placopecten magellanicus) on Georges Bank

Benthic detritus, bacteria, and settled phytoplankton are transported into the water column by resuspension, potentially providing a high quality food source to suspension feeders. Two aspects of resuspension must be considered in relation to food supplies for suspension feeders: the flux of particles from the sediments to the water column and its food value. Sediment resuspension rates on Georges Bank and the role of resuspended sediment in the diet of sea scallops (Placopecten magellanicus) were determined in laboratory flume experiments and shipboard feeding experiments, respectively. Resuspended carbon flux was estimated from flume bedload transport rates and the mass of organic carbon associated with the silt-clay fraction eroded from Georges Bank sediment during transport. A comparison of sand erosion thresholds with the frequency distribution of shear velocity estimated from field current meters indicated that tidal sediment resuspension will occur 62% of the time. Resuspended material had a carbon content of 4-8% and a C:N of 5-8. Rates of resuspension (33-229 mg C m -2 h -1 ) and settling rates indicate that resuspended sediment in a size range available to scallops (>5 pm) remains in suspension for periods of hours to days. Clearance rates of resuspended sediment by scallops were similar to those for water column particles, and filtration rates increased with increasing concentrations of resuspended material. Feeding experiments demonstrated that scallops absorbed organic matter from resuspended sediments with an efficiency of up to 40%. Therefore, in terms of particle retention, ingestion, and digestion, sea scallops are able to exploit resuspended organic matter from a continental shelf habitat. Furthermore, resuspension occurs with sufficient frequency, and resuspended sediment has long enough residence time in the water column to provide a consistent nutritional benefit to scallops.

Effect of scallop shells and sediment grain size on phytoplankton flux to the bed

Abstract A flume study was made of bed skin friction and phytoplankton (Thalassiosira weissf ogii) deposition about a sea scallop (Placopecten magellanicus) mimic on a coarse (mean grain dia. =1200 μm), medium (615 μm) and fine (170 μm) quarry sand. Bed skin friction immediately upstream, and at one shell diameter downstream of the mimic was 1.2–2.4 times higher than ambient values (8.1 × 10−2 Pa). Directly downstream of the mimic there was a region of near-zero skin friction. Bed diatom density was correlated with changes in skin friction; after 21 h, cell densities were 36–87% greater in the regions of high skin friction upstream and downstream of scallop than in control experiments. The pattern of bed diatom density about the scallop was similar in the coarse and medium sands, but deposition to the fine bed was not affected by the mimic. Diatom density was significantly correlated with grain diameter; deposition in the coarse bed was 2.3 times higher than in the medium sand, and 7.4 times higher than in the fine sand. A field experiment confirmed that the coarse sand was a greater sink of phytoplankton pigment than the fine sand. Previous studies and scaling arguments suggest that the differences in bed diatom density were controlled by the magnitude of interfacial solute fluxes. Regions of high skin friction about the mimic increased the porewater exchange, resulting in greater concentrations of diatoms retained within the bed. Similarly, the more permeable coarse sand would have a greater rate of porewater exchange than the fine sand, explaining the higher bed diatom density. Differences in the predicted volume of interstitial void space as a function of grain size closely matched the observed differences in bed diatom density. Results suggest that alteration of boundary layer flows by centimetre scale topography such as scallops, increases the flux of particulate organic matter toward the bed, but whether it is retained within the bed, and thus made available to the benthos is dependent on the sediment granularity.

Benthic oxygen consumption on continental shelves off eastern Canada

The consumption of phytoplankton production by the benthos is an important component of organic carbon budgets for continental shelves. Sediment texture is a major factor regulating benthic processes because fine sediment areas are sites of enhanced deposition from the water column, resulting in increased organic content, bacterial biomass and community metabolism. Although continental shelves at mid- to high latitudes consist primarily of coarse relict sediments (Piper, Continental Shelf Research, 11, 1013–1035), shelf regions of boreal and subarctic eastern Canada contain large areas of silt and clay sediments (Fader, Continental Shelf Research, 11, 1123–1153). We collated estimates of benthic oxygen consumption in coarse (<20% silt-clay, <0.5% organic matter) and fine sediments (20% silt-clay, 0.5% organic matter) for northwest Atlantic continental shelves including new data for Georges Bank, the Scotian Shelf, the Grand Banks of Newfoundland and Labrador Shelf. Estimates were applied to the areal distribution of sediment type on these shelves to obtain a general relationship between sediment texture and benthic carbon consumption. Mean benthic oxygen demand was 2.7 times greater in fine sediment than in coarse sediment, when normalized to mean annual temperature. In terms of carbon equivalents, shelf regions with minimal fine sediment (Georges Bank, the Grand Banks of Newfoundland-northeast Newfoundland) consumed only 5–8% of annual primary production. Benthos of the Gulf of Maine (100% fine sediment) and the Scotian Shelf (35% fine sediment) utilized 16–19% of primary production. Although 32% of the Labrador Shelf area contained fine sediments, benthic consumption of pelagic production (8%) was apparently limited by low mean annual temperature (2°C). These results indicate that incorporation of sediment-specific oxygen uptake into shelf carbon budgets may increase estimates of benthic consumption by 50%. Furthermore, respiration and production by large macrofauna allow an even greater proportion of primary production to enter benthic pathways. Fine sediment areas (shelf basins or “depocenters”) are postulated to be sites of enhanced biological activity which must be considered in the modelling of shelf carbon budgets and the role of the benthos in demersal fisheries.

Resuspendable organic matter in nova scotian shelf and slope sediments

Abstract Sediment samples from the continental shelf and slope off Nova Scotia were fractionated with a combination of elutriation and sieving to investigate the most easily resuspended sediment fraction and thus processes of organic deposition. Bulk sediments on the slope were enriched in carbon and nitrogen compared to those on the shelf. Fractionation indicated that

Swept Away: Resuspension of Bacterial Mats Regulates Benthic-Pelagic Exchange of Sulfur

Filaments and extracellular material from colorless sulfur bacteria (Beggiatoa spp.) form extensive white sulfur mats on surface sediments of coastal, oceanic, and even deep-sea environments. These chemoautotrophic bacteria oxidize soluble reduced sulfur compounds and deposit elemental sulfur, enriching the sulfur content of surface sediment fivefold over that of deeper sediments. Laboratory flume experiments with Beggiatoa mats from an intertidal sandflat (Nova Scotia) demonstrated that even slight erosion of sediment causes a flux of 160 millimoles of sulfur per square meter per hour, two orders of magnitude greater than the flux produced by sulfur transformations involving either sulfate reduction or sulfide oxidation by benthic bacteria. These experiments indicate that resuspension of sulfur bacterial mats by waves and currents is a rapid mechanism by which sediment sulfur is recycled to the water column. Benthic communities thus lose an important storage intermediate for reduced sulfur as well as a high-quality bacterial food source for benthic grazers.