Peter J. Cranford

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.

The influence of dilute clay suspensions on sea scallop (Placopecten magellanicus) feeding activity and tissue growth

Abstract Long and short-term laboratory experiments were conducted to examine biological responses of sea scallops ( Placopecten magellanicus ) to relatively dilute bentonite clay suspensions. The effect of bentonite addition (0, 2 and 10 mg·dm −3 treatments) to a natural diet of phytoplankton and organic detritus (seston) on somatic and reproductive tissue growth was examined over 68 days during the period of gametogenesis. Filtration, ingestion and pseudofaeces production rates and particle retention efficiency were measured for scallops which were fed mixtures of algae (dried Tetraselmis suecica ) or natural seston and bentonite (0 to 15 mg·dm −3 ). Pre-ingestive particle-selection capabilities were examined through chlorophyll a , organic content and carbon and nitrogen analyses of suspended particles and pseudofaeces. The results indicate a low tolerance to suspended bentonite. Extensive chronic mortalities and significant impacts on somatic and reproductive tissue growth were exhibited at 10 mg·dm −3 . Despite an inability to effectively retain clay-size particles, sea scallop feeding activity was altered by dilute bentonite suspensions. Animals fed natural seston without added bentonite displayed filtration rates twice as high as when 2 mg bentonite per dm 3 was added, but tissue growth in both groups was similar. When provided with a diet of Tetraselmis , a similar reduction in filtration rate required bentonite concentrations exceeding 6 mg·dm −3 ; levels lower than 1.0 mg·dm −3 enhanced filtration rates. Sea scallops displayed two feeding mechanisms for actively enhancing the quality of ingested matter when exposed to elevated levels of suspended clay. Scallops were less efficient at retaining bentonite particles at concentrations exceeding 2 mg·dm −3 than at lower levels. This shift in retention efficiency did not require a period of acclimation. Above a threshold bentonite concentration of 2 mg·dm −3 , scallops produced pseudofaeces and selectively ingested seston or Tetraselmis over clay particles. Selection efficiencies, determined from relative chlorophyll a , AFDW, POC and the PN-concentration of retained particles and pseudofaeces, were low compared with values published for other suspension-feeding bivalves.

In situ feeding and absorption responses of sea scallops Placopecten magellanicus (Gmelin) to storm-induced changes in the quantity and composition of the seston

Time-series of hourly clearance, ingestion and absorption rates and absorption efficiency were measured over 48 h for adult sea scallops (Placopecten magellanicus) held in situ in a coastal embayment in Nova Scotia, Canada, during a wind-induced resuspension event. Temporal variations in oceanographic variables, and seston quantity and composition (organic matter, organic carbon, nitrogen, chlorophyll a, and inorganic particle size spectra) were monitored during the study with moored instruments and hourly water sampling. Resuspension of bottom materials during the storm resulted in large changes in the amount (1 to 30 mg l−1 total particulate matter) and nutritional quality (25 to 50% organic content) of seston. High sedimentation rates after the storm were accelerated by flocculation, resulting in the rapid settling of resuspended particles and an increase in seston quality. Observed short-term (hourly) fluctuations in clearance rate were not related to storm- or tide-induced changes in seston characteristics but were directly related to flow velocity. Significantly lower clearance rates were observed at relatively low ( 9 cm s−1) flow speeds. The overall reduction in ingestion rates after the storm resulted from decreased food availability. Hourly absorption efficiency (AE) measurements were closely related to seston quality (total organic, organic C and N content) and AE declined exponentially with decreasing seston quality. Reductions in AE during the resuspension event were offset by the increased ingestion rate, resulting in no significant changes in absorption rates for organic matter, C, or N over the sampling period. As the low food quality of the resuspended matter was balanced by increased availability, any physiological regulation of food acquisition (i.e. clearance rate regulation) would have been irrelevant to maintaining food intake constant.

Chronic toxicity and physical disturbance effects of water- and oil-based drilling fluids and some major constituents on adult sea scallops (Placopecten magellanicus)

Abstract Adult sea scallops, Placopecten magellanicus, were exposed in the laboratory under environmentally representative conditions to different types and concentrations of used operational drilling fluids and their major constituents. Threshold waste concentrations causing reductions in somatic and/or reproductive tissue growth are: greater than 10 mg l−1 for used water-based mud (WBM); 2 mg l−1 for bentonite; and less than 0.5 mg l−1 for barite and used oil-based mud (OBM). Chronic exposure to OBM caused high mortalities at concentrations as low as 1.0 mg l−1. Non-nutritious particles in the food supply (all wastes) and chemical toxicity (OBM and perhaps barite) affected the growth rate and survival of sea scallops by altering physiological state (scope for growth) and nutritional condition (O:N ratio). The value of scope for growth (SFG) calculations for assessing the relative chronic toxicity of the drilling wastes was demonstrated by the close relation observed between SFG and actual growth measurements. These results show that chronic intermittent exposure of sea scallops to dilute concentrations of operational drilling wastes, characterized by acute lethal tests as practically non-toxic, can affect growth, reproductive success and survival.

Observations on the ecological importance of salt marshes in the Cumberland Basin, a macrotidal estuary in the Bay of Fundy

Abstract The Cumberland Basin, a 118 km 2 estuary at the head of the Bay of Fundy which has an average tidal range of about 11m, contains large tracts of salt marsh (15% of the area below highest high water). Low marsh (below about 0·9 m above mean high water) is composed almost exclusively of Spartina alterniflora while the vegetation on high marsh is more diverse but dominated by Spartina patens . Because of its higher elevation, high marsh is flooded infrequently for short periods by only extreme high tides. Low marsh is inundated much more frequently by water as much as 4m deep for periods as long as 4 h per tide. Temporal variability in the occurrence of extreme tides influences the flooding frequency of high marsh for any given month and year. Using a modification of Smalleys method, the mean annual net aerial primary production (NAPP) of low and high marsh is estimated to be 272 and 172 g C m −2 , respectively. Vegetation turnover times average 1·0 and 2·0 y for low and high marsh, respectively. Because of abundant tidal energy, much of the low marsh production appears to be exported and distributed widely about the estuary. Since high levels of turbidity suppress phytoplankton production, salt marshes produce approximately half of the carbon fixed photosynthetically in the Cumberland Basin. It is concluded that salt marshes play a major ecological role in the Cumberland Basin.

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.

Relationships between food quantity and quality and absorption efficiency in sea scallops Placopecten magellanicus (Gmelin)

Laboratory experiments, in which adult sea scallops (Placopecten magellanicus Gmelin) were fed rations of varying concentration and organic and elemental composition for periods of 15 and 56 days, were conducted to provide predictive relationships between the quantity and quality of the food supply and the efficiency and rate of absorption of dietary constituents. The absorption efficiency of particulate organic matter (AEPOM), organic carbon (AEPOC) and nitrogen (AEPN) decreased exponentially with a progressive increase in dietary inorganic content. Regression models indicated that between 74 and 84% of the variation in average AE values was attributed to differences in dietary quality, which was expressed as the POM, POC or PN content per unit dry weight of particulate matter. Changes in absorption efficiency were rapid after a change in seston quality and acclimation was complete within 6 days. During this time, the AEPOM of the lowest quality seston ration (39% organic content) was increased by a factor of 1.8 and AEPOC and AEPN progressively increased from negative values to about 60%. Scallops fed a seston diet that alternated between low and high quality (39 and 84% POM, respectively) at 12-h intervals acclimated to the lower quality diet. These results show that sea scallops respond to low-frequency reductions in food quality by rapidly maximizing the absorption of available resources. By maintaining this adapted state for some period greater than 12 h, the scallops are able to maintain a relatively high AE despite high-frequency, tidally forced variations in the food supply.

Detailed distribution of dissolved and particulate organic matter in the Arctic Ocean and comparison with other oceanic regions

Abstract Over 100 measurements of particulate organic carbon (POC), particulate nitrogen (PN) and dissolved organic carbon (DOC), based on bottle collections, were made through the ice in the Arctic Ocean as part of the Canadian CESAR expedition. Despite considerable scatter at all depths, the three variables show a general decrease with depth to the bottom (2100 m). The average PN concentrations are the lowest yet reported, as are the POC concentrations in the upper 500 m. DOC concentrations are similar to other oceanic regions. There is no relationship between any variable and water mass structure. The average C:N ratio (by weight) of particulate organic material is 11.5 in the surface layer and increases with depth. Compared to data collected from other oceans, the Arctic concentrations demonstrate similar depth gradients which are steepest for PN and least for DOC. There also is a broad scale relationship between surface primary production and POC and PN concentrations throughout the water column.

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.

Rapid digestion and assimilation of bait by the deep-sea amphipod Eurythenes gryllus

Scavenging lysianassoid amphipods (Eurythenes gryllus) were collected with a newly designed trap to measure digestion rates with timed exposure to bait in the Canada Basin of the Arctic Ocean (2075 m) and in the Nares (3521 m) and Sohm Abyssal (4978 m) Plains, northeast Atlantic Ocean. In feeding experiments up to 157 h long, water and organic matter content were not significantly different in anterior, central and hind gut regions of individual amphipods, showing that digestion in E. gryllus conforms to the batch reactor feeding model. Ingested bait was rapidly solubilized and water content increased from 90% within 69 h. Digestion rates, calculated from exponential curves fitted to decreases in gut contents for dry matter and various organic components, were very high (2.1–6.5% loss h−1) for sardines ingested during short (6–11 h) incubation periods in the Canada Basin. Mackerel ingested by amphipods trapped in the Nares and Sohm Abyssal Plains were digested at lower rates (0.4-1.0% loss h−1) in in situ experiments up to 123 h. Allometric regressions described relationships between body length and calories potentially available for assimilation from one meal by male and female instars. Rapid digestion in opportunistic scavengers like E. gryllus makes gut capacity available for additional feeding when food supply is unpredictable.