Donald C. Gordon

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.

A new dry combustion method for the simultaneous determination of total organic carbon and nitrogen in seawater

Abstract A new dry combustion method for the simultaneous determination of total organic carbon and nitrogen in seawater is presented. Concentrations of total organic carbon obtained using this method average about 2.0 mg/l in surface water and 1.5 mg/l in deep water in the Northwest Atlantic. These concentrations are significantly greater than those reported using wet oxidation and wet combustion methods, but are comparable to concentrations reported by Russian workers using a similar dry combustion method. Organic nitrogen concentrations average 0.44 mg/l in surface water and 0.12 mg/l in deep water, yielding CN ratios for total organic matter of 5 and 12 respectively.

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.

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.

Variability of particulate organic carbon and nitrogen along the Halifax—Bermuda section

Abstract Particulate organic carbon and nitrogen concentrations were determined in surface and deep water at five stations between Halifax and Bermuda on eight cruises between October 1971 and 1973. Both parameters appear to be log normally distributed in sea water. Geometric means decreased systematically with depth to at least 2000 m. In both surface and deep water, geometric means in different regions decreased in the following order: Shelf, Slope, Gulf Stream, and Sargasso Sea, Temporal variations in deep water during the observations were slight, but evidence is presented that concentrations in the northwestern Atlantic may have decreased during the last decade. Some of the variability can be attributed to microstructure. The data support the concept that particulate organic matter is gradually being utilized as it is transported downward to the sea floor.

Temporal variations and probable origins of hydrocarbons in the water column of Bedford Basin, Nova Scotia

Abstract Hydrocarbons in Bedford Basin seawater were analysed on 33 dates between January 1975 and April 1976 by fluorescence spectroscopy and gas chromatography. Crude oil equivalent concentrations averaged 2.5 and 1.8 μg l−1 at 2 and 50 m, respectively, while total n-alkanes averaged 200 and 180 ng l−1 at the same depths. Crude oil equivalent concentrations were slightly higher than generally found in oceanic regions while total n-alkane concentrations were comparable. Hydrocarbon concentrations tended to be highest in the winter and lowest in the summer. Biosynthesis does not appear to be an important source of hydrocarbons since the concentration of only one hydrocarbon (retention index of 2052 and suspected to be a polyunsaturated olefin reported to be common in marine phytoplankton) increased during the spring phytoplankton bloom. The majority of hydrocarbons appear to be of anthropogenic origin and the increased use of fuel oils in the urban area surrounding Bedford Basin during the colder months can explain the higher winter-time concentrations. If biosynthesis is not an important hydrocarbon source in Bedford Basin, it can be argued that it is also unimportant in the ocean as a whole.

Detailed observations on the distribution and composition of particulate organic material at two stations in the Sargasso Sea

Abstract The variability of particulate organic carbon (POC) and nitrogen (PN) concentrations in three different depth layers (10 to 25, 350 to 365, and 985 to 1000 m) was measured at two stations in the Sargasso Sea sampled frequently during periods of 3 and 6 days in February 1975. Within each layer, the concentration ranges were in excess of analytical error indicating measurable variability, although it was not so great as reported for other regions of the ocean with generally higher concentrations of POC and PN. There was no indication that samples with above average concentrations of POC and PN had higher than average concentrations of ATP (except in the surface layer) or zooplankton. The geometric means and ranges of POC and PN concentrations decreased with depth between layers along with ATP and particle volume.

Total mercury in sea water in the northwest Atlantic Ocean

Total mercury was determined using a shipboard method of analysis for sea-water samples collected on two identical cruises along a section between Halifax and Bermuda. Concentrations decreased seaward, but there were no significant vertical gradients. The concentrations, averaging 0·15 μg 1−1, are the highest yet reported for open ocean water. Experiments indicated that an average of 57% of the total mercury can be lost if samples are stored before analysis for 10 days at pH 1.