Bert Klein

Vertical Flux of Biogenic Carbon in the Ocean: Is There Food Web Control?

Models of biogenic carbon (BC) flux assume that short herbivorous food chains lead to high export, whereas complex microbial or omnivorous food webs lead to recycling and low export, and that export of BC from the euphotic zone equals new production (NP). In the Gulf of St. Lawrence, particulate organic carbon fluxes were similar during the spring phytoplankton bloom, when herbivory dominated, and during nonbloom conditions, when microbial and omnivorous food webs dominated. In contrast, NP was 1.2 to 161 times greater during the bloom than after it. Thus, neither food web structure nor NP can predict the magnitude or patterns of BC export, particularly on time scales over which the ocean is in nonequilibrium conditions.

Effects of pelagic food-web interactions and nutrient remineralization on the biogeochemical cycling of carbon: a modeling approach

Abstract The operation of the oceans biological CO2 pump depends on both the structure of the pelagic food web and remineralization processes in the water column. We have developed a novel pelagic ecosystem model to study the effects on carbon export of food-web interactions in the euphotic zone and remineralization processes over the entire water column. The one-dimensional model consists of 10 state variables that span the herbivorous and microbial food webs. It is forced by solar radiation, vertical mixing, and the nitrate concentration in deep water. According to the model, adjusted against a CJGOFS data set, up to 52% of the nitrate-based phytoplankton production is processed by the microbial food web before being exported from the euphotic zone. Remineralization of dissolved organic carbon and suspended particles in the water column is a key control on carbon export, and up to 77% of the total material exported from the euphotic zone is remineralized in a layer located between the bottom of the euphotic zone and the annual maximum depth of the surface mixed layer. Nitrification of ammonia released within this layer satisfies most of the biological demand for nitrate in the euphotic zone. This places limitations on the use of new production as usually determined at sea (i.e. based on the uptake of nitrate) to estimate carbon export towards the deep.

Trophic structure of macrobenthos in the Gulf of St. Lawrence and on the Scotian Shelf

Abstract The Gulf of St. Lawrence and Scotian Shelf provide a diversity of oceanographic conditions in a continental margin setting. Climate is markedly seasonal, and bathymetry and hydrodynamic conditions cover a broad range, significantly influencing the patterns of organic matter sedimentation and, potentially, benthic community dynamics. Samples for analysis of benthic macrofauna and sediment microorganisms were collected at six stations in the Gulf of St. Lawrence (GSL) and the Scotian Shelf during winter and summer cruises, as part of the Canadian Joint Global Ocean Flux Study. Multivariate analyses indicate significant site-related trends in trophic guilds, benthic assemblages, and microbial activity, some of which are related to geomorphological characteristics (bathymetry, topography, and substratum). Macrofaunal trophic guild data show that the stations with relatively deep settling basins (Cabot Strait and Emerald Basin), dominated by surface deposit feeders, were distinct from stations with sloping bottoms (Anticosti Gyre and Anticosti Channel), where subsurface deposit feeders dominated or surface and subsurface deposit feeders were equally abundant. Deposit feeders (surface and subsurface trophic groups) made up >60% of the benthic communities, except at the Scotian slope station where they represented 44% of the total benthic abundances. Based on the data collected in both the water column and the sediment at three deep stations in the GSL, we hypothesize that the proportion of surface and subsurface deposit feeders, and thus the nature of bioturbation activity, is related to the magnitude and pattern of organic matter supply from the euphotic zone.

Low light adaptation and export production in the deep chlorophyll maximum layer in the northern Indian Ocean

Abstract Phytoplankton standing stock and primary production rates were measured in the central northern Indian Ocean at 65°E and 18°N in the inter-monsoon period in May 1987. As the algal populations were dominated by minute forms difficult to assess by routine microscopic methods, main algal groups were identified by pigment patterns derived fromHPLC-analysis. Profiles of chlorophyll a showed a distinct maximum between 50 and 80 m depth, with up to 10-fold higher values than in the surface mixed layer. The main portion of this deep maximum was situated below rather than above the sharp chemocline at 50 m, which analyses of particulate C,N and P showed to be a biomass maximum as well. In the mixed surface layer cyanobacteria dominated phytoplankton biomass and primary production, whereas the deep chlorophyll maximum was composed of small successive layers of cyanobacteria, coccolithophorids, dinoflagellates and diatoms. Prochlorophytes could be detected by the presence of divinylchlorophyll a throughout the whole water column. Highest absolute rates of primary production were encountered at the nitracline at 50 m depth. Over nearly all of the photic zone, primary production rates were closely related to POC values, leaving the P/B ratio at about the same value from the 100% irradiation level at the surface down to about 1% at 60 m depth. This adaptational ability was achieved by an increased pigmentation with depth, indicated by constantly decreasing POC/Chl a ratios in the water column. It seemed to be enhanced by a strict vertical succession of different phytoplankton groups that, by means of different sets of accessory pigments, might have obtained an additional advantage in low light adaptation. The main ecological significance of the deep chlorophyll maximum layer at the time was its role as a source of export production while importing “new” nutrients from below the nitracline. HPLC and elemental analyses of sediment trap material proved this layer to be source of most of the sedimenting particles. Calculations of nitrogen fluxes suggested the import and export terms to be well balanced.

Temporal and spatial patterns in the surface-water biomass of phytoplankton in the North Water

Temperature, salinity, and in vivo fluorescence of surface seawater in the North Water were recorded continuously, using a CTD+fluorometer, in August 1997, April–July 1998 and August–October 1999. The phytoplankton bloom started in the polynya on the Greenland side in April. In April and May, high phytoplankton biomass coincided with saline water on the Greenland side, while biomass was low on the Ellesmere Island (Canada) side where a deep mixed layer prevailed. High phytoplankton biomass extended over the whole polynya in June, when surface temperature increased due to solar heating and salinity decreased due to freshwater input. The initiation of the bloom was about 2 months earlier on the Greenland than the Canadian side. In July and August, phytoplankton biomass became low in the southern survey area, indicating that the phytoplankton bloom had ended. In September, relatively saline and warm water occurred in the southeastern part of the study area where, consistent with the change in water properties, high concentrations of chlorophyll a were observed again. These results imply that both the earlier start of the algal bloom in spring and the eventual increase in phytoplankton biomass in summer contribute to the high annual primary production along the Greenland side, thus influencing the structure and biological productivity of the entire North Water ecosystem.

Algal pigment diversity in coastal sediments from Kerguelen (sub-Antarctic Islands) reflecting local dominance of green algae, euglenoids and diatoms

SummaryHigh Performance Liquid Chromatography analysis of algal pigments from inter- and subtidal (deep and shallow) sediments from the Kerguelen Islands showed clear differences in the pigment composition at the different stations. High concentrations of chlorophyll c and fucoxanthin were present at all locations, indicating significant diatom densities, chlorophyll b was detected at all sites. At one station the other green algal pigments were also present; here green algae contributed more to chlorophyll a concentrations than diatoms, as estimated by using pigment ratios and microscopic observations. At another location chlorophyll b was associated with a high concentration of diadinoxanthin, indicating an abundance of euglenoids. This indicates that chemotaxonomy can be powerful tool in microphytobenthos studies since enumeration of living cells are difficult as many algae are attached to sediment particles (epipsammic algae). Ways of diagenesis, carotenoid degradation and the role of grazing are briefly mentioned. Phaeophorbide a-like pigments were the most significant chlorophyll a degradation products, with concentrations up to 110 μg · g−1 dry weight sediment, i.e. 10 times the chlorophyll a concentration. Some taxonomic estimations, based on pigments ratios, and their limits, are discussed.

Growth and distribution of marine bacteria in relation to nanoplankton community structure

Abstract Bacterial productivity and biomass were investigated along with nanoplankton community structure and environmental variables at a number of sites in the Gulf of St. Lawrence (mid-April and mid to late-June) and at additional sites off the coast of Nova Scotia (late June), eastern Canada. Total bacterial cell concentrations were determined in conjunction with actively respiring cells (ARCs) visualized using a redox fluorochrome (5-cyano-2,3-ditolyl tetrazolium chloride, CTC). Bacterial growth rates were estimated by 3 H -thymidine uptake. There were strong seasonal differences in bacterial activity within the euphotic zone. The CTC assay indicated that the proportion of ARCs to total bacteria (BN) in the euphotic zone was lower in spring (1–4%) than summer (3–12%). In the aphotic zone bacterial growth (TdR-H3 uptake) was much lower than above and the proportion of ARCs was frequently