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The spring bloom on the Scotian Shelf is examined using a mooring array deployed from 18 March 2002 to 18 April 2002 to provide physical, chemical, and biological measurements with high temporal and vertical resolution. These measurements are complemented by the Atlantic Zone Monitoring Program (AZMP) biweekly occupations of a station near the mooring site (HL2). Results from AZMP sampling and Sea-viewing Wide Field-of-view Sensor (SeaWiFS) ocean color imagery in early March show that coastal upwelling played an important role in the initiation of the spring bloom near the coast. This was a period of very strong horizontal gradients in surface chlorophyll. The main bloom at HL2 was sustained for most of the mooring period with peak chlorophyll levels reaching 6 mg m-3. Following the drawdown of nutrients in the upper 20 m, the bloom continued for 9 d and then disappeared at the surface but remained at the depth of the nutricline (30-50 m). The onset and evolution of the spring bloom on the inner Scotian Shelf is a complex process in which nutrient inventory, vertical mixing, and coastal upwelling play roles of varying importance throughout its lifetime. Mesozooplankton biomass does not change significantly until the very end of the mooring period indicating the grazing by this component of the zooplankton did not have as important a role in the termination of the bloom as the exhaustion of near-surface nutrients.

The magnitude of the flux of biogenic particulate organic carbon (POC) exported from the surface waters of the world ocean and remineralized at depth is critical to constraining models of the global carbon cycle, yet remains controversial. The use of upper ocean sediment traps is still one of the primary tools for determining this export flux, although trap fluxes have been shown to vary significantly because of hydrodynamic and sample collection biases. Over the past decade, 234 Th increasingly has been used as a tracer to estimate POC export from the euphotic zone by multiplying the depth-integrated 234 Th flux by the POC/ 234 Th ratio of sinking particles. The accuracy of this technique is highly dependent on the natural variability in the POC/ 234 Th ratio and 234 Th flux, yet the significance of this variability to estimates of POC export remains uncertain. Based on an analysis of new 234 Th and POC data from the Labrador Sea and a review of 25 previous independent field studies, we report that POC export fluxes can vary 2-10 times or more solely because of variability in the POC/ 234 Th ratio and procedures used to estimate the 234 Th flux. Recommended improvements include studies of the biological, chemical, and physical mechanisms controlling 234 Th-organic matter interactions in seawater; detailed comparisons of POC/ 234 Th ratios in size-fractionated and sediment trap material; increased spatial and temporal sampling density of 234 Th; and more standardized procedures to calculate the 234 Th export flux.

Using ship-based phytoplankton fluorescence techniques on a 9-d transect of the North Atlantic Ocean, we have produced the first synoptic view of variations in the maximum quantum yield of photosynthetic energy conversion (4,) on an ocean-wide basis. Based on our 4, measurements, the North Atlantic can be divided into two basins. The southeastern basin (from the African coast to the midocean ridge) is characterized by relatively little horizontal variation in 4, over vast stretches of the ocean. In the northwestern basin (from the midocean ridge to the Canadian coast), we measured lower values of 4, than in the eastern basin, particularly in the upper water column. Lowest surface values were found in an area of strong horizontal flow. Our results suggest that nutrient supply limits phytoplankton productivity over vast stretches of the North Atlantic in this period (early summer). However, surface values of 4, across the North Atlantic appear to be independent of nutrient concentration and distance to nitrate supply. We propose that horizontal advection may play an important role in determining the local conditions to which phytoplankton productivity parameters respond. Our results also suggest that an apparent relationship between remotely sensed variables (ocean color, sea surface height) and phytoplankton photosynthetic parameters exists that may provide the basis for the development of new algorithms used to estimate phytoplankton productivity from satellites.

The concentration-dependent uptake of nitrate (NO,) and its inhibit ion by ammonium (NH,) were surveyed in surface waters of the North Atlantic Ocean. Low-level NO, determinations combined with conventional tracer methods provided the first comprehensive data set on NO, utlization kinetics at nanomolar concentrations. Uptake followed saturation kinetics described by the Michaeli s-Menten equation. The half-saturation parameter for uptake (KN) ranged 2-3 orders of magnitude, covarying with ambient NO, concentrations. KN concentrations in oceanic waters averaged -20-30 nM. NH, half-saturation parameters could only be approximated (i.e. KA + A), but observations suggested that KA and K,, were of similar magnitude in oceanic waters. Maximum uptake rates of nitrate, prntN), and ammonium, prncA), covaried, but prncA) almost always exceeded prncN); in oceanic waters, the disparity was an order of magnitude or greater. Most of the variability in A,,(~ and prncA) could be explained by variations in phytoplankton biomass and temperature. The slope of the uptake vs. concentration relationship, CX, was also investigated but was highly variable and could not be related to any of the oceanographic properties observed; cr, was generally greater than ayN. Kinetics analysis showed that NH4 is preferentially utilized over NO3 over the full spectrum of nitrogen concentrations, nanomolar to micromolar. The inhibition of NO3 uptake by NH,, was also parameterized using the Michaelis-Menten expression. The inhibition half-saturation parameter (KJ covaried with KN, but Kj concentrations in oceanic waters (N 4050 nM) always exceeded KW Maximum inhibition (1,) was rarely complete (i.e. I,,., < I), even at 2,000 nM ammonium. Overall, results suggest that nitrogen utilization parameters currently used in ecosystem models of the open ocean should be re-examined. The mechanisms of supply and the kinetics of utilization of dissolved inorganic nitrogen are believed to play a critical role in determining the productivity, size structure, and species succession of phytoplankton communities in much of the world’s oceans. Dugdale (1967) was the first to propose a theoretical framework for nitrogen acquisition by phytoplankton at steady state based on the concentration-dependent uptake of nitrate and ammonium and represented by the well-known Michaelis-Menten equation of enzyme kinetics

Uptake of inorganic (NO3‒, NH4+) and organic (urea) nitrogen compounds by arctic phytoplankton was linear for at least 30 h of incubation under natural temperature and light conditions. Extrapolation of linear fits of the data showed positive ordinal intercepts for NH4+, suggesting relatively more rapid uptake early in the incubation period. Short term uptake experiments confirmed this; rates computed from 20-min incubations were on the average 3-fold higher than 24-h uptake rates. The transient nature of these enhanced uptake rates resulted in their contributing little (

In the castcrn Canadian arctic the photosynthesis-irradiance curves of phytoplankton from the 50% and 1% light levels differ mainly in their susceptibility to photoinhibition. The photoinhibition parameters of deep poplllations and the intensity of the deep chlorophyll maximum were correlated with N2, the bulk stratification parameter of the water column. Comparison of data from the same region in different periods suggests that the characteristic susceptibility to photoinhibition requires 2-6 weeks to develop once a population is isolated below a pycnocline. Insensitivity to photoinhibition developed much more quickly when a population from the chlorophyll maximum was exposed to surface light. The threshold of photoinhibition increased rapidly, reaching surface intensities within 4-6 h. The adaptation is sufficiently rapid that phytoplankton from the chlorophyll maximum should be stressed only briefly when transported to the surface by a mixed-layer-deepening event.

Plankton biomass, material fluxes, e.g. 14C uptake, and specific growth rates are related quantities. In the course of comparing various methods of measuring these properties in September 1982 off Oahu, Hawaii, we found specific growth rates of l-2.d-I. Such rates approach the maximum expected values observed in laboratory cultures. Plankton production is reported to be low in subtropical oceanic gyres compared with coastal waters at similar latitudes (Steemann Nielsen 1955; Koblentz-Mishke et al. 1970). Standing stocks, e.g. chlorophyll concentrations, are lower in the gyres, and plant nutrient levels are scarcely detectable by standard methods. We report here on a