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The biological and physical processes contributing to planktonic thin layer dynamics were examined in a multidisciplinary study conducted in East Sound, Washington, USA between June 10 and June 25, 1998. The temporal and spatial scales characteristic of thin layers were determined using a nested sampling strategy utilizing 4 major types of platforms: (1) an array of 3 moored acoustical instrument packages and 2 moored optical instrument packages that recorded distributions and intensities of thin layers; (2) additional stationary instrumentation deployed outside the array comprised of meteorological stations, wave-tide gauges, and thermistor chains; (3) a research vessel anchored 150 m outside the western edge of the array; (4) 2 mobile vessels performing basin-wide surveys to define the spatial extent of thin layers and the physical hydrography of the Sound. We observed numerous occurrences of thin layers that contained locally enhanced concentrations of material; many of the layers persisted for intervals of several hours to a few days. More than one persistent thin layer may be present at any one time, and these spatially distinct thin layers often contain distinct plankton assemblages. The results suggest that the species or populations comprising each distinct thin layer have responded to different sets of biological and/or physical processes. The existence and persistence of planktonic thin layers generates extensive biological heterogeneity in the water column and may be important in maintaining species diversity and overall community structure.

The evidence for a qualitatively and quantitatively important trophic link between planktonic Protozoa and higher order metazoan consumers is reviewed. the available data are obtained primarily, but not exclusively, from laboratory studies of calanoid copepod consumers and tintinnid ciliate prey from marine estuarine and nearshore environments. the data indicates that the protozoan‐metazoan link is of similar magnitude and importance in the pelagic ecosystems of freshwaters. It is proposed that planktonic Protozoa constitute a high quality, nitrogen‐rich food in the diets of their metazoan consumers. Implications of die trophic link to the consumers, prey, and ecosystem are discussed.

Marine planktonic oligotrichs of the genera Strombidium and Strombidinopsis were collected from the Northwest Arm of Halifax Harbour, isolated, and maintained in monospecific batch culture for periods ranging from weeks to 10 mo. Successful laboratory culture was a function of appropriate phytoplankton food and the chemical composition of the oligotrich culture medium. Maximal reproductive rates, as doublings per day, were obtained when the oligotrichs were fed 2 dinoflagellate strains, Heterocapsa triquetra and Scrippsiella trochoidea, and when the organisms were grown in seawater medium containing 10-@ to 10-6 M Na,-EDTA and trace metals. Oligotrich cultures fed diatoms of the spinose genus Thalassiosira exhibited poor growth, regardless of the culture m e d~u m used. Losses of oligotrichs from field collections due to handling ranged from 20 to 6 0 % , depending on the methods of collection and concentration used.

The lobate ctenophore Mnemiopsis leidyi occurs throughout Narragansett Bay, Rhode Island, during warm summer months but is often undetectable in the central portion of the bay during winter months. During 2 yr of weekly sampling, we found that M. leidyi populations in a shallow embayment, Greenwich Cove, either overwintered or were only briefly absent during winter. The Greenwich Cove population reproduced weeks earlier and reached higher average and peak population concentrations than open-bay populations. Shallow embayment populations such as that in Greenwich Cove probably serve as source populations that inoculate the main region of the bay by advective transport in the spring months. We propose that earlier occurrences of M. leidyi during recent years are due to amplification of pulsed spring warming events that permit early reproduction in the shallow embayments that serve as source regions for M. leidyi in Narragansett Bay. We further suggest that the source-sink perspective we describe is relevant not only to Narragansett Bay but other temperate regions of the world persistently occupied by M. leidyi.The importance of Mnemiopsis leidyi as a planktonic predator has been documented by a large body of research on its feeding capabilities (Kremer 1975;Reeve et al. 1978;Waggett and Costello 1999) and trophic impacts (Kremer 1979;Shiganova et al. 2001;Sullivan et al. 2001). These predatory capabilities underlie the importance of recent range expansion patterns for M. leidyi. Invasion of regions outside its historical distributions have resulted in dramatic planktonic community alterations in regions such as the Black Sea (Shiganova et al. 2003) and Sea of Azov (Studenikina et al. 1991). Although perhaps less acclaimed than these spatial range expansions, records of temporal range expansion within its endemic range can also cause important changes in planktonic community dynamics (Sullivan et al. unpubl. data). For example, within Narragansett Bay, peak occurrence of M. leidyi has shifted approximately 2 months earlier than the historic mean (Sullivan et al. 2001). However, the historically dominant summer copepod, Acartia tonsa, has not experienced a similar phenological shift, with the result that the seasonal timing of predator (M. leidyi) and prey (A. tonsa) overlap differently than during the past. One result of this change is that A. tonsa has been almost eliminated from the plankton during recent summers in Narragansett Bay as a result of predation pressure from M. leidyi (Sullivan et al. unpubl. data). The long-term trophic consequences of near removal of copepods from Narragansett Bay during summer months, historically a period of high copepod abundance, are not yet clear. However, there is evidence of reduction in numbers of some species of larval fish in recent years (Keller et al. 1999) and increases in summer values of chlorophyll a (Chl a) (Sullivan et al. unpubl. data).Despite the potentially important consequences of phenological shifts by M. leidyi, the mechanisms underlying M. leidyi...

Grazing on chlorophyll by microzooplankton (<200 •um) and copepods was measured in the mixed layer of the high-latitude North Atlantic Ocean during May and August 1991. No significant grazing by microzøoplankton occurred in May during a spring bloom dominated by colonial Phaeocystis pouchettii and Nitzschia spp. As the bloom declined, the size distribution of chlorophyll shifted from dominance by the > 20 •um chlorophyll fraction to dominance by the <20 chlorophyll fraction. The impact of grazing by microzooplankton increased as the bloom declined, with microzooplankton consuming 100% of potential daily chlorophyll production following the bloom. In August,.when the phytoplankton was dominated by the <20 #m chlorophyll fraction, m•cr0zooplankton consumed 37-53% (mean = 41%_+11% s.d.)'of potential daily chlorophyll production. Averaged over all experiments, microzooplankton grazing accounted for 81% of daily chlorophyll production. The grazing impact of Calanus finmarchicus stages C4 and C5, which dominated mesozooplankton biomass in the upper euphotic zone in both spring and late summer, was concentrated on chlorophyll > 20/tin in both seasons; C. f in mar c h ic u s did not consume significant amounts of chlorophyll < 20 •um in either season. Compared to the microzooplankton, copepods did not consume a significant fraction of total chlorophyll in either season, accounting for only -1% of daily chlorophyll production. Introduction The objective of this study was to evaluate losses of chlorophyll due to grazing by microzooplankton and mesozooplankton in the mixed layer of the high-latitude North Atlantic Ocean under contrasting biological and hydrographic conditions during spring and summer. The research was conducted on RV Endeavor cruises EN224-3 and EN227 in spring and late summer 1991 under the aegis of the Marine Light Mixed Layers (MLML) Accelerated Research Initiative, a multidisciplinary effort focused on the biological, physical and optical dynamics of the mixed lay_cr. The MLML study site was occupied t'or 8 days •n May 1991 and 9 days in August/Scptcmber 1991. Paper number 94JC00983 0148-0277!95!94JC-00983505.00 tic Front, an area characterized by extreme seasonal forcing of biology and hydrography. Convective cooling of the water column during winter produces adccp mixed layer of at least several hundred meters, which provides new nutrients to the system [Robinson et al., 1979; Plueddemann et al., this issue]. Irradiance and phytoplankton standing stocks are low during winter when phytoplankton growth is believed to be very low. With the onset of stratification, typically in April, standing stocks of chlorophyll increase IWil!iams and Hopkins, 1974; Plueddemann et al., this issue; Stramska et al., this issue], and a strong diatom bloom reaching 2-5/•g L '•. Calanus finmarchicus and other copepod species appear in the euphotic zone at this time, but their grazing appears to have little impact on the bloom [Dam et al., 1993; Morales et al., 1991; Weeks et al., 1993]. Transition to the summer phytoplankto...

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Oceanographic regimes on the continental shelf display a great range in the time scales of physical exchange, biochemical processes and trophic transfers. The close surface-to-seabed physical coupling at intermediate scales of weeks to months means that the open ocean simplification to a purely pelagic food web is inadequate. Top-down trophic depictions, starting from the fish populations, are insufficient to constrain a system involving extensive nutrient recycling at lower trophic levels and subject to physical forcing as well as fishing. These pelagicbenthic interactions are found on all continental shelves but are particularly important on the relatively shallow Georges Bank in the northwest Atlantic. We have generated budgets for the 1 lower food web for three physical regimes (well mixed, transitional and stratified) and for three seasons (spring, summer and fall/winter). The calculations show that vertical mixing and lateral exchange between the three regimes are important for zooplankton production as well as for nutrient input. Benthic suspension feeders are an additional critical pathway for transfers to higher trophic levels. Estimates of production by mesozooplankton, benthic suspension feeders and deposit feeders, derived primarily from data collected during the GLOBEC years of 1995-1999, provide input to an upper food web. Diets of commercial fish populations are used to calculate food requirements in three fish categories, planktivores, benthivores and piscivores, for four decades, 1963-2002, between which there were major changes in the fish communities.Comparisons of inputs from the lower web with fish energetic requirements for plankton and benthos indicate that we obtained reasonable agreement for the last three decades, 1973 to 2002.However, for the first decade, the fish food requirements were significantly less than the inputs. This decade, 1963This decade, -1972, corresponds to a period characterized by a strong Labrador Current and lower nitrate levels at the shelf edge, demonstrating how strong bottom-up physical forcing may determine overall fish yields.