An ecosystem process model is developed and used to probe the hypothesis that grazing by zooplankton controls the standing stock of phytoplankton at Station P (50°N, 145'W) in the eastern subarctic Pacific Ocean The model attempts to reproduce seasonal variahons and integrated interactions of specific physical, chemical, and biological components of the subarctic pelagic ecosystem, and to simulate quantitatively the observed level of phytoplankton production. In its various versions the model describes the annual cycle of standing stock and production of phytoplankton, herbivorous microzooplankton, and the concentration of dissolved inorganic nitrogenous nutrients in a homogeneous mixed layer. Mesozooplankton (which include the large suspension-feeding copepods Neocalanus spp.) are assessed for their potential a s dominant grazers of phytoplankton and, alternatively, a s omnivorous consumers of phytoplankton and herbivorous rnicrozooplankton. Thls model indicates that a hypothesized trophic structure, in w h c h herbivorous rnicrozooplankton are the major grazers of phytoplankton and mesozooplankton are indmriminate omnivores, is plausible in theory and consistent with available but limited observations.

ABSTRACT. Grazing control and iron limitation have been portrayed as mutually exclusive alternative explanations of the high nutrientflow phytoplankton stock condition in the nutnent-rich areas of the open sea. To contrast clearly the underlying assumptions and speciflc consequences of the 2 mechanisms, a simple mathematical model of a chemostat contaming a pelagic food c h a~n with 1 to 4 trophic levels is used as a n analogue of the equatorial upwelling zone in the eastern Pacific. It shows that grazing control is essential to reproduce 3 conditions observed In the equatorial upwelling zone: low phytoplankton stocks, high concentrations of macronutrients, and phytoplankton specific growth rates that greatly exceed advective throughput rate in the mixed layer at the equator. Nevertheless, simultaneous grazing control and limitation by a trace nutrient such as iron could feas~bly account for observed phytoplankton specific growth rates that are less than the maximum rate possible under optimal growth conditions. In addition, release of an inefficiently grazed component of the phytoplankton assemblage from iron limitation seems to explain why escape from grazing control occurs under both experimental and natural iron ennchments.

From 3 to 6 November 2002, a colloquium was convened at the Benthos Laboratory of the Stazione Zoologica Anton Dohrn on Ischia, Italy, with the goal of evaluating the present status of the effects of diatoms on their main consumers, planktonic copepods, and to develop future research strategies to enhance our understanding of such interactions. These included (1) toxic effects of diatom metabolites on copepods, particularly reproduction, and (2) nutritional effects of diatoms on juvenile to adult copepods. Key issues involved in the impact of diatoms on the dynamics of natural plankton communities in situ were also addressed. During the plenary session, the most recent advances on this topic were presented. The plenary session was followed by 3 working groups on (1) production of aldehydes by phytoplankton, (2) toxic and nutritional effects of diatoms on zooplankton, and (3) the chemistry of diatom defense, as well as of their nutritional quality. These working groups focused on suggesting future research needs for the different topics. As a result, several recommendations were outlined, including experimental studies. It became evident that interdisciplinary efforts are needed, involving chemists, oceanographers and experimentalists, since many of the biological observations under controlled conditions and in situ require an integrated approach, including chemical causation. Extensive field observations based on common protocols are also recommended for investigation of the intrinsic variability of such effects and their environmental controls. Laboratory experiments are seen to be essential for the full understanding of environmentally occurring processes.

Daytime and nighttime vertically stratified zooplankton samples spanning the entire water column were obtained from Dabob Bay. Washington. USA, during several years. Vertical distributions of all the copepodid stages of Calanus pacificus and Metndia lucens were analyzed from 8 cruises representing the range of seasons, as well as several dates when inferred invertebrate or vertebrate predation pressures on zooplankton were strong or weak. Migration behaviors of similar-sized stages of C. pacificus and M. lucens differed. C. pacificus was closely associated with the surface waters; a large percentage of every stage was always in the surface 50 m at night except when the C5s were in diapause. During the day the different stages of C. pacificus showed varying degrees of avoidance of the surface layers, with the older, larger stages generally being deeper. The C4 and younger stages were particularly tied to the surface waters, with the majority of the population usually in the top 25 m during the day and night. M. lucens was less strongly associated with the surface layers. While the adult females, and usually the CSs, underwent a normal diel vetical migration (DVM), entering the surface 50 m at night, the vast majority of the adult males always stayed below 75 m. The C4 and younger stages showed more varied behavior. On some dates they underwent a reverse DVM, avoiding the surface 25 m at night, at other times they avoided the surface 10 m particularly during the day, while on still other dates the C3 and younger stages avoided the surface 25 m day and night. On most dates there were significant portions of all the stages in the deepest layers sampled. Differences in the 2 species' migration behavior may be due to differences in their susceptibility to predation, or some additional aspect of their biologies.

Benthic macrofauna of the coarse sediments of Browns Bank, off southwest Nova Scotia, Canada were sampled with a modified 0.5-m2 Van Veen grab; 29 stations from 1983 to 1985. Production was estimated from alcohol-stored biomass by multiplying by the annual turnover ratio, P:B, of each species. The latter was determined from an empirically derived relationship using known or estimated lifespans. Benthic macrofaunal production averaged 64 g wet weight∙m−2∙yr−1 on Browns Bank, markedly lower than the 193 g wet weight∙m−2∙yr−1 in the mixed and finer sediments of the Bay of Fundy. Other community characteristics, such as the number of polychaete and amphipod species for the two areas were similar, which we attribute to the similar geological ages of the sediments. Prey consumed by juvenile age 0 haddock (Melannogrammus aeglefinus) consist mostly of deposit-feeding macroinfauna which produce ~1.8 × 104 t wet biomass per year, representing ~8% of the total production of Browns Bank. Two linked hypotheses are proposed to account for the suitability of Banks as juvenile gadid feeding grounds: rapid rates of suspension of macrofauna by strong tidal currents; and drifting animals are of a suitable size and kind for juvenile haddock feeding.