Modeling the Effects of Doliolids on the Plankton Community Structure of the Southeastern US Continental Shelf

A model of the lower trophic levels that consists of a system of coupled ordinary differential equations was developed to investigate the time-dependent behavior of doliolid populations associated with upwelling features on the outer southeastern US continental shelf. Model equations describe the interactions of doliolids with two phytoplankton size fractions, five copepod developmental stages and a detrital pool. Additional equations describe nitrate and ammonia. Model dynamics are based primarily upon data obtained from field and laboratory experiments for southeastern US continental shelf plankton populations. Variations on a reference simulation, which represents average upwelling conditions without doliolids, were carried out to determine the effect of inclusion of doliolids, temperature and nutrient variations, and variations in ambient food concentrations on the basic plankton community structure. These simulations provide a measure of the role of environmental versus biological interactions in structuring the planktonic food web on the southeastern US continental shelf. Simulations show that the copepod population is significantly reduced when doliolids are present. This happens primarily as a result of direct predation of the doliolids on copepod eggs and juveniles as opposed to an increase in competition for phytoplankton, the primary food source. Additional simulations show that the cooler temperatures associated with the newly upwelled water temporarily decrease the growth rates of the doliolids and copepods, bestowing an even greater advantage on the rapidly reproducing doliolids

The Role of Feeding Behavior in Sustaining Copepod Populations in the Tropical Ocean

A fundamental question regarding marine copepods is how the many species coexist and persist in the oligotrophic environment (i.e. Hutchinson’s paradox). This question is addressed with a stochastic, object-oriented Lagrangian model that explicitly simulates the distinct foraging behaviors of three prominent tropical species: Clausocalanus furcatus, Paracalanus aculeatus and Oithona plumifera. The model also individually tracks all prey cells. Each particle’s motion combines sinking, turbulent diffusion and active swimming when applicable. The model successfully simulates observed size partitioned carbon uptake rates. Based on the model results, the wide-ranging translational ambit employed by C. furcatus is best suited for the acquisition of passive prey while the relatively stationary behavior of O. plumifera promotes the capture of larger, quickly sinking cells. The model results further suggest that the slow velocities and feeding current employed by P. aculeatus are best suited for acquiring the smallest cells though it also has a slight advantage over C. furcatus in acquiring the largest prey. A resource threshold, at a prey concentration of 530 cells mL–1,is consistently exhibited by all three modeled species. Overall, these results imply that the size-partition preferences due to their different foraging behavior contribute to the coexistence of these three species. (c) The Author 2005

Colloquium on diatom-copepod interactions

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, the 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