Ecology of the marine copepod genus Oithona

Copepods have a crucial role in the pelagic marine ecosystem, for their participation in the nutrient cycling and carbon export in biogeochemical cycles, and their role as link organisms between primary producers and higher trophic levels. Historically, marine zooplankton studies have focused on large organism (> 1mm), due to the use of relatively large mesh sizes in plankton nets, which has resulted in an underestimation of the importance of small copepods such as the genus Oithona. The study of small copepods, and especiall of Oithona, has raised special interest in recent years due to its great abundance and ubiquitous presence in both coastal and oceanic regions, and with a distribution that extends from polar to tropical . Besides their numerical dominance, Oithona also makes up a significant fraction of the biomass of copepods in certain regions. Their described low metabolic rates, coupled with an ambush feeding behavior and low mortality, are considered the clue of their success and of their capacity to maintain active populations throughout the year. Although new insights on the ecology of Oithona have been acquired in the past decade, knowledge on their vital rates is still very scarce. The lack of studies on the ecophysiology of Oithona contrasts with the large amount of studies conducted on calanoid copepods on aspects related to feeding, growth, and egg production. The main objective of this thesis was to contribute to a better knowledge and understanding of some biological and ecological aspects of the genus Oithona. For that purpose, we carried out laboratory experiments with cultured specimens, and experimental lab and fieldwork with natural populations in polar and tropical regions. In the laboratory we studied the effect of food concentration on ingestion and fecundity rates of Oithona davisae. The results obtained indicate that Oithona davisae is able to feed at very low food concentrations, which indicates its ability to exploit oligotrophic environments. Oithona capacity to reproduce continuously throughout the year, even in environments or periods of low food availability, was confirmed by the relatively high fertility rates observed in laboratory experiments at low food concentrations. The trophic role of Oithona, their natural diet and ingestion rates were studied in different pelagic ecosystems; as well as the fecudity of adult females in these ecosystems, and their relationship with environmental factors. We also investigated some aspects of the population ecology Oithona in polar regions (abundance, vertical distribution and migration patterns). The results of the in situ investigations were carried out in order to compare life strategies between Oithona species of contrasting habitats. In polar environments, winter reproductive activity of the adult females was observed, which highlights the importance of small copepods in high-latitude environments, especially when large calanoid are not present in the productive zone of the water column. Moreover, the strategy of Oithona life in polar area is different from that of large calanoid, and it benefits from its independence from the spring blooms of phytoplankton to maintain active populations throughout the year. The ability of Oithona to successfully survive when unfavourable conditions are present in the water could explain the success of this genus of copepods in marine environments around the world.