Strategies for success: Copepods in a seasonal world

Abstract

Amongst the zooplankton community, copepods display complex and diverse life history strategies, which could explain their wide success in the world ocean. Specically, in temperate and high latitude ecosystems, copepods are subject to \boom and bust" conditions where annual cycles are punctuated by a short, productive spring blooms, but with relatively little food and harsh conditions for the rest of the year. Due to their world-wide dominance in biomass, and their importance in the food webs, copepods are fairly well studied. However, the success of their complex life-history strategies remain open scientic questions, in particular, how these are attuned to environmental conditions, and how these may be compromised by climate change. Due to their ability to concentrate lipids in their small bodies, copepods are indeed of great ecological signicance as they are an important link between phytoplankton production and higher trophic levels such as sh, seas-birds and marine mammals. Their most striking life-history trait includes multiple moulting from egg to adult stages (energetically costly but allowing them to have the most ecient shape to swim relative to their size in water), overwintering at great depth and plastic behaviors such as switching between feeding modes and daily vertical migration allowing them to quickly adapt to local conditions. Some of their life history traits also vary widely amongst species, including how resources are allocated to reproduction and size at maturity. This thesis covers two of these life-history traits: diel vertical migration and the allocation of resource to reproduction. Diel vertical migration (DVM) strategies arise from a trade-o between feeding and predation risk { both of which tend to be maximized in the surface ocean. The latter is modulated on a day night cycle as the eciency of visual predators varies with ambient light. An ecient strategy is thus to migrate vertically, feeding at the surface at night, and taking refuge at depth during the day. The rst part of this thesis treats DVM with some observations and models. A rst study, made from a video plankton recorder in the West coast of Greenland, shows the wide range of migration patterns adopted in the zooplankton community but also within the Calanus copepods taxa (chapter 2). A second study shows the emergence of the DVM strategy in a game theory approach, not only of the organism, but also their predator (chapter 3). In addition to controlling trophic transfer in marine ecosystems, DVM also plays a potential role biogeochemistry of the worlds oceans. A simple modeling method taking behavior into account in large models is therefore needed, as ocean system model are already computationally intensive. The third study investigates how well a simple (myopic) optimization of DVM compares with life-history optimization using dynamic programming (chapter 4). The myopic optimizations shows surprisingly accurate predictive power over a large range of parameter space. Inspired by the copepods Calanus nmarchicus and C. hyperboreus, an individual based model is used to study the success of their reproduction strategy as a function of the feeding season duration. The rst of these two species is close to an income breeder, relying only on the incoming food supply, while the latter is a capital breeder, storing reserves to spawn at a time not directly dictated by food availability. Although from the same genus, their size at maturity and their distributions dier dramatically. Including behavior and energetic allocation, we thus investigate the eect of the resource allocation trait to reproduction coupled with the most suited maturity size as a function of the feeding season duration (chapter 5). Capital breeding favors large maturity size and is successful in short blooms, while the income breeders are favored in long blooms and benet from small size. As the output tness curve functions seem fairly standard, the model is simplied to an analytical approach that is used to investigate the same problem and gives similar results. The closed solutions are then used to explore the parameter space and the resulting pattern is shown to be robust (chapter 6). This thesis therefore focuses on two main aspects of the life-history of copepods: diel vertical migration and reproduction strategy. The results may be generalized and applied to other species living in a seasonal environment