Strategies for success: Copepods in a seasonal world

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