Zooplankton Hydrodynamics: An investigation into the physics of aquatic interactions

Zooplankton are hugely abundant organisms found in all aquatic environments and form an important part of the marine ecosystems. Most zooplankton swim in order to find food and mates, and to avoid predators. In spite of its advantages, swimming comes with trade-offs, it costs energy and creates flow disturbances that may attract predators.

The first part of this thesis attempts to quantify the trade-offs associated with the swimming behaviour of diverse zooplankton. We measured the swimming kinematics and flow fields around the 'jumping' copepod Acartia tonsa at various stages of its life cycle, and found qualitative differences in flow structures, energy expenditure, and swimming efficiency, between the early and later stages.

The spatial decay rate of flow disturbances was faster in the later stages, suggesting that those may be less vulnerable to predation. Broadening the scope, we then measured flows around a wide range of zooplankton which use a variety of swimming modes such as hovering, cruising, jumping, and breast stroke swimming.

We found that the spatial decay rate of the flow velocity is dictated by the swimming mode. The modes used for swimming only, such as jumping and breast stroke swimming, had much faster spatial decay as compared to the other modes, resulting in 'quiet' swimming. This motivated us to examine breast stroke swimming in more detail, for which flow velocity decayed spatially as one over distance cubed.

We employed a simple model using three point forces to represent the forces acting on the swimmer. Our analysis showed a configuration-dependent spatial decay of flow velocity. Arranging the propulsive forces close to the equator resulted in changing the far field velocity decay from one over distance squared to one over distance cubed, comparing well with the experimental observations.

To further investigate periodic swimming using breast stroke, we measured detailed swimming dynamics and induced flows for the cladoceran Podon intermedius. We estimated the propulsive forces acting on P. intermedius, which showed that the fast spatial decay in the induced flows was not explained by the three point force model.

We speculate that this is due to inertial effects in the flow, which seem to play an important role in the swimming of larger zooplankton. We also developed a simple model to mimic the dynamics of periodic swimming, which showed that non-linear drag terms are needed in the model to correctly capture the observed dynamics.

The second part of this thesis examines how size dictates transitions in life strategies, and thus acts as a structuring factor in marine life. To this end, we reviewed data on size-based scaling laws for resource acquisition, motility, sensing, and offspring size for all pelagic marine life, from bacteria to whales.

We also reviewed and developed theoretical arguments for the observed scaling laws and for the characteristic sizes at which transitions from one strategy to another take place. Based on our findings, we divided life in the ocean into seven major realms based on trophic strategy, physiology, and life history strategy.

Finally, we delve deeper into size based structuring of sensory strategies in the ocean. Survival in the open ocean requires effective collection of information from the surroundings via the use of various sensory modes. We studied how sensing modes and their respective ranges depend on body size. We investigated the physiological constraints on sense organs, together with the physics of signal generation, transmission, and reception.

Our analysis revealed a hierarchy of sensing modes - with increasing size, a larger battery of sensory modes becomes available and the sensing range increases. Our theoretical predictions of lower and upper size limits for various senses aligned well with the size ranges found in the literature. Although the scaling analyses and the size limits are only first order estimates, this work forms the first comprehensive analysis of the size based structuring of sensory modes used by marine life.