Wadhwa, Navish

Zooplankton er meget udbredte i havet og de ferske vande, og de udgør en vigtig del af det marine økosystem. De fleste zooplankton svømmer for at finde mad og mager, og for at undgå rovdyr, men der er omkostninger forbundet med svømning, da svømning koster energi og skaber strømningsforstyrrelser, som kan tiltrække rovdyr. Den første del af denne afhandling forsøger at kvantificere disse "trade-offs" for forskellige dyreplanktons svømning.Vi målte svømmekinematik og strømningsfelter omkring den "hoppende" vandloppe Acartia tonsa i forskellige stadier af dens livscyklus, og vi fandt kvalitative forskelle i strømningsstrukturer, energiforbrug og svømmeeffektivitet mellem tidlige og sene stadier. Det rumlige henfald af strømningsforstyrrelsen var hurtigere i de sene stadier, hvilket tyder på, at de kan være mindre sårbare over for prædation. For at perspektivere resultaterne målte vi derefter strømninger omkring en bred vifte af dyreplankton, der anvender forskellige svømmeformer såsom hovering, cruising, hop og brystsvømning. Vi fandt, at den rumlige henfaldshastighed af strømningshastigheden er dikteret af svømmeformen. De former, der udelukkende anvendes til svømning, såsom hop og brystsvømning, havde meget hurtigere rumlige henfald i forhold til de andre svømmeformer, hvilket resulterer i "stille" svømning.Disse observationer motiverede os til yderligere at undersøge brystsvømning, hvor strømningshastigheden henfaldt rumligt som en over afstanden i tredie. Vi anvendte en simpel model med tre punktkræfter til at repræsentere de kræfter, der virker på svømmeren. Vores analyse viste et positionsafhængigt henfald af strømningshastigheden. Placeres de fremaddrivende kræfter tæt på ækvator ændres henfaldet af fjernfeltet fra en over afstanden i anden til en over afstanden i tredie i god overensstemmelse med de eksperimentelle observationer. For yderligere at undersøge periodisk brystsvømning målte vi den detaljerede svømmedynamik og de inducerede strømninger for Podon intermedius. Vi estimerede de fremaddrivende kræfter, der virker på P. intermedius, og vi fandt, at det hurtige rumlige henfald i de inducerede strømningsforstyrrelser ikke blev forklaret af modellen baseret på tre punktkræfter. Vi formoder, at dette skyldes inertien i strømningen, som synes at spille en vigtig rolle for større dyreplanktons svømning. Endeligt udviklede vi en simpel model til at beskrive dynamikken for periodisk svømning, som viste, at ikke-lineære termer er nødvendige for at modellere den observerede dynamik korrekt.Den anden del af denne afhandling undersøger, hvordan størrelse bestemmer overgange i levestrategier, og dermed fungerer som en strukturerende faktor for det marine liv. Til dette formål har vi samlet og præsenteret data for størrelsesbaserede skaleringslove for ressourceoptag, motilitet, sansning, og størrelse af afkom for alle pelagiske livsformer, fra bakterier til hvaler. Vi har også revideret og udviklet teoretiske argumenter for de observerede skaleringslove og for de karakteristiske størrelser, hvor overgange fra en strategi til en anden finder sted. Baseret på vores resultater, har vi opdelt livet i havet i syv grupper baseret på trofisk strategi, fysiologi og livshistoriestrategi.Endelig har vi dykket dybere ned i størrelsesbaseret strukturering af sensoriske strategier i havet. Overlevelse i det åbne hav kræver effektiv indsamling af oplysninger fra omgivelserne ved hjælp af forskellige sanser. Vi har undersøgt, hvordan sanserne og deres respektive virkeområder afhænger af kropsstørrelse. Vi undersøgte de fysiologiske begrænsninger for sanseorganerne, sammen med fysikken for signalproduktion, transmission og modtagelse. Vores analyse afslørede et hierarki af sansetilstande - med voksende størrelse, bliver flere sanser tilgængelige og detektionsafstanden øges. Vores teoretiske forudsigelser af nedre og øvre grænse for størrelsen af forskellige sansers funktionsområder stemte godt med størrelsesintervaller i litteraturen. Selve skaleringsanalysen og størrelsesgrænserne er kun første ordens skøn. Dette arbejde udgør den første omfattende analyse af størrelsesbaseret strukturering af de marine organismers brug af sanser.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

Online Research Database In Technology

Zooplankton Hydrodynamics:An investigation into the physics of aquatic interactions

http://orbit.dtu.dk/ws/files/119786544/12_04531_32_PhD_thesis_Navish_Wadhwa.pdf_2274814_1_1.pdf

Zooplankton Hydrodynamics:An investigation into the physics of aquatic interactions

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