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Physicists have long examined the fluid dynamics of swimming at low Reynolds number, but the main scope has rarely been to understand the behavior and ecology of microorganisms. However, many ecological questions about the functioning of small aquatic organisms can only be addressed by the application of formal fluid physics. Here, I examine resource acquisition mechanisms in small aquatic organisms, ranging from uptake of dissolved molecules to feeding on suspended particulate prey, and examine how organism behaviors and morphologies may be shaped by the often non-intuitive small-scale fluid physics. [ABSTRACT FROM AUTHOR]

Bivalves are important grazers on phytoplankton in shallow waters. However, very little is known about their ability to capture actively moving zooplankton. We investigated the escape response and success of early and late nauplii of three copepod species (Acartia tonsa, Temora longicornis and Eurytemora affinis) in the flow field of a blue mussel, Mytilus edulis, using both video observations and incubation experiments. An empirical model was created to describe the spatial distribution of the fluid deformation rate. Nauplii responded with escape jumps at mean fluid deformation rates of 0.6–1.9 s-1. Escape success differed between taxa. T. longicornis was the poorest escaper, while A. tonsa and E. affinis were more efficient and similar to one another. Deformation rates differed in different parts of the flow field, which resulted in differences in escape success between the sectors. Nauplii were caught most often in the sector furthest away from the exhalent siphon, where the deformation rate was the weakest. There the nauplii were unable to detect an escape signal in time to react and flee. [ABSTRACT FROM AUTHOR]

Flocculation of phytoplankters into large, rapidly sinking aggregates has been implicated as a mechanism of vertical transport of phytoplankton to the sea floor which could have global significance. The formation rate of phytoplankton aggregates depends on the rate at which single cells collide, which is mainly physically controlled, and on the probability of adhesion upon collision (=coagulation efficiency, stickiness), which depends on physico-chemical and biological properties of the cells. We describe here an experimental method to quantify the stickiness of phytoplankton cells and demonstrate that three species of diatoms grown in the laboratory ( Phaeodactylum tricornutum, Thalassiosira pseudonana, Skeletonema costatum) are indeed significantly sticky and form aggregates upon collision. The dependency of stickiness on nutrient limitation and growth was studied in the two latter species by investigating variation in stickiness as batch cultures aged. In nutrient replete T. pseudonana cells stickiness is very low (< 5 × 10), but increases by more than two orders of magnitude as cell growth ceases and the cells become nutrient limited. Stickiness of S. costatum cells is much less variable, and even nutrient replete cells are significantly sticky. Stickiness is highest (> 10) for S. costatum cells in the transition between the exponential and the stationary growth phase. The implications for phytoplankton aggregate formation and subsequent sedimentation in the sea of these two different types of stickiness patterns are discussed. [ABSTRACT FROM AUTHOR]

Clearance rates on different sizes of spherically shaped algae were determined in uni-algal experiments for all developmental stages (NII through adult) of the copepod Acartia tonsa, and used to construct food size spectra. Growth and developmental rates were determined at 7 food levels (0 to 1 500 μg C l of Rhodomonas baltica). The lower size limit for particle capture was between 2 and 4 μm for all developmental stages. Optimum particle size and upper size limit increased during development from ∼7 μm and 10 to 14 μm for NII to NIII to 14 to 70 μm and ∼250 μm for adults, respectively. When food size spectra were normalized (percent of maximum clearance in a particular stage versus particle diameter/prosome length) they resembled log-normal distributions with near constant width (variance). Optimum, relative particle sizes corresponded to 2 to 5% of prosome length independent of developmental stage. Since the biomass of particulate matter is approximately constant in equal logarithmic size classes in the sea, food availability may be similar for all developmental stages in the average marine environment. Juvenile specific growth rate was exponential and increased hyperbolically with food concentration. It equaled specific female egg-production rate at all food concentrations. The efficiency by which ingested carbon in excess of maintenance requirements was converted into body carbon was 0.44, very similar to the corresponding efficiency of egg-production in females. On the assumptions that food availability is similar for all developmental stages, and that juvenile and female specific growth/egg-production rates are equal, female egg-production rates are representative of turnover rates (production/biomass) of the entire A. tonsa population and probably in other copepod species as well. Therefore, in situ estimates of female fecundity may be used for a rapid time- and site-specific field estimate of copepod production. This approach is shown to be fairly robust to even large deviations from the assumptions. [ABSTRACT FROM AUTHOR]

Marine plankton have been conspicuously affected by recent climate change, responding with profound spatial relocations and shifts in the timing of their seasonal occurrence. These changes directly affect the global carbon cycle by altering the transport of organic material from the surface ocean to depth, with consequences that remain poorly understood. We investigated how distributional and abundance changes of copepods, the dominant group of zooplankton, have affected biogenic carbon cycling. We used trait-based, mechanistic models to estimate the magnitude of carbon transported downward through sinking faecal pellets, daily vertical migration and seasonal hibernation at depth. From such estimates for over 200,000 community observations in the northern North Atlantic we found carbon flux increased along the northwestern boundary of the study area and decreased in the open northern North Atlantic during the past 55 years. These changes in export were primarily associated with changes in copepod biomass, driven by shifting distributions of abundant, large-bodied species. Our findings highlight how recent climate change has affected downward carbon transport by altering copepod community structure and demonstrate how carbon fluxes through plankton communities can be mechanistically implemented in next-generation biogeochemical models with size-structured representations of zooplankton communities.