Rodrigo J. Gonçalves

Flow disturbances generated by feeding and swimming zooplankton

Significance Plankton compromise their survival when they swim and feed because the fluid disturbances that they generate may be perceived by predators. Because the abundance and population dynamics of zooplankton in the ocean are governed by their access to food and exposure to predators, an important question is to what extent and how zooplankton may minimize the fluid disturbances that they generate. We show that when swimming and feeding are integrated processes, zooplankton generate fluid disturbances that extend much farther in the water than is the case for zooplankton that swim only to relocate. Quiet swimming is achieved through “breast swimming” or by swimming by jumping, whereas other propulsion modes are much noisier. This pattern applies independent of organism size and species. Interactions between planktonic organisms, such as detection of prey, predators, and mates, are often mediated by fluid signals. Consequently, many plankton predators perceive their prey from the fluid disturbances that it generates when it feeds and swims. Zooplankton should therefore seek to minimize the fluid disturbance that they produce. By means of particle image velocimetry, we describe the fluid disturbances produced by feeding and swimming in zooplankton with diverse propulsion mechanisms and ranging from 10-µm flagellates to greater than millimeter-sized copepods. We show that zooplankton, in which feeding and swimming are separate processes, produce flow disturbances during swimming with a much faster spatial attenuation (velocity u varies with distance r as u ∝ r−3 to r−4) than that produced by zooplankton for which feeding and propulsion are the same process (u ∝ r−1 to r−2). As a result, the spatial extension of the fluid disturbance produced by swimmers is an order of magnitude smaller than that produced by feeders at similar Reynolds numbers. The “quiet” propulsion of swimmers is achieved either through swimming erratically by short-lasting power strokes, generating viscous vortex rings, or by “breast-stroke swimming.” Both produce rapidly attenuating flows. The more “noisy” swimming of those that are constrained by a need to simultaneously feed is due to constantly beating flagella or appendages that are positioned either anteriorly or posteriorly on the (cell) body. These patterns transcend differences in size and taxonomy and have thus evolved multiple times, suggesting a strong selective pressure to minimize predation risk.

Characteristic Sizes of Life in the Oceans, from Bacteria to Whales

The size of an individual organism is a key trait to characterize its physiology and feeding ecology. Size-based scaling laws may have a limited size range of validity or undergo a transition from one scaling exponent to another at some characteristic size. We collate and review data on size-based scaling laws for resource acquisition, mobility, sensory range, and progeny size for all pelagic marine life, from bacteria to whales. Further, we review and develop simple theoretical arguments for observed scaling laws and the characteristic sizes of a change or breakdown of power laws. We divide life in the ocean into seven major realms based on trophic strategy, physiology, and life history strategy. Such a categorization represents a move away from a taxonomically oriented description toward a trait-based description of life in the oceans. Finally, we discuss life forms that transgress the simple size-based rules and identify unanswered questions.

Photorepair activity and protective compounds in two freshwater zooplankton species (Daphnia menucoensis and Metacyclops mendocinus) from Patagonia, Argentina

The impact of ultraviolet-B radiation (UV-B, 280-315 nm) on the cladoceran Daphnia menucoensis Paggi and the copepod Metacyclops mendocinus (Wierzejski) was determined in experiments designed to evaluate the effectiveness of the photorepair mechanism and the role of UV-absorbing compounds. In both species UV-B caused significant mortality at doses of approximately 40 kJ m(- 2) or higher. At lower UV-B doses, however, no significant mortality was detected in M. mendocinus; moreover, this species seems to have a threshold below which no UV-B induced mortality is determined. D. menucoensis, on the other hand, was very sensitive to UV-B, and significant mortality of 15% (p < 0.05) was observed when doses were as low as 10 kJ m(-2). Both species showed high efficiency for photorepairing UV-B-induced damage to the DNA molecule, with a significant decrease of mortality when the species were exposed to visible radiation, PAR (55 W m(-2)), in addition to UV-B. The higher resistance of M. mendocinus to UV-B as compared to that of D. menucoensis might be also related to the presence of mycosporine-like amino acids, MAAs (i.e., shinorine and porphyra-334), and carotenoids, which would add an adaptive advantage to the copepod.

Motility of Daphnia spinulata as affected by solar radiation throughout an annual cycle in mid-latitudes of Patagonia.

During an annual cycle, Daphnia spinulata collected from a plankton community of Patagonia was exposed outdoors to assess the impact of recently received solar radiation on motility (i.e. swimming speed and gravitaxis). Individual values of these parameters were obtained by video recordings and image analysis at different time intervals during the day. Initial swimming speed varied throughout the year, and changes in speed during exposure were not significantly affected by any waveband used in our experimental design (i.e. PAB, 280–700 nm; PA, 320–700 nm; and P, 400–700 nm). Overall, most of the individuals swam downwards, regardless of the radiation treatment imposed to the samples. We found that multifactor interactions (i.e. not a single parameter explained more than 40% of the observed variability) explained most of our observations on motility parameters. These factors include not only solar radiation, but other physical (underwater radiation field and wind intensity) and biological parameters (food availability, presence of predators and congeners). Our findings indicate that the plankton dynamics in the study site is likely to be governed by a sum of factors which must be taken into account when considering solar radiation effects on aquatic ecosystems.

Plankton dynamics and photosynthesis responses in a eutrophic lake in Patagonia (Argentina): influence of grazer abundance and UVR

Se estudio a lo largo del ano una comunidad planctonica natural de la laguna eutrofica Cacique Chiquichano de Patagonia-Argentina, para evaluar los cambios en parametros fotosinteticos como resultados de la exposicion a radiacion ultravioleta (RUV, 280-400 nm), abundancia de herbivoros y composicion taxonomica de la comunidad de fitoplancton. Tanto las variables fisicas (temperatura, radiacion solar) como biologicas (herbivoros, composicion taxonomica, parametros fotosinteticos) variaron a lo largo del estudio. Los crustaceos zooplanctonicos mostraron dominancia alternada entre un cladocero (Daphnia spinulata) y un copepodo (Metacyclops mendocinus). La comunidad de fitoplancton sufrio cambios concomitantes durante el ano, con cianobacterias y diatomeas alternando su dominancia. Sumado a esto, si bien la abundancia de copepodos no estuvo significativamente relacionada con cambios en el fitoplancton, la presencia de D. spinulata fue significativa en periodos de aguas mas transparentes, durante los cuales las diatomeas fueron dominantes. Por otra parte, durante los periodos de menor penetracion de la radiacion solar en la columna de agua, las cianobacterias dominaron la comunidad fitoplanctonica. La inhibicion fotosintetica debido a RUV disminuyo durante los periodos dominados por diatomeas. En cambio, la inhibicion aumento al incrementarse la proporcion de cianobacterias, probablemente como resultado de una aclimatacion a la baja irradiancia durante la fase de menor transparencia de la laguna. Mas aun, la presencia de D. spinulata estuvo asociada a una mayor penetracion de la radiacion en la columna de agua, por lo cual aumento indirectamente la inhibicion de la fotosintesis en cianobacterias. Los resultados sugieren que tanto la radiacion solar como la abundancia de herbivoros tienen una fuerte influencia en la dinamica y actividad fotosintetica del fitoplancton en la Laguna Cacique Chiquichano.