ALBERT

Description: Although copepods have been considered tolerant against the direct influence of the ocean acidification (OA) projected for the end of the century, some recent studies have challenged this view. Here, we have examined the direct impact of short-term exposure to a pCO 2 / pH level relevant for the year 2100 ( pH NBS , control: 8.18, low pH : 7.78), on the physiological performance of two representative marine copepods: the calanoid Acartia grani and the cyclopoid Oithona davisae . Adults of both species, from laboratory cultures, were preconditioned for four consecutive days in algal suspensions ( Akashiwo sanguinea ) prepared with filtered sea water pre-adjusted to the targeted pH values via CO 2 bubbling. We measured the feeding and respiratory activity and reproductive output of those pre-conditioned females. The largely unaffected fatty acid composition of the prey offered between OA treatments and controls supports the absence in the study of indirect OA effects (i.e. changes of food nutritional quality). Our results show no direct effect of acidification on the vital rates examined in either copepod species. Our findings are compared with results from previous short- and long-term manipulative experiments on other copepod species.

Description: We analyzed the genetic structure of the radiolarian morphospecies Larcopyle buetschlii from the surface to deep waters (up to 2000 m) in the Japan Sea using the internal transcribed spacer (ITS) regions of ribosomal RNA genes. Each individual had several ITS variants, but these polymorphisms show no vertical phylogeographic structure, suggesting a single biological species. Its rapid clonal reproduction suggested by high ITS variation likely plays a pivotal role in maintaining its wide vertical distribution.

Description: Phronima sedentaria is a hyperiid amphipod that diel migrates into a pronounced oxygen minimum zone (OMZ) in the Eastern Tropical North Pacific. In this study, oxygen consumption and lactate production were measured in P. sedentaria to estimate the aerobic and anaerobic contributions to total metabolism under conditions that mimic its day- (1% oxygen, 10°C) and night-time (20% oxygen, 20°C) habitat. When exposed to hypoxia and low temperature, the total metabolism of P. sedentaria was depressed by 78% compared with normoxic conditions. The metabolic enzymes citrate synthase (CS) and lactate dehydrogenase (LDH) were also measured as indicators of aerobic and anaerobic metabolism, and compared with specimens collected from the California Current and the North Atlantic to assess potential adaptations to low oxygen. LDH activity was not significantly different between regions. Significant differences in CS activity may be due to variation in food availability. Climate change is predicted to increase surface temperatures and cause the expansion of OMZs. This will result in vertical compression of the night-time range for P. sedentaria and is likely to have the same impact on other diel migrators. Habitat compression will reduce zooplankton contribution to carbon cycling and alter oceanic ecology, including predator–prey interactions.

Description: Many phytoplankton exploit phosphorus (P) from organic sources when dissolved inorganic P (DIP) is depleted. This process is, however, rarely considered in ecological and biogeochemical models. We present a mechanistic model describing explicitly the ability of phytoplankton to use dissolved organic P (DOP) when DIP is limiting, by synthesizing alkaline phosphatase (AP) that releases DIP from DOP. This model, applicable to any phytoplankton species expressing AP, is here specifically developed for the colony-forming Phaeocystis globosa. It describes the main processes related to P metabolism, including DIP transport, intracellular accumulation and assimilation. Model behaviour is explored in DIP-limiting batch-type conditions for different DOP ranging between 0 and 1.5 mmol P m –3 . Simulations show that the DOP-derived DIP increases the maximum biomass reached and extends the period of net growth. The magnitude of the enhanced biomass production is controlled by the DOP initially present as well as the released DOP, the latter being recycled by lysis of P. globosa cells. We also present a simplified model version derived from the mechanistic model, which involves fewer state variables and parameters. The latter is directly usable in both variable (quota-type) and fixed stoichiometry descriptions of phytoplankton growth.

Description: Zooplankton diel vertical migration (DVM) is often explained as a balance between predator avoidance and resource acquisition. However, recent studies suggest that ultraviolet radiation (UV) may also be important in driving zooplankton DVM in some systems. Williamson et al. ( Williamson et al ., 2011 ) proposed the "transparency-regulator hypothesis," which integrates UV into our current understanding of the drivers of DVM and predicts that the relative roles of UV and visual predation pressure will vary systematically across a gradient of lake transparency. To assess this hypothesis, we conducted in situ mesocosm experiments in five different lakes: two lakes without fish and three lakes with fish that spanned a range of UV and visible light transparency. We used an open-bottomed mesocosm design that allowed for the direct manipulation of UV that did not constrain visual predators or the amplitude or timing of natural migrations. Consistent with the transparency-regulator hypothesis, we found that UV is an important driver of Daphnia DVM in highly UV transparent lakes with and without fish but not in low transparency systems. Our results also suggest that UV and visual predation pressure may interact in systems of intermediate transparency.

Description: Competition for resources can lead to species exclusion. However, this exclusion may be avoided if species show differential adaptation to physical environment. Empirical studies on competition are difficult when species are phylogenetically close and have complex life cycles. This is the case of B. plicatilis and B. manjavacas , two cryptic rotifer species differing in their salinity niches and in life-history traits related to sex and diapause. These differences have been suggested to promote the stable co-occurrence observed in natural populations of these species. However, in a previous empirical study, the outcome of competition between both species was always exclusion. Here, we theoretically explored the effect of complex life-history traits and salinity fluctuations on the long-term competitive outcome of B. plicatilis and B. manjavacas . We developed a model and simulated ecological scenarios combining different growing period lengths, levels of crossed induction of sex between species and salinity regimes. Results show that a fluctuating salinity regime, an intermediate length of growing season and a low level of crossed induction of sex are essential conditions to take into account to explain coexistence.

Description: Colonization of new habitats through dispersal of phytoplankton cysts might be limited, if resident populations outcompete invaders during germination. We reciprocally transferred Gonyostomum semen (Raphidophyceae) cysts from three lakes into native and foreign waters originating from the respective habitats. Germination rate and germling growth were impacted by water origin, but there was no preference for native water. Gonyostomum semen 's ability to germinate in different conditions might explain its expansion in northern Europe.

Description: Measuring zooplankton biomass and physiological rates is of paramount importance in biological oceanography in order to assess the role of this community in, e.g. carbon fluxes. Classical methods (incubations) are very time-consuming and cannot match the frequency of physical and chemical measurements. Attempting to solve this, a variety of methods (e.g. egg production, RNA/DNA ratio or enzyme activities) have been developed over the last decades. These methods also show uncertainties and hitherto only incubation methods have been widely accepted. Predictive equations relating physiological processes and body weight (bw) and temperature are a rough alternative, normally used to ascertain the role of these organisms in the oceanic ecosystem. However, using imaging systems and empirical relationships to determine bw allows the application of physiological models to each individual, obtaining reliable estimates for taxonomic groups and size classes. In this study, we developed predictive equations suitable for growth and respiration estimations in subtropical regions. In addition, biomass and physiological rates assessed from empirical equations in combination with an image-based system (ZooImage) were compared with standard and enzymatic methods, respectively. We observed a consistent agreement between methodologies, the former resulting in an inexpensive and faster procedure for the appraisal of biomass and community carbon fluxes at large spatial and temporal scales.

Description: Carbon-specific prey clearance and ingestion rates of 1.5-mm tentaculate larvae of the ctenophore Mnemiopsis leidyi increased linearly between 6 and 25°C but declined between 25 and 30°C. Both absolute (length) and carbon-specific growth rate increased linearly with increasing temperature. The latter was 0.87 d –1 at 25°C. Extremely low or negative growth rates observed at 6 and 30°C help define the thermal limits to population growth of this successful biological invader.

Description: We have supplemented available, concurrent measurements of fresh weight ( W , g) and body carbon (C, g) (46 individuals, 14 species) and nitrogen (N, g) (11 individuals, 9 species) of marine gelatinous animals with data obtained during the global ocean MALASPINA 2010 Expedition (totalling 267 individuals and 33 species for the W versus C data; totalling 232 individuals and 31 species for the N versus C data). We then used those data to test the allometric properties of the W versus C and N versus C relationships. Overall, gelatinous organisms contain 1.13 ± 1.57% of C (by weight, mean ± SD) in their bodies and show a C:N of 4.56 ± 2.46, respectively, although estimations can be improved by using separate conversion coefficients for the carnivores and the filter feeders. Reduced major axis regression indicates that W increases isometrically with C in the carnivores (cnidarians and ctenophores), implying that their water content can be described by a single conversion coefficient of 173.78 gW(g C) –1 , or a C content of 1.17 ± 1.90% by weight, although there is much variability due to the existence of carbon-dense species. In contrast, W increases more rapidly than C in the filter feeders (salps and doliolids), according to a power relationship W = 446.68C 1.54 . This exponent is not significantly different from 1.2, which is consistent with the idea that the watery bodies of gelatinous animals represent an evolutionary response towards increasing food capture surfaces, i.e. a bottom-up rather than a top-down mechanism. Thus, the available evidence negates a bottom-up mechanism in the carnivores, but supports it in the filter feeders. Last, N increases isometrically with C in both carnivores and filter feeders with C:N ratios of 3.89 ± 1.34 and 4.38 ± 1.21, respectively. These values are similar to those of compact, non-gelatinous organisms and reflect a predominantly herbivorous diet in the filter feeders, which is confirmed by a difference of one trophic level between filter feeders and carnivores, according to stable N isotope enrichment data.