ALBERT

Description: Inducible defenses are a common phenotypically plastic response to a heterogeneous predation risk. Once induced, these defenses cannot only lose their benefit, but even become costly, should the predator disappear. Consequently, some organisms have developed the ability to reverse their defensive traits. However, despite extensive research on inducible defenses, reports on reversibility are rare and mostly concentrate on defensive behavior. In our study, we investigated the reversibility of morphological defenses in the freshwater crustacean Daphnia barbata . This species responds to Notonecta glauca and Triops cancriformis with two distinctively defended morphotypes. Within the numerous defensive traits, we found both trait- and predator-specific reversibility. Body torsion and tail-spine-related traits were highly reversible, whereas helmet-related traits remained stable, suggesting different physiological constraints. However, in general, we found the defenses against Triops mostly reversible, while Notonecta -induced defenses were persistent and grew further, even in the absence of a predator.

Description: Marine phytoplankton are a taxonomically and functionally diverse group of organisms that are key players in the most important biogeochemical cycles. Phytoplankton taxa show different resource utilization strategies (e.g. nutrient-uptake rates and cellular allocation) and traits. Therefore, acknowledging this diversity is crucial to understanding how elemental cycles operate, including the origin and dynamics of elemental ratios. In this paper, we focus on trait-based models as tools to study the role of phytoplankton diversity in the stoichiometric phenomenology observed in the laboratory and in the open ocean. We offer a compilation of known empirical results on stoichiometry and summarize how trait-based approaches have attempted to replicate these results. By contrasting the different ecological and evolutionary approaches available in the literature, we explore the strengths and limitations of the existing models. We thus try to identify existing gaps and challenges, and point to potential new directions that can be explored to fill these gaps. We aim to highlight the potential of including diversity explicitly in our modeling approaches, which can help us gain important knowledge about changes in local and global stoichiometric patterns.

Description: Highly intermittent spatial variability of phytoplankton is observed ubiquitously in marine ecosystems, especially when measurements are performed at the micro-scale level. Therefore, theoretical developments and new modelling tools are required to understand the observed small-scale vertical structure and its relationship to ecosystem behaviour. Nearly all current ecosystem models are formulated entirely based on the mean field approximation, ignoring sub-grid scale variability. Even if such approximation may be reasonable for meso-scales (and above), it cannot account for micro-scale dynamics, which may also impact macroscopic properties at the larger scale. To consider intermittency of variables in plankton ecosystem models, we apply a newly developed modelling approach called the closure approach. Detailed simulations were conducted, combining fluid-dynamics of the 1D water column with the nutrient-phytoplankton closure ecosystem model for application to a site in the northern North Sea. Compared with a control model, which does not account for such intermittency, the closure model produced substantially different spatio-temporal patterns of mean phytoplankton biomass and growth rate, which depended on the overall level of variability. In this study, we (i) seek to explore the effects of sub-scale variability coupled with physical transport and (ii) begin to address the yet unresolved question of how to consistently model the advection and diffusion of the variances and co-variances used to represent sub-scale variability in the closure approach. Our results suggest that it may be necessary to account explicitly for the intermittent distribution of plankton and nutrients, even in large-scale biogeochemical models.

Description: Seawater viscosity is influenced by temperature as well as through excretion of exopolymers by some plankton. We examined the role of viscosity on the movement patterns and sensory abilities of the dinoflagellate Heterocapsa triquetra , manipulating the viscosity of seawater to simulate a 10 ± 1.5°C temperature change. In a second treatment, we seeded the water with microbeads to examine swimming behaviours in the presence of a mechanical stimulus. Increased viscosity reduced distances between conspecifics 4.7-fold and increased distances between protists and microbeads by 3.4-fold. Increased viscosity also affected other aspects of motility, with an overall reduction in swimming speed of 2.0- and 7.0-fold for treatments with and without mechanical stimuli. Higher viscosities were associated with upward vertical migration, in both the presence and absence of microbeads. Cells were highly sensitive to disturbances to the velocity field, by as little as 1.5%, and different approach distances of H. triquetra to conspecifics over mechanical stimuli suggest sensory capacity to distinguish types of particles. Mediation of motility and migratory behaviours through viscosity implies ramifications for the distribution of protists and their encounters with resources, predators and conspecifics triggered by events such as temperature changes and phytoplankton bloom events.

Description: Parasites are not typically considered to be important components of polar marine ecosystems. It was therefore surprising when 18S rDNA surveys of protists in the West Antarctic Peninsula in winter revealed high abundances of parasite sequences. Parasite sequences made up, on average, over half (52%) of sequence reads in samples from deep water in winter. Winter surface water and sediment samples contained relatively fewer, but still strikingly high, parasite sequence reads (13 and 9%, respectively), while surface water samples in summer contained fewer parasite sequences (1.8%). A total of 1028 distinct parasite Operational Taxonomic Units were observed in winter, with the largest abundances and diversities within Syndiniales groups I and II, including Amoebophrya . Less abundant parasite sequence groups included Apicomplexa, Blastodinium , Chytriodinium , Cryptocaryon , Paradinium , Perkinsidae, Pirsonia and Ichthyophonae . Parasite sequence distributions suggested interactions with known hosts, such as diatom parasites which were mainly in the sediments, where resting spores of Chaetoceros spp. diatoms were abundant. Syndiniales sequences were correlated with radiolarian sequences, suggesting parasite–host interactions. The abundant proportions of parasite sequences indicate a potentially important role for parasites in the Antarctic marine ecosystem, with implications for plankton population dynamics, the role of the microbial loop, carbon flows and ecosystem responses to ongoing anthropogenic climate change.

Description: Metabarcoding (large-scale taxonomic identification of complex samples via analysis of one or few orthologous DNA regions, called barcodes) is revolutionizing analysis of biodiversity of marine zooplankton assemblages. Metabarcoding relies on high-throughput DNA sequencing (HTS) technologies, which yield millions of DNA sequences in parallel and allow large-scale analysis of environmental samples. Metabarcoding studies of marine zooplankton have used various regions of nuclear small- (18S) and large-subunit (28S) rRNA, which allow accurate classification of novel sequences and reliable amplification with consensus primers, but- due to their relatively conserved nature- may underestimate species diversity in a community. To discriminate species, more variable genes are needed. A limited number of metabarcoding studies have used mitochondrial cytochrome oxidase I (COI), which ensures detection of species-level diversity, but may require group-specific primers and thus result in inconsistent amplification success rates. Reference databases with sequences for accurately-identified species are critically needed to allow taxonomic designation of molecular operational taxonomic units (MOTU) and comparison with previous studies of zooplankton diversity. Potential and promising applications of metabarcoding include rapid detection of impacts of climate change, monitoring and assessment of ecosystem health, calculation of biotic indices, characterization of food webs and detection of introduced, non-indigenous species.

Description: In peninsular Florida, USA, rainfall is coupled with the Pacific Sea Surface Temperature Anomaly (SSTA), and rainfall affects mean depth and residence time of shallow lakes. We examined effects of two cycles of variation in rainfall using a 15-year data set from a shallow eutrophic lake dominated by small zooplankton, cyanobacteria and omnivorous fish. In high rainfall periods, the lake was deeper and cladoceran biomass was significantly higher than in dry periods. One factor was correlated with reduced biomass of cladocerans: a 3-fold higher biovolume of cyanobacteria. This led us to examine how variation in rainfall affects cyanobacteria. When cyanobacteria biovolume was high, the movement of water through the lake was low and invariant. Cyanobacteria grew unchecked. When cyanobacteria was reduced and cladocerans attained high biomass, there were intermittent flushing events that may have disrupted algal growth. Water color was elevated ~6-fold during the same time periods. Greater color may have made conditions less favorable for cyanobacteria by increasing light attenuation, and also more favorable for cladocerans, by reducing risk from fish. This study provides insight into how future variability in rainfall and drought, which may be exacerbated by global warming, could affect plankton in shallow subtropical lakes.

Description: We investigated the effects of temperature on weight-specific respiration rates ( R sp ) of the euryhaline calanoid copepod Pseudodiaptomus annandalei collected from the Matang Mangrove Forest Reserve (Malaysia). We employed a simple experimental approach consisting of a short temperature exposure time from 24 to 36°C. The relationship between temperature and R sp of acclimated copepods under conditions of excess food fitted an exponential function and indicated a complete lack of acclimation at the higher temperature range. Both fine-scale temperature measurements and piecewise linear regression enabled the detection of a breakpoint produced by a significant increase in respiratory rates. It is argued that thermally stressed copepods can be detected by both a higher Q 10 above 32°C and monitoring this breakpoint in the metabolic response ( R sp ). Similarly, the lower Q 10 (≤2) indicates some degree of independence of their metabolic rates between 26 and 32°C, which likely corresponds to their optimal thermal window. Results are discussed in relation to body mass, feeding status and gender.

Description: Restored ponds are uniquely suitable for the study of successional processes on aquatic communities. In a set of dune slacks of different restoration ages, we monitored monthly the environmental and community shifts of one newly restored pond over 4 years and compared it with older ponds. Questions were (i) how does the community change with time in the newly restored pond? (ii) Are these changes associated with the local environment? (iii) Was the pattern comparable to older ponds? Restoration exerted a strong effect in the pond, since the main changes were observed at the early stages of succession. Later on, the pond started to resemble older ponds in the area as it moved toward a more complex community.

Description: P-transport through the microbial food web was investigated in a mesocosm experiment in which orthophosphate was added to oligotrophic Cretan Sea water. As non-exclusive alternatives to traditional phytoplankton–zooplankton succession, two mechanisms for P-transport have previously been proposed: a stoichiometric shift in food quality (Tunnelling); and a transport through a predatory food chain from heterotrophic bacteria, bypassing phytoplankton (Bypass). Following P-addition, particulate C:P-ratio dropped from 436 to 44 (molar) within 1 day, and egg production increased after 2 days. This confirms the hypothesized stoichiometric shift and rapid copepod response of a "Tunnelling" scenario. Bacterial abundance responded positively to P-addition on Day 1, ciliates increased after Day 5 and new egg production peaks occurred on Days 5 and 9; a succession suggesting additional Bypass transport although the response expected in heterotrophic flagellate abundance was not confirmed. A small, but statistically not significant, increase in Chl a in the 0.6- to 2-µm size fraction also suggests possible additional P-transport through a phytoplankton–zooplankton succession. On the basis of the magnitude of the stoichiometric change and the short delay in egg production response, we consider the Tunnelling mechanism to be the most likely signal initiating egg production, but a Bypass, and possibly a traditional succession, may have contributed to maintain the elevated egg production.