ALBERT

Description: The population dynamics of Pseudocalanus elongatus have been investigated within the framework of the GLOBEC-Germany project to gain a better understanding of its life cycle and population dynamics and to estimate secondary production in the North Sea. During an intensive field study in the German Bight between February and October 2004, experiments on reproduction were performed and data on length of copepodids and abundance were collected to characterize the population in the southern North Sea. This data set was used to update the literature-based parameterization of a population model for P. elongatus to investigate the population dynamics, life history and production in the German Bight. The ability of data to improve population models is also discussed. Pseudocalanus elongatus was found to be a major contributor to carbon uptake contributing about one-third of copepod production. Though the spatial variability in field observations was not reflected by the model, the simulation matched data within one order of magnitude at most stations. The high-resolution field observations and experiments mainly improved the parameterization of the reproductive parameters. Mortality is found to be a critical parameter due to its influence on population size. Using constant rates, though based on observation-derived estimates, seems not to capture realistic variability. Our study confirms the need for experimental and field data to build a robust parameterization for concentration-based population models.

Description: Copepod grazing is a fundamental link between autotrophs and heterotrophs, affecting the particle size spectrum, modifying trophic interactions for a wide range of organisms, and playing a potentially significant role in the carbon cycle. A considerable amount of work has been done to quantify and understand feeding in copepods from a behavioral, physiological and ecological standpoint. An impressive body of knowledge has accumulated as a result. However, some of the methods used to experimentally determine copepod grazing have known, un-quantified problems. The small size of individual copepods requires the use of multiple animals, thereby introducing poorly understood variability. Digestion and gut dynamics in copepods are not fully understood either. Here we propose an approach to measure grazing that aims to be complementary to past contributions. Using a novel planar laser imaging technique, we have quantified grazing (pellet evacuation rates and gut clearance rates, K ) in feeding copepods ( Calanus pacificus ). Highly resolved time series up to 5 h long were obtained from individual copepods at three different temperatures. K was found to be temperature-dependent, to vary in time and among copepods, and to be significantly higher during feeding than non-feeding, invalidating some assumptions of previous methods. Previous studies not accounting for the observed variations in K may have underestimated copepod ingestion by 15–70%. This suggests that copepods may exert a greater grazing pressure on phytoplankton than previously estimated, implying a higher copepod-mediated vertical carbon transport, and a greater transfer rate of energy to higher trophic levels.

Description: Long-term changes in mesozooplankton and phytoplankton populations have been well documented in the North Atlantic region, whereas data for microzooplankton are scarce. This neglected component of the plankton is a vital link in marine food-webs, grazing on smaller flagellates and cyanobacteria and in turn providing food for the larger mesozooplankton. We use the latest tintinnid (Ciliophora, Protista) data from the Continuous Plankton Recorder (CPR) survey in the NE Atlantic and North Sea to examine the phenology, distribution and abundance of this important group of ciliates. Presence/absence data came from 167 122 CPR samples collected between 1960 and 2009 and abundance data from 49 662 samples collected between 1996 and 2009. In the North Atlantic the genus Dictyocysta spp. dominated and Parafavella gigantea showed an increase in abundance around Iceland and Greenland. In the North Sea higher densities of Tintinnopsis spp., Favella serrata and Ptychocylis spp. were found. The presence of tintinnids in CPR samples collected in the North Atlantic has increased over the last 50 years and the seasonal window of high abundance has lengthened. Conversely in the North Sea there has been an overall reduction in abundance. We discuss possible drivers for these long-term changes and point the way forward to more holistic studies that examine how ecosystems, rather than just selected taxa, are responding to climate change.

Description: We performed laboratory competition experiments with the invasive Daphnia lumholtzi and native North American Daphnia pulicaria at three temperatures. Daphnia pulicaria was the superior competitor at 20 and 24°C, while this dominance pattern shifted at 28°C. This observation of a temperature-mediated shift in competitive dominance suggests that a high thermal tolerance in D. lumholtzi has favoured its successful invasion in southern areas of North America. Combined with benefits of inducible defences, our work suggests a multi-faceted explanation for this invader's success.

Description: Calanus finmarchicus relies on dormancy to thrive in the seasonal environment of the boreal Atlantic. The lipid accumulation window (LAW) hypothesis proposes that a seasonal window of environmental conditions allows developing individuals to store enough lipids for dormancy to be safely initiated. Successful dormancy requires a sufficient amount of lipids to fulfil the reduced metabolic demand of the dormant individual and to sustain the final maturation process. We used a pattern-oriented modelling approach that implements the LAW hypothesis and employs a genetic algorithm for parameter estimation, in order to reproduce the observed phenology and demography of C. finmarchicus populations from the two contrasting regions, the Gulf of St. Lawrence (GSL) and the Gulf of Maine (GoM) in the northwest Atlantic shelf. In the GSL, the model reproduced the timing of dormancy, the abundance and individual condition of late copepodid stages. In the GoM, the model produced a semi-annual dormancy pattern, as no locally produced individual could last the 6–8 months of dormancy inferred from the available observations. Further testing requires extending demographic time series, including lipid condition of late copepodid stages in the GoM, and the implementation of a 3-D modelling framework that would explicitly address the complex interactions between circulation and population dynamics of C. finmarchicus over the entire northwest Atlantic shelf.

Description: Copepod feeding and digestion rates were measured using quantitative polymerase chain reaction (qPCR) to amplify the prey Thalassiosira weissflogii and Heterocapsa triquetra from the guts of Acartia tonsa . Using species-specific primers, prey 18S rDNA could be detected routinely and quantified in the guts and fecal pellets of A. tonsa . Recovery of gut contents DNA using two fixation methods was compared. Prey 18S copy numbers were 〉10-fold higher in copepods fixed in 95% ethanol (3260 ± 822 copies copepod –1 ) compared with anesthetized and frozen copepods (210 ± 19 copies copepod –1 ). Experiments using 95% ethanol fixation showed rapid prey DNA digestion rates during the initial 2 min after ingestion (0.7 min –1 ) after which they slowed ~10-fold. Chlorophyll pigment disappearance rates were slower (~0.015 min –1 ). Rates of gut filling measured by DNA and gut pigments differed, reaching 95% of the asymptote, I max , in 3 and 58 min, respectively, likely reflecting differences in rates at which biomarkers were digested. Gut fullness measured by DNA increased with prey concentration, reaching I max at 9760 copies copepod –1 and a critical concentration ( I crit ) at 1530 cells mL –1 . These results demonstrate that qPCR analysis of prey DNA in copepod guts can be used to provide a quantitative index of feeding rates.

Description: Copepod populations in the Gulf of Maine/Georges Bank region are dominated by warm water species in late summer/autumn, which are specifically adapted to lower food conditions and higher temperatures and metabolic costs. Better knowledge of their physiological processes and behavior is needed to estimate the impact of changes in environmental conditions on their seasonality and spatial distribution. We investigated the population dynamics of the warm water species Centropages typicus in this region using a copepod population model coupled with a three-dimensional ecosystem model. Although growth and development of copepods are well-investigated processes and well represented in population models, the importance of other life-history traits (e.g. cannibalism, feeding and migration strategies) and their impact on seasonal dynamics and spatial distributions are recognized but difficult to quantify. In this study, species-specific life-history traits were included to investigate their influence on the spatio-temporal patterns. The results confirmed our assumption that cannibalism is a possible constraint controlling the abundances on Georges Bank. Omnivorous feeding by C. typicus on both phytoplankton and microzooplankton is important for maintaining the population in the deeper basins, while vertical aggregation in zones of high-food concentration is a secondary mechanism sustaining the population in low-food seasons. Finally, advective influx from the upstream region on the Scotian Shelf is critical in sustaining the Gulf of Maine population.