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Aquatic ecologists are integrating mixotrophic plankton – here defined as microorganisms with photosynthetic and phagotrophic capacity – into their understanding of marine food webs and biogeochemical cycles. Understanding mixotroph temporal and spatial distributions, as well as the environmental conditions under which they flourish, is imperative to understanding their impact on trophic transfer and biogeochemical cycling. Mixotrophs are hypothesized to outcompete strict photoautotrophs and heterotrophs when either light or nutrients are limiting, but testing this hypothesis has been hindered by the challenge of identifying and quantifying mixotrophs in the field. Using field observations from a multi-decadal northern North Atlantic dataset, we calculated the proportion of organisms that are considered mixotrophs within individual microplankton samples. We also calculated a “trophic index” that represents the relative proportions of photoautotrophs (phytoplankton), mixotrophs, and heterotrophs (microzooplankton) in each sample. We found that the proportion of mixotrophs was positively correlated with temperature, and negatively with either light or inorganic nutrient concentration. This proportion was highest during summertime thermal stratification and nutrient limitation, and lowest during the North Atlantic spring bloom period. Between 1958 and 2015, changes in the proportion of mixotrophs coincided with changes in the Atlantic Multi-decadal Oscillation (AMO), was highest when the AMO was positive, and showed a significant uninterrupted increase in offshore regions from 1992-2015. This study provides an empirical foundation for future experimental, time series, and modeling studies of aquatic mixotrophs.

 

Abstract The phenology, distribution, and size composition of plankton communities are changing rapidly in response to warming. This may lead to shifts in the prey fields of planktivorous fish, which play a key role in transferring energy up marine food chains. Here, we use 60 + years of Continuous Plankton Recorder data to explore temporal trends in key taxa and community traits in the prey field of planktivorous lesser sandeels (Ammodytes marinus) in the North Sea, the Faroes and southern Iceland. We found marked spatial variation in the prey field, with Calanus copepods generally being much more common in the northern part of the study area. In the western North Sea, the estimated amount of available energy in the prey field has decreased by more than 50% since the 1960s. This decrease was accompanied by declining abundances of small copepods, and shifts in the timing of peak annual prey abundances. Further, the estimated average prey community body size has increased in several of the locations considered. Overall, our results point to the importance of regional studies of prey fields, and caution against inferring ecological consequences based only on large-scale trends in key taxa or mean community traits. 

Abstract. Plankton form the base of the marine food web and are sensitive indicatorsof environmental change. Plankton time series are therefore an essentialpart of monitoring progress towards global biodiversity goals, such as theConvention on Biological Diversity Aichi Targets, and for informingecosystem-based policy, such as the EU Marine Strategy Framework Directive.Multiple plankton monitoring programmes exist in Europe, but differences insampling and analysis methods prevent the integration of their data,constraining their utility over large spatio-temporal scales. The PlanktonLifeform Extraction Tool brings together disparate European planktondatasets into a central database from which it extracts abundancetime series of plankton functional groups, called “lifeforms”, according toshared biological traits. This tool has been designed to make complexplankton datasets accessible and meaningful for policy, public interest, andscientific discovery. It allows examination of large-scale shifts inlifeform abundance or distribution (for example, holoplankton beingpartially replaced by meroplankton), providing clues to how the marineenvironment is changing. The lifeform method enables datasets with differentplankton sampling and taxonomic analysis methodologies to be used togetherto provide insights into the response to multiple stressors and robustpolicy evidence for decision making. Lifeform time series generated with thePlankton Lifeform Extraction Tool currently inform plankton and food webindicators for the UK's Marine Strategy, the EU's Marine Strategy FrameworkDirective, and for the Convention for the Protection of the MarineEnvironment of the North-East Atlantic (OSPAR) biodiversity assessments.The Plankton Lifeform Extraction Tool currently integrates 155 000 samples,containing over 44 million plankton records, from nine different planktondatasets within UK and European seas, collected between 1924 and 2017.Additional datasets can be added, and time series can be updated. The PlanktonLifeform Extraction Tool is hosted by The Archive for Marine Species andHabitats Data (DASSH) at https://www.dassh.ac.uk/lifeforms/ (last access: 22 November 2021, Ostle et al., 2021). The lifeform outputs are linked to specific, DOI-ed, versions of thePlankton Lifeform Traits Master List and each underlying dataset.