Exploring the Trophic Spectrum: Placing Mixoplankton Into Marine Protist Communities of the Southern North Sea

Abstract

While traditional microplankton community assessments focus primarily on phytoplankton and protozooplankton, the last decade has witnessed a growing recognition of photo-phago mixotrophy (performed by mixoplankton) as an important nutritional route among plankton. However, the trophic classification of plankton and subsequent analysis of the trophic composition of plankton communities is often subjected to the historical dichotomy. We circumvented this historical dichotomy by employing a 24 year-long time series on abiotic and protist data to explore the trophic composition of protist communities in the Southern North Sea. In total, we studied three different classifications. Classification A employed our current knowledge by labeling only taxa documented to be mixoplankton as such. In a first trophic proposal (classification B), documented mixoplankton and all phototrophic taxa (except for diatoms, cyanobacteria, and colonial Phaeocystis) were classified as mixoplankton. In a second trophic proposal (classification C), documented mixoplankton as well as motile, phototrophic taxa associated in a principle component analysis with documented mixoplankton were classified as mixoplankton. In all three classifications, mixoplankton occurred most in the inorganic nutrient-depleted, seasonally stratified environments. While classification A was still subjected to the traditional dichotomy and underestimated the amount of mixoplankton, our results indicate that classification B overestimated the amount of mixoplankton. Classification C combined knowledge gained from the other two classifications and resulted in a plausible trophic composition of the protist community. Using results of classification C, our study provides a list of potential unrecognized mixoplankton in the Southern North Sea. Furthermore, our study suggests that low turbidity and the maturity of an ecosystem, quantified using a newly proposed index of ecosystem maturity (ratio of organic to total nitrogen), provide an indication on the relevance of mixoplankton in marine protist communities.

Modeling mixoplankton along the biogeochemical gradient of the Southern North Sea

“…Although the aim was to run the AEM with all five PFTs, it was difficult to parameterize NCMs for this AEM application using literature and data-based knowledge. Unfortunately, NCMs are not sampled in the routine monitoring program of the SNS ( Schneider et al, 2020 ), so there is a lack of knowledge about the distribution of NCMs in the SNS. Furthermore, there is still a lack of physiological understanding of NCMs ( Hansen et al, 2019 ).…”

“…In a recent analysis of routine monitoring data on the protist community of the Dutch Southern North Sea (from here on referred to as the Southern North Sea - SNS), Schneider et al (2020) showed that CMs occur mostly in inorganic nutrient depleted, highly transparent, stratified environments. Eutrophied, turbid, mixed environments were devoid of CMs.…”

“…Eutrophied, turbid, mixed environments were devoid of CMs. Schneider et al (2020) hypothesized that the environmental factors which exhibit a biogeochemical gradient drive the trophic composition of protist communities in the SNS. However, Schneider et al (2020) were not able to elucidate which environmental factor – the dissolved inorganic nutrient or the suspended sediment gradient – governed the occurrence of CMs.…”

“…We validated PROTIST using 11 1D-vertical (1D-V) models that mimic 11 routine monitoring location classes located in the estuaries, coasts and offshore regions of the Dutch SNS ( Schneider et al, 2020 ). The 11 1D-V models were forced with timeseries of inorganic nutrients and suspended sediments sampled at the 11 location classes.…”

“…Using the 11 1D-V models, we were also able to further explore the hypothesis put forward by Schneider et al (2020) that the biogeochemical gradient drives the trophic composition of protist communities. We conducted a sensitivity analysis to determine whether the inorganic nutrients or the suspended sediment concentration has more effect on the occurrence of CMs.…”