Marine Microbial Food Web Networks During Phytoplankton Bloom and Non-bloom Periods: Warming Favors Smaller Organism Interactions and Intensifies Trophic Cascade

Abstract

Microbial food web organisms are at the base of the functioning of pelagic ecosystems and support the whole marine food web. They are very reactive to environmental changes and their interactions are modified in response to different productive periods such as phytoplankton bloom and non-bloom as well as contrasted climatic years. To study ecological associations, identify potential interactions between microorganisms and study the structure of the microbial food web in coastal waters, a weekly monitoring was carried out in the Thau Lagoon on the French Mediterranean coast. The monitoring lasted from winter to late spring during two contrasting climatic years, a typical Mediterranean (2015) and a year with an extreme warm winter (2016). Correlation networks comprising 110 groups/taxa/species were constructed to characterize potential possible interactions between the microorganisms during bloom and non-bloom periods. Complex correlation networks during the bloom and dominated by negative intraguild correlations and positive correlations of phytoplankton with bacteria. Such pattern can be interpreted as a dominance of competition and mutualism. In contrast, correlation networks during the non-bloom period were less complex and mostly dominated by tintinnids associations with bacteria mostly referring to potential feeding on bacteria, which suggests a shift of biomass transfer from phytoplankton-dominated food webs during bloom to more bacterioplankton-based food webs during non-bloom. Inter-annual climatic conditions significantly modified the structure of microbial food webs. The warmer year favored relationships among smaller group/taxa/species at the expense of large phytoplankton and ciliates, possibly due to an intensification of the trophic cascade with a potential shift in energy

Environmental drivers of plankton communities in coastal waters: the central role of temperature revealed by co-occurrence networks

“…2016 was an unusually warm year 19 , and water temperature could also have accelerated nutrient remineralisation. The present results are in accordance with those of a previous study suggesting that the warm conditions of 2016 in Thau Lagoon favoured the co-occurrence of smaller taxa, including heterotrophic nano agellates, viruses and bacteria 22 .…”

“…Trombetta et al (2019, 2020) 19,22 found that in Thau Lagoon, water temperature triggers phytoplankton blooms and suggested that interannual warming favours interactions among small organisms and trophic cascades. The ndings of the present study suggest for the rst time that water temperature, rising from winter to spring, is the most important environmental factor regulating the composition, succession (Figs.…”

“…Weekly sampling for nutrients and all organisms was carried out in the morning between 09 am and 10 am. Some of the data used in the present study were already published in previous manuscripts 19,22 . Thus, details on acquisition methods can also be found in these manuscripts, as speci ed below.…”

Effects of experimental warming on small phytoplankton, bacteria and viruses in autumn in the Mediterranean coastal Thau Lagoon

“…The simultaneous decrease in growth rates of both bacterial groups with the increase in those of some phytoplankton groups observed in the present study might suggest an accentuation in competition among these groups under warming. Competition for resources among bacteria and between bacteria and phytoplankton are commonly described in natural plankton communities (Løvdal et al 2007;Trombetta et al 2020), and it was shown that warming can accentuate competition for resources (Goldman and Ryther 1976;Kordas et al 2011).…”

“…Microphytoplankton are mainly represented by diatoms and dinoflagellates are generally less abundant, excepted during exceptional blooms (Derolez et al 2020b). Thus, the phytoplankton community of Thau Lagoon is generally dominated by small phytoplankton (Trombetta et al 2020). Bacteria are also highly abundant in the lagoon, their abundances ranging from 2.0 to 10.0 9 10 6 cells cm 3 (Vidussi et al 2011;Mostajir et al 2015b).…”