Geographical and Seasonal Thermal Sensitivity of Grazing Pressure by Microzooplankton in Contrasting Marine Ecosystems

Abstract

Grazing pressure, estimated as the ratio between microzooplankton grazing and phytoplankton growth rates (g:μ), is a strong determinant of microbial food-web structure and element cycling in the upper ocean. It is generally accepted that g is more sensitive to temperature than μ, but it remains unknown how the thermal dependence (activation energy, Ea) of g:μ varies over spatial and temporal scales. To tackle this uncertainty, we used an extensive literature analysis obtaining 751 paired rate estimates of μ and g from dilution experiments performed throughout the world’s marine environments. On a geographical scale, we found a stimulatory effect of temperature in polar open-ocean (∼0.5 eV) and tropical coastal (∼0.2 eV) regions, and an inhibitory one in the remaining biomes (values between −0.1 and −0.4 eV). On a seasonal scale, the temperature effect on g:μ ratios was stimulatory, particularly in polar environments; however, the large variability existing between estimates resulted in non-significant differences among biomes. We observed that increases in nitrate availability stimulated the temperature dependence of grazing pressure (i.e., led to more positive Ea of g:μ) in open-ocean ecosystems and inhibited it in coastal ones, particularly in polar environments. The percentage of primary production grazed by microzooplankton (∼56%) was similar in all regions. Our results suggest that warming of surface ocean waters could exert a highly variable impact, in terms of both magnitude and direction (stimulation or inhibition), on microzooplankton grazing pressure in different ocean regions.

Functional Stability of a Coastal Mediterranean Plankton Community During an Experimental Marine Heatwave

“…However, during the second part of the heatwave, l increased more than µ, and the µ:l ratio was negatively affected by the heatwave. This is in line with a meta-analysis of the µ to grazing ratio dataset estimated in temperate coastal areas that reported a negative effect of temperature on this ratio (Cabrerizo and Marañón, 2021). The µ:l ratio is a major factor controlling the fate of newly produced organic matter in the ocean and has important implications for element cycling.…”

Microbial Ecology of Oxygen Minimum Zones Amidst Ocean Deoxygenation

“…Acidification will likely decrease the abundance of any calcium test-bearing microeukaryotes in OMZs like it does for coccolithophores (D'Amario et al, 2020). Further, rising temperatures have the potential to increase grazing rates of bacteria by microeukaryotes in oxic waters (Gu et al, 2020;Cabrerizo and Marañón, 2021), but it is unclear whether this can overcome the observed negative effect low oxygen concentrations have on grazing. As for viruses, warming has the potential to increase decay rates, especially in tropical regions (Danovaro et al, 2011), and acidification seems to have the ability to select for particular coccolithophore virus strains over others (Highfield et al, 2017) but whether this is a general trend for all viruses has not been ascertained.…”