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.
Highlights
Temperature is a key environmental driver controlling the rates at which energy and materials flow through food webs and ecosystems (Gillooly et al, 2001; Cross et al, 2015)
We conducted a subsequent filtering of the data of μ and g retrieved, whereby data were excluded in the following cases: (1) the linear regression fit between dilution factor and phytoplankton growth rate had a determination coefficient
Our results show that the temperature dependence of grazing pressure is highly variable among biomes and regions
Summary
Temperature is a key environmental driver controlling the rates at which energy and materials flow through food webs and ecosystems (Gillooly et al, 2001; Cross et al, 2015). Predictions by the metabolic theory of ecology (MTE, Brown et al, 2004) and evidence from terrestrial (Allen et al, 2005; Allen and Gillooly, 2009) and aquatic (López-Urrutia et al, 2006; Liu et al, 2019; Barton et al, 2020) ecosystems show that the thermal dependence [or activation energy (Ea)] of heterotrophic processes [e.g., grazing rates (g)] is higher than that of autotrophic ones [e.g., phytoplankton growth. Recent experimental studies show that the thermal dependence of phytoplankton growth and microzooplankton grazing can change seasonally, with Ea of μ exhibiting variations two- to four-fold larger than those found in Ea of g (Liu et al, 2019). If nutrient limitation decreases Ea of μ (without altering Ea of g), we could expect a lower Ea of g:μ in nutrient-rich, coastal, and upwelling areas than in the open ocean, where nutrient limitation of phytoplankton growth is most prevailing (Moore et al, 2013)
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