Abstract
Solar radiation and nutrient pulses regulate the ecosystem’s functioning. However, little is known about how a greater frequency of pulsed nutrients under high ultraviolet radiation (UVR) levels, as expected in the near future, could alter the responses and interaction between primary producers and decomposers. In this report, we demonstrate through a mesocosm study in lake La Caldera (Spain) that a repeated (press) compared to a one-time (pulse) schedule under UVR prompted higher increases in primary (PP) than in bacterial production (BP) coupled with a replacement of photoautotrophs by mixotrophic nanoflagellates (MNFs). The mechanism underlying these amplified phytoplanktonic responses was a dual control by MNFs on bacteria through the excretion of organic carbon and an increased top-down control by bacterivory. We also show across a 6-year whole-lake study that the changes from photoautotrophs to MNFs were related mainly to the frequency of pulsed nutrients (e.g. desert dust inputs). Our results underscore how an improved understanding of the interaction between chronic and stochastic environmental factors is critical for predicting ongoing changes in ecosystem functioning and its responses to climatically driven changes.
Highlights
Nutrients and light are two key factors that regulate the ecosystem’s functioning
The temporal dynamics from 2010–2015 showed that phytoplankton predominated over bacteria in terms of biomass and reflected two clear response patterns related to the intensity and frequency of aerosol inputs (i.e. Aerosol Index [AI])
Partially contrasting with the findings over the shorter term, our results show that the press or pulse input counteracted (e.g. C:P ratio, bacterial production (BP), bacterial growth efficiency (BGE)) or reversed the negative ultraviolet radiation (UVR) effects, except for the %PEGA, for which a pulse input unmasked the negative effects mentioned above
Summary
Nutrients and light are two key factors that regulate the ecosystem’s functioning. Nutrient pulses are stochastic events of brief resource availability that have strong effects on a huge variety of ecological processes (e.g. growth, primary production [PP], consumer-resource interaction) in a range of ecosystems worldwide[1,2]. An increasing number of studies show the prevalence of mixotrophs in natural communities, in surface waters of oligotrophic ecosystems (i.e. freshwater and marine), where they tend to predominate due to having an advantage on photoautotrophic phytoplankton by obtaining limiting nutrients (e.g. P) through phagotrophy[31,32,33,34] In addition to their flexible nutrition, mixotrophs seem to be favoured by high light conditions[28,35], and it would be expected that, under stabler and shallower upper mixed layers (UMLs) due to global change situation, they would be strongly benefited with respect to photoautotrophic groups by greater exposure to radiation. Other studies contend that nutrient pulses can attenuate the damage that UVR inflicts on PP and BP44,28,45
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