Algal growth, bumblebee colony and individual development, bee behavior and yield of oilseed rape under a trophic cascade and extreme weather

Trophic cascades in the aquatic environment constitute important
mechanisms for improving water quality. However, how
the presence or non-presence of these
trophic cascades may affect interactions across the aquatic-terrestrial
interface remains poorly investigated. Pollinators such as bees may be
especially vulnerable to changes in water resource quality induced by
trophic cascades. Understanding how aquatic trophic cascades affect bees
and pollination becomes even more pressing under ongoing climate change
due to increased physiological demands for water under extreme weather
events.In a novel field experiment combining terrestrial and aquatic
mesocosms, we aimed to test how changes in water quality induced by an
aquatic trophic cascade affected foraging and growth of bumblebee
colonies as well as foraging of solitary bees. While we expected fish
predation to reduce top-down control of zooplankton on phytoplankton and
thereby, indirectly, induce increased growth of toxic cyanobacteria, we
instead found the trophic cascade to induce the formation of algal surface
mats that bumblebees used to access water under a severe heat wave and
drought. This access to water was associated with higher bumblebee colony
reproductive success, growth and weight compared to control colonies with
no trophic cascade induced (and hence no algal surface mats). We also
found marginal but non-significant effects on
oilseed rape yield, but surprisingly with higher yields in the control
treatment where bumblebees could not access water.Our results provide new
insights on how aquatic trophic cascades can lead to unpredicted
ecological interactions across the aquatic-terrestrial interface
facilitated by climate change. Our study highlights the importance of
water for the fitness of terrestrial ecosystem service providers under
altered environmental conditions.