Dataset on how mesopredator-mediated trophic cascade can break persistent phytoplankton blooms in coastal waters

Managing eutrophied systems using only nutrient decreases to impose
bottom-up control can be economically and ecologically challenging.
Top-down controls through increased consumption have sometimes effectively
controlled phytoplankton blooms. However, mechanistic insights, especially
on possible trophic cascades, are less understood in brackish,
species-poor coastal waters, where large cladocera are absent. In this
study, we set up large mesocosms for three consecutive years during the
growing season. One set of mesocosms contained mesopredators (gobies and
shrimps), whereas the other mesocosms had no such mesopredator present.
The results were standardized to monitoring data from the ecosystem to
track possible differences between treatments and the system. We found
that mesopredator mesocosms showed lower turbidity, phytoplankton biomass,
and nutrients compared to no-mesopredator mesocosms, and compared to the
ecosystem. This decrease allowed macrophytes to colonize water depths only
sparsely colonized in the ecosystem. Rotifer biomass increased in
mesopredator mesocosms compared to the ecosystem and to the
no-mesopredator mesocosms. Likewise, copepod biomass that potentially
grazes upon rotifers and other microzooplankton decreased in mesopredator
mesocosms. No-mesopredator mesocosms were colonized by an omnivorous
mesograzer (Gammarus tigrinus), potentially creating additional pressure
on macrophytes and increasing grazing-mediated nutrient release.
Zooplankton was not able to control the non-nutrient-limited
phytoplankton. We propose a new mechanism, where a higher mesopredator
density will increase grazing on phytoplankton by promoting
microzooplankton capable of grazing on picophytoplankton. This proposed
mechanism would contrast with freshwater systems, where a decrease of
zooplanktivorous fish would promote larger phytoplankton grazer like
cladocerans. Biomanipulation in such species-poor eutrophic coastal waters
may be more successful, due to fewer trophic pathways, that can cause
complex top-down controls like in other systems. Stocking eutrophic
coastal waters with gobies and shrimps may be an alternative
biomanipulative approach rather than selectively removing large
piscivorous or omnivorous fish from eutrophic coastal waters.