Food web interactions determine energy transfer efficiency and top consumer responses to inputs of dissolved organic carbon
Food web interactions determine energy transfer efficiency and top consumer responses to inputs of dissolved organic carbon
0 R. Degerman R. Lefe ́bure U. Ba ̊mstedt S. Larsson A. Andersson Umea ̊ Marine Science Centre , 905 71 Ho ̈rnefors , Sweden
1 R. Degerman R. Lefe ́bure P. Bystro ̈m U. Ba ̊mstedt S. Larsson A. Andersson (&) Department of Ecology and Environmental Science, Umea ̊ University , 901 87 Umea ̊ , Sweden
2 Handling editor: Jonne Kotta
Climate change projections indicate increased precipitation in northern Europe, leading to increased inflow of allochthonous organic matter to aquatic systems. The food web responses are poorly known, and may differ depending on the trophic structure. We performed an experimental mesocosm study where effects of labile dissolved organic carbon (DOC) on two different pelagic food webs were investigated, one having zooplankton as highest trophic level and the other with planktivorous fish as top consumer. In both food webs, DOC caused higher bacterial production and lower food web efficiency, i.e., energy transfer efficiency from the base to the top of the food web. However, the top-level response to DOC addition differed in the zooplankton and the fish systems. The zooplankton production increased due to efficient channeling of energy via both the bacterial and the phytoplankton pathway, while the fish production decreased due to channeling of energy mainly via the longer and less efficient bacterial pathway. We conclude that the added DOC either acted as a subsidy by increasing the production of the top trophic level (mesozooplankton), or as a sink causing decreased top consumer production (planktivorous fish).
Food web efficiency; Carbon transfer; Allochthonous dissolved organic carbon; Mesocosm; Planktivorous fish
Introduction
Knowledge about pathways and constraints of energy
transfer through food webs is fundamental for the
understanding of ecosystem function
(e.g., Dickman
et al., 2008; Wollrab et al., 2012)
. The complexity of
food web dynamics and structure inherently means
that both changes in basal production and top–down
control can create alternative pathways of energy flow
up to the highest trophic level, resulting in patterns of
energy transfer and productivity commonly deviating
from expectations based on classical food chain theory
(Vadeboncoeur et al., 2004, Hulot et al., 2014)
. For
example, the presence or absence of classic trophic
cascades in marine food webs has been suggested to be
dependent on the size structure of the phytoplankton
community
(Stibor et al., 2004)
.
In aquatic systems, phytoplankton (autotrophs) and
bacteria (heterotrophs) are basal producers, acting as
energy source for higher trophic levels, and thus shape
the food webs depending on their production and
composition
(Azam et al., 1983; Legendre &
Rassoulzadegan, 1995, Jansson et al., 2007)
. The balance
between autotrophs and heterotrophic bacteria is
governed by both bottom–up factors such as nutrient
availability, and top–down effects, e.g., trophic
interactions via top predators
(Carpenter et al., 1985;
Hairston & Hairston, 1993, Vanni & Layne, 1997)
.
These food webs are complex, as many consumers
feed on organisms from both the phytoplankton and
the bacterial pathway. Both phytoplankton and
bacteria are osmotrophic organisms competing for
inorganic nutrients, nevertheless phytoplankton utilize
inorganic forms of carbon while heterotrophic bacteria
are in many systems dependent on autochthonous
organic carbon produced by phytoplankton
(Cole
et al., 1988)
. However, in aquatic systems influenced
by allochthonous dissolved organic carbon (ADOC),
bacteria can be decoupled from autochthonous
production
(Karlsson et al., 2002, Stibor et al., 2004)
.
Accordingly, in systems with high inputs of ADOC,
heterotrophic bacteria tend to contribute substantially
to the total basal production
(Pace et al., 2004;
Berglund et al., 2007, Jansson et al., 2007)
.
Phytoplankton are often directly consumed by primary
consumers like mesozooplankton, whereas bacteria
are too small to be readily eaten by such organisms.
Instead, bacteria are consumed by protozoans which in
turn are consumed by mesozooplankton
(Hessen &
Andersen, 1990, Brett et al., 2009)
. Hence, the
heterotrophic-based pathway, i.e., the microbial food
web, will have a more complex pattern of energy
transfer compared to systems dominated by
autotrophic production
(Sommer et al., 2002; Berglund
et al., 2007)
.
At the top of the food web, interactions between
predator and prey may also yield strong effects on food
web function and structure, but the nature and
composition of top consumer organisms in the food
web will exert different top–down impacts on lower
trophic levels
(Hairston & Hairston, 1993, Vanni &
Layne, 1997, Hulot et al., 2014)
. For instance, in food
webs where mesozooplankton function as top
predator, consumption rates on phytoplankton and ciliates
have been shown to be high
(Johan (...truncated)