K.D. Hambright and S.C.
Blumenshine. Filter feeding fishes: a link between water quality and fisheries
management? In Press for Freshwater Biology
Abstract
Seasonal plankton consumption by the
filter-feeding planktivore Sarotherodon galilaeus from Lake Kinneret was examined
experimentally by monitoring changes during 24 h in plankton assemblages in 5m3 mesocosms containing varying densities of fish.
Taxon-specific grazing rates ranged from 0–17 mg gfish-1d-1,
with mean total consumption of 1.6 % fish body weight per day. During the spring bloom of
the dinoflagellate Peridinium gatunense, S. galilaeus consumed mostly
(94%) netphytoplankton (=20 µm). The remaining 6% consisted mostly of nanophytoplankton
(<20µm). During the summer and fall, net- and nanophytoplankton accounted for 54 and
42%, respectively, of the diet of S. galilaeus. Zooplankton and flagellated and
ciliated protozoans made up the remaining 5%. Bioenergetics modeling indicated that
consumption rates were near maximum in spring (90%Cmax), while consumption was
reduced in summer-fall (59%Cmax). Sarotherodon galilaeus obtains
sufficient energy through filter-feeding year-round though most growth (>=60%) occurs
during the spring Peridinium bloom. Despite efficient feeding on Peridinium and
nanophytoplankton by S. galilaeus, instantaneous plankton mortality estimates due
to ingestion were two orders of magnitude smaller than maximum potential plankton growth
rates. Thus the potential for the S. galilaeus population in Lake Kinneret to
positively affect water quality through algal suppression is low.
S.C.
Blumenshine, and K.D. Hambright. Top-down control in pelagic systems: a role for
invertebrate predation. In Press for a special issue of Hydrobiologia; a
tribute to Ramesh Gulati.
Abstract
Limnologists have long
recognized the importance of predation in freshwater communities. The majority of study of
predator effects has involved vertebrate predators, with emphasis on planktivorous fish.
Documented effects of planktivorous fish have been so dramatic that manipulations of their
populations are seen by many as potential tools in lake management. However, the success
of such manipulations is often less than desired due to the ubiquitous complexity of food
webs and the pervasiveness of compensatory responses to food web manipulation. Recently,
enormous effort has been applied to the Lake Kinneret pelagic food web in effort to
reduced the abundance of the planktivorous Kinneret bleak Acanthobrama terraesanctae
and thereby increase the biomass of herbivorous zooplankton in the hopes of increasing
water clarity. We compared potential predation pressure on Lake Kinneret herbivorous
zooplankton by bleak and the other major zooplankton predators in the lake, the cyclopoid
copepods Mesocyclops ogunnus and Thermocyclops dybowskii. We
found that, despite having much lower biomass, cyclopoid copepods accounted for a greater
portion of the predation mortality on herbivorous zooplankton than bleak. Our results
suggest that not only will reductions in predation pressure by bleak not yield subsequent
increases in herbivorous zooplankton biomass, the opposite effect is likely. Reductions in
bleak predation pressure may allow for increases in cyclopoid copepod abundance and
thereby a net increase in predation pressure on herbivorous zooplankton.