New Zealand Journal of Marine and Freshwater Research abstracts
Responses of two trophic levels to patch enrichment along a New Zealand stream
continuum
BARRY J. F. BIGGS
NIWA-Ecosystems
National Institute of Water and Atmospheric
Research Ltd
P.O. Box 8602
Christchurch, New Zealand
REX L. LOWE
Department of Biological Sciences
Bowling Green State University
Bowling Green, OH 43403
USA
Abstract We carried out a benthic survey and two experiments in
runs at eight sites down the Kakanui River (South Island, New Zealand) during
summer low flows, to investigate the interaction between nutrients, periphyton,
and macro-grazers. Benthic periphytic biomass was generally low (< 20
mg m-2 chlorophyll a) at most sites, but high densities of macro-grazers
(mainly snails) were observed at six of the eight sites. Chlorophyll a
and cellular P concentrations were generally higher on artificial substrates in
the first- to third-order tributaries, compared with downstream. Macro-grazer
densities (mainly snails) were also highest in the second- and third-order
tributaries. Enrichment of patches with N and P did not translate into
significant increases in chlorophyll a concentrations. Instead there was
a general increase in macro-grazers, and an increase in the relative abundance
of Cocconeis placentula. In a second experiment, the chlorophyll
a level was five-fold higher on the substrates where macro-grazers were
excluded and there was no significant response of chlorophyll a to
nutrient addition on these substrates. On the grazed substrates, densities of
snails and caddis-larvae were two-fold higher with N+P enrichment. These
experiments provided evidence for a tight coupling between first and second
trophic levels, and strong grazer control of periphyton, in this river.
Keywords periphyton; stream ecology; invertebrate grazers;
trophic levels; patch dynamics; nutrients; top-down control; bottom-up
control
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