Forskning ved Københavns Universitet - Københavns Universitet


Qualitative importance of the microbial loop and plankton community structure in a eutropic lake during a bloom of Cyanobacteria

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Plankton community structure and m~or pools and fluxes of
carbon were observed before and after culmination of a bloom of cyanobacteria
in eutrophic Frederiksborg Slotsso, Denmark. Biomass changes of
heterotrophic nanoflagellates, ciliates, microzooplankton (50 to 140 urn),
and macrozooplankton (larger than 140 Urn) were compared to phytoplankton
and bacterial production as well as micro- and macrozooplankton
ingestion rates of phytoplankton and bacteria. The carbon budget was used
as a means to examine causal relationships in the plankton community.
Phytoplankton biomass decreased and algae smaller than 20 um replaced
Aphanizornenon after the culmination of cyanobacteria. Bacterial net production
peaked shortly after the culmination of the bloom (510 ug C liter- 1
d-') and decreased thereafter to a level of approximately 124 gg C liter-'
d -~. Phytoplankton extracellular release of organic carbon accounted for
only 4-9% of bacterial carbon demand. Cyclopoid copepods and smallsized
cladocerans started to grow after the culmination, but food limitation
probably controlled the biomass after the collapse of the bloom. Grazing
of micro- and macrozooplankton were estimated from in situ experiments
using labeled bacteria and algae. Macrozooplankton grazed 22% of bacterial
net production during the bloom and 86% after the bloom, while microzooplankton
(nauplii, rotifers and ciliates larger than 50 Urn) ingested low
amounts of bacteria and removed 10-16% of bacterial carbon. Both macroand
microzooplankton grazed algae smaller than 20 urn, although they did
not control algal biomass. From calculated clearance rates it was found
that heterotrophic nanoflagellates (40--440 ml -t) grazed 3--4% of the bacterial
production, while ciliates smaller than 50 um removed 19-39% of
bacterial production, supporting the idea that ciliates are an important link
between bacteria and higher trophic levels. During and after the bloom of
Aphanizornenon, major fluxes of carbon between bacteria, ciliates and crustaceans were observed, and heterotrophic nanoflagellates played a minor
role in the pelagic food web
TidsskriftMicrobial Ecology
Sider (fra-til)253-272
StatusUdgivet - 1990

ID: 295018