Outdoor phytoplankton
continuous culture in a marine fish–phytoplankton–bivalve integrated
system: combined effects of dilution rate and ambient conditions on growth
rate, biomass and nutrient cycling
S. Lefebvre, I. Probert, Ch. Lefrançois, J.
Hussenot-2004
Aquaculture, 240(1-4): 211-231
Abstract:
Natural phytoplankton populations were cultured in
outdoor continuous cultures using fish-farm effluents as the source of
nutrients. The dilution rate was assumed to be the integrating factor of
phytoplankton growth and biomass development (flux and stock). In this
context, the combined effects of (i) dilution rates of the outdoor culture
and (ii) ambient conditions were tested on phytoplankton growth, biomass and
cycling of the major nutrient elements (C, N and P). Experiments were
carried out in outdoor polyester tanks (0.7 m deep), homogenised by gentle
aeration. Si/P ratio was balanced at around 5 in the inflow in order to
induce diatom domination while maintaining high N and P assimilation by
phytoplankton. Nutrient cycling was assessed through analyses of the
different forms of particulate and dissolved nutrients in the inflow and the
outflow. Culture dilution rates determined the longevity of the culture and
the assimilation efficiency of nutrients. Dissolved phosphorus was the most
limiting nutrient. The optimal dilution rate was approximately 0.5 day−1
at 10 °C and 1.5 day−1 at 20 °C with a mean diatom
biomass of 9 μM P. Under these conditions, 80% of the dissolved
nutrients provided to the tanks were transformed, a production of 8 g C m−2
day−1 and an assimilation rate of 0.3 g P m−2
day−1 were recorded. Assimilation by diatoms was the major
pathway of nutrient cycling. During the experiment, a bottom sediment
developed progressively and this also played an important role in
denitrifying the excess dissolved nitrogen in the fish-farm effluent.
However, the results showed that diatom biomass can collapse and we
hypothesize that this was the consequence of an increase in cellular sinking
rates due to cell aggregation under nutrient or light stress. Modelling
approaches are needed in future research in order to determine optimal
dilution rates taking into account phytoplankton growth rates, nutrient
inputs and ambient conditions (e.g. light and temperature).
(Laboratoire
de Biologie et Biotechnologies Marines, Université de Caen, esplanade de la
paix, 14032 Caen cedex, France, e-mail: s.lefebvre@ibfa.unicaen.fr)