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Abstracts - Posters
P26. Nitrogen addition in semi-extensive aquaculture enclosures: effects
on phytoplankton and zooplankton composition
Blanda E. 1 , Bruno E. 2 , Jakobsen H. H. 3 , Jepsen P. M. 1 , Mahjoub M. S. 4 , Pedersen A. T. 5 , Rayner T.
A. 1 & Hansen B. W. 1
1 Department of Environmental, Social, and Spatial Change, Roskilde University, 4000 Roskilde,
Denmark eblanda@ruc.dk, pmjepsen@ruc.dk, bhansen@ruc.dk
2 DTU – Danmarks Tekniske Universitet, 2920 Charlottenlund, Denmark elebr@aqua.dtu.dk
3 Department of Bioscience, Aarhus University, 4000 Roskilde, Denmark hhja@dmu.dk
4 DHI-NTU Research and Education Hub, 128388 Singapore mma@dhi.com.sg
5 Maximus A/S, 7755, Bedsted Thy, Denmark
The aim of this project is to study the effect of inorganic nitrogen in a semi-extensive aquaculture
system in northern Jutland, Denmark.
We hypothesize that a higher availability of nutrients, in particular nitrogen in the form of NO3, will
affect the food chain providing a higher availability of food for fish larvae. The final goal is in fact
to increase the survival and development of the turbot larvae Scophthalmus maximus where the
crucial factor is the appropriate diet.
A manipulation experiment was performed in 280 m 3 open top tanks in late summer 2012 (middle
august-early September). 2x3 tanks were spiked with inorganic Nitrogen (full- and pulse dose) and
3 tanks were used as controls. The initial NO3 concentration of the full dose treatment was 15
µmol/L while in the pulsed dose treatment the same amount was divided in 3 doses and added
weekly during the campaign.
Inorganic nutrients and phytoplankton concentration were analysed every third day, phytoplankton
and zooplankton composition were monitored every week. Fatty acids (FA) of all trophic levels in
the pelagic food web were analysed. This effort was established to set up budgets for energy and
matter from phytoplankton to zooplankton to measure what is available in terms of copepod food
and energy for fish larvae.
The nutrient addition caused phytoplankton biomass to increase: chlorophyll a was less than 3,8 µg
L -1 in the control enclosures, while it increased up to 50 µg L -1 in enclosures where nitrogen was
added. As a consequence the abundance of zooplankton, in particular of copepods, increased
significantly. Additionally the productivity and fecundity of the copepods in the full dose treatments
was higher than the pulsed one or the control.
In conclusion we can assume that inorganic nitrogen controls the productivity of the classical food
chain and increase the phyto- and zooplankton biomass, along with the fecundity of the copepods.
We suggest disposing of nutrient enriched green-water tanks to produce copepods for life feed in
aquaculture and use the stock for feeding fish larvae to enhance the survival and development of
turbot fry.
Keywords: Nutrients; Phytoplankton; Zooplankton; Turbot larvae survival
Program 17. danske havforskermøde, 143