Plankton composition and biomass development: a seasonal study of a semi-intensive outdoor system for rearing of turbot

Abstract

Plankton food web dynamics were studied during a complete production season in a semi-intensive land-based facility for rearing of turbot (Scophthalmus maximus) larvae. The production season was divided into three production cycles of 3-5 weeks. Phytoplankton biomass (using chlorophyll a as biomass proxy) peaked in each production cycle. However, the maximum biomass decreased from spring (18 lg chlorophyll a L À1 ) to fall (ca. 7 lg chlorophyll a L À1 ), simultaneous with a decline in the concentration of dissolved nitrogen in the inoculating water. During the three production cycles, we observed decreasing copepod nauplii concentration from spring to fall in the rearing tanks. The decreasing nitrogen gave increasing carbon to chlorophyll a ratio in the seston (from 23 in spring to 73 in fall). The pool of free amino acids in seston was constant. We suggest that the decreasing nitrogen input in the inoculating water reduces the quality of the phytoplankton and thus the growth potential of the prey for fish larvae, copepods.

Inorganic nitrogen addition in a semi-intensive turbot larval aquaculture system: effects on phytoplankton and zooplankton composition

“…These values are not higher than what has been reported in the past for this rearing system in non‐nitrogen treated tanks (Jakobsen et al . ). Moreover, the values reported in the present study (0.29 ± 0.04 per day for A. tonsa and 0.11 ± 0.05 per day for C. hamatus ) are quite below maximum rates found in laboratory studies for A. tonsa (>0.4 per day, Berggreen et al .…”

“…90% of the total phytoplankton biomass at the end of the experiment (Jakobsen et al . ). Also P. minimum have been reported to cause deleterious food web effect including inhibition of growth in tintinnid ciliates (Rosetta & McManus ) and a toxin had been identified in senescent laboratory cultures of this algal species (Grzebyk, Denardou, Berland & Pouchus ).…”

“…However, we focused on the quantity of nitrogen rather than the ratios between the nutrients available for the system; the reason behind this choice was given by the conclusion that nitrogen was the only limiting nutrient of MAXIMUS system (Jakobsen et al . ; E. Blanda and P.M. Jepsen pers. comm.).…”

“…We therefore hypothesize that phytoplankton and copepod production in temperate latitude fish rearing tanks are limited by inorganic nitrogen, especially during late summer (Jakobsen et al . ). Although physical boundaries differs among the tropical and the temperate system, nitrogen additions nevertheless increase phytoplankton abundance which in turn cascades into elevated copepod biomass, and finally into increased fish larvae survival, development and growth (Naas, van der Meeren & Aksnes ).…”

Ontogenetic development of attack behaviour by turbot larvae when exposed to copepod prey

“…When the microalgae are at a high density in the tanks, copepods are inoculated, and when a pelagic food web is established, yolk sac turbot larvae are added. They grow for 3 to 5 weeks until metamorphosis to benthic juvenile larvae, and are hereafter collected and weaned to formulate diet in indoor facilities (Jakobsen et al., ). In intensive systems, the larval production is conducted in indoor facilities, where the abiotic factors are controlled.…”