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

Abstract

A nitrogen manipulation experiment was conducted in a semi‐intensive outdoor system where turbot larvae feed on copepods. Nitrogen addition is hypothesized to stimulate a cascade reaction increasing phytoplankton biomass, copepods’ productivity and larval fish survival. Triplicates were established for three treatments: a control with no additional nitrogen, a pulsed dose where nitrogen was added in three doses over time and a full dose where all nitrogen, equal to the total of the pulse dose treatment, was added initially. In the control, chlorophyll a averaged 3.3 ± 1.5 μg L−1 and phytoplankton was dominated by diatoms, while the pulsed and full dose treatments showed chlorophyll a at 28.6 ± 9.9 and 47.7 ± 10.0 μg L−1, respectively, with dinoflagellates as the main phytoplankton group. Due to photosynthesis, pH increased >9 in both the nitrogen treatments compared to the control (8.5). Potential toxic dinoflagellates, including Alexandrium pseudogonyaulax and Prorocentrum spp., became dominant in the nitrogen treatments and might have arrested zooplankton recruitment. Laboratory experiments with a toxic strain of A. pseudogonyaulax proved that Acartia tonsa reproduction and naupliar survival were affected negatively at realistic fish tank concentrations of 100 and 20 cells mL−1, respectively. Compared to the control, pulsed and full dose treatments reached higher copepod biomass and showed a shift over time in species composition from Centropages hamatus to Acartia spp. However, high pH levels and dinoflagellate blooms had a negative effect on larval fish survival, suggesting management improvements on water quality and separation between copepods and fish production tanks.

Outdoor rearing facilities of free spawning calanoid copepods for turbot larva can host a bank of resting eggs in the sediment

“…Hence, some predation on the nauplii took place in the control due to relatively high turbot larval survival (20 % survival rate). However, the copepods were not diminished significantly by the fish according to Blanda et al (2015) and their availability was sufficiently high to sustain turbot larvae growth until the end in the control treatment. On the other hand, we cannot prove that the absence of fish had an effect on the nutrients treatments revealing a higher copepod biomass; yet, we can confirm that copepods reproduction increased by the end of the manipulation experiment, keeping copepods abundances high independently of the presence-absence of fish larvae .…”

“…Between 8 August and 6 September 2012, a turbot fry production cycle fed by concurrent copepods of approximately 4 weeks was conducted in nine outdoor tanks that were challenged with three different nutrient addition treatments Blanda et al 2015). The purpose of the experiment was to investigate whether the addition of inorganic nutrients could stimulate the pelagic food web, increase copepod production, and eventually improve the success of turbot fry production.…”

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

“…The production of turbot larvae usually takes place in either semi‐intensive systems or in intensive systems. In a typical semi‐intensive system, estuarine water is pumped into 100's–1000's m 3 enclosures, followed by a bloom of naturally occurring microalgae (Blanda et al., ; Engell‐Sørensen, Støttrup & Holmstrup, ). 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.…”

“…In the semi‐intensive production of turbot larvae, the species composition and size of live prey varies greatly through a season, and through the production of each batch of turbot larvae (Blanda et al., ; Engell‐Sørensen et al., ; Sørensen, Drillet, Engell‐Sørensen, Hansen & Ramløv, ). The classical approach to evaluate the effect of the varying sizes of live prey is to examine the gut content of the turbot larvae (Cunha & Planas, , ; Van der Meeren, ; Witt et al., ).…”

“…The prey composition in the surrounding environment is typically compared to the gut content of the larvae to evaluate which sizes of prey that the turbot larvae select through their ontogenetic development (Van der Meeren, ). A potential miss‐match between the dominant size of prey in a production and the optimal prey size of the larvae might contribute to the low survival (<20%) that is typically observed for turbot larvae in semi‐intensive systems (Blanda et al., ; Jepsen et al., ). The mean size of prey in the guts of the turbot larvae is often compared to the jaw size of the larvae, as the jaw size is considered the deciding factor for the maximal size of prey that the turbot larvae are able to ingest, and therefore interpreted as the optimal prey size (Cunha & Planas, ).…”