Broad-scale distribution of the winter protozooplankton community in the North Sea.

Protozooplankton (PZP) (here size range: 12–200 μm) are rarely sampled over a broad scale, especially in ecosystem monitoring programs, despite their trophodynamic importance as grazers in the microbial loop and as prey for larger zooplankton and early life stages of fish. In this study we sampled PZP from Dutch, French,German and Norwegian research vessels taking part in the annual ICES coordinated International Bottom Trawl Survey (IBTS) which provides data on fish stock abundances and status for the entire North Sea. The abundance,biomass, composition and distribution of PZP were examined at 39 stations across the North Sea (from 3.2°W to 7.6°E and 50.5 to 59.8°N) in mid-winter (January–February 2014), a period of the year which is under-investigated so far. Twenty four taxa of dinoflagellates and ciliates were identified. Two groups comprised 89% of the total abundance of PZP: Gymnodinium spp. and other athecate dinoflagellates (68%) and Strombidium spp. and other naked ciliates (21%). The biomass of PZP at each station ranged between 0.08 and 2.4 μg C L−1, which is much lower than that reported for spring or summer (≥100 μg C L−1) in the North Sea. Relatively small-sized (< 40 μm) PZP contributed 46% of the total biomass. No significant spatial pattern in the composition of the PZP community was found, although the total abundance of tintinnids was highest in the southern North Sea, an important over-wintering area for marine fish larvae. Using this fish survey (IBTS) as a sampling platform allowed us to obtain a synoptic view of the PZP community over a large area. The present collaborative effort provides an example of how existing monitoring platforms can be augmented in the future to collect relevant data and potential ecological indicators needed to advance the ecosystem-based approach to managing marine systems.Broad-scale distribution of the winter protozooplankton community in the North Sea.publishedVersio

Role of protozooplankton in the diet of North Sea autumn spawning herring (Clupea harengus) larvae

The role of small prey (< 200 µm) in larval marine fish nutrition is largely understudied. Here, we explore the contribution of protozooplankton (PZP 20–200 µm) to larval fish diets, compared to metazoan microzooplankton (MZP 55–200 µm). More specifically, we tested whether the contribution of PZP increased during the low productivity season and decreased as larvae grow. We used North Sea autumn spawning herring (Clupea harengus) as a case study, as it is a key species with high commercial and ecological importance. In autumn and winter, the potential PZP and MZP prey was dominated by cells < 50 µm (mainly Gymnodiniales, Pronoctiluca pelagica,Tripos spp. and Strombidium spp.), while copepod nauplii and copepodites where more abundant in autumn than in winter. Based on their trophic enrichment (∆15 N), larvae preferentially grazed on small MZP < 50 µm rather than PZP both in autumn and winter. Larvae of different body size (range 8–14 mm standard length) fed at the same trophic level but on different prey (similar δ15N but different δ13C). Growth rates (based on RNA/DNA estimates) were similar in autumn and winter, suggesting that growth was not affected by station-specific differences in the composition of the prey field. Our results not only underscore the important role of MZP on larval herring diets both in autumn and wintertime, but also emphasize the limitations of bulk stable isotope analysis. Given the current low recruitment in North Sea herring, these results provide significant information for future monitoring approaches relevant to stock assessment of this species

Role of protozooplankton in the diet of North Sea autumn spawning herring (Clupea harengus) larvae

The role of small prey (<200 µm) in larval marine fsh nutrition is largely understudied. Here, we explore the contribution of protozooplankton (PZP 20–200 µm) to larval fsh diets, compared to metazoan microzooplankton (MZP 55–200 µm). More specifcally, we tested whether the contribution of PZP increased during the low productivity season and decreased as larvae grow. We used North Sea autumn spawning herring (Clupea harengus) as a case study, as it is a key species with high commercial and ecological importance. In autumn and winter, the potential PZP and MZP prey was dominated by cells<50 µm (mainly Gymnodiniales, Pronoctiluca pelagica, Tripos spp. and Strombidium spp.), while copepod nauplii and copepodites where more abundant in autumn than in winter. Based on their trophic enrichment (∆15 N), larvae preferentially grazed on small MZP<50 µm rather than PZP both in autumn and winter. Larvae of diferent body size (range 8–14 mm standard length) fed at the same trophic level but on diferent prey (similar δ15N but diferent δ13C). Growth rates (based on RNA/ DNA estimates) were similar in autumn and winter, suggesting that growth was not afected by station-specifc diferences in the composition of the prey feld. Our results not only underscore the important role of MZP on larval herring diets both in autumn and wintertime, but also emphasize the limitations of bulk stable isotope analysis. Given the current low recruitment in North Sea herring, these results provide signifcant information for future monitoring approaches relevant to stock assessment of this species

Role of protozooplankton in the diet of North Sea autumn spawning herring (Clupea harengus) larvae

The role of small prey (<200 µm) in larval marine fsh nutrition is largely understudied. Here, we explore the contribution of protozooplankton (PZP 20–200 µm) to larval fsh diets, compared to metazoan microzooplankton (MZP 55–200 µm). More specifcally, we tested whether the contribution of PZP increased during the low productivity season and decreased as larvae grow. We used North Sea autumn spawning herring (Clupea harengus) as a case study, as it is a key species with high commercial and ecological importance. In autumn and winter, the potential PZP and MZP prey was dominated by cells<50 µm (mainly Gymnodiniales, Pronoctiluca pelagica, Tripos spp. and Strombidium spp.), while copepod nauplii and copepodites where more abundant in autumn than in winter. Based on their trophic enrichment (∆15 N), larvae preferentially grazed on small MZP<50 µm rather than PZP both in autumn and winter. Larvae of diferent body size (range 8–14 mm standard length) fed at the same trophic level but on diferent prey (similar δ15N but diferent δ13C). Growth rates (based on RNA/ DNA estimates) were similar in autumn and winter, suggesting that growth was not afected by station-specifc diferences in the composition of the prey feld. Our results not only underscore the important role of MZP on larval herring diets both in autumn and wintertime, but also emphasize the limitations of bulk stable isotope analysis. Given the current low recruitment in North Sea herring, these results provide signifcant information for future monitoring approaches relevant to stock assessment of this species