The microzooplankton community during winter in NE Atlantic waters and its potential impact on condition and growth of larval Atlantic herring (Clupea harengus)

Abstract

Microzooplankton is the planktonic size fraction between 20 and 200 μm consisting of unicellular (protozooplankton) and metazoan organisms, such as the early life stages of copepods and other invertebrate larvae. Protozooplankton (PZP) is a key player in the marine carbon cycle as part of the so-called microbial loop. Within the microbial loop, carbon is recycled through a “bacteria - small phytoplankton - PZP – loop” back into the classical food chain. During periods of low productivity, such as the winter in the NE Atlantic, more carbon is cycled via the microbial loop compared to the classical, linear food chain. While future scenarios predict more beneficial conditions for small-sized cells and, hence, a larger contribution of microbial loop dynamics, it is important to investigate trophodynamics occurring within these planktonic size fractions. For instance, PZP as well as metazoan microzooplankton, serve as a food source for early life stages of fish. Despite the recognition of the importance of microzooplankton the microzooplankton-ichthyoplankton link remains poorly studied. This thesis elucidates the autumn and winter community composition of PZP in the NE Atlantic and uses the larvae of autumn spawning Atlantic herring (Clupea harengus) as a case study to investigate the impact of the components of this small plankton size fraction on the distribution, condition and growth of overwintering ichthyoplankton. Manuscript 1 describes the findings of a broad scale field study on the winter PZP community in the North Sea. The focus was set on the taxonomic composition of the main PZP components, the hetero- and mixotrophic taxa from the phyla Ciliophora (ciliates) and Dinophyta (dinoflagellates). The dominance of small sized cells (<50 μm) in the community suggested that the microbial loop played a pronounced role in fueling the productivity of higher trophic levels. The maximum PZP carbon biomass found was approximately 50 times lower than during bloom conditions. However, highest biomass was observed in southern areas, which are known to be important winter spawning grounds of certain fish species. This work highlights how this type of sampling can be added to a pre- existing survey for fishery assessment. The potential importance of the microzooplankton-ichthyoplankton link to autumn spawning herring in the Irish Sea was highlighted in Manuscript 2 using a combination of field sampling, taxonomic plankton identification, biochemical analysis and physiological-based modelling. In contrast to plankton distribution patterns in spring and summer the highest PZP biomass was found within the main herring spawning sites. Spatial differences in the taxonomic composition of the protozoan community were strongly related to hydrographic characteristics. Statistical analysis indicated a relationship of small copepods as well as dinoflagellates with larval herring abundance and, thus, it can be assumed that grazing dynamics were complex in the lower trophic food web. The small prey size (<200 μm) was implemented for the first time into a foraging and growth model. Comparing the modelled with observed in-situ growth rates indicated an important role of PZP for larval growth. To further resolve the role of microzooplankton for larval fish, the main potential prey sources and their impact on larval nutritional condition (RNA/DNA) and growth was determined using stable isotope analysis (δ13C and δ15N) (Manuscript 3). A comparison of early feeding larvae (8-14 mm) and their potential prey in two different autumn and winter spawning grounds in 2013 and 2014 in the North Sea showed differences in the isotopic composition, depending on season, geographic location or on the sampling year. The trophic level of the autumn as well as the winter spawned larvae suggested a diet based predominantly on prey bigger than 50 μm. The results of the present thesis demonstrated the potential importance of microzooplankton, particularly PZP, to the winter carbon cycle. Additionally, the abundance and composition of this plankton size fraction may directly or indirectly impact on the condition and growth of larval herring and, thus, potentially influence survival and recruitment dynamics. The results presented in this thesis demonstrate the need of implementing this kind of data into ecosystem and food web models. Furthermore, to unravel and better understand processes influencing the growth and survival of in early life stages of fish, and potentially recruitment success, and to advance the ecosystem approach to fisheries management, further emphasize should be placed on investigating the microzooplankton-ichthyoplankton link.