Background: With changing seawater temperatures, the bloom dynamics and succession patterns of monocellular algal species change. As they comprise the base of the food pyramid, the complex trophic structures in the pelagial may also change, which will ultimately affect fishes, birds and mammals. In the North Sea, several plankton regime shifts have been observed in relation to temperature (see Alvarez-Fernandes et al. 2012). In a cold period that occurred in the late 1970s, the phytoplankton bloom was comparatively small, it occurred late in the season and early diatom species were lacking (Edwards et al. 2002). Wiltshire and Manly (2004) showed that, associated with warming of the water since the late 1980s, the diatom bloom had been delayed at  the Helgoland time series station. Alvarez-Fernandez et al. (2012), who have analyzed recent continuous plankton recorder data from the entire North Sea, have detected that dinoflagellate abundance decreased over the last decade. All this implies that zooplankton organisms, which feed primarily on these algal groups, have to cope with severe changes in timing of occurrence (match/mismatch), quantity and quality of their food items.

 

In the North Sea, calanoid copepods dominate the zooplankton community for most of the year (Fransz et al. 1991). Most of these species rely on continuous food supply, due to rapid turnover of biomass and low capacity to accumulate lipid reserves (Båmstedt 1986, Helland et al. 2003, Kreibich et al. 2008, 2011). Their growth and reproductive activity is therefore closely linked to bloom events (e.g. Ianora 1998, Halsband and Hirche 2001, Halsband-Lenk et al. 2004) and when algae are scarce, some of the copepods such as Temora longicornis feed on heterotrophic prey to sustain their metabolic needs (e.g. Gentsch et al., 2009). Seasonally, meroplanktonic larvae may occur in high abundances (Fransz et al. 1991) and they also have only limited capacity to sustain food limitation. Changes in phytoplankton availability and nutritional quality thus directly influence zooplankton population dynamics. 

 

Objectives and work programme: The proposed MARES Ph.D. project, for which Bremen University (BU), AWI and Ghent University (GU) will merge their complementary expertise and infrastructure, will focus on the effects of a changing food regime on the performance of dominant copepod and meroplankton species (e.g. polychaete larvae, decapod larvae). Objectives are (i) to quantify the energy budget in relation to food quality and temperature and, thus, tackle the question whether and how late or failed diatom spring blooms affect growth and reproduction and (ii) to elucidate the allocation of dietary components. The project will be organized in four alternating work units including (1) field and experimental work either onboard or at the AWI field station at Helgoland, (2) analyzes of fatty acid compositions, (3) stable isotope analyzes and (4) determination of enzyme activities and patterns. Zooplankton sampling and experimental work (work unit 1) will be conducted at AWI studying the response of selected zooplankton species from the North Sea to different food sources. We will follow the uptake, digestion, assimilation and excretion of dietary material by means of a combination of classical and modern methods. Grazing, fecal pellet and egg production rates will be measured in incubation experiments. Respiration rates will be determined via optodes. Algal cells will be enriched with stable isotopes (δ13C) and fed to the copepods. With assimilation, δ13C will accumulate in the zooplankton organisms - a process that can be followed by compound-specific stable isotope analysis, which will be performed in cooperation with Ghent University (work unit 2). Changes in fatty acid compositions due to feeding will be analyzed at Bremen University (work unit 3) and the activities of metabolic and digestive enzymes (e.g. citrate synthase, lipases, proteinases) will be carried out at AWI. Each of the work units will cover approximately six months. In combining these methods we will aim at a thorough understanding of processes involved in the digestion and assimilation of different dietary compounds and, thus, elucidate the physiological capacity of dominant zooplankton species to adapt to a changing food regime.

 

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