Description: We conducted a year-round mesozooplankton study in the Arctic Kongsfjord from August 1998 until July 1999 to investigate seasonal abundance and vertical as well as stage distributions of the prevalent taxa. It is the first investigation in Kongsfjord that covers the Arctic winter season and provides reasonable estimates also of small-sized copepod species. Abundant smaller copepods comprised Oithona similis, Pseudocalanus minutus, Microcalanus spp., Triconia borealis and Acartia longiremis. Among the larger copepods, Calanus finmarchicus, C. glacialis, C. hyperboreus and Metridia longa dominated. The thecosome pteropod Limacina helicina was also an important component. Abundance maxima occurred in November (988,669 ind. m−2) with one to two orders of magnitude higher numbers as compared to all other months (39,832–200,067 ind. m−2). The summers of 1998 and 1999 were characterized by intrusions of Atlantic water, but the community was not entirely dominated by advected boreal species. During winter, the majority of the mesozooplankton occurred below 100 m. Advection is the most likely reason for the accumulation of zooplankton at depth in winter, but local production may also contribute to high overwintering numbers. Much lower abundances of most species in spring suggest high winter mortality and emphasize the importance of sufficient reproductive success during the previous summer to ensure enough winter survivors as seed stock for the coming reproductive season. This study was conducted prior to the recent warming trend in the Arctic. Therefore, it provides valuable baseline data and allows comparing present and future states of the zooplankton community in Kongsfjord.

Description: The Humboldt Current Upwelling System (HCS) is the most productive eastern boundary upwelling system (EBUS) in terms of fishery yield on the planet. EBUSs are considered hotspots of climate change with predicted expansion of mesopelagic oxygen minimum zones (OMZs) and related changes in the frequency and intensity of upwelling of nutrient-rich, low-oxygen deep water. To increase our mechanistic understanding of how upwelling impacts plankton communities and trophic links, we investigated mesozooplankton community succession and gut fluorescence, fatty acid and elemental compositions (C, N, O, P), and stable isotope (δ13C, δ15N) ratios of dominant mesozooplankton and microzooplankton representatives in a mesocosm setup off Callao (Peru) after simulated upwelling with OMZ water from two different locations and different N:P signatures (moderate and extreme treatments). An oxycline between 5 and 15 m with hypoxic conditions (〈50 µmol L−1) below ∼10 m persisted in the mesocosms throughout the experiment. No treatment effects were determined for the measured parameters, but differences in nutrient concentrations established through OMZ water additions were only minor. Copepods and polychaete larvae dominated in terms of abundance and biomass. Development and reproduction of the dominant copepod genera Paracalanus sp., Hemicyclops sp., Acartia sp., and Oncaea sp. were hindered as evident from accumulation of adult copepodids but largely missing nauplii. Failed hatching of nauplii in the hypoxic bottom layer of the mesocosms and poor nutritional condition of copepods suggested from very low gut fluorescence and fatty acid compositions most likely explain the retarded copepod development. Correlation analysis revealed no particular trophic relations between dominant copepods and phytoplankton groups. Possibly, particulate organic matter with a relatively high C:N ratio was a major diet of copepods. C:N ratios of copepods and polychaetes ranged 4.8–5.8 and 4.2–4.3, respectively. δ15N was comparatively high (∼13 ‰–17 ‰), potentially because the injected OMZ source water was enriched in δ15N as a result of anoxic conditions. Elemental ratios of dinoflagellates deviated strongly from the Redfield ratio. We conclude that opportunistic feeding of copepods may have played an important role in the pelagic food web. Overall, projected changes in the frequency and intensity of upwelling hypoxic waters may make a huge difference for copepod reproduction and may be further enhanced by varying N:P ratios of upwelled OMZ water masses.

Description: In this chapter, the effects of temperature change—as a main aspect of climate change—on marine biodiversity are assessed. Starting from a general discussion of species responses to temperature, the chapter presents how species respond to warming. These responses comprise adaptation and phenotypic plasticity as well as range shifts. The observed range shifts show more rapid shifts at the poleward range edge than at the equator-near edge, which probably reflects more rapid immigration than extinction in a warming world. A third avenue of changing biodiversity is change in species interactions, which can be altered by temporal and spatial shifts in interacting species. We then compare the potential changes in biodiversity to actual trends recently addressed in empirical synthesis work on local marine biodiversity, which lead to conceptual issues in quantifying the degree of biodiversity change. Finally we assess how climate change impacts the protection of marine environments.