The role of biodiversity for ecosystem functions in polar sea-ice ecosystems

Abstract

In the Arctic Ocean, changes caused by global climate warming have been the focus of research for about two decades. Especially the decline and loss of sea ice, but also the increasing inflow of Atlantic Water into the Arctic Ocean or the northward shift of sea-ice formation, are studied processes related to climate change. Those physical changes will undoubtedly affect the Arctic marine ecosystem. Our knowledge of this unique ecosystem, though, is still incomplete, which makes it difficult to assess the consequences of ongoing climate change. More recently, the research focus has been on the importance of ice algae and phytoplankton for the Arctic marine food web. As the main primary producers, they constitute an important food source for many ice-associated (sympagic) species that rely on the ice-algal bloom in spring and the phytoplankton bloom in summer. Sympagic species are important transmitters of carbon from the sea-ice to pelagic and benthic communities. In order to assess the consequences of environmental changes for sympagic communities, we need to broaden our basic understanding of underlying ecosystem functions, such as biomass or carbon cycling.

Following this goal, we used a unique approach to measure physical parameters and sample sympagic organisms of the sea-ice and under-ice environment in the Eurasian Basin. We combined sea-ice coring and trawling with the Surface and Under-ice Trawl. The latter is equipped with an array of sensors to measure environmental parameters, e.g., sea-ice thickness, water temperature, salinity, and chlorophyll a concentration, whilst collecting fauna.

The overarching aim of this study is to further our understanding of the Arctic sympagic ecosystem regarding its biodiversity and related ecosystem functions in the Eurasian Basin. Specific objectives are to 1) generate a quantitative inventory of the biodiversity, community structure, and abundance of sea-ice meiofauna and under-ice fauna, 2) characterize physical habitat properties over large scales to identify environmental parameters of the sympagic environment that structure both sympagic communities, 3) assess ecosystem functions (biomass, production, consumption) of the sympagic communities with a focus on carbon budgets and food web efficiency.

Chapter I adresses objectives 1) and 2) as it provides a quantitative inventory of sea-ice meiofauna and under-ice fauna taxa in the Eurasian Basin in spring 2015. The dominating taxa were Harpacticoida for the sea-ice and Calanus species for the under-ice community. Except for a hyperabundance event, also other changes in community composition compared to earlier studies could be detected. Geographical regions were identified as the most relevant parameter in structuring both sympagic communities. Chapter II focuses on objective 2). It provides a set of large-scale physical and biological sea-ice and under-ice water properties of both hemispheres. The results of the two relevant expeditions for this thesis constituted the base for the assessment of correlations between environmental parameters and community composition of the sympagic fauna for Chapter I and Chapter IV. Chapter III addresses objective 3) as it provides a compilation of the carbon budget (carbon biomass, demand, and production rates) of the sympagic community in the Eurasian Basin in spring 2015. The carbon produced by ice algae and phytoplankton was more than sufficient to cover the demand of the sympagic grazers. The under-ice fauna, especially Calanus species, was the main contributor to the cryo-pelagic carbon flux, whereas sea-ice meiofauna played a minor role. The secondary production was hardly sufficient for the carnivorous under-ice fauna, mainly Chaetognatha, in this study and could therefore force them to evade to deeper layers to fulfill their carbon demand. Chapter IV complements objective 3) as it summarizes the findings of another SUIT study from the Eurasian Basin in summer 2012 and gives estimations on the total carbon budget of the under-ice fauna. Geographical regions were identified as the main parameter in structuring the under-ice community, which addresses objective 2) and supports our findings in Chapter I. Chapter V addresses objective 3). The generation of important allometric equations of key taxa of the sympagic ecosystem constituted the groundwork of Chapter V, which was used for some of the biomass estimations in Chapter III and Chapter IV. The equations can be used in future studies and warrant accurate biomass estimates.

In summary, our results show that the sympagic fauna comprises a mixture of taxa of sea-ice, pelagic, or benthic origin. The under-ice fauna indicated a switch to predominantly boreal Atlantic species attributed to the ongoing Atlantification. Loss of former abundant or dominant taxa was detected for both sympagic communities, likely caused by environmental changes due to climate warming. The sympagic community was mainly structured by geographical regions, but also sea-ice thickness played a role. Primary production was sufficient to fulfill the carbon needs of the sympagic grazers and sea-ice algae constituted an important carbon source. The secondary production was hardly sufficient for the demand of carnivorous under-ice taxa, leading to the assumption that predators need to evade to deeper layers to fulfill their carbon demand. Due to the close connectivity of the sympagic and the pelagic communities, the mentioned changes will consequently impact the entire ecosystem.